U.S. patent application number 15/436432 was filed with the patent office on 2017-08-17 for techniques for on-demand production of medical isotopes such as mo-99/tc-99m and radioactive iodine isotopes including i-131.
The applicant listed for this patent is Global Medical Isotope Systems LLC. Invention is credited to Francis Yu-Hei Tsang.
Application Number | 20170236607 15/436432 |
Document ID | / |
Family ID | 44911763 |
Filed Date | 2017-08-17 |
United States Patent
Application |
20170236607 |
Kind Code |
A1 |
Tsang; Francis Yu-Hei |
August 17, 2017 |
TECHNIQUES FOR ON-DEMAND PRODUCTION OF MEDICAL ISOTOPES SUCH AS
MO-99/TC-99M AND RADIOACTIVE IODINE ISOTOPES INCLUDING I-131
Abstract
A system and method for radioisotope production uses
fast-neutron-caused fission of depleted or naturally occurring
uranium targets in an irradiation chamber. Fast fission can be
enhanced by having neutrons encountering the target undergo
scattering or reflection to increase each neutron's probability of
causing fission (n, f) reactions in U-238. The U-238 can be
deployed as one or more layers sandwiched between layers of
neutron-reflecting material, or as rods surrounded by
neutron-reflecting material. The gaseous fission products can be
withdrawn from the irradiation chamber on a continuous basis, and
the radioactive iodine isotopes (including I-131) extracted.
Inventors: |
Tsang; Francis Yu-Hei; (Las
Vegas, NV) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Global Medical Isotope Systems LLC |
Henderson |
NV |
US |
|
|
Family ID: |
44911763 |
Appl. No.: |
15/436432 |
Filed: |
February 17, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14627977 |
Feb 20, 2015 |
9576691 |
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15436432 |
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12944694 |
Nov 11, 2010 |
8989335 |
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14627977 |
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61405605 |
Oct 21, 2010 |
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61265383 |
Dec 1, 2009 |
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61260585 |
Nov 12, 2009 |
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Current U.S.
Class: |
376/186 |
Current CPC
Class: |
G21G 1/08 20130101; G21G
4/02 20130101; G21G 2001/0042 20130101; G21G 1/06 20130101; G21G
2001/0036 20130101; G21G 2001/0063 20130101; G21G 1/001
20130101 |
International
Class: |
G21G 1/08 20060101
G21G001/08; G21G 1/00 20060101 G21G001/00 |
Claims
1. A method for producing radioisotopes comprising: introducing
non-enriched uranium ("NEU") material into a an irradiation
chamber, the irradiation chamber having one or more walls formed of
neutron-reflecting material; irradiating the NEU material with
neutrons having energies above 800 keV to cause fast fission
reactions to occur in the NEU material and generate fission
products, wherein: at least some neutrons from the irradiating are
reflected from at least one of the one or more walls, thereby
increasing the path length over which those neutrons are in the NEU
material, and the increased path length increases the probability
that those neutrons in the NEU material will cause fast fission
reactions; and extracting the fission products from the NEU
material.
2. The method of claim 1 wherein one of the fission products
extracted comprises at least one of molybdenum-99 (Mo-99) and
technetium-99m (Tc-99m).
3. The method of claim 1 wherein one of the fission products
extracted comprises at least one of iodine 131 (I-131) and iodine
132 (I-132).
4. The method of claim 1 wherein the NEU material in the
irradiation chamber occupies a single spatially contiguous
region.
5. The method of claim 1 wherein the NEU material in the
irradiation chamber occupies multiple spatially disjoint
regions.
6. The method of claim 1 wherein the one or more walls formed of
neutron-reflecting material comprise at least one internal wall of
the irradiation chamber.
7. The method of claim 1 wherein the one or more walls formed of
neutron-reflecting material comprise an outer wall that surrounds
all the NEU material in the irradiation chamber.
8. The method of claim 1 wherein the one or more walls formed of
neutron-reflecting material comprise: at least one internal wall of
the irradiation chamber; and an outer wall that surrounds all the
NEU material in the irradiation chamber.
Description
[0001] INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS
[0002] Any and all applications for which a foreign or domestic
priority claim is identified in the Application Data Sheet as filed
with the present application are hereby incorporated by reference
under 37 CFR 1.57.
BACKGROUND OF THE INVENTION
[0003] Field of the Invention
[0004] The present invention relates generally to the generation of
unstable, i.e., radioactive, nuclear isotopes (often referred to as
radioisotopes), and more particularly to techniques for generating
medical isotopes such as molybdenum-99 (Mo-99) and its decay
daughter technetium-99m (Tc-99m), and radioactive iodine such as
iodine-131 (I-131).
[0005] Description of the Related Art
[0006] Radioisotopes, in very small doses, are widely used in
clinical therapy (radiation treatments) for such diseases as cancer
and hyperthyroidism, as well as diagnostics using the ability to
image regions where radioisotopes concentrate in the subject's
body. Currently, nearly 80% of all nuclear imaging procedures
utilize Tc-99m, making it a very important isotope for diagnostic
medicine. Molecules and proteins that concentrate in specific areas
of the body can be tagged with Tc-99m, which decays to a ground
state through emission of a low energy gamma-ray, and observed from
outside the body using gamma-ray cameras or detectors. This method
allows "active" areas, or regions where the Tc-99m tagged compound
concentrates, to be observed in 3-D from outside the body.
[0007] With a high demand for medical procedures involving the use
of Tc-99m and radioactive iodine such as 1-131, a demand that is
only expected to increase as the U.S. population ages, reliability
of the Tc-99m and I-131 supplies is critical. A major obstacle to a
reliable source is the fact that 100% of the U.S. supply is
imported from foreign reactors. The U.S. supply is sourced almost
entirely from the NRU Reactor in Canada (Chalk River) and the HFR
in the Netherlands, and both reactors are over 40 years old. The
rapid decay of the Mo-99 means that product must be shipped and
used immediately with no long term storage possible. Any
interruptions in supply, even brief periods such as a reactor
shutting down for maintenance, can cause shortages and patient
treatment delays. Real shortages have occurred as recently as 2007
and 2008 when the NRU Reactor and HFR, respectively, were shut down
for a period of time.
[0008] The thyroid is a gland in the neck that produces two
hormones that regulate all aspects of the body's metabolism, the
chemical process of converting food into energy. When a thyroid
gland is overactive, it produces too much of these thyroid
hormones, accelerating the metabolism. The condition is called
hyperthyroidism, and can be caused by Graves' disease, which causes
the entire thyroid to be overactive, or by nodules within the
thyroid which are locally overactive.
[0009] Radioactive iodine I-131 (sometimes referred to as 131I or
radioiodine I-131) is another important radioisotope. It has a
half-life of 8.02 days and is used for a variety of applications.
These include diagnostic and therapeutic thyroid applications (in
either a solution or capsule form), industrial tracers, and various
research applications such as antibody labeling. I-131 is also used
to label antibodies for therapeutic applications in the treatment
of cancers.
[0010] Traditional methods rely on thermal fission of targets made
of highly enriched uranium (HEU). HEU is uranium that has been
processed to greatly increase the percentage of fissionable U-235
above the approximately 0.7% level, found in naturally occurring
uranium, to levels above 93%. Thermal fission refers to the
irradiation of a target by low-energy ("thermal") neutrons, causing
fission to occur. Currently, the U.S. exports more than 50 kg of
HEU having more than 93% U-235 to at least five foreign nuclear
reactors for irradiation and extraction of the Mo-99/Tc-99m and
other medical isotopes. The proliferation potential and hazards
associated with shipping fresh HEU and spent HEU are obvious, but
current production of Mo-99/Tc-99m relies on this process.
[0011] The spent HEU target material is also a threat because only
between 1-3% of the U-23 5 in the HEU target is burned up and the
remaining target material can still contain 92% enriched U-235.
Also, because of the low burn-up, after three-year storage, the HEU
target materials can be essentially contact handled, meaning that
due to its relatively low burn up, the amount of long-lived fission
products in the spent HEU target material are minimal. The spent
HEU target can be handled and processed relatively easy with
minimal shielding materials to protect the proliferators.
[0012] Alternative techniques have been proposed, but they are
thought to be significantly less cost effective and many technical
challenges remain. One such proposal is to transition to a lower
level of enrichment of the U-235 target (LEU), say below 20% U-235,
but this still presents the same problems as HEU, including the
need for a nuclear reactor. Another method proposed is to utilize
neutron capture in Mo-98, which can be mined as ore. However,
natural molybdenum contains on the order of 24.1% Mo-98, so targets
are likely to require enrichment prior to irradiation.
[0013] Other techniques proposed for production of Mo-99 include
causing (p, 2n) or (p, pn) reactions in an Mo-100 target using a
proton accelerator (cyclotron). Again, natural molybdenum contains
on the order of 9.64% of Mo-100, so targets for cyclotrons are also
likely to require enrichment prior to irradiation. It has also been
proposed to cause (.gamma., f) reactions on U-235 or LEU, or U-238
or a combination of all three materials via bremsstrahlung
radiation produced from a high-energy electron accelerator.
[0014] The proposed alternative methods using particle accelerators
all have similar problems: [0015] They all require large and
enriched isotopic targets. [0016] They all require heat removal
from the targets during irradiation, which represents a technical
challenge. [0017] The Mo-99 produced must be purified to remove
unused molybdenum isotopes and other fission products and
activation by-products. [0018] They all require development of fast
dissolution methods for the metallic targets. [0019] Treatment and
disposal of the waste fission products and waste uranium present
significant challenges (for LEU). Summary of the Invention
SUMMARY OF THE INVENTION
[0020] Embodiments of the present invention provide techniques for
the production of radioactive iodine isotopes. Radioisotopes that
are useful in the field of medicine are sometimes referred to as
medical isotopes, although some stable isotopes have potential
medical uses and are sometimes referred to as stable medical
isotopes. The techniques provided by the invention overcome at
least some of the problems discussed above. Embodiments do not rely
on a nuclear reactor far from the delivery site but can be
implemented as relatively small stand-alone devices that can be
widely distributed.
[0021] In describing embodiments of the invention, the following
terms are sometimes used: [0022] Non-enriched uranium ("NEU");
[0023] Neutron-reflecting material; [0024] Fast neutrons; [0025]
Fast neutron fission reactions; and [0026] Neutron generator.
[0027] As will now be discussed, these terms are defined
broadly.
[0028] The term "non-enriched uranium" ("NEU") is intended to cover
naturally occurring uranium or depleted uranium, in addition to any
uranium that contains at least as much U-238 as naturally occurring
uranium (99.27%) and no more U-235 than naturally occurring uranium
(0.72%). Depleted uranium is normally understood to mean uranium
that has less than the naturally occurring amount of U-235 (0.72%),
but depleted uranium that is used for commercial and military
purposes more commonly has less than 0.3% U-235.
[0029] The definition of NEU is not limited to any form of the
uranium, so long as the isotope content meets the above criteria.
The NEU material, also referred to as the NEU feedstock or the NEU
target, can be in the form of bulk solid material, crushed solid
material, metallic shavings, metallic filings, sintered pellets,
liquid solutions, molten salts, molten alloys, or slurries. The
NEU, whatever its form, can also be mixed with other material that
is compatible with the intended use.
[0030] The term "neutron-reflecting material" is intended to cover
material that reflects or scatters neutrons. While it is preferred
that the scattering be elastic, or largely so, this is not
necessary for the definition. Further, while some of the
embodiments use neutron-reflecting material formed into solid
structural shapes such as plates, spherical shells, cylindrical
shells, tubes, and the like, the term is intended to cover material
that includes small particles such as powders, pellets, shavings,
filings, and the like.
[0031] The term "fast neutron" is often used to distinguish thermal
neutrons, which Wikipedia characterizes as having energies of "of
about 0.025 eV." Wikipedia also characterizes fast neutrons as
having energies "greater than 1 eV, 0.1 MeV or approximately 1 MeV,
depending on the definition." For present purposes, the term "fast
neutron" will mean a neutron with an energy above 800 keV (i.e.,
0.8 MeV), which is a threshold for fission in U-238. However,
embodiments of the present invention can use neutrons of higher
energies, say 10-20 MeV, or possibly 12-16 MeV. Higher-energy
neutrons, say in the 20-100 MeV range, can also be used.
[0032] The term "fast neutron fission" is intended to cover the
fission reactions that are caused by neutrons with energies that
are above the threshold of 800 keV. The reaction representation (n,
f) is used for simplicity.
[0033] The term "neutron generator" is intended to cover a wide
range of devices and processes for generating neutrons of the
desired energies. Wikipedia defines neutron generators as follows:
[0034] Neutron generators are neutron source devices which contain
compact linear accelerators and that produce neutrons by fusing
isotopes of hydrogen together. The fusion reactions take place in
these devices by accelerating either deuterium, tritium, or a
mixture of these two isotopes into a metal hydride target which
also contains either deuterium, tritium or a mixture. Fusion of
deuterium atoms (D+D) results in the formation of a He-3 ion and a
neutron with a kinetic energy of approximately 2.45 MeV. Fusion of
a deuterium and a tritium atom (D+T) results in the formation of a
He-4 ion and a neutron with a kinetic energy of approximately 14.1
MeV. Fusion of a triton and a tritium atom (T+T) results in the
formation of a He-4 ion and two neutrons. These two neutrons can
have an energy range from below 0.1 eV to .about.9.33 MeV.
[0035] As used in this application, however, the term "neutron
generator" is defined more broadly to include any device that would
provide a sufficient number of neutrons of the desired energies.
This could include, for example, but is not limited to, the
following. [0036] A dense plasma focusing device can use deuterium
or tritium plasma to produce 2.45 MeV neutrons, 14.1 MeV neutrons,
or neutrons covering a broad spectrum (below 0.1 eV to .about.9.33
MeV). [0037] An electron accelerator can be used to send high
energy electrons from an electron beam onto a converter material,
e.g., tantalum (Ta), tungsten (W), etc., thereby converting the
electron energy into bremsstrahlung radiation. This bremsstrahlung
radiation can then be used to interact with neutron-rich materials
to produce neutrons via (y, n) interactions. For example,
irradiating the beryllium isotope Be-9 with y rays can produce a
beryllium isotope with a lower atomic mass and one or two neutrons
(the reactions being denoted Be-9(.gamma., n)Be-8, or Be-9(.gamma.,
2n)Be-7). [0038] A proton accelerator such as a cyclotron can be
used to send high energy protons into materials such as carbon,
beryllium, or lithium, for example, to produce neutrons via C-12(p,
n)N-12, or Be-9(p, n)B-9 reactions, for example.
[0039] In short, embodiments of the present invention use
fast-neutron-caused fission of depleted or naturally occurring
uranium targets in an irradiation chamber. A generic term for such
uranium is non-enriched uranium ("NEU"). U-238, is fissionable in
that it can be made to fission when struck by fast neutrons, i.e.,
neutrons having energies above a fission threshold. It is not
fissile in that it cannot sustain a chain reaction. This is because
when U-238 undergoes fission, neutrons resulting from the fission
are generally below the energy threshold to cause more U-238
fission.
[0040] The purpose of causing fission is to generate and extract
fission products that are, or decay to, desired radioisotopes.
Embodiments will be described in the context of extracting
radioactive iodine isotopes, but this is exemplary. Since the
fission products include Mo-99/Tc-99m, some embodiments can also
extract Mo-99/Tc-99m as well.
[0041] In this application, the term "fission product" will be used
as set forth in the NRC glossary
(http://www.nrc.gov/reading-rm/basic-ref/glossary.html), which
defines "fission products" as "[t]he nuclei (fission fragments)
formed by the fission of heavy elements, plus the nuclide formed by
the fission fragments' radioactive decay." Thus the term is used
more broadly than a definition that would cover only the nuclei
resulting directly from the fission reaction.
[0042] This interpretation is consistent with the glossary at
http://www.nuclearglossary.com ("The Language of the Nucleus"),
which defines "fission product" as "[a]residual nucleus formed in
fission, including fission fragments and their decay daughters."
The term "fission fragment" is defined as "[a]nucleus formed as a
direct result of fission. Fission products formed by the decay of
these nuclides are not included." The term "primary fission
product" is said to be a synonym for "fission fragment."
[0043] In an aspect of the invention, a method for producing
radioactive iodine comprises introducing uranium into an
irradiation chamber, the irradiation chamber having a gas inlet and
a gas outlet; irradiating the uranium material with neutrons to
cause fission reactions to occur in the uranium material, wherein
at least some of the fission reactions lead to the production of
radioactive iodine, and at least some of the radioactive iodine
sublimates in the irradiation chamber; introducing a carrier gas
into the gas inlet to mix with the sublimated radioactive iodine
and form a gas mixture; withdrawing the gas mixture from the gas
outlet; and separating at least some of the radioactive iodine from
other components of the gas mixture.
[0044] In some embodiments, the method further includes allowing a
period of time to elapse so that at least one short-lived
radioactive iodine isotope substantially decays to a non-iodine
substance; and separating the non-iodine substance from the
remaining radioactive iodine.
[0045] In some embodiments, the uranium material is non-enriched
uranium material (NEU). In some embodiments, the neutrons are
provided at an energy above a fast fission threshold for U-238. In
some embodiments, at least a portion of the gas mixture is returned
to the gas inlet after separation of the radioactive iodine. In
some embodiments, separating at least some of the radioactive
iodine includes using a silver zeolite trap.
[0046] In some embodiments, the uranium material is removed from
the irradiation chamber after an irradiation period, the uranium
material is dissolved in a solvent, thereby releasing additional
sublimated iodine as part of an additional gas mixture, and at
least some of the additional radioactive iodine is separated from
other components of the additional gas mixture.
[0047] In another aspect of the invention, a method for producing
radioactive iodine comprises providing a volume of non-enriched
uranium ("NEU") material in an irradiation chamber, the irradiation
chamber having a gas inlet and a gas outlet; providing one or more
neutron-reflecting regions disposed in or around the irradiation
chamber; irradiating the uranium material with neutrons to cause
fission reactions to occur in the uranium material, wherein the
neutrons are provided at an energy above a fast fission threshold
for U 238, the one or more neutron-reflecting regions are
configured to increase the path length traveled by at least some of
the neutrons before those neutrons leave the irradiation chamber,
at least some of the fission reactions lead to the production of
radioactive iodine, and at least some of the radioactive iodine
sublimates in the irradiation chamber. The method also comprises,
during irradiation, introducing a carrier gas into the gas inlet to
mix with the sublimated radioactive iodine and form a gas mixture;
withdrawing the gas mixture from the gas outlet; and separating at
least some of the radioactive iodine from other components of the
gas mixture.
[0048] In some embodiments, at least a portion of the gas mixture
is returned to the gas inlet after separation of the radioactive
iodine. In some embodiments, separating at least some of the
radioactive iodine includes using a silver zeolite trap. In some
embodiments, the method further includes, after an irradiation
period, removing the NEU material from the irradiation chamber;
dissolving the NEU material in a solvent, thereby releasing
additional sublimated iodine as part of an additional gas mixture;
and separating at least some of the additional radioactive iodine
from other components of the additional gas mixture.
[0049] In another aspect of the invention, apparatus for producing
radioactive iodine isotopes comprises a neutron generator; an
irradiation chamber that provides a number of regions configured to
receive uranium material for irradiation by neutrons generated by
the neutron generator, and the irradiation chamber includes a gas
inlet and a gas outlet. During irradiation, fission reactions occur
in the uranium material, at least some of the fission reactions
lead to the production of radioactive iodine, and at least some of
the radioactive iodine sublimates in the irradiation chamber. The
gas inlet is configured to allow a carrier gas to be introduced
into the gas inlet to mix with the sublimated radioactive iodine
and form a gas mixture, and the gas outlet is configured to allow
withdrawing the gas mixture from the gas outlet. The apparatus also
includes a mechanism for separating at least some of the
radioactive iodine from other components of the gas mixture.
[0050] In some embodiments, the uranium material is non-enriched
uranium material, and the neutrons are provided at an energy above
a fast fission threshold for U-238. In some embodiments, one or
more neutron-reflecting regions are disposed in or around the
irradiation chamber, with the one or more neutron-reflecting
regions being configured to increase the path length traveled by at
least some of the neutrons before those neutrons leave the
irradiation chamber. The one or more neutron-reflecting regions may
form one or more walls of the irradiation chamber formed of
neutron-reflecting material.
[0051] In another aspect of the invention, apparatus for producing
radioactive iodine isotopes comprises a neutron generator; an
irradiation chamber, the irradiation chamber providing a number of
regions configured to receive uranium material for irradiation by
neutrons generated by the neutron generator, the irradiation
chamber having a gas inlet and a gas outlet. During irradiation,
fission reactions occur in the uranium material, at least some of
the fission reactions lead to the production of radioactive iodine,
and at least some of the radioactive iodine sublimates in the
irradiation chamber. The apparatus further includes a mechanism for
introducing a carrier gas into the gas inlet to mix with the
sublimated radioactive iodine and form a gas mixture; a mechanism
for withdrawing the gas mixture from the gas outlet; and a
mechanism for separating at least some of the radioactive iodine
from other components of the gas mixture.
[0052] Embodiments of the present invention operate to enhance fast
fission in NEU targets by having neutrons undergo scattering or
reflection after passing through a region of NEU. This is
accomplished by having the target material interspersed with what
is referred to as "neutron-reflecting" material, which reflects or
scatters neutrons so that the neutrons travel a longer path before
leaving the target material. This provides more opportunities for
the neutrons to cause fission reactions with the NEU target
material.
[0053] Thus, a given neutron tends to have multiple interactions
(e.g., scattering events) with U-238 nuclei before it causes a
fission reaction or leaves the region or regions occupied by NEU
target material without being absorbed by the U-238. After a number
of scattering events within the NEU target material, a neutron's
energy will drop below the fission threshold. The target material
can be interspersed with neutron-reflecting material according to a
number of different geometrical arrangements.
[0054] The particular form of feedstock typically depends on the
geometry of the irradiation chamber, the characteristics of the
fission products that are to be extracted, and the manner of
extracting the fission products. A preferred feedstock is in the
form of depleted uranium. Depleted uranium is a byproduct of
uranium enrichment and contains over 99.7% of U-238 as compared to
natural uranium, which contains about 99.3% of U-238.
[0055] As well as maintaining the neutron energy above a fast
fission threshold, it is preferred to maintain a sufficiently high
neutron energy to minimize neutron absorption by U-238. When a
neutron with slower energy is captured by U-238, Pu-239 is produced
after subsequent decay of the excited U-238 atom. Using fast
neutron sources instead, the probability of fission of U-238
becomes orders of magnitude higher than the probability of capture,
resulting in greatly reduced production of Pu-239.
[0056] In one set of embodiments, the NEU and neutron-reflecting
material are formed as alternating layers of NEU and
neutron-reflecting material. The layers can take the form of
spherical shells, cylindrical shells, flat plates and the like,
with a fast neutron generator disposed near the center. In these
embodiments, the irradiation chamber is generally spherical,
generally cylindrical, or generally rectangular. Other geometries
such as polygonal cylinders and polyhedrons are also possible, and
may allow easier fabrication.
[0057] In another set of embodiments, the NEU occupies a plurality
of parallel elongate regions with each region surrounded by
neutron-reflecting material. The surrounding neutron-reflecting
material can be formed as a plurality of tubes, and solid rods or
crushed NEU can be disposed in the tubes. The NEU and surrounding
neutron-reflecting material can occupy a cylindrical region with a
hollow center for the fast neutron generator. Thus, in this set of
embodiments, the irradiation chamber is generally cylindrical
(circular or polygonal base).
[0058] In other embodiments, the NEU and neutron-reflecting
material can both be formed as relatively small objects (say a few
centimeters in size), and mixed in solid form or in a slurry. The
slurry can be circulated in tubes surrounding the neutron
generator. Embodiments where the NEU is in molten form, whether or
not interspersed with neutron-reflecting material, can also be
circulated. Circulating the NEU results in even irradiation of all
the NEU in the chamber.
[0059] A further understanding of the nature and advantages of the
present invention may be realized by reference to the remaining
portions of the specification and the drawings, which are intended
to be exemplary and not limiting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0060] FIG. 1 is a stylized high-level schematic of a radioisotope
generator according to an embodiment of the present invention.
[0061] FIG. 2 shows the decay products of Mo-99 after it has been
generated by fast fission of U-238.
[0062] FIG. 3 is a graph of U-238's fission cross section and
neutron capture cross section as functions of neutron energy.
[0063] FIG. 4 is a simplified oblique view of a radioisotope
generator having a spherical irradiation chamber according to an
embodiment of the present invention.
[0064] FIG. 5 is a simplified oblique view of a radioisotope
generator having a cylindrical irradiation chamber according to an
embodiment of the present invention.
[0065] FIG. 6 is a simplified cross-sectional view that applies to
either of the radioisotope generators shown in FIGS. 4 and 5 and is
taken along line 6-6 in FIGS. 4 and 5.
[0066] FIG. 7 is a simplified oblique view of a radioisotope
generator having a rectangular-prism-shaped irradiation chamber
according to an embodiment of the present invention.
[0067] FIG. 8 is a simplified cross-sectional view of the
radioisotope generator shown in FIG. 7 and is taken along line 8-8
in FIG. 7.
[0068] FIG. 9 is a simplified cross-sectional view of a
radioisotope generator where the NEU is in the form of rods.
[0069] FIG. 10 is a process schematic for the generation of
radioisotopes according to an embodiment of the present
invention.
[0070] FIG. 11A is a perspective view of a radioisotope generator
according to a specific embodiment having a cylindrical irradiation
chamber with parallel NEU rods.
[0071] FIG. 11B is a perspective, cutaway view of the radioisotope
generator shown in FIG. 11A with the cover and the outer cylinder
removed.
[0072] FIG. 11C is a perspective cutaway view of the radioisotope
generator shown in FIG. 11A taken from a different view to show
additional details.
[0073] FIG. 12 shows the decay products of I-131 after it has been
generated by fast fission of U-238.
[0074] FIG. 13 is a simplified cross-sectional view of a
radioisotope generator such as that shown in the sectional view of
FIG. 6, with additional details relating to the circulation of gas
through the irradiation chamber.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Overview Of Embodiments
[0075] FIG. 1 is a stylized high-level schematic of a radioisotope
generator 10 according to an embodiment of the present invention,
and is used to illustrate salient features that can be viewed as
generic to the various embodiments discussed below. The main
components of radioisotope generator 10 include an irradiation
chamber 15, a fast neutron generator 20, and, for preferred
embodiments, one or more neutron-reflecting elements 25. The
irradiation chamber is configured to accept a charge of what is
referred to as non-enriched uranium ("NEU").
[0076] As mentioned above, the NEU can be in any suitable form,
including elemental metal, salt, alloy, molten salt, molten alloy,
slurry, or other mixture, and can assume any one of a number of
shapes and states, as will be described below. For purposes of
generality, the NEU is shown as a plurality of arbitrary-shaped
bodies 30 (stippled for clarity). The irradiation chamber is
generally provided with mechanisms for introducing NEU into, and
removing NEU from, the irradiation chamber, such as one or more
fill ports 35a and one or more empty (drain) ports 35b. In some
embodiments, it is desired to flow gas through the irradiation
chamber, and to this end the chamber can be provided with one or
more gas inlet ports 40a and one or more gas outlet ports 40b.
[0077] FIG. 1 also shows a neutron-absorbing outer containment
chamber 45 surrounding the irradiation chamber. In representative
embodiments, this outer containment chamber is an entirely separate
structure from the irradiation chamber and can be built on site
prior to installation of the radioisotope generator. Containment
chamber 45 can be built as a concrete vault or bunker and lined
with special neutron-absorbing material such as borated
polyethylene. It is preferred that the outer containment chamber be
significantly larger than the irradiation chamber so that the
irradiation chamber subtends a relatively small solid angle at any
point on walls of the outer containment chamber. This tends to
reduce the likelihood of any slow neutrons scattered by the outer
containment chamber walls getting back inside the irradiation
chamber.
[0078] The walls of irradiation chamber 15 preferably include at
least a layer of neutron-reflecting material, and so the
combination of irradiation chamber 15's layer of neutron-reflecting
elements, and neutron-reflecting elements 25 can be considered to
define one or more neutron-reflecting regions disposed in or around
the irradiation chamber. Some of the embodiments to be described
below include separate neutron-reflecting elements inside the
chamber, which may have the effect of subdividing the chamber into
disjoint regions occupied by NEU material, while other embodiments
provide a single contiguous region occupied by NEU material.
[0079] For generality, FIG. 1 shows the NEU as separate disjoint
bodies, but the case of a single contiguous body of NEU material
should be considered to fall within the scope of the invention. In
keeping with the schematic nature of FIG. 1, the neutron-reflecting
elements and the NEU bodies are shown as sparsely populating the
irradiation chamber. In most embodiments, however, the regions of
NEU material will generally fill hollow regions within the
irradiation chamber so as to be surrounded by neutron-reflecting
material. However, this does not preclude their being hollow
portions of the chamber having neither neutron-reflecting material
nor NEU material.
[0080] Embodiments of the present invention are not limited to any
particular type of neutron generator. Representative systems can
use a portable and compact accelerator that can accelerate and
direct charged particles, or neutral particles, or deuterons, or
tritons to targets that can be used to produce neutrons with
energies that are above the 800 keV fast neutron fission threshold
energy of U-238. The target materials can be elements, compounds,
or solutions. Other target materials also can contain materials or
compounds that are enriched with tritium atoms. Specifically, the
fusing of deuterium atoms with tritium atoms produces 14.1-MeV
neutrons which are used to cause fast fission reactions in the
uranium atoms. When tritons are accelerated and fused with targets
enriched with tritium atoms, the produced neutrons can encompass
energies from below 0.1 eV to .about.9.3 MeV.
[0081] Suitable neutron generators are commercially available, for
example from Adelphi Technology Inc., 2003 East Bayshore Rd,
Redwood City, Calif. 94063, Halliburton, 10200 Bellaire Blvd.,
Houston, Tex. 77072, and Schlumberger Technology, 300 Schlumberger
Dr., Sugar Land, Tex. 77478.
[0082] The operation of radioisotope generator 10 can be summarized
as follows. Neutron generator 20 provides neutrons above a fission
threshold for U-238. The purpose of causing fission is to generate
and extract one or more fission products that are, or decay to,
desired radioisotopes.
[0083] FIG. 2 shows the decay products of Mo-99 (Z=42), which is
one of the primary fission products, and is also produced through
the decay chain. Mo-99 constitutes .about.6% of the fission yield.
Mo-99 will decay, with a half-life of 66 hours, to Tc-99 (12.5%)
and Tc-99's metastable isomer Tc-99m (87.5%) (both Z=43). Both
Mo-99 and Tc-99m can be recovered with chemical extraction
processes and/or electrochemical separators. Embodiments will be
described in the context of extracting Mo-99/Tc-99m. The neutrons
are preferably maintained above energies where the
neutron-absorption cross section for U-238 remains negligible.
Thus, the NEU target does not breed plutonium (Pu-239). Other
fission products include I-129, I-131, I-132, and I-133, which can
also be recovered with chemical extraction processes and/or
electrochemical separators.
[0084] The purpose, and operation, of the neutron-reflecting
material is to increase the path length traveled by at least some
neutrons from the neutron generator before those neutrons leave the
irradiation chamber. Thus, as a neutron exits a region of NEU
material, possibly having undergone one or more scattering
interactions therein, it is reflected or scattered to enhance the
likelihood that it will encounter additional NEU material and
therefore have an increased chance to initiate a fission reaction.
Iron is an example of an element that can act as neutron-reflecting
material. In specific embodiments, stainless steel is used both for
its structural and neutron-reflecting properties. For neutron
energies above 800 keV, each scattering event with iron causes the
neutron to lose about 0.56 MeV in energy. Thus, after a sufficient
number of scattering events (depending on initial energy), the
neutron will fall below the fast fission threshold.
Overview of Relevant Properties of Uranium and Fission
Reactions
[0085] The literature concerning the properties of uranium and the
physics of nuclear fission is vast, and is well understood by those
skilled in the nuclear physics and engineering fields. For the sake
of completeness, a short overview of the relevant aspects of this
vast store of knowledge will be outlined to provide context for the
description of embodiments of the present invention.
[0086] As discussed above, embodiments of the present invention use
depleted or naturally occurring (i.e., non-enriched uranium or NEU)
targets. Naturally occurring uranium is about 99.27% U-238, 0.72%
U-235, and 0.0055% U-234. Depleted uranium is the by-product of the
process of enriching naturally occurring uranium to achieve a
higher proportion of U-235. Thus the depleted uranium contains
significantly less U-235 and U-234 than natural uranium (say less
than a third as much). There is no fundamental reason why
embodiments of the present invention could not use pure U-238, but
as a practical matter, that would be much more expensive.
[0087] Further, as discussed above, embodiments of the present
invention irradiate NEU, largely containing U-238, with fast
neutrons to cause fission. The most common nuclear reactors, on the
other hand, irradiate U-235 with thermal neutrons to cause fission.
Both U-235 and U-238 will undergo fission when struck with fast
neutrons, but the characteristics are different.
[0088] First is the difference in fission cross section as a
function of neutron energy for the two isotopes. U-235's fission
cross section for fast neutrons is at least 250 times lower than
for thermal neutrons. That is the reason why a nuclear reactor is
used to produce Mo-99 using targets enriched with U-235, since the
neutrons within a nuclear reactor are typically thermalized.
Thermal neutrons cannot cause fission in U-238.
[0089] Second is the nature of the reaction. U-235 is fissile,
meaning that it can sustain a chain reaction when a critical mass
is present, since the neutrons resulting from the fission reactions
have energies where U-235's fission cross section is high. U-238,
on the other hand, while fissionable, is not fissile. While U-238
can be made to fission when struck with fast neutrons, most of the
neutrons resulting from the fission reactions have energies that
are not sufficiently high to cause additional U-238 fission.
[0090] FIG. 3 is a graph of U-238's fission cross section and
neutron capture cross section as functions of neutron energy. This
graph was generated using the tools provided at
http://atom.kaeri.re.kr/cgi-bin/endfplot.pl. U-238 also has a
reaction path where it absorbs a neutron and, after two beta
decays, becomes Pu-239. While this reaction path may be desirable
in breeder reactors, embodiments of the invention seek to reduce
the probability of neutron absorption by maintaining neutron
energies in a range where the cross section for fission far exceeds
the cross section for neutron absorption. For example, as can be
seen in the graph of FIG. 3, the two cross sections are equal at a
neutron energy of .about.1.3 MeV, while at 2.2 MeV, the fission
cross section is .about.20 times larger. Accordingly, it is
preferred to minimize or eliminate materials that would act as
moderators.
[0091] As can be seen from the graph of U-238's fission cross
section as a function of neutron energy, the fission cross section
plateaus between 1 and 2 MeV, the neutron capture cross section is
rapidly falling off in this range of energies.
[0092] It should be understood that a fast neutron can transverse
its entire path through the NEU without causing a fission reaction.
This fast neutron also can be scattered or reflected from U-238
nuclei. The neutron-reflecting or scattering material is used to
enhance the probability that a fast neutron will ultimately
interact with a U-238 nucleus and cause a fission reaction before
its energy drops below the 800 keV threshold. Once this fast
neutron interacts with U-238 and causes a fission reaction, this
neutron essentially is gone. There will be 2-3 neutrons born after
the fission reaction (prompt neutrons), and some of these neutrons
can cause some addition fission in the NEU if their energies can
stay above the 800 keV threshold.
Overview of Representative Geometrical Configurations
[0093] FIG. 4 is a simplified oblique view of a radioisotope
generator 10.sub.sph having a spherical irradiation chamber
according to an embodiment of the present invention. Fill port 35a
is shown, but drain port 35b would be hidden from view. The chamber
is preferably fabricated in at least two sections, which is
signified by an equatorial line (the hidden half being shown in
phantom). The equatorial line also denotes the intersection of a
horizontal plane that is designated by a section line 6-6. For
simplicity, gas inlet port(s) 40a and gas outlet port(s) 40b are
not shown.
[0094] FIG. 5 is a simplified oblique view of a radioisotope
generator 10.sub.cyl having a cylindrical irradiation chamber
according to an embodiment of the present invention. Again, fill
port 35a is shown, but drain port 35b would be hidden from view.
Also, gas inlet port(s) 40a and gas outlet port(s) 40b are not
shown. Shown in phantom is the intersection of a horizontal plane
that is designated by a section line (also 6-6). The cylindrical
wall of irradiation chamber 15 need not be fabricated in multiple
sections, although it can be. In such a case, the phantom line
designating the intersection of the horizontal plane could be drawn
in solid on the visible portion of the cylindrical wall.
[0095] FIG. 6 is a simplified cross-sectional view that applies to
either of radioisotope generators 10.sub.sph and 10.sub.cyl, and is
taken along line 6-6 in FIGS. 4 and 5. For the spherical embodiment
10.sub.sph, neutron-reflecting elements 25 are formed as a
plurality of spaced concentric spherical shells that surround
neutron generator 20. For the cylindrical embodiment 10.sub.cyl,
neutron-reflecting elements 25 are formed as a plurality of spaced
coaxial cylindrical shells that surround neutron generator 20.
[0096] In both embodiments, NEU bodies or regions 30 partially or
fully occupy the spaces between neutron-reflecting shells 25. The
drawing is simplified in that it doesn't show holes in the
neutron-reflecting shells that allow NEU material introduced in one
region to find its way to other regions. Further, it is
contemplated that there may be bulkheads, again not shown, that
maintain the spacing between the shells and provide additional
structural strength.
[0097] For both embodiments, the outermost neutron-reflecting
shell, which is shown as being thicker than the radially inward
shells, at least partially defines irradiation chamber 15. While a
plurality of regions for NEU are shown, some embodiments can have
only one region (spherical or cylindrical shell as the case may
be). Furthermore, the outermost neutron-reflecting shell need not
be thicker than the inner one or ones.
[0098] Radially outward of the outermost neutron-reflecting shell
is a biological shield 50, which is used to block ionizing
radiation such as alpha particles, electrons, and gamma rays that
might leak out of the neutron-reflecting shell. Biological shield
50 also can be considered to partially define irradiation chamber
15. Such a shield can be made of materials such as lead, iron,
borated polyethylene, or a combination of any or all of such
materials.
[0099] While specific dimensions do not form a part of the
invention, some representative dimensions, or at least factors that
can be considered in specifying particular dimensions will be
discussed. For example, the NEU can be formed in a single layer on
the order of 30 to 50 cm thick, or a combination of multiple layers
on the order of 10 cm thick and separated by stainless steel layers
on the order of 0.5 cm thick. In this way, when the neutron energy
falls below 1 MeV after multiple scattering events, the neutron
will leak out of the outermost layer of the NEU. A distance of
containment chamber 45's walls from irradiation chamber 15 on the
order of 2.5 meters is representative.
[0100] FIG. 7 is a simplified oblique view of a radioisotope
generator 10.sub.rect having a rectangular-prism-shaped irradiation
chamber according to an embodiment of the present invention. Again,
fill port 35a is shown, but drain port 35b would be hidden from
view. Also, gas inlet port(s) 40a and gas outlet port(s) 40b are
not shown. Shown in phantom is the intersection of a horizontal
plane that is designated by a section line 8-8.
[0101] FIG. 8 is a simplified cross-sectional view of the
radioisotope generator shown in FIG. 7 and is taken along line 8-8
in FIG. 7. In this embodiment, neutron-reflecting elements 25 are
formed as a plurality of spaced parallel plates, some of which are
cut away or shortened to provide a cavity for neutron generator 20.
NEU bodies or regions 30 partially or fully occupy the spaces
between neutron-reflecting plates 25.
[0102] FIG. 9 is a simplified cross-sectional view of a
radioisotope generator 10.sub.rod where the NEU is in the form of
rods 55, one of which is shown enlarged. As can be seen, each rod
comprises a tubular shell of neutron-reflecting material 25
surrounding a bore 44 containing NEU 30. The rods are shown as
circular in cross section, but polygonal tubes can also be used. A
circular cross section is generally preferred since that is
generally the most common and would be more economical to
manufacture and operate. A circular cross section has the
additional advantages of the greatest structural integrity and
efficiency for the propagation of fast neutrons.
[0103] The tubes are shown disposed in a cylindrical irradiation
chamber, but there is no requirement. As schematically drawn, rods
are laid out in an octagonal array, and so an octagonal irradiation
chamber could also be used. The rods could also be distributed
along a set of concentric circles so that their axes would lie in
concentric cylindrical surfaces.
Process Overview
[0104] FIG. 10 is a process schematic for the generation of
radioisotopes according to an exemplary embodiment of the present
invention. Each block in the schematic represents an operation, but
many of the blocks also represent a physical piece of apparatus.
The connector lines represent logical flow as well as material
flow, so the figure can also be viewed as a flowchart.
[0105] The production can be considered to begin with providing an
initial supply of NEU (operation 60), which in this exemplary
embodiment is depleted uranium. This exemplary embodiment uses NEU
in granular form, and so NEU is then subjected to a grinding
operation 65 and a sorting operation 70 that rejects undersized
pieces of NEU. The pieces meeting the desired size criteria are
sent to an inventory 75 of NEU in a suitable form or state, and the
undersized pieces are diverted to bypass the inventory and are
subjected to further processing as will be described below.
Grinding and sorting operations 65 and 70 can also be used for
recycling NEU as will be described below.
[0106] For the sake of this exemplary embodiment, a suitable form
would be NEU that had been ground or crushed to "pebbles" of
desired nominal size, say on the order of no less than 0.64 cm
(1/4-inch) in the smallest dimension and no more than 12.7 cm
(1/2-inch) in the largest dimension. The NEU is loaded into the
radioisotope generator, say by gravity through fill port(s) 35a
(not shown in FIG. 10), and irradiated for a suitable period of
time with neutrons of a suitable energy (operation 80). In one
exemplary embodiment, the NEU is irradiated with
3.5.times.10.sup.13 neutrons/second for 20 hours using 14.1-MeV
neutrons. This provides a balance between production and decay. For
the example of a single 30-cm spherical shell of NEU having an
outer diameter of 182.88 cm (6 feet), the mass of the NEU would be
on the order of 22,000 kg.
[0107] After irradiation, the NEU, which now contains fission
products, including the desired fission products and other fission
products, is removed from the radioisotope generator, say by
gravity through drain port(s) 35b (not shown in FIG. 10), subjected
to a radioisotope recovery operation 85 to provide the desired
fission product, in this case Mo-99. The recovered Mo-99 is then
subjected to a quality control testing operation 90, is used to
charge a Tc-99m generator (operation 95), and the Tc-99m generator
is set for shipment to an end user (operation 100). By way of
example, a single generator may contain 6 curies and would be used
to prepare a large number of individual patient doses that might be
on the order of 10-30 microcuries per patient.
[0108] The NEU from which the Mo-99 has been recovered is subjected
to a separate recovery operation 105 to remove the other fission
products, some of which may be desirable radioisotopes, and is then
subjected to yet another recovery operation 110 to recover the NEU
for recycling. As will be described below, the recovery operations
can use ionic liquids, and more specifically room-temperature ionic
liquids (RTILs).
[0109] The recovered NEU provided by recovery operation 110 is
returned to be subjected to grinding and sorting operations, which
can be the same grinding and sorting operations 65 and 70 used for
the NEU that is originally provided to the system. As for the
originally provided NEU, the sorting operation rejects undersized
pieces of NEU. The pieces meeting the desired size criteria are
returned to the NEU inventory, and the undersized pieces are
diverted to bypass the inventory and irradiation chamber.
[0110] The irradiation and fission can also give rise to various
fission products in a gaseous state. These gaseous fission products
include fission products that are themselves gases (e.g., xenon and
krypton), and iodine (e.g., I-129, I-131, I-132, I-133, etc.),
which is a solid, but easily sublimates. In some traditional
systems using HEU, the HEU target elements are encapsulated. Thus,
these gaseous fission products would be trapped in the encapsulated
target elements, and the gaseous fission products would be captured
after irradiation in connection with the Mo-99 recovery.
Additionally, to the extent that gaseous fission products leaked
out of the target, the iodine would dissolve in the water that
acted as a coolant and moderator.
[0111] In this exemplary embodiment, the gaseous fission products
(i.e., fission gases and sublimated iodine) are extracted during
irradiation. Thus, while the Mo-99 is recovered on a batch basis,
the gaseous fission products can be collected on a continuous
basis. As will be discussed below, some of the fission gases and
iodine remain trapped within the NEU matrix and are recovered on a
batch basis.
[0112] In this exemplary embodiment, the irradiation chamber is
provided with one or more gas inlet ports 40a and one or more gas
outlet ports 40b (shown schematically in FIG. 1). Further,
provision is made for fluid communication between the gas inlet and
outlet ports and the NEU in the irradiation chamber. An inert
carrier gas (e.g., argon) is introduced into an inlet port and
circulated through the irradiation chamber where it mixes with the
gaseous fission products. The gas exiting an outlet port of the
irradiation chamber is subjected to a scavenging operation 115 to
remove the gaseous fission products before the gas is reintroduced
into an inlet port of the irradiation chamber.
[0113] The gases removed by scavenging operation 115 are subjected
to one or more recovery operations 120, one of which is shown. This
can be a standard chemical extraction process or a standard
electrochemical separation. In this exemplary embodiment, it is
desired to extract iodine (with I-131 often being the radioisotope
of greatest interest), which can be captured in a silver zeolite
trap, and the remaining gaseous fission products captured in HEPA
filters for disposal. The iodine (including I-131) is then
subjected to a quality control testing operation 125, packaged in
suitable quantities (operation 127), and set for shipment to an end
user (operation 130).
[0114] References Burger_2004 ("HWVP Iodine Trap Evaluation"),
Chapman 2010 ("Radioactive Iodine Capture in Silver-Containing
Mordenites through Nanoscale Silver Iodide Formation"), and
Wang_2006 ("Simulating Gaseous .sup.131I Distribution in a Silver
Zeolite Cartridge Using Sodium Iodide Solution") provide additional
background for the iodine recovery.
[0115] The field of isotope extraction and separation is well
developed, and Mo-99 recovery process 85 could use techniques such
as chemical extraction processes and/or electrochemical separation
processes. For example, generalized procedures for the recovery of
Mo-99 from HEU have been developed in connection with
nuclear-reactor-based operations. The HEU is normally encapsulated
in a dispersion-type target with aluminum cladding, and the HEU can
take the form of mini fuel plates or pins. After irradiation
(typically 10-12 days), the targets are removed from the reactor
and cooled for several hours in the pool adjacent to the reactor
before being transported to the processing hot cell.
[0116] The targets are then dissolved in nitric acid, with the
possible addition of mercury (II) nitrate (Hg(NO.3)2) to assist the
dissolution of the aluminum. Following dissolution, the solution is
fed to an alumina or polymer column, and the Mo-99 is adsorbed on
the column with minor amounts of other components including heavy
metals. Once the column is loaded with the Mo-99m, the column is
washed with nitric acid and then water, and then Mo-99 is stripped
from the column using an ammonium-hydroxide solution. Purification
is carried out to remove as much of the heavy metals as possible.
Some producers have to carry out many purification steps in order
to reduce the heavy metal concentrations to the level to meet FDA
requirements.
[0117] Chapter 2 of Reference NRC_2004 ("Medical Isotope Production
without Highly Enriched Uranium") provides a description of
Molybdenum-99/Technetium-99m production and use, with a description
of the dissolution and Mo-99 recovery at pages 25-30.
[0118] In this exemplary embodiment, Mo-99 recovery process 85 uses
ionic liquids, and more specifically room-temperature ionic liquids
(RTILs). The recovery process includes a series of sub-processes,
as will now be described. Initially, the NEU (including the fission
products) that is unloaded from the irradiation chamber is
dissolved in an RTIL (operation 135), and the Mo-99 is recovered
from the solution (operation 140). Recovery operation 140, for this
exemplary embodiment, entails electrodepositing the Mo-99 onto an
anode. The recovered Mo-99 is then removed from the anode
(operation 145). For sacrificial anodes, this can entail dissolving
or otherwise destroying the anode with a higher charge. In the case
of a permanent anode, this can include techniques such as
scraping.
[0119] References Pemberton_2008 ("Solubility and Electrochemistry
of Uranyl Carbonate in a Room Temperature Ionic Liquid System") and
Pemberton_2009 ("Solubility and Electrochemistry of Uranium
Extracted into a Room Temperature Ionic Liquid") provide additional
background.
[0120] The above description of the iodine recovery was somewhat
simplified, and will be explained in greater detail below. In many
circumstances, some of the fission gases and some of the iodine
fission product remain trapped in the NEU, and are released during
Mo-99 recovery. To recover desired radioisotopes, provision is made
to scavenge fission gases and sublimated iodine released during the
Mo-99 recovery, to extract the iodine (including I-131), to subject
the recovered iodine to quality control testing, to package the
iodine, and to set the packaged iodine for shipment. This is shown
schematically in phantom blocks associated with the NEU dissolution
(operation 135).
[0121] These blocks correspond generally to scavenging operation
115, recovery operation(s) 120, quality control testing operation
125, packaging operation 127, and setting for shipment operation
130 that are performed during irradiation of the NEU. While these
blocks represent operations that are performed at different times,
one or more may be implemented using the same apparatus that is
used to perform these operations during irradiation. This is
denoted by the legend "(One or more could be shared with
irradiation chamber)." That possibility is also denoted
schematically by a dashed arrow from NEU dissolution operation 135
to the gas scavenging operation 115 that is associated with
irradiating the NEU (operation 80).
Specific Embodiment with Cylindrical Irradiation Chamber with
Parallel NEU Rods
[0122] FIG. 11A is a perspective view of a radioisotope generator
according to a specific embodiment having a cylindrical irradiation
chamber in which are disposed parallel cylindrical NEU rods. FIG.
11B is a perspective, cutaway view of the radioisotope generator
shown in FIG. 11A with the cover and the outer cylinder removed,
exposing the tube assemblies, neutron reflecting regions 25a and
25b, and neutron generator. FIG. 11C is a perspective cutaway view
of the radioisotope generator shown in FIG. 11A taken from a
different view to show additional details of the tube assembly and
the neutron generator.
Generation and Recovery of Radioactive Iodine Isotopes Including
I-131
[0123] As mentioned above, the irradiation and fission give rise to
various fission products, and some of these are in gaseous states.
Radioactive iodine 131 (sometimes referred to as .sup.131I,
radioiodine 131, or simply I-131) is not a fission gas, but readily
sublimates, and so is one of these gaseous fission products, and is
an important radioisotope to be recovered. Embodiments of the
invention are designed with the production and recovery of I-131
and other radioactive iodine isotopes in mind.
[0124] An iodine isotope of major interest is I-131, but the
fission products include a number of other radioactive iodine
isotopes and other elements that decay to radioactive iodine.
Properties and Uses of I-131
[0125] I-131 (atomic number Z=53, 78 neutrons) has a half-life of
8.02 days and is used for a variety of applications. These include
diagnostic and therapeutic thyroid applications (in either a
solution or capsule form), industrial tracers, and various research
applications such as antibody labeling. I-131 is also used to label
antibodies for therapeutic applications in the treatment of
cancers.
[0126] Examples of its use in radiation therapy include the
treatment of thyrotoxicosis and thyroid cancer. When a small dose
of I-131 is swallowed, it is absorbed into the bloodstream in the
gastrointestinal (GI) tract and concentrated from the blood by the
thyroid gland, where it begins destroying the gland's cells.
Diagnostic tests exploit the mechanism of absorption of iodine by
the normal cells of the thyroid gland. As an example I-131 is one
of the radioactive isotopes of iodine that can be used to test how
well the thyroid gland is functioning.
[0127] FIG. 12 shows the decay products of I-131, which is one of
the primary fission products, and constitutes on the order of 3% of
the total fission yield. In short, the I-131 decays to xenon 131,
or Xe-131 (Z=54, 77 neutrons), emitting a beta particle
(.beta..sup.-, a gamma (.gamma.) ray, and a neutrino (v) in the
process. The primary emissions of I-131 decay are 364-keV .gamma.
rays (81% abundance) and 606-keV .beta..sup.- particles (89%
abundance).
[0128] As shown in more detail in FIG. 12, the decay is actually a
two-step process where the I-131 first decays by beta decay to one
of a number of excited states of Xe-131, emitting a .beta..sup.-
particle and a neutrino in the process, and Xe-131 in the excited
state falls to a metastable state (Xe-131m), emitting a y ray in
the process. The first step occurs with a half-life of about 8
days, while the second step is, for present purposes,
immediate.
[0129] FIG. 12 is simplified in that only two of the excited states
are shown, a first that is 637 keV above the metastable state, and
a second that is 364 keV above the metastable state. The beta decay
to the first state results in beta particles having a range of
energies between zero and a maximum of 333 keV, while the beta
decay to the second state results in beta particles having a range
of energies between zero and a maximum of 606 keV. The remaining
energy is carried off by the neutrino. In 82% of the decays to the
metastable state, a 364-keV gamma ray is emitted, and in 7% of
decays the decays to the metastable state, a 637-keV gamma ray is
emitted. Other decay mechanisms make up the other 11% of decays to
the ground state.
[0130] The metastable isomer Xe-131 has a half-life of 11.93 days,
and undergoes an isomeric transition to the stable isotope Xe-131
by the mechanism of internal conversion, ejecting a single 164-keV
electron in the process. Xe-131 is one of xenon's nine stable
isotopes, and constitutes 21.2% of naturally occurring xenon.
[0131] FIG. 12 shows I-131 as a fission product, which includes
some I-131 nuclei which are primary fission product (fission
fragments) and also includes some nuclei that are decay products of
other fission products. For example, fission fragments and fission
products in the chain include: [0132] indium 131 (In-131, Z=49, 82
neutrons), which beta decays with a half-life of less than a second
to tin 131 (Sn-131, Z=50, 81 neutrons); [0133] Sn-131, which beta
decays with a half-life of less than a minute to antimony 131
(Sb-131, Z=51, 80 neutrons); [0134] Sb-131, which beta decays with
a half-life of 23 minutes to two isomers of tellurium 131 (Te-131
and Te-131*, Z=52, 79 neutrons); [0135] Te-131, which beta decays
with a half-life of 25 minutes to I-131 (Z=53, 78 neutrons); and
[0136] (Te-131*, which undergoes an isomeric transition with a
half-life on the order of 30 hours to Te-131, which beta decays to
I-131 as above). [0137] The total of the I-131 fission fragments
and the I-131 decay products make up on the order of 3% of the
total fission yield.
Properties and Uses of Other Radioactive Iodine Isotopes
[0138] As noted above, the fission products include a number of
iodine isotopes in addition to I-131. The longer-lived radioactive
fission products include the following (also shown are half-lives
and fission yield): [0139] I-129 (1.59 million years, 0.54%);
[0140] I-131(8.042 days, .about.3%); [0141] I-132 (2.29 hours
(metastable isomer 1.4 hours), 4.31%); [0142] I-133 (20.8 hours
(metastable isomer 9 seconds), 6.77%); [0143] I-134 (52.6 minutes,
7.87%); and [0144] I-135 (6.6 hours, 6.54%).
[0145] At least some of these isotopes have applications in imaging
and/or medical therapy (the most useful are believed to be I-131,
I-132, and I-133).
[0146] Embodiments of the present invention also can produce
radioactive iodine isotopes up to 1-142. Depending on the
application, it is believed that the radioactive iodine produced by
embodiments of the present invention will have lower dose
requirements than pure I-131 produced by other techniques. I-130
(12.4 hours (metastable isomer 8.9 minutes)) is not a fission
product since it is not a fission fragment and would only be
produced in a decay chain from Te-130, except that Te-130 has a
half-life on the order of 2.5.times.1021 years. Radioactive iodine
isotopes below I-127 are not fission fragments and are not decay
chain products since they are blocked by stable elements.
[0147] Other radioactive iodine isotopes are short-lived (hours or
minutes) and occur in very small amounts, and can be ignored as a
practical matter. I-129 accounts for 0.54% of the primary fission
yields and has a half-life of 15.9 million years, thus being
essentially stable. It is possible to separate these radioactive
iodine isotopes, and depending on the application, there may be
reasons to do so.
I-132 and I-133 are additional radioactive iodine isotopes that are
of interest. The I-132 production scheme is as follows. [0148]
indium 132 (In-132, Z=49, 83 neutrons), which beta decays with a
half-life of less than a second to tin 132 (Sn-132, Z=50, 82
neutrons); [0149] Sn-132, which beta decays with a half-life of 40
seconds to two isomers of antimony 132 (Sb-132 and Sb-132*, Z=51,
81 neutrons); [0150] Sb-132 and Sb-132*, which beta decay with
respective half-lives of 4.2 minutes and 2.8 minutes to tellurium
132 (Te-132, Z=52, 80 neutrons); [0151] Te-132, which beta decays
with a half-life of 3.2 days to two isomers of iodine 132 (I-132
and I-132*, Z=53, 79 neutrons); [0152] I-132, which beta decays
with a half-life of 2.28 hours to xenon 132 (Xe-132,
[0153] Z=54, 78 neutrons); and [0154] (I-132*, which undergoes an
isomeric transition with a half-life of 1.4 hours to I-132, which
beta decays to Xe-132 as above). [0155] The total of the I-132
fission fragments and the I-132 decay products make up on the order
of 4.31% of the total fission yields.
[0156] The I-133 production scheme is as follows. [0157] indium 133
(In-133, Z=49, 84 neutrons), which beta decays with a half-life of
less than a second to tin 133 (Sn-133, Z=50, 83 neutrons); [0158]
Sn-133, which beta decays with a half-life of 1.4 seconds to
antimony 133 (Sb-133, Z=51, 82 neutrons); [0159] Sb-133, which beta
decays with a half-life of 2.5 minutes to two isomers of tellurium
133 (Te-133 and Te-133*, Z=52, 81 neutrons); [0160] Te-133, which
beta decays with a half-life of 12.4 minutes to two isomers of
iodine 133 (I-133 and I-133*, Z=53, 80 neutrons); [0161] (Te-133*,
which undergoes an isomeric transition with a half-life of 55.4
minutes to Te-133); [0162] I-133, which beta decays with a
half-life of 20.8 hours to two isomers of xenon 133 (Xe-133 and
Xe-133*, Z=54, 79 neutrons); [0163] (I-133*, which undergoes an
isomeric transition with a half-life of 9 seconds to I-133, which
beta decays to Xe-133 and Xe-133* as above; [0164] Xe-133, which
beta decays with a half-life of 5.24 days to cesium 133 (Cs-133,
Z=55, 78 neutrons); and [0165] (Xe-133*, which undergoes an
isomeric transition with a half-life of 2.19 days to Xe-133, which
beta decays to Cs-133 as above). [0166] The total of the I-133
fission fragments and the I-133 decay products make up on the order
of 6.7% of the total fission yields.
[0167] Where the end result of the iodine decay is an inert isotope
of xenon (e.g., Xe-131, Xe-132, and Xe-133), there is no problem.
Otherwise, the processing may entail additional operations. If the
end result is not a stable xenon isotope, it may be desirable to
separate it out, for example using electrochemical techniques or
ion-exchange chromatography (ion chromatography). This would be the
case for relatively long-lived radioactive substances or for
undesirable stable substances such as barium, cerium, and
cesium.
[0168] Some short-lived radioactive substances can be addressed by
allowing the extracted iodine additional time so the radioactive
end result substance can decay to a stable substance or a
radioactive substance that is susceptible of separation. For
example, I-133 decays to stable Cs-133, but I-135 and I-137 decay
to radioactive cesium isotopes, which are considered undesirable
for both imaging and therapeutic applications.
[0169] Since the irradiation cycle is on the order of 20 hours, one
approach is to let the collected radioactive iodine decay for about
a day (-4 half-lives for I-135, and more than 1000 half-lives for
I-137) so that the radioactive cesium can be electrochemically
separated or separated through ion-exchange chromatography from the
iodine solution. As a result, the resulting iodine solution would
contain mainly I-127 (non-radioactive), I-129, I-131, I-132, and
I-133, which could be used for both therapeutic and imaging
applications.
[0170] I-132 has a relatively short half-life--2.29 hours with an
isomeric transition of I-132* of 1.39 hours. Since I-132's
half-life is short, that means it decays quickly within the body,
so that there is no lingering radioactivity after the procedure and
the dosage is much lower than other iodine imaging isotopes.
Radioisotope Generator Tailored for Generation of I-131 and Other
Radioactive Iodine Isotopes
[0171] Any of the above irradiation chamber designs can be adapted
to enhance the extraction of the gaseous fission products
(including I-131 and other radioactive iodine isotopes, which
sublimate to a gaseous state). In particular, as mentioned above,
it is desired to withdraw the gaseous fission products from the
irradiation chamber during irradiation by introducing an inert
carrier gas (e.g., argon, which is inert and relatively cheap due
to its large natural occurrence), circulating it through the
irradiation chamber to mix with the fission gases, and exhausting
the gas mixture for further processing.
[0172] FIG. 13 is a simplified cross-sectional view of a
radioisotope generator such as that shown in the sectional view of
FIG. 6 (e.g., radioisotope generator 10.sub.sph or 10.sub.cyl),
with additional details relating to the circulation of gas through
the irradiation chamber. Shown explicitly are gas inlet port(s) 40a
and gas outlet port(s) 40b. Gas venting layers 150 (shown in
heavier solid lines) are provided along surfaces of the NEU layers,
and radially extending gas venting channels 155 provide gas
communication paths between gas venting layers 150 and the inlet
and outlet ports.
[0173] Additional ways to increase the circulation of the carrier
gas in the irradiation chamber include providing apertures in
bulkheads and other structural elements. For embodiments using NEU
in tubes, the tube walls can be provided with holes that are
generally smaller than the smallest expected size of the NEU
granules.
[0174] A pump 160 exhausts the gases from irradiation chamber 15
and the gases are subjected to the scavenging and iodine recovery
operations described above. The irradiation chamber is preferably
maintained at a slight negative pressure during operation.
[0175] As discussed above, some of the iodine and gaseous fission
products can remain trapped in the uranium matrix, and are
recovered in connection with the recovery of Mo-99 and other
materials after the NEU is removed from the irradiation chamber.
Providing the NEU in a granular form tends to increase the amount
of iodine and fission gases that can escape from the uranium matrix
during irradiation and be recovered on a continuous basis.
REFERENCES
[0176] The following references are incorporated by reference.
[0177] 1) Burger_2004--L. L. Burger, R. D. Scheele, "HWVP Iodine
Trap Evaluation," Pacific Northwest National Laboratory Report
PNNL-14860 (September 2004) [0178] 2) Chapman_2010--Karena W.
Chapman, Peter J. Chupas, and Tina M. Nenoff, "Radioactive Iodine
Capture in Silver-Containing Mordenites through Nanoscale Silver
Iodide Formation," J. Am. Chem. Soc., 2010, 132 (26), pp 8897-8899
(publication date (web) Jun. 15, 2010) DOI: 10.1021/ja103110y
[0179] 3) NRC_2009--"Medical Isotope Production without Highly
Enriched Uranium," Nuclear and Radiation Studies Board, Division of
Earth and Life Studies, National Research Council of the National
Academies, The National Academies Press, Washington, D.C. (2009).
Dissolution and Mo-99 Recovery are discussed at pages 25-30.
http://www.nap.edu/openbook.php?record id=12569 [0180] 4)
Pemberton_2008--Wendy J. Pemberton, Kenneth R. Czerwinski, David
Hatchett, "Solubility and Electrochemistry of Uranyl Carbonate in a
Room Temperature Ionic Liquid System," presented Sep. 25, 2008 in
the Radiochemistry in the Advanced Nuclear Fuel Cycle session of
the 42nd Western Regional Meeting of the American Chemical Society,
Las Vegas, Nev. (Sep. 23-27, 2008) [0181] 5) Pemberton_2009--Wendy
J. Pemberton, Kenneth R. Czerwinski and David H Hatchett,
"Solubility and Electrochemistry of Uranium Extracted into a Room
Temperature Ionic Liquid," Actinides 2009, San Francisco, Calif.,
July 2009 [0182] 6) Wang 2006--Wei-Hsung Wang, Kenneth L. Matthews,
II, "Simulating Gaseous .sup.131I Distribution in a Silver Zeolite
Cartridge Using Sodium Iodide Solution," Health Physics: May
2006--Volume 90--Issue 5--pp S73-S79 DOI:
10.1097/01.HP.0000203812.30182.7b
Conclusion and Potential Advantages
[0183] In conclusion it can be seen that embodiments of the present
invention can provide safe, efficient, economical techniques for
producing medical isotopes. Embodiments of the present invention
can be characterized by one or more of the following attributes,
alone or in any combination: [0184] Using neutron-reflecting
material maximizes the neutron population above the fast fission
threshold of U-238 within the NEU layer or layers, enhancing the
fast fission process in the NEU material. [0185] Maintaining the
neutron energy above .about.1 MeV while in the NEU minimizes
neutron capture, and hence the decay to Pu-239. [0186] U-238 can be
used as a primary fissionable material rather than enriched U-235,
which is used by traditional nuclear-reactor-based methods.
Depleted uranium, a byproduct from the enrichment process that is
already stored at the Department of Energy (DOE) sites, can be
utilized efficiently. This greatly reduces the cost of Mo-99/Tc-99m
production and I-131 production due the more relaxed regulatory
requirements concerning natural uranium or depleted uranium. [0187]
The radioisotope generator according to embodiments of the present
invention can be widely deployed, thereby allowing radioisotope
generation closer to the end users for use as diagnostic,
therapeutic, and research medical radioisotopes in imaging centers,
hospitals, and medical research institutions. [0188] Embodiments of
the present invention eliminate or reduce the need to export HEU to
foreign nuclear reactors and subsequently import radioisotopes such
as Mo-99/Tc-99m and radioactive iodine isotopes. [0189] A suite of
radioactive iodine radioisotopes is produced. [0190] The integrated
iodine dose from all the iodine radioisotopes produced is larger
than systems producing only I-131. [0191] Because some of the
iodine radioisotopes have much shorter half-lives than I-131, the
produced radioisotope iodine potentially has a broader
applicability than I-131 alone--lower dose.
[0192] While the above is a complete description of specific
embodiments of the invention, the above description should not be
taken as limiting the scope of the invention as defined by the
claims.
* * * * *
References