GB2105697A

GB2105697A - Methods of preparing sinterable uranium dioxide powder

Info

Publication number: GB2105697A
Application number: GB08225109A
Authority: GB
Inventors: Veldon D Holaday; Thomas A Thornton
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1981-09-11
Filing date: 1982-09-03
Publication date: 1983-03-30
Also published as: CH654820A5; ES515439A0; IT8209510A0; ES8403347A1; SE8205138D0; AU8813382A; SE8205138L; IT8209510A1; IT1192507B; AT385267B; BE894371A; KR880002699B1; FR2512801A1; DE3232867A1; IL66706A0; CA1197069A; DK406882A; ATA335682A; NO823079L; BR8205375A

Abstract

In a method of preparing a sinterable uranium dioxide powder for use in nuclear fuel preparation, microwave radiation is used and microwave induction furnaces replace conventional electrical resistance furnaces, radiant heat transfer dryers and kilns. Specifically, a starting material comprising uranyl nitrate hexahydrate, ammonium diuranate or ammonium uranyl carbonate is heated in a microwave induction furnace until it decomposes. The decomposed material is heated in a microwave induction furnace in a reducing atmosphere until it is decomposed to uranium dioxide powder, the powder being cooled in a reducing atmosphere.

Description

SPECIFICATION Methods of preparing sinterable uranium dioxide powder The present invention relates to methods of preparing sinterable uranium dioxide powder for use in nuclear fuel preparation.

Uranium dioxide is the fuel most commonly used in present day nuclear power reactors.

Generally, uranium dioxide powder is pressed and sintered to form pellets which are loaded into and sealed in slender metal tubes called fuel rods. it is a plurality of such fuel rods that establishes an accumulation of fissionable material in sufficient concentration to support sustained fission reactions within the core of a nuclear power reactor.

A number of methods have been developed for preparing a sinterable uranium dioxide powder, generally the starting compound in a nuclear fuel pellet preparation process, the most common of which methods involves the decomposition and reduction of ammonium diuranate (ADU) and is known as the ADU method. ADU is produced by precipitation from a solution of uranyl fluoride by the addition of ammonia and the ADU formed in this manner has a very fine particle size which carries through to uranium dioxide powder after thermal drying, decomposition and reduction in an electrical resistance furnace, radiant heat transfer dryer, kiln or a combination thereof.

Another common method for the manufacture of uranium dioxide powder is the ammonium uranyl carbonate or AUC method. The AUC is produced by precipitation from a solution of uranyl fluoride by the simultaneous addition of NH3 and CO2; the AUC precipitated thereby is separated from the mother liquor by filtering and washing, and the uranium dioxide powder is formed by thermal decomposition of the AUC and subsequent reduction of the resulting U308 to UO2 in a reducing atmosphere. The thermal decomposition of the AUC and the reduction of the oxide into uranium dioxide powder in hydrogen or other reducing gas is normally carried out in an electrical resistance furnace or in two such units, such as the so-called vortex-bed furnaces.

Still another, method for the manufacture of uranium dioxide powder is the uranyl nitrate hexahydrate or UNH method. The UNH method proceeds by starting with uranyl nitrate hexahydrate, UO2, (NO3)2.6 H2O, then heating and decomposing the compound in an electrical resistance furnace to form UO3, oxides of nitrogen and water vapour. The UO3 is then heated in an electrical resistance furnace in a hydrogen reducing atmosphere to form uranium dioxide powder and water vapour.

The prior art processes for preparing uranium dioxide powder have in common the use of standard electrical resistance or combustion-fired heating furnaces during the decomposition and reduction steps, that is, the decomposition to UO3 or U308 followed by reduction to uranium dioxide powder. Alternatively, uranium bearing compounds are processed primarily by methods utilising radiant heat transfer dryers and kilns.

Heretofore, microwave induction has been used as a heating mechanism almost entirely via the susceptance of the water molecule to microwave radiation, that is, the use of microwaves for the heating of materials has been centred on the effects that microwaves have no water molecules. Microwaves cause rapid changes in the polarisation of the water molecule, thereby generating heat. Embodiments of the invention described hereinbelow disclose that uranyl nitrate hexahydrate, ammonium diuranate and ammonium uranyl carbonate also suscept to microwave radiation, generating heat.

Accordingly, the above-discussed electrical resistance furnace, radiant heat transfer dryer and kiln can be replaced by microwave induction furnaces during the preparation of uranium dioxide powder via the ADU, AUC and UNH powder preparation processes.

According to the present invention there is provided a method of preparing a sinterable uranium dioxide powder for use in nuclear fuel preparation, the method comprising the steps of: heating a starting material in a microwave induction furnace for a period of time sufficient to decompose the starting material, the starting material comprising a compound selected from the group consisting of uranyl nitrate hexahydrate, ammonium diuranate and ammonium uranyl carbonate; heating the decomposed compound in a microwave induction furnace in a reducing atmosphere for a period of time sufficient to reduce the decomposed compound to uranium dioxide powder; and cooling the uranium dioxide powder in a reducing atmosphere.

Embodiments of the present invention described hereinbelow can overcome or at least alleviate many of the shortcomings of the prior art methods by decreasing material heatup time, allowing a greater range of processing temperatures, shortening processing times, lowering fluoride impurity levels, improving the ease in handling gelatinous ADU or AUC filter cakes, conserving energy by generating heat entirely within the target material, finding greater utility in remote locations required for nuclear fuel processing and providing a ceramically active, sinterable uranium dioxide powder product.

The invention may be more readily understood from the following description, given by way of illustrative and non-limiting example, of embodiments thereof.

A sinterable uranium dioxide powder to be used in a nuclear fuel preparation process is produced by first selecting a commercially available starting material from the group of compounds comprising uranyl nitrate hexahydrate, ammonium diuranate and ammonium uranyl carbonate. The selected starting material or compound is then heated in a microwave induction furnace for a period of time sufficient to decompose the material, the composition of which may have a uranium oxide stoichiometric range of from U03 to U308. The preferred decomposition end product is U 308 and the decomposition may be conducted in either an oxidising atmosphere (air, 2 or the like), a mixed air-steam atmosphere, or an inert atmosphere.

The decomposition step is conducted at a heating temperature in the range of about 4000C to 6000C when uranyl nitrate hexahydrate is selected as the starting compound. The decomposition heating temperature is conducted in the range of about 3500C to 4500C when either ammonium diuranate or ammonium uranyl carbonate is selected as the starting compound.

The decomposed compound is then heated in a microwave induction furnace in a reducing atmosphere consisting essentially of a hydrogennitrogen gas mixture, or the like, for a period of time sufficient to reduce the decomposed compound to uranium dioxide powder; the reduction step is conducted at a heating temperature in the range of about 4500C to 5500C, notwithstanding the starting material selected from the aforementioned compound group. The uranium dioxide powder is then cooled in a reducing atmosphere to approximately room temperature. After cooling the powder is ready for use in a nuclear fuel preparation process.

Uranyl nitrate hexahydrate, ammonium diuranate and amonium uranyl carbonate were each subjected to microwave radiation in a microwave induction furnace at approximately 2450MHz the frequency of the standard kitchentype microwave oven, to determine the susceptance of each compound to microwave radiation. It should be understood that, while a conventional microwave oven was selected for use because of its ready availability, other microwave induction furnaces operating at different frequencies would also be operable.

Additionally, one oven or a plurality of ovens could be used for the decomposition and reduction processes. Each uranium compound readily suscepted, heating rapidly. However, other materials, such as niobia, alumina, silica, and graphite, while suscepting when exposed to microwave radiation, did not exhibit the rapid heating found to be characteristic of the above compounds of uranium.

Uranyl nitrate hexahydrate crystals suscepted to microwave radiation in a microwave induction furnace in an oxidising atmosphere by first forming a liquid as the hydrated water molecules were released, then decomposing in a 4000C to 6000C temperature range, progressively drying, releasing nitrous oxide gas and water vapour and forming uranium trioxide (UO3). The U03 was then heated to a temperature in the 450"C to 5000C range in a microwave induction furnace in a reducing atmosphere wherein water vapour was released and the U03 was reduced to uranium dioxide powder which was then cooled in the reducing atmosphere to about room temperature.

Ammonium diuranate, available as a filter cake, suscepted to microwave radiation in a microwave induction furnace in an oxidising atmosphere by first releasing water and drying in the microwave field and then by decomposing in a 3500C to 4500C temperature range, releasing ammonia gas and water vapour and forming U308. The U308 was then heated to a temperature in the 450 CC to 550"C range in a microwave induction furnace in a reducing atmosphere wherein water vapour was released and the U308 was reduced to uranium dioxide powder which was then cooled in the reducing atmosphere to about room temperature.

Ammonium uranyl carbonate, subjected to the conditions imposed upon ammonium diuranate, decomposed in much the same manner as did the ammonium diuranate, releasing gases of ammonia and water vapour with the additional release of carbon dioxide gas and forming U3O8.

The reduction of U308 to uranium dioxide powder, followed by cooling, proceeded as did the reduction and cooling of ammonium diuranate.

Uranyl nitrate hexahydrate, ammonium diuranate and ammonium uranyl carbonate decomposition and reduction in a microwave induction furnace or furnaces is accomplished in processing times in the order of minutes rather than the hours customarily associated with the use of conventional electrical resistance furnaces.

Additionally, the processing of a glossy or gelatinous filter cake does not hinder the microwave decomposition-reduction processes.

(The presence of such cakes lengthens process times in conventional furnaces and affects finished product quality). Uranyl nitrate hexahydrate, ammonium diuranate and ammonium uranyl carbonate processed in a microwave field produce a finished product of sinterable uranium dioxide powder suitable for use in a nuclear fuel preparation process.

Claims

1. A method of preparing a sinterable uranium dioxide powder for use in nuclear fuel preparation, the method comprising the steps of: heating a starting material in a microwave induction furnace for a period of time sufficient to decompose the starting material, the starting material comprising a compound selected from the group consisting of uranyl nitrate hexahydrate, ammonium diuranate and ammonium uranyl carbonate; heating the decomposed compound in a microwave induction furnace in a reducing atmosphere for a period of time sufficient to reduce the decomposed compound ot uranium dioxide powder; and cooling the uranium dioxide powder in a reducing atmosphere.

2. A method according to claim 1, wherein the first-mentioned heating is conducted in an oxidising atmosphere.

3. A method according to claim 1 ,wherein the first-mentioned heating is conducted in a mixed air-steam atmosphere.

4. A method according to claim 1, wherein the first-mentioned heating is conducted in an inert atmosphere.

5. A method according to any one of the preceding claims, wherein the starting compound decomposition is in the range of from U03 to U3O8.

6. A method according to any one of claims 1 to 5, wherein the first-mentioned heating is conducted at a temperature in the range of about 4000C to 6000 C.

7. A method according to any one of claims 1 to 5, wherein the first-mentioned heating is conducted at a temperature in the range of about 350 C to 4500C.

8. The method according to any one of the preceding claims, wherein the second-mentioned heating is conducted at a temperature in the range of about 450 C to 55O0C.

9. A method according to any one of the preceding claims, wherein the uranium dioxide powder is cooled to about room temperature.

10. A method according to claim 1, substantially as herein described.

11. A uranium dioxide pellet formed by sintered uranium dioxide powder produced by a method according to any one of the preceding claims.