U.S. patent application number 09/847510 was filed with the patent office on 2002-01-10 for melting crucible.
Invention is credited to Quayle, Brian Edwin.
Application Number | 20020004017 09/847510 |
Document ID | / |
Family ID | 9891020 |
Filed Date | 2002-01-10 |
United States Patent
Application |
20020004017 |
Kind Code |
A1 |
Quayle, Brian Edwin |
January 10, 2002 |
Melting crucible
Abstract
A melting crucible is described, the crucible having a
composition comprising in weight %: 24-32 Cr; 8-11 Fe; 0.15-0.25 C;
1.8-2.4 Al; 0.1-0.2 Ti; 0.05-0.12 Y; 0.010-0.10 Zr; 0.1 max. Mn;
0.5 max. Si; 0.1 max. Cu; Balance Ni apart from incidental
impurities.
Inventors: |
Quayle, Brian Edwin;
(Cumbria, GB) |
Correspondence
Address: |
REISING, ETHINGTON, BARNES, KISSELLE
LEARMAN & McCULLOCH, P.C.
5291 COLONY DRIVE NORTH
SAGINAW
MI
48603
US
|
Family ID: |
9891020 |
Appl. No.: |
09/847510 |
Filed: |
May 2, 2001 |
Current U.S.
Class: |
420/449 |
Current CPC
Class: |
F23G 2204/204 20130101;
C03B 5/005 20130101; C22C 19/058 20130101; F27B 14/10 20130101;
F23G 2209/18 20130101; F27B 14/061 20130101; C03B 5/021 20130101;
Y02P 10/25 20151101; F23G 2202/20 20130101; G21F 9/305 20130101;
C03B 5/06 20130101; Y02P 10/253 20151101 |
Class at
Publication: |
420/449 |
International
Class: |
C22C 019/05 |
Foreign Application Data
Date |
Code |
Application Number |
May 6, 2000 |
GB |
0010861.3 |
Claims
1. A melting crucible, the crucible having a composition comprising
in weight %: 24-32 Cr; 8-11 Fe; 0.15-0.25 C; 1.8-2.4 Al; 0.1-0.2
Ti; 0.05-0.12 Y; 0.01-0.10 Zr; 0.1 max. Mn; 0.5 max. Si; 0.1 max.
Cu; Balance Ni apart from incidental impurities.
2. A melting crucible according to claim 1 wherein the chromium
content lies in the range from 26 to 32 wt %.
3. Use as a melting crucible of an alloy comprising in weight %:
24-32 Cr; 8-11 Fe; 0.15-0.25 C; 1.8-2.4 Al; 0.1-0.2 Ti; 0.05-0.12
Y; 0.01-0.10 Zr; 0.1 max. Mn; 0.5 max. Si; 0.1 max. Cu; Balance Ni
apart from incidental impurities.
4. Use as a melting crucible of an alloy according to claim 3
wherein the chromium content lies in the range from 26-32 wt %.
5. A melting crucible substantially as hereinbefore described with
reference to the accompanying description and drawing.
Description
[0001] The present invention relates to a melting crucible and
metal alloys for use as crucibles for the melting of material for
the encapsulation of waste materials particularly, though not
exclusively, radioactive waste material encapsulated by a so-called
vitrification process where the waste is mixed with glass forming
materials which are melted.
[0002] The so-called vitrification process for the encapsulation of
high level radioactive waste (HLW) in particular, involves the
mixing of the waste material with glass forming material and the
subsequent melting of the glass forming material in a crucible so
as to dissolve the waste therein. The waste results mainly from the
dissolution of irradiated fuel assemblies which contain, in
addition to the uranium fuel and fission products per se, large
quantities of iron, zirconium and chromium from the fuel cans,
mainly Zircaloy (trade name) alloy and from the fuel assembly
stainless steel fittings. The waste is mainly in the form of oxides
formed from the calcining of the nitrates resulting from the
dissolution process (the so-called "PUREX Process", for
example).
[0003] The temperatures at which the glass and waste are processed
is in excess of 1050.degree. C., the molten material forming an
aggressive corrosive medium with respect to the crucible material.
The current alloy used for the manufacture of crucibles is known as
Inconel 601 (trade name) (UNS N6601) which comprises in weight %:
23 Cr; 60 Ni; 1.3 Al; <0.5 Co; 0.01 Si; <0.1 C; <1.0 Mn;
<1.0 W; <1.0 Cu; Balance Fe. Each crucible produces about 25
kg of the product waste per hour and has a life of about 4000 hours
consisting of 500 pours on an eight hour cycle before needing to be
replaced. This alloy suffers from chromium depletion in the surface
region in both the alloy above the molten glass level, i.e. in air,
and also in the alloy below the glass level, i.e. leaching of the
chromium by the molten glass. Owing to the nature of the material
being processed, the proximity of operators and direct handling of
the used life-expired crucibles is precluded. Consequently, the
replacement of crucibles is a difficult and time consuming process
as it must be undertaken by remote handling equipment. Furthermore,
the used crucible itself also constitutes active waste which
further adds to the waste which must be dealt with by processing
and storing. Thus, any measure which will extend the service life
of the melting crucible will provide valuable economic and
ecological advantages.
[0004] It is an object of the present invention to provide a
crucible of an alloy having a longer service life for the
processing of HLW waste than known crucible alloys.
[0005] According to a first aspect of the present invention, there
is provided a melting crucible, the crucible having a composition
comprising in weight %: 24-32 Cr; 8-11 Fe; 0.15-0.25 C; 1.8-2.4 Al;
0.1-0.2 Ti; 0.05-0.12 Y; 0.010-0.10 Zr; 0.1 max. Mn; 0.5 max. Si;
0.1 max. Cu; Balance Ni apart from incidental impurities.
[0006] According to a second aspect of the present invention there
is provided use as a melting crucible of an alloy having a
composition comprising in weight %: 24-32 Cr; 8-11 Fe; 0.15-0.25 C;
1.8-2.4 Al; 0.1-0.2 Ti; 0.05-0.12 Y; 0.01-0.10 Zr; 0.1 max. Mn; 0.5
max. Si; 0.1 max. Cu; Balance Ni apart from incidental
impurities.
[0007] Preferably, the chromium content lies in the range 26-32 wt
%.
[0008] The melting operation for the encapsulation of HLW is
generally carried out by induction melting under an air atmosphere,
the crucible being induction heated and the waste charge being
heated by radiation and conduction from the crucible wall. Most
alloys used for high temperature oxidation resistant applications
depend upon the formation of a continuous and stable oxide film on
their surface, the oxide films generally comprising chromia
(Cr.sub.2O.sub.3) and/or alumina (Al.sub.2O.sub.3). Alumina is
generally the more stable oxide at temperatures above 1050.degree.
C.
[0009] During the vitrification process for HLW, the glass forming
materials are selected specifically for their ability to dissolve a
wide range of oxides including chromia and alumina which are
contained in the calcined waste prior to mixing with the glass
forming materials. The alumina results mainly from aluminium added
as a metallurgical addition to the so-called "Magnox" or uranium
metal fuel. Alumina dissolves very easily in the glass whereas
chromia dissolves more slowly. The prior art crucible material,
Alloy 601, is a chromia former, which type of alloys incidentally,
are not normally used for glass processing applications since the
chromia turns the glass green, however, since the waste itself
contains chromia this is of little importance.
[0010] The alloy for use in the crucible according to the present
invention has compositional similarities to Inconel 601 but
contains increased aluminium and further has an addition of yttrium
to improve oxidation resistance due to the formation of a
continuous and stable alumina film in contrast to the chromia film
of Inconel 601. The alloy also contains higher chromium and carbon
to provide a dispersion of chromium carbide precipitates throughout
the microstructure and which produces a significant improvement in
high temperature mechanical strength and creep resistance. Inconel
601 does not contain any second phases to improve high temperature
mechanical properties.
[0011] Testing of alloys used for the crucible according to the
present invention indicate that a service life in the region of
6000 hours will be achieved, i.e. an increase of 50% in the service
life.
[0012] Thus, since the alloy for the crucible of the present
invention is an alumina former, it is surprising that the life and
properties are greatly improved over the prior art alloy. It has
been found that, in use, above the molten glass surface level,
alumina forms as expected providing oxidation resistance in the air
atmosphere pertaining above the glass level, whilst below the glass
surface level, chromia forms, the chromium carbide dispersion
effectively providing a reservoir of chromium. Thus, surprisingly
the crucible alloy according to the present invention provides both
superior oxidation resistance above the glass level in air and
superior corrosion resistance below the glass whilst providing
improved high temperature mechanical properties in both zones which
may allow a greater degree of acceptable corrosion/oxidation before
the crucible needs replacing.
[0013] In order that the present invention may be more fully
understood, an example will now be given with reference to the
accompanying drawing which shows a schematic cross section through
a crucible according to the present invention.
[0014] The drawing shows the crucible 10 of a waste melting plant.
The main melting part 12 of the crucible is surrounded by induction
heating means 14 and a lower neck portion 16 has separate induction
heating means 18 therearound. Waste from a calciner (not shown) is
led into the crucible indicated by arrow 20 as is glass forming
frit, known as "crizzle" in the industry, indicated by arrow 22.
The mixture is melted in the main part 12 of the crucible. A plug
26 of solid vitrified material is left in the neck portion 16 from
a previous melt. Once the charge 24 is fully molten and at the
correct temperature, the plug 26 is melted and the molten charge 24
is run out into a stainless steel container 30 for long term
storage.
[0015] In the drawing, the crucible 10 is made from an alloy having
a composition in weight %: 25 Cr; 10 Fe; 2.1 Al; 0.2 C; 0.1-0.2 Ti;
0.05-0.12 Y; 0.01-0.1 Zr; <0.5 Si; <0.1 Mn; Balance Ni. Above
the molten surface level 28 (and on the outside of the crucible)
the oxide film (not shown) on the surface of the crucible is
predominantly alumina whereas the oxide film on the crucible
surface below the surface 28 is predominantly chromia. The alumina
above the glass level is undepleted and the chromia below the glass
level is also substantially undepleted for much longer than the
Inconel 601.
[0016] Thus the crucible made from and using the metal alloy
according to the present invention provides a large and surprising
advantage over crucibles made from known alloys.
* * * * *