DE2921146A1 - METHOD FOR PRODUCING CERAMIC FUEL TABLETS FOR CORE RACTORS - Google Patents

Description

NüKEM GmbH
6450 Hanau 11

Process for the production of ceramic fuel pellets for nuclear reactors.

The invention relates to a method of manufacture of ceramic fuel pellets for nuclear reactors, which convert the fissile and / or breeding material into 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, but it is also used in limited quantities in LWR reactors instead of U-235. Thorium can also be used as breeding material. In addition to oxidic fuel, fuel carbides _Z um can also be used in SNR reactors.

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

Oxide fuel tablets are usually made from uranium oxide powder (UO ₂ ) or from mixed powder oxides UO ₂ and PuO ₂ by pressing, sintering and grinding. The powdery oxides required for this

Above all, T5 must have special sintering properties, which can only be achieved through complex manufacturing processes (e.g. DE-PS 1 592 468, DE-AS 1 592 477 or DE-OS 2 623 977).

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. Like the oxidic fuel, this is made into tablets by pressing, sintering and grinding (U.S. Patent 3,236,922).

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

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The dimensional tolerances of the tablets achieved in this way nevertheless required costly post-processing. Therefore this process has achieved no technical importance.

_1Λ addition is "known to produce the fuel pellets from gel particles. The fabricated on nassehemischem paths gel particles are compressed into tablets and then by sintering the compacted target density (for example, DE-OS 2713108).

.._ The gel particles are then solidified the formation of spherical drops either by removing water from the drops (sol-gel process), by solidification in the gas phase and / or in a liquid phase with ammonia (gel

^ _n precipitation process), or by internal decomposition of auxiliary substances dissolved in the drop, which release ammonia.

In another known method, fuel particles are deposited produced by the fact that a sight

_ ,. metal nitrate solution with curable organic substances is added, and the drops formed from this mixed solution in a hot liquid 70 - 95 ° C (OE-PS 267 007 and DE-PS 1 283 199). The problem is that a

2Q premature reaction of the mixed solution can be prevented got to. This is usually achieved by adding inhibitors to extend the reaction time of the mixed solution may be added, e.g., primary alcohols such as methanol or ethanol. As curable organic sub-

oc punch is preferably a synthetic resin from a mixture used by two components, namely

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preferably resorcinol (or urea) and formaldehyde (or another aldehyde). The mixed solution will injected through a drip nozzle into the hot liquid, the drops are then after a few seconds solidified.

DE-OS 2 822 387 describes the production of spherical fuel particles by dripping a liquid mixture of nuclear fuel and synthetic resin components, with subsequent hardening of the drops falling to the ground in a gaseous medium by supplying energy through radiation and through subsequent Heat treatment and sintering of the particles thus formed.

The previous processes for the production of fuel tablets with subsequent sintering to achieve the required density have a series of disadvantages.

The quality of the powder or the gel particles there are high requirements in terms of compressibility, Sinterability, evenness and purity. These requirements can be only meet 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 are mainly waste products as contaminated aqueous solutions in the form of ammonium nitrate, sodium uranium or equivalent Fluorine compounds. These waste materials have to be disposed of using complex procedures. When handling plutonium and thorium

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Breeding uranium-233 has to take special safety precautions because of its radiation hazard and toxicity are taken, which are very expensive and therefore increase the manufacturing costs in addition.

'"In addition, the sintering process is a relatively complex one The work step is followed by grinding the tablets. "Usually this occurs an inevitable scrap due to flaking. In addition, there is waste from grinding the grinding fluid has to be recovered. This increases the manufacturing costs. On the other hand, the precipitation of the heavy metal salt solutions results and washing out the particles or the precipitate for the purpose of ammonium salt removal to inevitable

Loss of fissile material.

It has also been proposed (P 28 42 402) to produce fuel tablets from oxidic powders, optionally in the presence of graphite powder, by forming a pre-press of 30 to 60 % theoretical density with subsequent impact compression in a die under vacuum. Working with powders, however, brings some benefits with this method

Trouble with yourself.
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The invention is therefore based on the object of a method for producing fuel tablets for nuclear reactors, which develop the fissile and / or breeding material in oxide or carbidic form.

hold by blowing a pre-pressed part

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c in a die in which the aforementioned difficulties omitted, and the production of fuel tablets largely economical without scrap recycling and without the accumulation of aqueous waste products.

The object is inventively achieved in that the green compact is formed from spherical particles opfen by curing of T _r, consisting of an aqueous solution of the nitrates of the cleavage and / or breeding

_ις materials and the components of a curable synthetic resin

ο at temperatures above 100 C and subsequent heat treatment at temperatures between 200 and 850 C. getting produced. The particles thus formed are compressed into tablets in a known manner and after

Heating compressed to final dimension to temperatures above 1500 C ⁰ abruptly, wherein the compression and expulsion of the tablets is preferably carried out in less than 100 milliseconds. The impact compaction can be carried out under vacuum as well as under normal pressure or increased pressure.

With the inventive method a number of benefits achieved.

on When producing particles, there are no aqueous waste solutions containing nitrate or fluorine. That Heavy metal is in the form of nitrate solution during drop hardening through polymerization or polycondensation for example, phenols with formaldehyde absorbed by the solidified particles. With this type of particle generation, no solidification occurs in the droplets

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Ammonium salts. There is therefore no need for any washing process to remove ammonium salts.

The mixture solution used to pour the drops In addition to the dissolved heavy metal nitrate, it only contains the two necessary for the formation of the hardenable synthetic resin Reaction components, preferably a phenol and an aldehyde. The aldehyde is, preferably as an aqueous formaldehyde solution, only immediately mixed in before casting so that no premature synthetic resin formation occurs before dripping.

Mixture solution is advantageously brought to temperatures cooled below OC. As a result, the addition of inhibitor substances is unnecessary and still sufficient Security against the risk of premature synthetic resin formation guaranteed. The missing of the inhibitor substances has the further advantage that the formation of vapor bubbles mentioned in OE-PS 267 006 from the low-boiling alcohols used as inhibitors avoided during drop hardening will. This effectively eliminates the risk of the particles tearing open. It also becomes the manufacturing process is significantly improved in terms of waste reduction,

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

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Hardening above 100 C results in better heat transfer and, as a result, faster Heating of the drops achieved within seconds. The rapid heating of the drops to temperatures above 100 C can also be caused by the absorption of radiant energy such as infrared radiation or microwaves will. Microwaves are particularly suitable here.

Those containing the heavy metal nitrate solidified Synthetic resin particles are then subjected to a heat treatment at 200 to 850 C. This The process known as calcination takes place, for example, initially at temperatures of up to 600 C under inert gas, heavy metal oxides, carbon and nitrous gases are formed. This is part of the carbon 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 produced during the calcination, provided they are not used for the Carbon oxidation is consumed, recycled by absorption in water and conversion to nitric acid by using the acid again to dissolve the "fuel".

The residual carbon still present on the particles after calcination is used in the case of the production of Oxydic fuel burned in air at temperatures up to 85O ° C. For the production of carbide Fuel becomes the residual carbon through treatment

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c with COo gas "at 85O ⁰ C so far that it is sufficient for the subsequent carbothermal reaction for carbide formation. This heat treatment creates a material that can be formed into pre-pressed parts without difficulty.

IQ ability of the spherical particles makes processing easier when pre-pressing the fuel pellets, reduces the formation of dust and thus considerably reduces the dose exposure of the environment. The pressing of the heat-treated particles into tablets

•, ς takes place at a relatively low pressure, the is dimensioned in such a way that the pre-pressings obtained can only be handled. Demands for tight Tolerances for density and dimensions of the pre-pressed parts omitted because the final dimensions in the press die at

2Q Heisschlagverdxchten can be achieved.

The pre-pressings with a density of 30 to 60 % of the theoretical density are heated in a hydrogen or inert gas atmosphere or in a vacuum to temperatures above 1000 C in order to obtain the final state of the chemical composition as heavy metal oxide or carbide.

These pretreated compacts are at temperatures 3Q ren, for example, heated in vacuum, quickly transferred into a steel die, is suddenly compressed to the final dimension and ejected above 1500 ⁰ C. The compression and ejection takes place within 3 to 100 milliseconds, the vacuum is preferably less than 10 ~ bar. In the

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- Impact compaction becomes a very exact dimensional accuracy achieved, so that a subsequent post-processing by grinding the ceramic body is completely eliminated can.

iQ This omission of the otherwise usual grinding process and the grinding waste occurring during grinding results in a significant simplification of the tablet production. A further simplification results in the complete omission of the otherwise customary sintering treatment in the previous one the final density of the pressed ceramic body. was obtained. The short period of time in the range of milliseconds, in which the compact is in contact with the press die during impact compaction the multiple use of a steel die.

The extremely good distribution of fissile material in the breeding material, given by the mixed solution of the starting materials, leads to a complete temperature when heated to the temperature required for hot impact compaction Mixed crystal formation. The presence of the mixed crystal ensures complete solubility of the fuel guaranteed in pure nitric acid and thus the dissolution during the reprocessing burned off 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.

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

Example 1: Production of U0 ₂ fuel tablets

A uranyl nitrate solution with 300 g / l, which contained resorcinol as phenol component in an amount of 30 g / l, was used as the starting solution for the particle production. This solution was cooled to -5 ° C. For the polycondensation hardening, a formaldehyde solution cooled to + 5 ° C. was used, which was mixed with the cooled starting solution in an amount of 200 ml per liter before the droplets were generated, a temperature of -3 ° C. being established. The drop formation was carried out in per se "known manner by jet breakup of the liquid at the outflow from a nozzle. The droplets of diameter 0.9 mm fell into a tube with Perchloräthylendampf of 121 ⁰ C and solidified there within one second to spherical particles. After After curing, the solidified particles were ^{calcined under argon up to 500 °} C. with a holding time of 3 hours, then the residual carbon was ^{burned in air at 800 °} C., free-flowing particles of IL ₇ O _{0 being} formed.

J O

To produce the tablets, the particles were pressed at a specific pressure of 80 MN / m to a density of k.2 g / cm. The pellets with a diameter of 12 mm and a height of 16 mm were placed in a reducing hydrogen atmosphere at a heating rate of 250 / h

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heated to 1300 C and held for one hour. The U_Og was reduced to UO ₂ . After this treatment, the compacts had a density of less than 70 "Jc of the theoretical density. These porous tablets were heated in a vacuum from about 0.1 bar to 2300 ° C. and transferred to a steel die, between

two rams compressed to final dimension at a pressure of 400 MN / m and ejected. Compression and ejection took place in 7 milliseconds. The tablets, cooled to room temperature, had a diameter of 9 ₅ 1 mm and a height of 9> 4 mm. The density was 10.46 g / cm, which corresponds to 95 »4 % of the theoretical density.

Example 2: Production of UO ₂ - ThOg fuel tablets

A starting solution was used to produce particles for the production of fuel tablets Uranyl nitrate with 180 g U / l and thorium nitrate with

120 g Th / l and resorcinol in an amount of 345 g / l.

The heavy metal content was thus 60% by weight U and 40% by weight of theory. After adding 180 ml of cooled 40% formaldehyde solution to 1 1 starting solution of As in Example 1, solidified particles became -5 ° C

generated. These particles are made up of resorcinol-formaldehyde resin, in which the uranyl and thorium nitrate solution contain Avar. By calcining under nitrogen to 600 ⁰ C with a holding time of

4 hours all the nitrate was decomposed and in 35

converted to nitrous gases. The residual carbon out

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the decomposed resin component was then burned in air at 850 C, whereby free-flowing Particles formed from a mixture of uranium and thorium oxide.

To produce the fuel tablets, the particles were pressed at a specific pressure of 120 MN / m to a density of 1.1 g / cm 2. The pre-presses with a diameter of 12 mm and a height of 15, 5 mm were then as in Example 1 in a hydrogen atmosphere to UO ₀ · ThO re- ₀ <l <z

dues. The porous tablets with a density of less than 70 % of the theoretical density were heated in a vacuum of about 0.1 bar to 230O ⁰ C, transferred to a steel die and then placed between two rams at a specific pressure of ι.

500 MN / no. Compacted and ejected on the final dimension.

Compression 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, which corresponds to 9 % of theoretical density.

Example 3: Production of UO ₉ · PuO _p -Brennstoff- - tablets

Analogously to Example 2, a starting solution was used for the particle production which contained uranyl nitrate with 180 g U / l and plutonium nitrate with 120 g Pu / l and 3 ^ 5 g / l resorcinol. Except for a lower temperature during hot impact compaction of 2000 ⁰ C

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instead of 23OO C "all other manufacturing companies remained the same. The finished tablets were composed of 6O Gev.% UO ₀ and 40 Gev.% PuOp and had the following properties:

9.1 mm diameter

Height 8.5 mm

Density 10.80 g / cm, accordingly

96.8 % theoretical density

Example 4: Manufacture of UC fuel tablets

The starting solution for the resin part contained 300 g U / l as uranyl nitrate and 380 g / l resorcinol. This solution was cooled to -7.degree. C., then mixed with cooled 40% formaldehyde solution and heated to -5.degree. Drops of 0.6 mm diameter were produced from the solution, which were solidified within 1.5 seconds in a vertical graphite tube of 100 mm diameter heated ^{to 800 ° C. under argon.} Then the particles were in the Ar stream at a maximum of 500 C and one

o ₀ holding time of 3 hours calcined. The chemical composition of these particles was 47.6 wt. $ U, 38 wt.% C and 14.4 wt.% 0, according to the composition UO _0, _ · To required for the stoichiometric carbon Karboreduktion

oc to adjust the amount, the particles were treated _{in the CO 0} stream at 850 ^{0 C.} After that, the

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c C content 11.6 % by weight and the O / U ratio 2.08. From the particles "were carried out at a pressure of ■ 200 MN / m tablets with a density of 3.8 ^: g./bnr ί ■ = - Pressed Tablets were heated finally presented on ^r ~ 175O ° C, it was the.. UO _{2 Q8} through carbon

IQ reduction to UC implemented. The final compaction took place by hot impact compaction at a temperature of 2100 C and a specific pressure of 400 MN / m within 10 milliseconds. the tablets cooled to room temperature

■ ic the following properties:

Diameter: 7/9 inches Height: 9> 5 mm Density: 12.93 g / cm "^> , accordingly

⁽⁹⁵ '·'

C content: 4.64 wt. ^ C (free): 80 ppm

Frankfurt / Main, May 9, 1979
Dr.Br.-Bi

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Claims (6)

2321U6 NÜKEM GmbH 6450 Hanau 11 Process for the production of ceramic fuel pellets for nuclear reactors. 15 PATENT CLAIMS 1. Process for the production of ceramic fuel pellets for nuclear reactors, which the fissile and / or breeding material in oxidic "_ Or carbidic form contained by Senlagverdichten a pre-pressed part in a die, characterized in that the pre-pressed part is formed from spherical particles, which by hardening droplets, consisting of a 2Q aqueous solution of the nitrates of the fission and / or breeding materials and the components of a curable synthetic resin, at temperatures above 100 C and subsequent heat treatment at temperature ι
will. temperatures between 200 and 850 ⁰ C produced 030048/0449 2321146 - 2 - 2. The method according to claim 1, characterized in that the impact compaction at temperatures above 1
follows. half 1500 ⁰ C in less than 100 milliseconds 3. The method according to claims 1 and 2, characterized characterized in that the droplets are produced from an aqueous solution which was cooled ^{to temperatures below O 0 C before the droplet was generated.} 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. The method according to claims 1 to 4, characterized in that the mean diameter of the spherical particles is between 0.1 and 1 um. 6. The method according to claims 1 to 5> thereby characterized in that the pre-pressed oxidic fuel compacts are heated in a reducing atmosphere prior to impact compaction. 030048/0449