DE2921146C2 - Process for the production of ceramic fuel pellets for nuclear reactors - Google Patents

Description

The invention relates to a method for the production of ceramic fuel tablets for nuclear reactors, which the fission and / or breeding material in oxidic or carbide form contained by impact compaction of a pre-press in a die.

The fuel elements for light water reactors (LWR) and fast sodium-cooled reactors (SNR) contain the fuel and / or breeding material mainly in the form of sintered tablets made of uranium oxide or a mixture of uranium oxide and plutonia oxide. Plutonium is mainly used in SNR reactors ^: n limited quantities, however, it is also used in LWR reactors instead of U-235. Thorium can also be used as a breeding material. In addition to oxidic fuel, fuel carbides can also be used in SNR reactors.

While the LWR fuel tablets generally have a height of about 12 mm, a diameter of approx. 10 mm and a weight of approx. 10 g are the corresponding data for SNR tablets normally 6 mm, 5 mm and 1.3 g.

Oxydic fuel tablets are usually made from uranium oxide powder (UO2) or mixed Powder oxides UO2 and PUO2 by pressing, sintering and Loops made. The pulverulent oxides required for this must above all have special sinter properties have shafts that can only be achieved through complex manufacturing processes (e.g. DE-PS 92 468, DE-AS 15 92 477 or DE-OS 26 23 977).

In the manufacture of carbidic fuel, uranium oxide powder, or a Mixture of uranium plutonia powder, with carbon by carbo-reduction to form uranium carbide or to Uranium-plutonium-carbide converted and then the reaction product to fine sinterable powder ground. Analogous to the oxidic fuel, this is made by pressing, sintering and grinding Tablets manufactured (US-PS 32 36 922).

Attempts have also been made to produce oxidic and carbidic fuel tablets by hot pressing. At the required temperatures of more than 1300 ^° C., the material problems for the die and punch could not be solved economically. The dimensional tolerances of the tablets achieved in this way nevertheless required extensive post-processing. Therefore, this process has not achieved any technical importance

It is also known to produce the fuel tablets from gel particles. In doing so, the Gel particles manufactured by wet chemical means are compressed into tablets and then sintered the target density is compressed (ζ. Β. DE-OS 27 13 108). Included solidification of the gel particles takes place either after the formation of spherical drops by removing water from the droplets (sol-gel process), by solidifying in the gas phase and / or in a submitted liquid phase with ammonia (gel precipitation process), or by internal decomposition of im Drops of dissolved excipients that release ammonia. These gel particles produced can be admittedly well Compress tablets, but after sintering they leave their original shape in the sintered body, so that a kind of "blackberry structure" arises which leads to unsatisfactory properties

In a further known method fuel particles are prepared by a heavy metal nitrate solution is mixed with curable organic substances, and the drops formed from this mixture solution to be solidified in a hot liquid at 70-95 ⁰ C (AT-PS 2 67 007 DE-PS 12 83 199). The problem here is that premature reaction of the mixed solution must be prevented. This is usually achieved by adding inhibitors to the mixed solution to extend the reaction time, e.g. B. primary alcohols such as methanol or ethanol. A synthetic resin composed of a mixture of two components is preferably used as the curable organic substance, preferably resorcinol (or urea) and formaldehyde (or another aldehyde). The mixed solution is injected into the hot liquid through a drop nozzle, the drops then solidify after a few seconds

DE-OS 28 22 387 describes the production of spherical fuel particles by dripping one liquid mixture of nuclear fuel and synthetic resin components, with subsequent hardening of the Drops falling to the ground in a gaseous medium by means of energy supplied by radiation and by subsequent heat treatment and sintering of the particles thus formed.

The previous process for the production of fuel tablets with subsequent sintering for Achieving the required density has a number of disadvantages.

The quality of the molding powder or the gel particles are subject to high requirements with regard to the Compressibility, sinterability, evenness and

Purity made These requirements can only be met with complex processes for powder or particle production. Especially with the gel particles produced by wet chemical means or In the case of pressed powder, waste products fall mainly as contaminated aqueous solutions in the form of ammonium nitrate, sodium nitrate or the corresponding Fluorine compounds. These waste materials have to be disposed of using complex procedures. In the Handling of plutonium and uranium-233 hatched from thorium requires special safety precautions to be taken due to their radiation hazard and toxicity, which are very expensive and therefore make the manufacturing costs more expensive.

In addition, the sintering process is a relatively complex work step, which is followed by grinding of the tablets. Usually there is an unavoidable scrap due to flaking. In addition, waste is created during grinding that must be recovered from the grinding fluid. This increases the manufacturing costs. On the other hand, the precipitation of the heavy metal salt solutions and washing out of the particles or the precipitate for the purpose of ammonium salt removal leads to inevitable losses of fissile materials.

It has also already been proposed (P 28 42 402) to carry out fuel tablets from oxidic powders, possibly in the presence of graphite powder Forming a pre-pressed part of 30 to 60% theoretical density with subsequent impact compaction in a die under vacuum. However, working with powders is beneficial in this process some trouble with it.

The invention is therefore based on the object To find a process for the production of fuel pellets for nuclear reactors that have the fission and / or Contain breeding material in oxidic or carbidic form, by impact compaction of a pre-pressed part in a die in which the difficulties mentioned do not apply, and the production of Brennsiofftabletten in an economical manner largely without Allows scrap rack management and without accumulation of aqueous waste products

The object is achieved according to the invention in that the pre-pressed part is formed from spherical particles, which are formed by hardening droplets consisting of an aqueous solution of the nitrates of the fissile and / or brood matter and the components of a hardenable synthetic resin at temperatures above 100 ° C and then Heat treatment at temperatures between 200 and 850 ⁰ C can be produced. The particles thus formed are pressed in a known manner to tablets and compacted by heating to temperatures of more than 1500 ⁰ C abruptly to final dimension, wherein the compaction and ejection of the tablets preferably in less than 100 milliseconds is carried out, the impact compaction can be both under vacuum and under atmospheric pressure or increased pressure.

A number of advantages are achieved with the method according to the invention

No aqueous nitrate or fluorine-containing waste solutions are produced during particle production. That Heavy metal is absorbed by the solidified particles in the form of nitrate solution during drop hardening through polymerization or polycondensation of, for example, phenols with formaldehyde. at this type of particle generation arise in the Drops when solidifying no ammonium salts. There is therefore no need for washing to remove ammonium salts.

The mixed solution used to pour the drops contains, in addition to the dissolved heavy metal nitrate, only the two reaction components required to form the curable synthetic resin, preferably a phenol and an aldehyde. The aldehyde is preferably an aqueous solution of formaldehyde, until just prior to pouring admixed so that no premature resin formation even before dripping occurs Advantageously, the mixture solution is cooled to temperatures below 0 ⁰ C As a result, an addition of inhibitor substances unnecessary while a sufficient safety against the risk The lack of inhibitor substances has the further advantage that the formation of vapor bubbles from the low-boiling alcohols used in the case of drop hardening, as mentioned in AT-PS 2 67 006, is avoided. This effectively eliminates the risk of the particles tearing open. In addition, the manufacturing process is significantly improved in terms of waste reduction

A special feature of the invention is that the Drops generated from the reaction mixture in a manner known per se at temperatures above 100.degree are cured, preferably in the hot steam of a boiling chlorinated hydrocarbon, such as perchlorethylene, or in superheated steam of 150 ° C, for example

By curing above 100 "C a better heat transfer and consequently a faster heating of the droplets achieved within seconds The rapid heating of the droplets Temperatures above 100 ° C can also be caused by the absorption of radiant energy, such as infrared radiation or microwaves. Microwaves are particularly suitable

Since the solidification of the drops through the reaction of two organic synthetic resin components above 100 "C is triggered, there are no gel microspheres, but synthetic resin particles in which the fuel-fertilizer solution is absorbed

The Schwertnetallnitrat containing the solidified resin particles are then subjected to a heat treatment at 200-850 ⁰ C. This process, known as calcination, takes place, for example, initially at temperatures of up to 600 ^° C. under inert gas, with heavy metal oxides, carbon and nitrous gases being formed. Part of the carbon is oxidized by the nitrous gases and removed from the particles in the form of carbon oxide

It is of particular advantage that the nitrous gases formed during the calcination, provided they are not Consumed for carbon oxidation, by absorption in water and conversion to nitric acid, can be recycled by the acid again is used to dissolve the fuel.

The still present after the calcination in the particles residual carbon is in the case of the preparation up to 850 ⁰ C in air burned by oxidic fuel at temperatures For the preparation of carbidic fuel, the residual carbon is so far reduced by treatment with CO ₂ gas at 850 ⁰ C that it is sufficient for the subsequent carbothermal reaction to form carbide

■ Can deform difficulties into pre-pressings. the Particles produced in this way have a relatively low density and can be used like powder granules pressing without creating a "blackberry structure" in the test specimen. The pourability of the spherical Particles facilitate processing when pre-pressing the fuel tablets, reducing ο the formation of dust and thus considerably reduces the dose burden on the environment. The pressing of the heat-treated Particles into tablets takes place at a relatively low compression pressure, which is such that the obtained Pre-pressed items are only manageable. Demands for tight tolerances for density and dimensions the pre-pressings are omitted because the final dimensions are achieved in the press die during hot impact compaction.

The green compacts having a density of 30 to 60% of the theoretical density are heated in a hydrogen or inert gas atmosphere or in a vacuum at temperatures above 1000 ⁰ C to the final state of the chemical composition to obtain a heavy metal or -karbid.

These pretreated compacts who, for example, Jen heated to temperatures above 1500 ⁰ C in vacuo quickly transferred into a steel die, is suddenly compressed to the final dimension and ejected 2s. The compression and ejection takes place within 3 to 100 milliseconds, the vacuum is preferably less than 10 ~ ⁶ bar. With impact compaction, very exact dimensional accuracy is achieved, so that subsequent reworking by grinding the ceramic body can be completely dispensed with.

This elimination of the otherwise usual grinding process and the grinding waste that occurs during grinding results in a significant simplification of the tablet production. This gives a further simplification Complete elimination of the otherwise customary sintering treatment, in which previously the final density of the pressed ceramic body was obtained. The short period of time in the range of milliseconds in which the compact when Schlagverdicliten is in contact with the press die, enables the multiple use of a steel die.

The extremely good distribution of fissile material in the breeding material, given by the mixture solution of the Starting materials, when heated to the temperature required for hot impact compaction, results in a complete mixed crystal formation. The presence of the mixed crystal ensures complete solubility of the Fuel is guaranteed in pure nitric acid and thus the dissolution during reprocessing spent fuel much easier.

All of these advantages are particularly important when re-manufacturing nuclear fuel from reprocessing plants, so that a remanufacturing is made considerably easier.

The following examples are intended to explain the process according to the invention in more detail:

example 1
Manufacture of UC> 2 fuel tablets

60

A uranyl nitrate solution with 300 g U / l, which contained resorcinol as phenol component in an amount of 380 g / l, was used as the starting solution for the particle production. This solution was cooled to -5 ° C. Polykondensationshärtung to a cooled to + 5 ⁰ C solution of formaldehyde was used, which was mixed before ml per drop generator in an amount of 200 liters with the cooled starting solution, with a temperature of - 3 ° C ceased. The droplets were formed in a manner known per se by the disintegration of the jet of the liquid as it flowed out of a nozzle. The drops with a diameter of 0.9 mm fell into a tube with perchlorethylene ^{vapor of 12 10} C uid solidified there within a second to form spherical particles. After hardening, the solidified particles were ^{calcined under argon up to 500 °} C. with a holding time of 3 hours. Subsequently, the residual carbon was burned in air at 800 ⁰ C, said free-flowing particles formed from U3O8. To produce the tablets, the particles were pressed at a specific pressure of 80 MN / m ² to a density of 4.2 g / cm ³ . The compacts with a diameter of 12 mm and a height of 16 mm were heated to 1300 ° C. in a reducing hydrogen atmosphere at a heating rate of 250 ° / h and held for one hour. The U3O8 was reduced to UO _2. After this treatment, the compacts had a density of less than 70% of the theoretical density. These porous tablets were heated in vacuum of about 0.1 bar to 2300 ⁰ C, transferred to a steel die, in between two press punches at a pressure of 400 MN / m ² is compressed to final dimension and ejected. Compression and ejection took place in 7 milliseconds. The tablets, cooled to room temperature, had a diameter of 9.1 mm and a height of 9.4 mm. The density was 10.46 g / cm ³ , which corresponds to 95.4% theoretical density.

Example 2
Manufacture of UO ₂ · ThO ₂ fuel tablets

One was used to produce particles for the production of fuel tablets. Starting solution of uranyl nitrate with 180 g U / l and thorium nitrate with 120 g Th / 1 and resorcinol in an amount of 345 g / l. The heavy metal content was therefore 60% by weight U and 40% by weight of theory. After adding 180 ml of cooled 40% formaldehyde solution to 1 liter of starting solution at -5 ° C., solidified particles were produced as in Example 1. These particles were made up of resorcinol-formaldehyde resin, which contained the uranyl and thorium nitrate solution. By calcining under nitrogen to 600 ^° C. with a holding time of 4 hours, all of the nitrate was decomposed and converted into nitrous gases. The residual carbon from the decomposed resin component was then burned in air at 850 ⁰ C, said free-flowing particles formed from a mixture of uranium and thorium.

To produce the fuel tablets, the particles were pressed at a specific pressure of 120 MN / m ² to a density of 4.1 g / cm ³ . The pre-pressings with a diameter of 12 mm and a height of 15.5 mm were then reduced _{to UO 2} · ThO _{2 as in Example 1 in a hydrogen atmosphere.} The porous tablets with a density of less than 70% of the theoretical density were heated in a vacuum from about 0.1 bar to 2300 ° C, transferred to a steel die and then placed between two rams at a specific pressure of 500 MN / m ² to final dimensions condensed and expelled. The fouling and ejection took about 20 milliseconds. The tablets, cooled to room temperature, had a diameter of 9.1 mm and a height

of 9.0 mm. The density was 9.97 g / cm ³ , which corresponds to 94.1% of the theoretical density.

Example 3
Manufacture of UO2 · PuO ₂ fuel tablets

Analogously to Example 2, a starting solution was used for particle production which contained uranyl nitrate with 180 g U / l and plutonium nitrate with 120 g Pu / I and 345 g / l resorcinol. With the exception of a lower temperature during hot impact compaction of 2000 ° C instead of 2300 ° C, all other manufacturing parameters remained the same. The finished tablets were composed of 60% by weight of UO ₂ and 40% by weight of PuO ₂ and had the following properties:

diameter 9.1 mm height 8.5 mm density 10.80 g'cm ³ , accordingly 96.8% theoretical density

Example 4
Manufacture of UC fuel tablets

The starting solution for the resin particles contained 300 g U / l as uranyl nitrate and 380 g / l resorcinol. This solution was cooled to -7 ° C, then mixed with cooled 40% formaldehyde solution and heated to -5 ° C. _{Drops with a diameter of 0:} 6 mm were produced from the solution and solidified within 1.5 seconds under argon in a vertical graphite tube 100 mm in diameter heated to 800 ° C. The particles were then calcined in a stream of Ar at a maximum of 500 ° C. and a holding time of 3 hours. The chemical composition of these particles was 47.6% by weight U, 38% by weight C and 14.4% by weight O, corresponding to the composition UO ₂ , 4j Particles treated in the CO ₂ -StTOm at S50 ° C. Thereafter, the C content was 11.6% by weight and the O / U ratio was 2.08. Tablets with a density of 3.8 g / cm ^{3 were} pressed from the particles at a pressure of 200 MN / m ^2. The tablets were then heated to 1750 ° C., _{during which the U0 2} .os was converted to UC by carbo-reduction. The final compaction was carried out by hot impact compaction at a temperature of 2100 ° C. and a specific pressure of 400 MN / m ² within 10 milliseconds. The tablets cooled to room temperature had the following properties:

diameter 7.9 mm height 9.5 mm density 12.93 g / cm ³ , corresponding (95%) C content 4.64 wt% C (free) 80 ppm

230244/441

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Claims (6)

Patent claims: 1. Process for the production of ceramic fuel pellets for nuclear reactors that use the Contains fissile and / or breeding material in oxidic or carbidic form, by means of impact compaction of a preprint in a die characterized in that the preform is made spherical particles shaped by Hardening of drops, consisting of an aqueous solution of the nitrates of the cleavage and / or Brutstoff and the components of a curable synthetic resin, at temperatures above 100 ° C and subsequent heat treatment at temperatures between 200 and 850 ° C. 2. The method according to claim 1, characterized in that the impact compaction takes place at temperatures above 1500 ⁰ C in less than 100 milliseconds 3. The method according to claims 1 and 2, characterized in that the drops from a aqueous solution can be prepared, which before the drop generation to temperatures below 0 ° C was cooled. 4. The method according to claims 1 to 3, characterized in that the drops are hardened in a steam atmosphere and the hardening takes place within a period of 0.1 to 3 s 5. Process according to claims 1 to 4, characterized in that the mean diameter of the spherical particles between 0.1 and 1 mm 6. The method according to claims 1 to 5, characterized in that the pre-pressed oxidic Fuel compacts are heated in a reducing atmosphere prior to impact compaction.