U.S. patent application number 10/547813 was filed with the patent office on 2006-08-03 for process for separating metals.
Invention is credited to Zara Banfield, David John Hebditch.
Application Number | 20060169590 10/547813 |
Document ID | / |
Family ID | 9954059 |
Filed Date | 2006-08-03 |
United States Patent
Application |
20060169590 |
Kind Code |
A1 |
Hebditch; David John ; et
al. |
August 3, 2006 |
Process for separating metals
Abstract
An electrorefining apparatus is capable of operating in
continuous mode, and includes a criticality control mechanism,
preferably a geometric criticality control mechanism, for example,
to control the dimensions of the apparatus. Electrochemical cells
include a large surface area per unit volume, preferably in the
form of thin, flat plates. Cells include a crusting liquid
cathodes, a cast cathode, a fluidised cell or pulsed bed, a moving
belt cathode, a consolidating cathode, or a liquid anode and a
plate cathode. A continuous process for the isolation of metals,
typically uranium, from spent nuclear fuels includes
electrochemically treating the spent nuclear fuels in the
apparatus.
Inventors: |
Hebditch; David John;
(Gloucestershire, GB) ; Banfield; Zara; (Cheshire,
GB) |
Correspondence
Address: |
MYERS BIGEL SIBLEY & SAJOVEC
PO BOX 37428
RALEIGH
NC
27627
US
|
Family ID: |
9954059 |
Appl. No.: |
10/547813 |
Filed: |
March 3, 2004 |
PCT Filed: |
March 3, 2004 |
PCT NO: |
PCT/GB04/00911 |
371 Date: |
September 2, 2005 |
Current U.S.
Class: |
205/46 |
Current CPC
Class: |
C25C 3/34 20130101; C25C
7/005 20130101 |
Class at
Publication: |
205/046 |
International
Class: |
C25C 1/22 20060101
C25C001/22; C25C 3/34 20060101 C25C003/34 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 4, 2003 |
GB |
0304884.0 |
Claims
1. An electrorefining apparatus, the apparatus comprising an
electrorefining cell having an anode and a cathode, wherein the
apparatus is adapted for operation in continuous mode, and includes
means for criticality control, the means for criticality control
comprising electrochemical cells having a large surface area per
unit volume.
2. An apparatus as claimed in claim 1 wherein the means for
criticality control comprises a thin flat plate.
3. An apparatus as claimed in claim 2 wherein the thin flat plate
is horizontally or vertically disposed.
4. An apparatus as claimed in claim 1 further comprising means for
the removal of deposited material from the apparatus, following
operation.
5. An apparatus as claimed in claim 4 wherein the means for removal
of deposited material comprises sweeping means, conveyor means or
means by which a section of the apparatus may be removed.
6. An apparatus as claimed in claim 1 further comprising a vessel,
an anode and a crusting liquid cathode.
7. An apparatus as claimed in claim 6 wherein the crusting liquid
cathode comprises a liquid metal cathode or liquid alloy
cathode.
8-10. (canceled)
11. An apparatus as claimed in claim 1 further comprising a vessel,
an anode and a cast cathode.
12. An apparatus as claimed in claim 11 wherein the cast cathode
comprises a metal or alloy cathode with a lower melting point than
uranium metal.
13. An apparatus as claimed in claim 12 wherein the cast cathode is
introduced to an electrorefiner cell as a melted liquid cathode
which is then frozen to form a solid metal or alloy cathode as a
horizontal thin slab.
14-15. (canceled)
16. An apparatus as claimed in of claim 1 further comprising a
fluidised cell or pulsed bed.
17. An apparatus as claimed in claim 16 wherein the fluidised cell
or pulsed bed comprises a vessel including an anode and a cathode
which comprises cathode beads or particulates.
18. An apparatus as claimed in claim 16 further comprising a
ceramic divider.
19. An apparatus as claimed in claim 17 wherein the cathode beads
or particulates are formed from graphite or uranium.
20. (canceled)
21. An apparatus as claimed in claim 1 which comprises a vessel, an
anode and a moving belt cathode.
22. An apparatus as claimed in claim 21 wherein the moving belt
cathode is mounted on a pair of rollers.
23. An apparatus as claimed in claim 21 wherein the moving belt
cathode comprises a refractory metal belt.
24-25. (canceled)
26. An apparatus as claimed in claim 1 further comprising a vessel,
an anode and a consolidating cathode.
27. An apparatus as claimed in claim 26 wherein the consolidating
cathode comprises at least one pair of rotating cylinders, the
cylinders being suspended vertically.
28. (canceled)
29. An apparatus as claimed in claim 27 wherein the cylinders are
formed from uranium metal.
30. (canceled)
31. An apparatus as claimed in claim 1 further comprising a vessel,
a liquid anode and a plate cathode.
32. An apparatus as claimed in claim 31 wherein the plate cathode
is arranged in sections or modules.
33. An apparatus as claimed in claim 31 wherein the liquid anode
comprises spent fuel pre-dissolved or slurried in liquid metal or
alloy.
34. An apparatus as claimed in claim 31 wherein the plate cathode
comprises a horizontal thin slab positioned above the anode in
parallel plate arrangement.
35. An apparatus as claimed in claim 31 wherein the plate cathode
is comprised of uranium.
36. A continuous process for the isolation of metals from spent
nuclear fuels, the process comprising electrochemically treating
the spent nuclear fuels in an apparatus according to claim 1.
37. A process as claimed in claim 36 further comprising the
electrochemical separation of uranium metal from spent nuclear
fuel, the spent nuclear fuel comprising reduced metal oxides.
38-40. (canceled)
41. A process as claimed in claim 36 wherein an electrolyte is
introduced into the apparatus, the electrolyte comprising molten
salt or an ionic liquid.
42-46. (canceled)
47. An apparatus as claimed in claim 5, wherein the sweeping means
comprises a scraper.
48. An apparatus as claimed in claim 1, wherein the anode is
adapted for use in a continuous process.
Description
FIELD OF THE INVENTION
[0001] This invention relates to processes for the separation of
metals from compositions containing metals. The invention includes
processes for the treatment of spent nuclear fuel forming part of a
process for reprocessing, conditioning and/or partitioning nuclear
fuels. Reference will be made hereinafter mainly to nuclear fuels
but it should be understood that the invention is not restricted to
any particular type of material and has application outside the
nuclear industry. The processes involve the purification of a
substance which is liquid at its operating temperature and at this
temperature is comprised wholly or largely of ionic species. Such
substances generally fall within one of two main classes, ionic
liquids and molten salts. Ionic liquids typically have a relatively
low melting point and usually contain an organic cation, whereas
molten salts are generally totally inorganic and most commonly have
a melting point of at least several hundred degrees Centigrade.
Molten salts typically comprise eutectic mixtures, the melting
points of which can be significantly depressed from those of the
individual components.
BACKGROUND TO THE INVENTION
[0002] In the metals recovery and refining industry generally, the
type of metal recoverable from a solvent is dependent upon the size
of the electrochemical window of the solvent in which the metal is
dissolved, and from which purification and recovery is taking
place. In aqueous solutions, this is governed by the
electrochemical window of water or supporting electrolyte. This
limits the recovery, purification and electroplating of metals on
to surfaces from aqueous solution to those metals whose electrode
reduction potentials are more positive than the cathodic limit of
the aqueous solution. In acidic aqueous solution, metal ions would
not be recoverable where their electrode reduction potentials are
more negative than that of the H.sub.3O.sup.+ ion. Recovery of
metals with electrode reduction potentials more negative than
H.sub.3O.sup.+, means that non-aqueous (aprotic) solvents are
required. There are a number of aprotic solvents which are used.
These are often molten salts and, for instance, aluminium is
industrially purified electrochemically by electrolysis of
Al.sub.2O.sub.3 dissolved in molten cryolite Na.sub.3AlF.sub.6.
Other aprotic media include the organic solvents, such as
acetonitrile, benzene and toluene.
[0003] Molten salts are often used as media in the nuclear
industry. Such salts may be eutectic mixtures of salts and comprise
chloride salts such as sodium or lithium chloride. These molten
salts are typically liquid only at high temperatures.
Alternatively, as previously noted, ionic liquids may be employed;
the said term may refer to a salt, a mixture of salts, a mixture of
components which produce a salt or salts which melts below or just
above room temperature. (As used herein, the term "salt" means an
entity comprising entirely of cationic and anionic species). The
liquids are known as "ionic liquids" although this term is
sometimes used for salts which melt at relatively high
temperatures. In this specification, the term "ionic liquid"
essentially refers to a salt which melts at a relatively low
temperature. Ionic liquids free of molecular solvents were first
disclosed by Hurley and Wier in a series of U.S. Pat. Nos.
(2,446,331, 2,446,349, 2,446,350).
[0004] Common features of ionic liquids include a near zero vapour
pressure at room temperature, a high solvation capacity and a large
liquid range (for instance, of the order of 300.degree. C.). Known
ionic liquids include aluminium(III) chloride in combination with
an imidazolium halide, a pyridinium halide or a phosphonium halide.
Examples include 1-ethyl-3-methylimidazolium chloride,
N-butylpyridinium chloride and tetrabutylphosphonium chloride. An
example of a known ionic liquid system is a mixture of
1-ethyl-3-methylimidazolium chloride and aluminium (III)
chloride.
[0005] Internationally there are two well developed molten salts
processes for the reprocessing/waste conditioning of irradiated
nuclear fuel. A process developed by the Dimitrovgrad SSC--RIAR
process uses high temperature (1000K) eutectic molten salt mixtures
as solvents for the fuel and also as electrolyte systems. In this
Russian system the solvent is typically an eutectic mixture of
NaCl/KCl or CsCl/KCl. The process uses chemical oxidants (chlorine
and oxygen gases) to react with powdered UO.sub.2 fuel, or mixtures
of UO.sub.2 and PuO.sub.2, to form higher oxidation state compounds
such as UO.sub.2Cl.sub.2 which are soluble in the molten salt. At
the cathode the uranium and, if applicable, plutonium compounds are
reduced to UO.sub.2, or UO.sub.2--PuO.sub.2, which form crystalline
deposits. However, after a period of use the molten salt becomes
loaded with fission products which not only begin to affect the
quality of the product, but also result in too much heat generation
within the salt. These fission products are commonly, but not
exclusively, highly active lanthanide or actinide elements which
may need to be isolated in a suitable form for immobilisation as a
waste.
[0006] In the process developed by Argonne National Laboratory
(ANL) in the USA, molten LiCl/KCl eutectic mixtures containing some
UCl.sub.3 are generally used, rather than systems containing sodium
or caesium salts, and a high temperature (around 773K) is again
employed. However, single salts, such as LiCl, are suitable if
higher temperatures are required, for example in the
electrochemical reduction of fuel oxides. The process treats the
spent nuclear fuel by flowing a current to oxidise a uranium anode
and form uranium ions in the molten salt electrolyte. At the
cathode the uranium is reduced and deposited as uranium metal. The
ANL process is, unfortunately, a batch process, since the uranium
is collected in a receptacle at the bottom of the apparatus,
requiring that the process is interrupted in order that the
receptacle may be withdrawn and the product recovered. In addition,
the operation of the process is mechanically intense, involving the
use of rotating anodes which are designed to scrape the product off
the cathodes; difficulties are encountered on occasions due to the
seizure of this mechanism.
[0007] The ANL process requires a metal feed. If oxide fuels are to
be treated, it is necessary to reduce the uranium oxide (usually
UO.sub.2 pellets) to the metal. This reduction process may be
carried out chemically, using lithium metal in a LiCl or LiCl/KCl
molten salt at 500 to 700.degree. C. or, preferably, may be
achieved by means of a direct electroreduction process.
Alternatively, a salt transport process can be used involving a
Cu--Mg--Ca alloy and molten CaCl.sub.2 salt. However, in these
reduction methods the by-products, Li.sub.2O and CaO respectively,
need to be recovered from the molten salt phase by an electrolysis
step. Effectively this means a two stage process.
[0008] A disadvantage of the lithium reduction process for
producing a metallic feed from an oxide is the production of
Li.sub.2O by-product. This requires recycle to make the process
economic, and this is done by an electrolytic recovery of lithium
metal. Hence this is a two stage process, comprising a reduction
step followed by a lithium recovery stage.
[0009] EP-A-1055240 discloses a method for reprocessing spent
nuclear fuel which comprises dissolving the spent fuel or
constituent parts of the spent fuel in an ionic liquid to
substantially separate fissile material from other components of
irradiated fuel. Also disclosed is the subsequent treatment of the
resulting ionic liquor, either by solvent extraction or
electrochemical treatment to recover the dissolved uranium and
plutonium. However, these processes for the clean up of the ionic
liquid are disadvantageous from an economic point of view.
[0010] Furthermore, whilst the methods described in EP-A-1055240
are technically suitable for general use and, in particular, for
use in nuclear fuel reprocessing, it has previously been thought
that an electrorefining process, which avoids the need for an
initial chemical dissolution step, requires the use of a high
temperature molten salt electrolyte. If fuel is chemically
oxidatively dissolved, there is less control over the species which
are dissolved during this step. All those species which will be
oxidised by the oxidising agent added will enter into the solution.
Because the oxidising agents and conditions are aggressive, most
species will dissolve, with the possible exception of species such
as the noble metals.
[0011] EP-A-1212756 discloses a method for separating a metal from
a composition including the said metal, the method comprising
forming an electrolytic cell having an anode, a cathode and an
electrolyte, wherein the anode comprises a composition including
the metal and the electrolyte comprises an ionic liquid, and
applying a sufficient potential difference between the anode and
the cathode to cause the metal to transfer from the anode to the
cathode and to be deposited thereon.
[0012] However, when this method is applied to a composition which
comprises a metal or metal compound comprising a uranium or a
transuranic element, problems of criticality may arise, since ionic
liquids serve as moderators in such systems. In such circumstances,
the difficulties may be obviated by placing a limit on the
allowable dimensions of the electrochemical cell.
[0013] WO-A-02/066712 is concerned with the particular advantages
that may be achieved by increasing the surface area per unit volume
of the anode and cathode and reducing the electrode separation,
thereby maximising the cell current. In addition, this document
describes the benefits of providing high electrolyte velocities,
which result in a scouring action, helping to remove dendritic
growth on the cathode, and also provide turbulent flow, thus
reducing the boundary layer and subsequently increasing the mass
transfer coefficient. The advantages initially observed with
uranium and transuranic elements were also applicable in the case
of a wide range of other metals and metal compositions, and the
process was found to be equally advantageous when using
electrolytes comprising molten salts, rather then ionic
liquids.
[0014] The provision of the metal in a form which exhibits a large
surface area per unit volume facilitates increased efficiency of
the electrorefining process and enables the cell to be simply
constructed, thereby leading to lower costs. The application of a
suitable potential difference between the anode and the cathode
results in electrochemical oxidation of the metal at the anode,
causing it to enter into the liquid electrolyte medium. The soluble
metal species is then electro-transported to the cathode where a
reduction process occurs, which results in the deposition of the
metal at the cathode.
[0015] The process may be applied to a variety of metal fuel feeds;
typically the metal composition to be treated is irradiated nuclear
fuel and the metal to be separated is uranium. Uranium or a uranium
compound, and possibly other transuranic metals or compounds are
deposited at the cathode in a purified form; the cathode generally
comprises a solid cathode in the case of uranium, and a liquid
cathode for mixtures of uranium, plutonium and other transuranics.
Any fission products which are oxidised from the anode together
with the uranium remain in the electrolyte. After the uranium
electrorefining operation, and electrorecovery of transuranics, if
desired, the electrolyte is subjected to further processing if
fission product removal is required.
[0016] The process is analogous to the ANL process, wherein a
metallic fuel feed is electrorefined and a uranium metal product is
collected on a cathode. However, the process shows significant
advantages over the earlier method in that it is free from the
moving components, such as rotating anodes, which often cause
problems with the ANL process. Nevertheless, although the system of
WO-A-02/066712 also shows benefits over the ANL process in
providing for semi-continuous operation, it is still essentially
limited in applicability to batch electrorefiners. Additionally, it
suffers from the disadvantage of limited size, and the geometry of
the system can cause difficulty in achieving a uniform potential
field. Consequently, the system is not well suited to scale-up.
[0017] The present invention, therefore, seeks to address these
shortcomings in the prior art, and to provide a fully continuous
process, which allows for maximised throughput, with the attendant
economic advantages which thereby accrue.
[0018] EP-A-1240647 discloses a single step process for reducing to
metallic form a metal oxide present in spent nuclear fuel, the
process comprising cathodically electrolysing the oxide in the
presence of a molten salt electrolyte, the potential of the cathode
being controlled so as to favour oxygen ionisation over deposition
of the metal from the cations present in the molten salt.
[0019] The process thereby involves the use of a single
electrochemical process to reduce the metal oxide fuel to a
metallic form, with oxygen produced as the only by-product. The
potential of the cathode is maintained and controlled so that only
oxygen ionisation occurs and not the deposition of the cations (eg
Ca ions) in the fused salt. Typically, the oxide comprises an
actinide oxide, such as uranium oxide or irradiated uranium
oxide.
[0020] WO-A-02/099815 provides a process for reducing to metallic
form oxides of uranium, or metals more noble than uranium, present
in spent nuclear fuel comprising a mixture of metal oxides, the
process comprising cathodically electrolysing the oxide in the
presence of a molten salt electrolyte, the potential of the cathode
being controlled so as to favour oxygen ionisation over deposition
of metal from the cations present in the molten salt, and to ensure
that reduction of metals other than uranium or metals more noble
than uranium does not occur. The process is particularly applicable
to the production of actinides, specifically uranium and metals
more noble than uranium, from actinide oxides present in irradiated
nuclear fuels. These methods are applicable to the treatment of
irradiated fuels for producing actinides in metallic form suitable
for use as feeds in subsequent electrorefining processes.
[0021] In the nuclear industry, it is often necessary to separate
metals from a mixture of metal oxides such as occurs in spent
nuclear fuel. Thus mixtures of uranium and plutonium oxides,
together with the oxides of other actinide metals, may additionally
be contaminated with oxides of other, chemically active, metals
such as, for example, those associated with zircalloy cladding. The
method of WO-A-02/099815 provides a method for the treatment of
irradiated fuel which allows for the separation of uranium, and
metals more noble than uranium, from such mixtures as found in
spent nuclear fuel, and provides these metals in a form suitable
for use as the feed in a molten salt electrorefining process,
whilst ensuring that other, more electropositive, metals remain in
the form of oxides.
[0022] The invention provides a single electrochemical process to
reduce the metal oxide fuel to a metallic form, with oxygen
produced as the only by-product. The potential of the cathode is
maintained and controlled so that only oxygen ionisation occurs and
not the deposition of the cations (eg Ca ions) in the fused salt,
and also to ensure that, whilst reduction of uranium or metals more
noble than uranium occurs smoothly, the less noble metals are not
reduced and remain in the anode as oxides. Typically, the mixture
of oxides includes an actinide oxide, such as uranium oxide or
irradiated uranium oxide, or mixed uranium/plutonium oxides. The
uranium oxide is commonly uranium dioxide.
[0023] After electrolysis the irradiated fuel is left in the form
of a metal/metal oxide solid mixture at the cathode, with uranium
and more noble metals having been reduced to the metallic form,
whilst the less noble metals remain in the form of their oxides.
This metallic/metal oxide product, which contains fission products,
can be removed and used directly as the feed for an electrorefining
process. The remaining components of the cell may be re-used
immediately without the need for any cleaning.
[0024] In the electrorefining step, a potential is applied to the
metal/metal oxide mixture at the anode such that only uranium metal
enters the salt, whilst the less noble metals remain behind as
oxides. Insufficient potential is applied to encourage the
dissolution of metals more noble than uranium.
[0025] The advantage of the process is that it is effectively a
single stage process. It is used for the treatment of irradiated
mixed metal oxide nuclear fuel, possibly in the form of pellets
and, most particularly, is applied to fuels which contain uranium
oxide, and mixed uranium and plutonium fuels.
[0026] Whilst the technology of the prior art has found much
success and wide applicability in the nuclear power industry,
however, the industry is always mindful of safety and environmental
issues, and there are constant efforts to increase standards in
these areas. An issue of particular concern is criticality, and
improvements in criticality control continue to be sought. It is
this aspect of the technology that is addressed by the present
invention, which seeks to provide a method and apparatus for the
geometrically safe electrochemical separation of uranium metal from
spent nuclear fuel.
[0027] Criticality safety is concerned with the balance of neutrons
produced by fission of fissile atoms, and the control of this
balance, in order that both critical and supercritical states may
be prevented, thus avoiding the possibility of a self-sustaining
neutron chain reaction. One approach to criticality safety is
asymmetric control, one dimension of the system is restricted, so
that a critical mass of fissile material cannot be obtained. Thus,
by adapting the geometry of the system, and limiting certain
dimensions, it is possible to achieve criticality safety such that
a critical mass of fissile material cannot be achieved. However,
the known prior art does not provide a single example of metal
electrorefiner or oxide electrolyser that displays such geometric
safety and is capable of providing an industrial scale throughput.
Hence, this is the problem addressed by the present invention,
which seeks to provide this elusive combination of industrial scale
viability and criticality safety.
[0028] Of the prior art previously discussed, only WO-A-02/066712
addresses the issue of criticality control. The application
discloses that the electrorefiner is preferably designed to have
designed to have significantly greater length and width than depth,
providing a large, thin planar device which maximises the available
surface area of the electrodes and is beneficial in delivering the
criticality constraints imposed on the system. However, no further
attention is paid to this matter, and no other options are
considered. The present invention addresses this omission and
proposes novel cell designs with geometric criticality control
having particular application in the electrochemical separation of
uranium metal from spent nuclear fuel.
STATEMENTS OF INVENTION
[0029] Thus, according to a first aspect of the present invention,
there is provided an electrorefining apparatus, said apparatus
being capable of operating in continuous mode, and including means
for criticality control.
[0030] Said apparatus is capable of operating in fully continuous
mode, rather than being restricted to operation in semi-continuous,
or batchwise, mode as is the case with the apparatus of the prior
art.
[0031] In order to facilitate efficient operation in continuous
mode, it is preferred that said apparatus should additionally
comprise means for the removal of deposited material from the
apparatus, following operation. Said means for removal of deposited
material may comprise, for example, sweeping means, conveyor means
or means by which a section of the apparatus may be removed.
[0032] Preferably, said means for criticality control comprises
means for geometric criticality control. Most preferably said means
for geometric criticality control comprises means for control of
the dimensions of said apparatus. It is desired that said means for
geometric criticality control should be fully compatible with other
elements of the apparatus, in particular the means for removal of
deposited material from the apparatus. Thus, for example, sweeping
means should not be hindered by the specific dimensions of the
apparatus.
[0033] Particularly preferred means for geometric criticality
control comprise electrochemical cells having a large surface area
per unit volume. Such means most preferably comprises a thin flat
plate which may conveniently be horizontally or vertically
disposed.
[0034] Said electrorefining apparatus may be utilised, for example,
for the electrochemical separation of uranium metal from spent
nuclear fuel. Alternatively, said electrorefining apparatus may be
utilised for the reduction of a metal oxide fuel to a metallic
form, with oxygen produced as the only by-product.
[0035] Typically, the apparatus according to the invention
comprises an electrorefining cell having an anode and a cathode; in
operation, an electrolyte is introduced into said electrorefining
cell. Preferably, said electrolyte comprises a molten salt or an
ionic liquid.
[0036] A second aspect of the present invention envisages a
continuous process for the isolation of metals from spent nuclear
fuels, said process comprising electrochemically treating said
spent nuclear fuels in an apparatus according to the first aspect
of the invention in the presence of an electrolyte.
[0037] Said continuous process preferably comprises the
electrochemical separation of uranium metal from spent nuclear
fuel, or the electrochemical reduction of a metal oxide fuel to a
metallic form, with oxygen produced as the only by-product.
[0038] The application of a suitable potential difference between
the anode and the cathode results in electrochemical oxidation of
the metal at the anode, causing it to enter into the liquid
electrolyte medium. The soluble metal species is then
electro-transported to the cathode where a reduction process
occurs, which results in the deposition of the metal at the
cathode.
DETAILED DESCRIPTION OF THE INVENTION
[0039] A first embodiment of the apparatus according to the first
aspect of the present invention comprises a crusting liquid
cathode. Generally, said crusting liquid cathode is comprised in a
horizontal thin slab configuration. Preferably, said crusting
liquid cathode comprises a liquid metal cathode or liquid alloy
cathode. During the electrochemical separation process for the
separation of uranium metal from spent nuclear fuel, uranium metal
is deposited on the cathode surface, and this metal deposit and
residual amounts of cathode metal and molten salt are then removed
from said cathode surface, for example by intermittent scraping or
scooping. Initially, a high potential and current density may be
employed in order to promote crusting of the molten metal surface
by solid uranium. The metal deposit recovered at the conclusion of
the process generally requires purification in order to remove
contaminants, such as residual cathodic material. Typically, the
anode in said apparatus comprises a basket containing spent fuel,
which is adapted for use in a continuous process. Thus, for
example, said basket may form part of a conveyor belt of such
baskets, which may be successively introduced into the apparatus in
order to provide a continuous process.
[0040] A second embodiment of the apparatus according to the first
aspect of the present invention comprises a cast cathode.
Preferably, said cast cathode comprises a metal or alloy cathode
with a lower melting point than uranium metal, the melted cathode
being introduced to an electrorefiner cell as a liquid which is
then frozen to form a solid metal or alloy cathode as a horizontal
thin slab. In operation during the separation of uranium metal from
spent nuclear fuel, uranium metal is deposited on the cathode
surface, and the cathode is then melted and transferred out of the
cell, transporting with it the uranium metal deposit as a slurry.
The anode in said apparatus is adapted for use in a continuous
process, and preferably comprises a horizontal basket containing
spent fuel, which may be continuously fed and discharged
transversely.
[0041] A third embodiment of the apparatus according to the first
aspect of the present invention comprises a fluidised cell or
pulsed bed. Preferably said fluidised cell or pulsed bed comprises
cathode beads or particulates, preferably formed from graphite or
uranium, and said electrorefiner cell is divided by a ceramic
non-conducting membrane, forming a vertical thin slab. In operation
during the separation of uranium metal from spent nuclear fuel, a
charge is applied across the anode and cathode as though parallel
plates and a molten salt containing dissolved uranium ions is
pumped up through the cathode bed, resulting in the formation of
uranium metal deposits on the bead or particulate surface.
Typically, the anode in said apparatus comprises a basket
containing spent fuel, which is adapted for use in a continuous
process.
[0042] A fourth embodiment of the apparatus according to the first
aspect of the present invention comprises a moving belt cathode.
Preferably, said cathode comprises a refractory metal belt, forming
a horizontal thin slab, conveying in and out of a molten salt and,
in operation during the separation of uranium metal from spent
nuclear fuel, the uranium metal deposits on the belt; once the belt
is conveyed out of the salt, heat is applied to the uranium deposit
to melt or soften it for scraping off the belt. Generally, the
anode in said apparatus comprises a basket containing spent fuel or
a liquid metal or alloy containing dissolved spent fuel, and
adapted for use in a continuous process.
[0043] A fifth embodiment of the apparatus according to the first
aspect of the present invention comprises a consolidating cathode.
Preferably, said consolidating cathode comprises at least one pair
of rotating cylinders, said cylinders being suspended vertically in
a molten salt; in operation during the separation of uranium metal
from spent nuclear fuel, uranium metal deposits on the cathode
cylinder surface. Thereafter, rotation of the cylinders against
each other causes the uranium deposit to compact and consolidate,
thereby forming a plate of uranium metal. The cathode cylinders may
then be scraped off line or scraped in situ to recover uranium
metal; alternatively, the cylinders are formed from uranium metal
and are melted down offline in order to facilitate removal of the
salt and recovery of the cast uranium metal. Potentially, uranium
deposits may break off and settle on the base of the vessel, and
these deposits may be screw conveyed out of the apparatus for
treatment. In general, the anode in said apparatus comprises a
basket containing spent fuel, adapted for use in a continuous
process.
[0044] A sixth embodiment of the apparatus according to the first
aspect of the present invention comprises a liquid anode and plate
cathode. Preferably, said liquid anode comprises spent fuel
pre-dissolved or slurried in liquid metal or alloy. Preferably,
said plate cathode comprises a horizontal thin slab, and said plate
cathode is positioned above said anode in parallel plate
arrangement. Optionally, said cathode is comprised of uranium. In
operation during the separation of uranium metal from spent nuclear
fuel, uranium metal is electrodeposited on to the cathode plate and
said cathode plate is removed periodically to allow for collection
of said uranium metal or, preferably, said cathode plate is
arranged in sections or modules, which may then conveniently be
removed individually to allow for collection of the uranium metal
in the context of the continuous electrorefining process.
[0045] In the process according to the second aspect of the
invention, the spent nuclear fuel typically comprises a metal fuel
assembly of fuel pins; alternatively, the spent nuclear fuel may
comprise reduced metal oxides. Assemblies of fuel pins are firstly
dismantled to single pins for feeding to an electrorefiner cell.
Typically, said pins may then be cropped into small sections by
means of a cropping machine, shredded using a bulk shredder, or
ground to a small particle size. The sections of fuel, which should
be as small as possible in order to allow the electrolyte to act on
the composition, may then be loaded into an anode basket.
[0046] The electrolyte for use in the process according to the
second aspect of the invention may comprise a molten salt or an
ionic liquid. When the electrolyte comprises a molten salt, it may
comprise any molten salt well known to those skilled in the art.
Thus, for example, a LiCl/KCl molten salt eutectic mixture may be
used, for example a LiCl/KCl eutectic melt comprising 41.5 mol. %
KCl, with m.p. 361.degree. C. If higher process temperatures, for
example in the region of 600-700C, are required, such as in direct
electrochemical reduction processes, LiCl alone may be employed as
the molten salt electrolyte.
[0047] Alternatively, the electrolyte may comprise an ionic liquid,
such as 1-ethyl-3-methylimidazolium chloride, or a mixture of two
or more ionic liquids. In the case of electrolytes which comprise
ionic liquids, the electrorefiner cell may be operated at lower
temperatures, but the temperatures have to be kept sufficiently
high to ensure that the liquid is maintained above its melting
point; a suitable heating medium is employed for this purpose.
[0048] The process of the invention can be applied to a variety of
metal fuel feeds. Uranium or a uranium compound, and possibly other
transuranic metals or compounds, will be deposited at the cathode
in a purified form. Any fission products and transuranics,
including plutonium, which are oxidised from the anode together
with the uranium, will remain in the electrolyte. After the uranium
electrorefining operation has been carried out, the electrolyte is
subjected to further processing if plutonium removal is
required.
[0049] By contrast with a process involving chemical dissolution,
in an electrochemical process there can be much greater selectivity
of the species to be dissolved. The potential at the anode can be
controlled, such that metals which are more electropositive than
uranium, and with larger negative Gibbs free energies associated
with the species formed in solution, are the only metals which
dissolve at the anode. This is the first separation step, as many
of the more noble metals will remain behind in an anodic sludge.
The electrolyte now contains a solution of metal ions including
uranium and those of more electropositive species. A suitable
potential is applied at the cathode, whereby uranium and metals
less electropositive than uranium are electrodeposited. This should
only include uranium, as those less electropositive metals have not
been anodically dissolved.
[0050] The process of the present invention is analogous to the ANL
process, wherein a metallic fuel feed is electrorefined and a
uranium metal product is collected on a cathode. However, the
process of the present invention shows significant advantages over
this prior art method in that it is a continuous process, and is
well suited to scale up as a consequence of the criticality safe
nature of the apparatus of the invention. The present process also
allows for the use of ionic liquids, as well as molten salts, in
its operation.
DESCRIPTION OF THE DRAWINGS
[0051] The apparatus and method of the present invention will now
be illustrated, though without limitation, by reference to the
accompanying drawings, in which:
[0052] FIG. 1 is a side elevation representation of a crusting
liquid cathode according to the first embodiment of the first
aspect the invention;
[0053] FIG. 2 is a side elevation representation of a cast cathode
according to the second embodiment of the first aspect of the
invention;
[0054] FIG. 3 is a side elevation representation of a fluidised
cell according to the third embodiment of the first aspect of the
invention;
[0055] FIG. 4 is a side elevation representation of a moving belt
cathode according to the fourth embodiment of the first aspect of
the invention;
[0056] FIG. 5 is a plan representation of a consolidating cathode
according to the fifth embodiment of the first aspect of the
invention; and
[0057] FIG. 6 is a side elevation representation of a liquid
anode/plate cathode according to the sixth embodiment of the first
aspect of the invention.
[0058] Referring firstly to FIG. 1, there is seen an apparatus
comprising a vessel 1 containing an anode 2 and a crusting liquid
cathode 3. In operation, an electrolyte 4 is introduced into the
vessel 1, and metal is deposited at the crusting liquid cathode 3;
this may be removed by the action of the scraper 5, moving along
the surface of the cathode. The vessel has a width and a length of
1 m, giving a base area of 1 m.sup.2; the depth of the vessel is
0.1 m. The electrodes have a length and width of 1 m. The volume of
electrolyte employed is 0.1 m.sup.3.
[0059] Turning to FIG. 2, there is shown an apparatus comprising a
vessel 6 containing an anode 7 and a cast cathode 8. In operation,
an electrolyte 9 is introduced into the vessel 6, and metal is
deposited at the cast cathode 8; this may be removed by the action
of the scraper 10, moving along the surface of the cathode. The
vessel has a width and a length of 1 m, giving a base area of 1
m.sup.2; the depth of the vessel is 0.1 m. The electrodes have a
length and width of 1 m. The volume of electrolyte employed is 0.1
m.sup.3.
[0060] FIG. 3 shows an apparatus consisting of a fluidised cell
comprising a vessel 11 including an anode 12 and a cathode 13 which
comprises cathode beads or particulates 14. The vessel comprises
sections separated by ceramic divider 15. In operation, electrolyte
16 is introduced in the direction of arrow A, and metal is
deposited on the surfaces of the beads or particulates comprising
the cathode 13. The vessel has a width of 0.1 m and a length of 2
m, giving a base area of 0.2 m.sup.2; the depth of the vessel is
0.5 m. The volume of electrolyte employed is 0.1 m.sup.3.
[0061] In FIG. 4, there is shown an apparatus comprising a vessel
17 including an anode 18 and a moving belt cathode 19 mounted on a
pair of rollers 20, 21 and moving in the direction of arrow B. In
operation, electrolyte 22 is introduced into the vessel and metal
is deposited on the moving belt cathode 19. The metal is carried in
the direction of arrow B and the application of heat facilitates
its collection by the action of scraper 23. The vessel has a width
and a length of 1 m, giving a base area of 1 m.sup.2; the depth of
the vessel is 0.1 m. The electrodes have a length and width of 1 m.
The volume of electrolyte employed is 0.1 m.sup.3.
[0062] In FIG. 5, there is seen an apparatus comprising a vessel 24
including an anode 25 and a cathode comprising two pairs of
rotating cylinders 26, 27 and 28, 29. In operation, electrolyte 30
is introduced into the vessel, cathode cylinders 26, 27, 28 and 29
rotate in the directions of arrows C, D, B and F, respectively, and
metal is deposited on the cathode cylinders. The vessel has a
length and a depth of 1 m, and the width of the vessel is 0.1 m.
The electrodes have a length and depth of 1 m, and the width of the
anode basket is 0.05 m. The volume of electrolyte employed is 0.1
m.sup.3.
[0063] FIG. 6 shows an apparatus comprising a vessel 31 including a
liquid anode 32 and a plate cathode 33, arranged in sections or
modules. In operation, electrolyte 34 is introduced into the vessel
and metal is deposited on the plate cathode 33, sections of which
are removed and replaced periodically to allow for recovery of the
metal. The vessel has a width of 1 m and a length of 3 m, giving a
base area of 3 m.sup.2; the depth of the vessel is 0.1 m. The
electrodes have a length of 3 m and a width of 1 m. The volume of
electrolyte employed is 0.3 m.sup.3.
* * * * *