GB2056155A - A process for the production of ceramic fuel tablets for nuclear reactors - Google Patents

Abstract

A process for the production ceramic fuel tablets for nuclear reactors comprises the shock compaction of a corresponding preform in a cavity, wherein the preform is formed by the spherical particles which are produced by hardening droplets consisting of an aqueous solution of the nitrates of the fissible and/or fertile materials and the components of a hardening synthetic resin at temperatures above 100 DEG C, followed by heat treatment at temperatures in the range from 200 to 850 DEG C. The process operates largely without the recycling of scrap and the accumulation of aqueous waste.

Description

SPECIFICATION A process for the production of ceramic fuel tablets for nuclear reactors This invention relates to a process for the production of ceramic fuel tablets for nuclear reactors, which contain the fissile and/or fertile material in oxidic or carbidic form, by the shock compaction of a preform in a cavity.

The fuel elements for light-water reactors (LWR) and fast sodium-cooled reactors (SNR) contain the fuel and/or fertile material largely in the form of sintered tablets of uranium oxide or a mixture of uranium oxide and plutonium oxide.

The plutonium is predominantly used in SNR reactors, although it is also used to a limited extent instead of U-235 in LWR-reactors. Thorium is also a suitable fertile material. In addition to oxide fuel, fuel carbides may also be used in SNRreactors.

Whereas LWR fuel tablets are generally about 12 mm in length, about 10 mm in diameter and weigh approximately 10g, the corresponding data for SNR tablets are normally 6 mm, 5 mm and 1.3 9, respectively.

Oxidic fuel tablets are usualiy made from uranium oxide powder (UO2) or from mixed powder oxides UO2 and PuO2 by pressing, sintering and grinding. The powder-form oxides required for this purpose are required above all to have special sintering properties which can only be obtained by elaborate production processes (cf.

for example German Patent No. 1,592,468: German Auslegeschrift No. 1,592,477 or German Offenlegungsschrift No. 2,623,977).

In the production of carbidic fuel, uranium oxide powder or a mixture of uranium/plutonium oxide powder is normally first reacted with carbon by carboreduction to form uranium carbide or uranium/plutonium carbide and the reaction product thus obtained subsequently subjected to fine grinding to form the sinterable powder.

Tablets are produced from the powder thus obtained by pressing, sintering and grinding in the same way as for the oxidic fuel (cf. US Patent No.

3,236,922).

Attempts have also been made to produce oxidic and carbidic fuel tablets by hot-pressing.

The high temperatures of more than 1 3000C required pose material problems for the cavity and punch which it has not been possible to solve economically. In addition, the dimensional tolerances of the tablets are such that the tablets have to be subsequently machine finished at considerable cost. Accordingly, this process has never acquired any technical significance.

In addition, it is known that fuel tablets can be produced from gel particles. In this case, the gel particles which are produced by wet chemical methods are compressed into tablets and subsequently consolidated to the required density by sintering (cf. for example German Offenlegungsschrift No. 2,713,108). The gel particles are solidified after the formation of spherical droplets either by removing water from the droplets (sol-gel process), by solidification ire the gas phase and/or in a liquid phase introduced beforehand using ammonia (gel precipitation process) or by the internal decomposition of ammonia-releasing auxiliary substances dissolved in the droplets.

In another known process, fuel particles are produced by adding hardening organic substances to a heavy metal nitrate solution and solidifying the droplets formed from this mixed solution in a hot liquid at temperatures in the range from 70 to 950C (Austrian Patent No. 267,007 and German Patent No. 1,283,199). The problem here is to prevent the mixed solution from reacting prematurely. In most cases, this is achieved by adding inhibitors, for example primary alcohols such as methanol or ethanol, to the mixed solution in order to lengthen the reaction time. The hardening organic substance used is preferably a synthetic resin of a mixture of 2 components, preferably resorcinol (or urea) and formaldehyde (or another aldehyde).The mixed solution is sprayed through a drop-forming nozzle into the hot liquid in which the drops harden after a few seconds.

German Offenlegungsschrift No. 2,822,387 describes the production of spherical fuel particles by converting a liquid mixture of nuclear fuels and synthetic resin components into drops, subsequently hardening the drops as they sink to the bottom in a gaseous medium by applying energy in the form of radiation and then subjecting the particles thus formed to heat treatment and sintering.

Conventional processes for the production of fuel tablets which end with a sintering step to obtain the necessary density are attended by a number of disadvantages.

The quality of the pressing powder and the gel particles has to meet stringent requirements in regard to pressability, sinterability, uniformity and purity. These requirements can only be satisfied bs elaborate processes for producing the powder and the particles. It is particularly in cases where gel particles produced by wet chemical methods or pressing powders are used that waste products accumulate, predominantly in the form of contaminated aqueous solutions of ammonium nitrate, sodium nitrate or corresponding fluorine compounds. These waste products have to be eliminated by elaborate processes. In the handling of plutonium and thorium-bred uranium-233, special safetly measures have to be taken on account of the radiation risk and the toxicity factor. These safety measures are very expensive and increase the production costs.

In addition, the sintering step is a fairly expensive operation and is followed by grinding of the tablets. As a rule, some of the particles are inevitably chipped during grinding and have to be rejected. In addition, waste accumulates during grinding and has to be recovered from the grinding liquid. This adds to the production costs. On the other hand, precipitation of the heavy metal salt solutions and washing of the particles or rather the deposit to remove ammonium salts leads to unavoidable losses of fissile materials.

It has also been proposed (British Specification 2,032,899 A) to produce fuel tablets from oxidic powders, optionally in the presence of graphite powder, by forming a preform having a density of from 30 to 60 % of the theoretical, followed by shock compaction in vacuo in a cavity. However, the use of powders gives rise to a few problems in this process.

Accordingly, an object of the present invention is to provide a process for the production of fuel tablets for nuclear reactors, which contain the fissile and/or fertile material in oxidic or carbidic from, by the shock compaction or a preform in a cavity which eliminates the difficulties referred to above and which enables fuel tablets to be economically produced largely without any need to recycle scrap and without any accumulation of aqueous waste products.

According to the invention the preform is formed from spherical particles which have been produced by hardening droplets of an aqueous solution of the nitrates of the fissile and/or fertile materials and the components of a hardening synthetic resin at a temperature above 100 C, followed by heat treatment at a temperature in the range from 200 to 8500 C. The particles thus formed are pressed into tablets in known manner and, after heating to a temperature above 1 5000C, are shock-compacted to their final dimensions, the compaction and ejection of tablets preferably taking place in less than 100 milliseconds. Shock compaction may be carried out in vacuo, under normal pressure or elevated pressure.

The process according to the invention affords a number of advantages.

No aqueous nitrate-containing or fluorine containing waste solutions accumulate during production of the particles. The heavy metal is taken up by the solidified particles in the form of nitrate solution during hardening of the droplets by the polymerisation or polycondensation of, for example, phenols with formaldehyde. Where the particles are produced in this way, no ammonium salts are formed in the droplets during their solidification. Accordingly, there is no need for any washing to remove ammonium salts.

Apart from the dissolved heavy metal nitrate, the mixed solution used for casting the droplets contains only the two reaction components required for forming the hardening synthetic resin, preferably a phenol and an aldehyde. The aldehyde is not added until immediately before casting, preferably in the form of an aqueous formaldehyde solution, to ensure that no premature resin formation occurs before the drops are formed. The mixed solution is advantageously cooled to a temperature below OOC. As a result, there is no need to add inhibitors, inspite of which adequate safety against the risk of premature resin formation is guaranteed. The absence of initiators has the further advantage that the formation of steam bubbles from the low boiling alcohols used as inhibitors, as mentioned in Austrian Patent No.

267,006, is avoided during hardening of the droplets. The danger of the particles splitting open is thus effectively eliminated. In addition, the production process is considerably improved in regard to the reduction of waste.

One particular feature of the invention is that the droplets produced from the reaction mixture in known manner are hardened at a temperature above 100 C, preferably in the hot vapour of a boiling chlorinated hydrocarbon, such as perchloroethylene, or in superheated steam at for example 1 500 C.

Hardening at a temperature above 1000C provides for better heat transfer and, hence, for quicker heating of the droplets in a matter of seconds. The rapid heating of the droplets to temperatures above 1 000C may also be obtained by the absorption of radiation energy, such as infra-red radiation or microwaves. Microwaves are particularly suitable.

The solidified synthetic resin particles containing the heavy metal nitrate are then subjected to a heat treatment at temperatures of from 200 to 8500C. This step, which is known as calcination, is carried out for example initially at a temperature of up to 6000C in an inert gas atmosphere, resulting in the formation of heavy metal oxides, carbon and nitrous gases. Some of the carbon is oxidised by the nitrous gases and removed from the particles in the form of carbon monoxide.

It is a particular advantage that the nitrous gases formed during calcination, providing they are not used up in oxidising the carbon, may be recycled by absorption in water, and conversion into nitric acid, the acid being re-used to dissolve the fuel.

In the production of oxidic fuel, the residual carbon still present after calcination of the particles is burnt in air at a temperature of up to 8500C. To produce carbidic fuel, the residual carbon content is reduced by treatment with CO2 gas at 8500C to such an extent that it is sufficient for the subsequent carbothermal reaction by which the carbides are formed. This heat treatment results in the formation of a material which may be converted without difficulty into preforms. The free flow properties of the spherical particles facilitate processing during preforming of the fuel tablets, reduce dust formation and, hence, pollution of the environment. The heat-treated particles are pressed into tablets under a relatively low pressure which is gauged in such a way that the preforms obtained are easy to handle. There is no longer any need for narrow tolerances regarding the density and dimensions of the preforms because the final dimensions are obtained in the cavity during shock compaction at elevated temperature.

The preforms having a density of from 30 to 60% of the theoretical density are heated to a temperature beyond 1 0000C in a hydrogen or inert gas atmosphere or in vacuo in order to obtain the final state of the chemical composition as a heavy metal oxide or carbide.

These pretreated pressings are heated to a temperature above 1 5000 C, for example in vacuo, and quickly transferred to a steel cavity where they are shock-compacted to their final dimensions and ejected. Compaction and ejection take place in 3 to 100 milliseconds, the vacuum preferably amounting to less than 10-6 bar. Very high dimensional stability is obtained during shock compaction, with the result that there is no need whatever for the ceramic particles to be finished by grinding.

This elimination of the otherwise usual grinding step and of the waste accumulating during grinding simplifies production of the tablets to a considerable extent. Further simplification is obtained by the complete elimination of the otherwise usual sintering treatment during which the final density of the pressed ceramic particles has hitherto been obtained. The short period of time of the order of milliseconds, in which the pressing is in contact with the cavity during shock compaction, enables a steel cavity to be repeatedly used.

The extremely good distribution of fissile material in the fertile material attributable to the mixed solution of the starting materials leads to complete mixed crystal formation during heating to the temperature required for hot shock compaction. The presence of the mixed crystal guarantees complete solubility of the fuel in pure nitric acid and, hence, considerably simplifies dissolution during the reprocessing of burnt-up fuel elements.

All these advantages are particularly noticeable in the event of refabrication of nuclear fuel from reprocessing installations, so that refabrication is made very much easier.

The process according to the invention is illustrated by the following examples.

EXAMPLE 1 Production of UO2-fuel tablets The starting solution used for production of the particles was a uranyl nitrate solution containing 300 g of uranium per litre in which resorcinol was present as the phenol component in a quantity of 380 g/l. This solution was cooled to -50C. A formaldehyde solution cooled to +50C was used for hardening by polycondensation. Before formation of the droplets, it was mixed in a quantity of 200 ml per litre with the cooled starting solution, the mixture thus formed assuming a temperature of-30C. Droplet formation was carried out in known manner by atomisation of the liquid using a spray nozzle. The droplets 0.9 mm in diameter fell into a tube filled with perchloroethylene vapour at 121 0C where they hardened in 1 second to form spherical particles.After hardening the solid particles were calcined in argon for 3 hours at up to 5000C. The residual carbon was then burnt in air at 8000C, resulting in the formation of free-flowing particles of U306.

To produce the tablets, the particles were compressed under a specific pressure of 80 MN/m2 to a density of 4.2 g/cc. The pressings 12 mm in diameter and 1 6 mm long were heated to 1 3000C in a reducing hydrogen atmosphere at a heating rate of 2500C per hour and were kept at that temperature for 1 hour, during which the U308 was reduced to UO2. After this treatment, the pressings had a density of less than 70% of the theoretical density. These porous tablets were heated to 23000C in a vacuum of about 0.1 bar and then transferred to a steel cavity block in which they were compacted to their final dimensions between two force plugs under pressure of 400 MN/m2 and ejected. Compaction and ejection took 7 milliseconds.The tablets cooled to room temperature had a diameter of 9.1 mm and a length of 9.4 mm. Their density amounted to 10.46 g/cc, corresponding to 95.4% of the theoretical density.

EXAMPLE 2 Production of UO2. ThO2-fuel tablets: The starting solution used for forming the particles required for the production of fuel tablets was a solution of uranyl nitrate (180 g of U/1) and thorium nitrate (120 g of Th/1) containing resorcinol in a quantity of 345 g/l. Accordingly, the heavy metal content amounted to 60% by weight of U and 40% by weight of Th. Following the addition of 180 ml. of a cooled 40% formaldehyde solution to 1 litre of starting solution at --50C, solid particles were produced in the same way as in Example 1. These particles consisted of resorcinol/formaldehyde resin in which the uranyl and thorium nitrate solution was contained. By calcination under nitrogen for 4 hours at up to 6000C, all the nitrate was decomposed and converted into nitrous gases.

The residual carbon from the decomposed resin component was then burnt in air at 8500C, resulting in the formation of free-flowing particles of a mixture of uranium and thorium oxide.

To produce the fuel tablets, the particles were compressed under a specific pressure of 1 20 MN/m2 to a density of 4.1 g/cc. The preforms 12 mm in diameter and 15.5 mm long were then reduced to UO2.ThO2 in a hydrogen atmosphere in the same way as in Example 1. The porous tablets having a density of less than 70% of the theoretical density were heated to 23000C in a vacuum of approximately 0.1 bar and then transferred to a steel cavity block in which they were compacted to their final dimensions between two force plugs under a pressure of 500 MN/m3 and ejected. Compaction and ejection took about 20 milliseconds. The tablets cooled to room temperature had a diameter of 9.1 mm and a length of 9.0 mm. Their density amounted to 9.97 g/cc., corresponding to 94.1 % of the theoretical density.

EXAMPLE 3 Production of UO2.PuO2-fuel tablets: Similarly to Example 2, a solution of uranyl nitrate (180 g of U/1) and plutonium nitrate (120 g of Pu/l) containing 345 g/l or resorcinol was used as the starting solution for production of the particles. Apart from a lower temperature during hot shock compaction (20000C as opposed to 23000C), all the other production parameters were the same. The tablets obtained consisted of 60% by weight of Us, and 40% by weight of PuO2 and had the following properties: Diameter 9.1 mm Length 8.5 mm Densityl 0.80 g/cc., corresponding to 96.8% of the theoretical density EXAMPLE 4 Production of UC-fuel tablets: The starting solution for the resin particles contained 300 g of U/i in the form of uranyl nitrate and 380 g/l of resorcinol.This solution was cooled to -70C, subsequently mixed with cooled 40% formaldehyde solution and tempered to -50C.

Droplets 0.6 mm in diameter were produced from the solution and were hardened in 1.5 seconds in a vertical argon-filled graphite tube 100 mm in diameter heated to 8000C. The particles were then calcined in argon for 3 hours at a maximum temperature of 5000 C. The chemical composition of these particles was 47.6% by weight of U, 38% by weight of C and 14.4% by weight of 0, corresponding to the composition Us2 43. In order to adjust the stoichiometric quantity of carbon required for carboreduction, the particles were treated in a stream of CO2 at 8500C. The C content then amounted to 11.6% by weight and the O/U ratio to 2.08. Tablets having a density of 3.8 g/cc were pressed from the particles under a pressure of 200 MN/m2. The tablets were then heated to 1 7500C, the UO2,0B being converted into UC by carboreduction. Final compaction was obtained by hot shock compaction for 10 milliseconds at a temperature of 21 000C and under a specific pressure of 400 MN/m2. The tablets cooled to room temperature had the following properties: Diameter: 7.9 mm Length: 9.5 mm Density: 12.93 g/cc., corresponding to 95% C-content: 4.64% by weight C (free): 80 ppm.,

Claims (8)

1. A process for the production of ceramic fuel tablets for nuclear reactors, which contain fissile and/or fertile material in oxidic or carbidic form, by the shock compaction of a preform in a cavity, the preform being formed from spherical particles which have been produced by hardening droplets of an aqueous solution of the nitrates of the fissile and/or fertile materials and the components of a hardening synthetic resin at a temperature above 1 000C followed by heat treatment at a temperature in the range from 200 to 8500C.

2. A process as claimed in Claim 1 wherein shock compaction is carried out in less than 100 milliseconds at a temperature above 1 5000 C.

3. A process as claimed in Claim 1 or 2 wherein the droplets are obtained from an aqueous solution which was cooled to a temperature below OOC before formation of the droplets.

4. A process as claimed in any of Claims 1 to 3, wherein the droplets are hardened in a steam atmosphere over a period of from 0.1 to 3 s.

5. A process as claimed in any of Claims 1 to 4, wherein the spherical particles have a mean diameter of from 0.1 to 1 mm.

6. A process as claimed in any of Claims 1 to 5, wherein before shock compaction, the oxidic fuel preforms are heated in a reducing atmosphere.

7. A process for the production of ceramic fuel tablets substantially as described with reference to any of the examples.

8. Ceramic fuel tablets when produced by a process as claimed in any of Claims 1 to 8.