DE1696687C - Process for coating nuclear fuel and / or nuclear fuel particles with carbon - Google Patents

Description

The invention relates to a method for coating of nuclear fuel and / or nuclear material particles with carbon in the fluidized bed by thermal Decomposition of hydrocarbons, if necessary in the presence of an inert carrier gas.

Coated particles - so-called coated particles - used. These particles, mostly they are globules with a Diameters between 100 and 1000 µm are used to prevent the radioactive fission gas from escaping dense, firmly adhering, ceramic, metallic Materials or - particularly advantageous - with pyrolytic Coated with carbon, d. H. coated. Coating with carbon takes place in most of them Felling in a fluidized bed with a mixture of hydrocarbons (e.g. methane, propane, acetylene) and a carrier gas (e.g. noble gas) and is at temperatures carried out between 1200 and 2000 C.

However, this previously practiced procedure has a number of disadvantages, some of which are fundamental:

1. Large amounts of carrier gas are required to maintain the vortex state. That is of considerable influence on the economic viability of the process.

2. The total pressure in the reaction space is always greater than atmospheric pressure, i. li., it amounts to always more than 760 mm of mercury. This is also a disadvantage because of the quality of the layer depends on both the total pressure and the partial pressure of the hydrocarbon gas.

3. With the procedure practiced up to now, pyrolytic carbon layers are obtained which have considerable Gaseinschlur.se (originating from the cracking process) contain.

4. The layers produced in this way always contain a considerable proportion of soot.

Both are undesirable because the gas content of the layer slowly escapes when the reactor is in operation, so that a porous layer remains, which has only a low retention capacity for fission gases. In addition, the proportion of soot in the layer under the influence of the irradiation shrinks more than the pyrolytic carbon, so that in this case too a porous layer remains. Furthermore The previous procedure brings other procedural niches Difficulties with yourself:

5 Higher methanaase concentrations and higher deposition rates lead to soot launches in the apparatus.

6. The "deposition of a per se desired A layer with a coarse-grained structure causes considerable difficulties in carrying out the reaction.

It has now been found that the difficulties mentioned can be overcome in that the coating in the fluidized bed by thermal decomposition \ on hydrocarbon gas. optionally in the presence of inert carrier gas, at a total pressure from 10 to 650 torr.

It is known per se (British patent specification y43 719). a coating of nuclear fuel particles at I to 200 Torr in a rotary kiln. Such a coating in a rotary kiln has the disadvantage that the de ;, Particle-forming surface layer is uneven, in particular it is not guaranteed that alk-particles have a surface layer with a uniform thickness.

In a fluidized bed, the isotropy of the surface layer is clearly guaranteed because the particles from the hydrocarbon gas or carrier gas completely be flowed through evenly. Also step :, during continuous pumping in a rotary kiln Sealing problems.

The coating can be applied both in fluidized beds with conical nozzle bottoms and over bottoms. Frits, sieves or bell bottoms can be carried out.

The method according to the invention offers the following advantages:

1. The required amount of carrier gas becomes considerable reduced.

2. As a result of the reduced pressure: the pyrolytic carbon layer does not contain any significant gas inclusions.

3. Soot deposits in the layer practically no longer occur.

4. The retention capacity for fission gases is considerably better.

5. The setting of certain layer structures - including those with a coarse-grained structure - is possible without major difficulties.

6. The amount of soot in the reaction chamber is significantly reduced, so that no disruptions during the Coating occur.

The following examples are intended to explain the advantages of the method according to the invention.

example 1

(Comparison of both methods with the same gas composition and same charging temperature)

1 kg of UC ₂ particles was coated in a fluidized bed at a total pressure of 200 mm Hg with an argon-methane mixture of 65% argon and 35% methane at a temperature of 18 (X) ⁰ C until the layer thickness was pyrolytic Carbon was 100 μm. Examination of the layer revealed:

3 4

a) Density of the layer 1.98 g / ccm

V) Compressive strength 2.4 kg / part.kel

c) Surface contamination 2 · 10-% U of the total U-content

released Xe _, ₁ " _β

d) Xenon »» release, total amount of Xe

e) Gas content of the layer 4 ppm H ₂ and 20 ppm CO

In contrast, the normal pressure method under the same conditions (gas composition, temperature) get the following values for the shift:

a) Density of the layer 1.5 g / ccm

released Xe_ ₌ . ,. ".,

d) Xenon »3 release total amount Xe '

e) Gas content of the layer 30 ppm H ₂ and 100 ppm CO

It can be seen from this that the values for the density of the layer, the compressive strength, the surface curve cleaning as well as for the xenon release rate and the gas content during coating under vcrmmdcitem Pressure are considerably better than those obtained with the normal pressure process.

Example 2
(Comparison of the two processes in the production of a layer with the same coherent [coarse-grained] structure) 1 ke ThCWUC, (mixed crystal of 82% thorium carbide and 18% uranium carbide) particles were in the fluidized bed at a total pressure of 10 (1 mm Hg at 18 (MTC so coated for a long time until the corner of the shaft was 100 α. The total gas consumption was 0.91 m ^{3 of} argon and 1.2 m ^{3 of} methane.

a) Columnar structure (coarse-grained)

b) Density of the layer W g / «™

c) Compressive strength 2.7 kg / park

d) Surface contamination - · ° ^{iu / 0}

released Xe ^ ^ _ ₅

e) Xenon release rate "total amount Xe" "'

f) Gas content of the layer 4 ppm H ₂ and 15 p _P m CO

In contrast, in the normal printing process to produce a layer with the same structure,

aleicher 'temperature a gas consumption determined by:

24.8 m ³ argon and 1.26 m ³ methane

The investigation of this layer T gave:

a) Columnar structure (coarse-grained)

b) Density of the layer 2.0 g / g ^^ .^

c) Druckiestigke.t - f _'10 _ _{30 / ö of the} total U-content

d) surface ^{contamination llvt lv} " ^{/ 0}

released Xe _{= 1} ^ _ _1Q _ ₄

e) Xenon release rate total amount Xe

f) gas content of the

_Schichl 8 ppm H ₂ and 60 ppm CO

Process and 24.8 m ^{3 of} argon at normal pressure from Example 2, it can be seen that the process.)

generation of the same layer structures in the case of the invention. Furthermore, it can be seen that the values for the

process according to the invention under reduced pressure density, for compressive strength, etc. when the invention saves considerable amounts of the carrier gas. (Gas-based processes are better than the previous consumption of 0.91 m ^{; l of} argon in the normal-pressure process according to the invention.

Claims (3)

Patent claims: 1. Process for coating nuclear fuel and / or nuclear fuel particles with carbon in the fluidized bed by thermal decomposition of hydrocarbon gas, if necessary in the presence of inert carrier gas, characterized in that that the coating takes place at a total pressure of 10 to 650 Torr. 2. The method according to claim 1, characterized in that that the negative pressure is maintained by continuously pumping out the gas. 3. The method according to claim 1, characterized in that that the feed is carried out under reduced pressure in fluidized beds with conical nozzle bottoms or by bathing from fries. Sieve or bubble cap trays is carried out.