GB2330684A - Production of nuclear fuel pellets - Google Patents

Abstract

A method of producing nuclear fuel pellets containing mixed metals comprises mixing urania powder with porous spheres containing mixed metals and obtainable by gel precipitation. The mixed metals may be in the form of oxides of uranium and plutonium or thorium.

Description

PRODUCTION OF NUCLEAR FUEL PELLETS This invention relates to methods of producing nuclear fuel pellets and is particularly concerned with the use of gel precipitation in connection with such methods.

Gel precipitation was introduced in the early 1970's primarily as a method of producing fast reactor fuel that could be loaded directly into the fuel pin. There are two principle methods. internal gelation involves the formation of droplets in an immiscible liquid (silicone oil or a chlorinated hydrocarbon) or in air'.

Homogeneous precipitation occurs because hexamethylene tetramine in the feed solution undergoes thermal decomposition and releases ammonia internally as the sphere drops in the hot gelation media.

External gelation, or polymer supported precipitation, involves the addition of a water soluble organic polymer to a nitrate feed solution (eg a solution of Pu(NO3)4 and Uo2(No3)2)2s. This additive maintains the structure of the droplet as it precipitates when contacted with ammonia gas or an ammonium solution.

More recently a process called total gelation has been developed, which in effect combines the internal and external routes6,7.

The gel spheres are typically washed in hot water to remove reaction products from the gelation stage and then dried, for example by extracting the water into hexanol or another solvent. The spheres are calcined in, usually, a carbon dioxide atmosphere to debond them, i.e. to remove polymer and convert the spheres to microspheres of porous metal oxide.

Gel precipitation offers a number of advantages over traditional powder fabrication methods. The absence of dust, with its associated high plant maintenance requirements and operator dose levels, is often quoted as the most significant improvement. The benefits, however, of co-processing to give a homogeneous oxide product, the continuous operation of a process capable of remote operation and the ability to pour the inherently free flowing fully-sintered spheres directly into fuel pins are all recognised as important further advantages.

Gel sphere routes were initially developed to provide spheres for vibro packing into pins for use in fast reactors but this type of fuel could not withstand the very high ratings used. The spheres were later used as press feed for pelleting but failed to produce pellets of adequate quality. More recently gel sphere routes have been reinvestigated for thermal MOX. MOX fuels have a relatively high ratio of uranium to plutonium. Accordingly, the gel sphere process, applied to a MOX blend, involves the treatment of a relatively large volume in the form of the plutonium stream.

According to the present invention, there is provided a method of producing nuclear fuel pellets, comprising the step of mixing uranium powder with porous spheres containing mixed metals and obtainable by gel-precipitation.

The mixed metals are normally oxides and often consist of UO2 and PuO2. In pellets, these oxides typically form a thermal MOX blend. Alternatively, the mixed oxides may contain additional oxides to UO2 and PuO2, e.g. oxides of Np or Am. The invention also contemplates the preparation of mixed nitride pellets, mixed carbide pellets, and Th/U pellets such as, for example, ThO2-25% UO2.

Although the invention is applicable to mixed fuels other than (UO2 + PuO2), the following description relates specifically to (UO2 + Put2) fuels.Included in the present invention, therefore, is a method of producing nuclear fuel pellets containing both uranium and plutonium in which the ratio of uranium to plutonium is relatively high, comprising the steps of:1. forming gel-precipitated spheres containing uranium and plutonium in which the ratio of uranium to plutonium is relatively low; and 2. mixing said spheres with urania powder to increase the ratio of uranium to plutonium to a relatively high ratio.

A uranium/plutonium mixture in which the proportion of plutonium is relatively high is the Masterblend mixture which typically has 30% plutonium and 70% uranium. If the gel sphere process is applied to a Masterblend composition and the resultant microsphere product is then blended down to MOX enrichment with urania, the volume of the plutonium stream requiring finishing is reduced by a factor of 5 compared with that involved in the treatment of an initial MOX composition.

Furthermore the blending down stage will improve the pressing properties and quality of the pellets.

A method in accordance with the present invention minimises dry powder processing by eliminating milling and conditioning stages in fuel fabrication. Powder is solely handled as urania.

The gel-precipitation process used in forming the microspheres is not critical to the invention. The skilled reader will not need instruction as to gel-precipitation techniques, but one technique which may be mentioned is the so-called KEMA internal gelation route, originally devised in 19741 but further developed subsequently. An alternative technique is an external gel-precipitation method developed by the UK Atomic Energy Authority2~5. This latter process is summarised in the flow diagram:

Pu(N03)4 + U02(N03)2 Formamide Polyacrylamide Feed preparation (mixing of solutions) Drop formation t Precipitation in ammonia Washing Drying Removal of polymer (calcination) Pressing into pellets Sintering The gel spheres are calcined/debonded to form microspheres containing uranium and plutonium, e.g. using known processes. The microspheres are mixed, suitably by tumbling, with urania powder and formed into pellets. Conventional pressing techniques may be used to form green pellets which are then sintered. The pellets may be subjected to one or more further process to form a fuel rod or other product.

In one class of processes uranyl nitrate and plutonium nitrate are fed directly to the gel-precipitation process from a reprocessing plant, as a co-processed Masterblend nitrate feed. This has the advantage that the plutonium is never separated from the uranium in reprocessing and so the process is more diversion resistant and has improved criticality control.

Alternatively uranyl nitrate and plutonium nitrate may be supplied separately and then be blended to the desired Masterblend ratio. This process avoids the high energy mixing of UO2 and PuO2 as practised in dry powder routes to MOX to achieve required homogeneity.

In any event, the feed is gel-precipitated using any method. The sphericity and size of the particles does not have to be as tightly controlled as for vibro fuel, the prior art Vipak fast reactor fueP, since the particles are just a press feed.

In practice, it will be necessary to control the density of the calcined/debonded spheres. Low density spheres (e.g. 1.4 gem~3) produced by debonding at low temperatures (e.g. 500 "C) are very deformable during pressing but contain a lot of residual carbon. These can be pyrophorically unstable and, if necessary or desirable, will in practice be subjected to an oxidative heat treatment prior to sintering to remove the carbon. The calcined spheres preferably have a diameter of from 200 to to 600 clam, although this size range is not critical.

High density spheres produced conventionally by debonding at higher temperatures (r800 OC) have low residual carbon levels. These, however are generally brittle due to the onset of sintering and tend to splinter on pressing. The final fuel pellets tend to contain undesirable relic structures from these pre-sintered spheres.

As is known, control of various process parameters, e.g. sphere ageing, drying conditions and calcination temperature, produces a stable and pressable sphere over a density range of at least 1.2-6.5 gum~3.

The calcined spheres are blended, for example by gentle tumbling, with sufficient urania powder to achieve the required final MOX Pu enrichment. A lubricant (e.g. zinc stearate or stearic acid) can be added at this stage although, due to the spherical nature of the spheres, inter-particle friction and die wall friction should be low. This should be especially true if a free flowing and readily pressable UO2 powder type is selected, e.g. ammonium uranyl carbonate ("AUC") derived powder or another powder with broadly comparable flowability and pressability. It is preferred to use UO2 obtained by a precipitation process, since these generally produce powder with better pressing characteristics and reactivity. Precipitation routes include AUC, ADU, peroxide and hydrothermal synthesis. However UO2 powder obtained by thermal denitration (TDN) may be suitable, especially if a modified production route is used to increase powder reactivity, eg microwave or plasma heating, reduced pressure processing atmosphere or additives in feed solution.

If necessary or desirable, the UO2 powder, the porous, debonded microspheres or both may be treated with a binder to increase the green strength of the pellets.

A conventional gel sphere route, therefore, has the following stages:- a) mixing of solutions b) drop formation c) precipitation within drops by ammonia to form soft spheres d) washing with water e) drying of spheres f) calcination (de-bonding, i.e. removal of polymer) g) pressing into pellets h) sintering.

The drying may be performed by extracting the water into an organic solvent such as hexanol, for example, and then removing the solvent by evaporation.

A process in accordance with the present invention may include the following steps a) mixing of solutions b) drop formation c) precipitation within drops by ammonia to form soft spheres d) washing with water e) removal of water f) calcination g) blending with free flowing urania h) pressing into pellets i) sintering the fuel, by for instance, low temperature oxidative sintering.

In a possible variant of the process in accordance with the present invention the soft spheres from stage c) are passed directly into a non-fluidised bed drier and then to the calcination stage. In this way the number of treatment columns may be reduced from four to one and the solvent waste stream eliminated.

Example In the Example thorium has been used as a representative simulant for plutonium.

(U-30% Th)O2 gel spheres were made at three different sizes ( > 100, 200 and 400 llm equivalent sinter diameter) in separate batches by an external gelation route. Each of these batches was sub-divided and each debonded at different temperatures to give a range of debond densities (e.g. for 100 llm sphere 1.5, 2.5 and 3.1 gcm-3). These sub batches were further divided and diluted down to simulated MOX enrichment by mixing with three different types of UO2: BNFL's Integrated Dry Route material (IDR), powder prepared by ammonia precipitation (ADU) and powder prepared by the ammonium uranyl carbonate route (AUC) involving precipitation by ammonia and carbon dioxide. Each of these was pressed over a range of pressing pressures (#231-617 MPa) and sintered in Ar/4% H2.

Two specific examples are detailed in protocols 1 and 2 below: 1. (U-30% Th)O2 100 m spheres were prepared by external gelation and debonded to a density of 3.09 gcm-3, These were blended by tumbling with 5.02 times as much AUC UO2 and 0.1 wt% Zn stearate. These were pressed into pellets, green density > 5 gcm-3 and sintered to a density > 9 gcm~3- 2. (U-30% Th)O2 200 m spheres, debond density 2.56 gcm-3, were blended with 5.02 times as much ADU UO2 to simulate a 5% fissile content for MOX.

Pellets were pressed to green densities > 5 gcm-3 and sintered to densities > 9.7gcm-3.

REFERENCES 1. J.B.W. Kanij, A.J. Noothout and 0. Votocek. The KEMA (U(Vr)-process for the production of microspheres. IAEA-161 (1974) p185-195.

2. GB-A-2023110.

3. US 4284593.

4. B. Stringer, P.J. Russell, B.W. Davies and K.A. Danso. Basic aspects of the gel-precipitation route to nuclear fuel. Radiochimica Acta 36(1984)31.

5. GB-A-1575300.

6. Zhichang et al, The First Pacific Rim International Conference on Advanced Materials and Processing, Hanzhou, China, June 23-27, 1992.

7. Zhichang et al, the preparation of UO2 ceramic microspheres with an advanced process (TGU), China Nuclear Science and Technology Report, CHIC0073, 1994.

Claims (21)

1. CLAIMS 1. A method of producing nuclear fuel pellets containing mixed metals, comprising the step of mixing urania powder with porous spheres containing mixed metals and obtainable by gel-precipitation.
2. 2. A method of Claim 1, wherein the mixed metals are mixed metal oxides.
3. 3. A method of Claim 2, wherein the mixed metal oxides comprise ThO2 and uo2.
4. 4. A method of Claim 2, wherein the mixed metal oxides comprise UO2 and PuO2.
5. 5. A method of any of Claim 4, wherein the spheres are obtainable from a Masterblend feed.
6. 6. A method of any of Claims 1 to 5, wherein the urania powder is free flowing and readily pressable.
7. 7. A method of any of Claims 1 to 6, wherein the spheres are mixed with the urania powder by tumbling.
8. 8. A method of any of Claims 1 to 7, wherein the mixing step includes mixing of a lubricant with the urania powder and the spheres.
9. 9. A method of any of Claims 1 to 8, wherein the product obtained by the mixing step is formed into fuel pellets by pressing it into green pellets and sintering the green pellets.
10. 10. A method of Claim 9, wherein a binder is included in the green pellets.
11. 11. A method of producing nuclear fuel pellets containing both uranium and plutonium in which the ratio of uranium to plutonium is relatively high, comprising the steps of: i) forming gel-precipitated spheres containing uranium and plutonium in which the ratio of uranium to plutonium is relatively low; and ii) mixing said spheres with urania powder to increase the ratio of uranium to plutonium to a relatively high ratio.
12. 12. A method according to Claim 11 in which the gel spheres are formed by precipitation within drops by ammonia to form soft spheres.
13. 13. A method according to Claim 12 in which the soft spheres are passed directly into a non-fluidised bed dryer and calcined.
14. 14. A method according to Claim 12, wherein the spheres are washed, dried, and then calcined.
15. 15. A method according to Claim 13 or Claim 14 in which, following calcination, the spheres are blended with urania.
16. 16. A method according to any of Claims 13 to 15, wherein the calcined spheres have a density of from 1.2 to 6.5 gcm3.
17. 17. A method of producing nuclear fuel pellets, comprising forming gel spheres containing a mixture of UO2 and PuO2; washing the gel spheres with water; removing the water; calcining the gel spheres to form porous spheres of UO2 and PuO2; blending the porous spheres with urania powder; pressing the resultant blend into pellets; and sintering the resultant pellets.
18. 18. A method of any of Claims 11 to 16 which further includes the feature(s) recited in one or more of Claims 3 or 6 to 10, or a method of Claim 17 which further includes the feature(s) recited in one or more of Claims 3, 6 to 10 or 16.
19. 19. A method of any of Claims 1 to 18, wherein the gel-precipitation process uses uranyl nitrate and plutonium nitrate fed directly to the process from a reprocessing plant.
20. 20. A method of any of Claims 1 to 19, wherein the fuel pellets are subjected to one or more processes, for example to form them into fuel rods.
21. 21. A product comprising a mixture of urania powder and porous spheres containing a mixture of fissile metal oxides.