DE2036545A1 - Fuel particles with layers without property gradients - Google Patents

Description

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N ϋ Κ Ε Μ

Nuclear Chemistry and Metallurgy Society m.b.H.

Wolfgang b. Hanau

Fuel particles with layers without property gradients.

Fuel assemblies for high-temperature reactors contain the fuel usually in the form of coated particles. There are spherical fuel particles made of uranium and Thorium oxide or carbide produced with layers of pyrolytic carbon alone or in combination with silicon carbide be enveloped. The diameter of such particles is between 100 and 1,000, u. The coating is usually carried out in fluidized beds through thermal decomposition of hydrocarbons. The task of these layers is to keep the fuel and the fission products produced during the burn-up in the to hold back individual particles themselves. This results in the requirements that these layers are used both in the production of fuel assemblies survive and suffer no damage during the burn-off.

It has been shown that the dose of fast neutrons in particular has a considerable influence on the layers Has pyrocarbon; the pyrocarbon shrinks, the fuel The core expands and tensions occur, which lead to the formation of cracks in the layer. You tried to get through Applying successive individual layers of different properties to overcome this difficulty. E.g. Two-layer carbide particles, three-layer oxide particles made of pyrolytic carbon and oxide particles with five layers of pyrolytic carbon and silicon carbide (Bickerdicke, Itanson, Vivante, D.P.R. 139, 1963).

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It was next recognized that pyrocarbon layers were underneath Neutron bombardment even without interaction with the inner part of the particle break, namely when they are strongly anisotropic are. Model calculations have shown that internal stresses occur in such layers, which lead to breakage. (Literature e.g. J.W. Prados and J.L. Scott, Nuclear Applications Vol. 2 (1966), page 402).

The degree of anisotropy of the layers of a batch of particles is indicated by the Bacon's vision anisotropy factor, the is measured on the layers on graphite platelets that are added to the relevant batch of particles during the coating process was. The anisotropy factor increases with increasing anisotropy and is equal to 1.0 for isotropic layers. In Irradiation tests have now shown that a low Baconian anisotropy factor is necessary for good irradiation behavior, but not sufficient, i.e. one finds e.g. among particle types with a Bacon factor -'- 1.0S and other identical properties both those,

22 which at a neutron dose of 1.2 · 10 are still healthy than also those whose layers are torn (literature: irradiation experiment in the DPR Dounreay). The resulting Investigations led to the present invention, which is based on the knowledge that the specification of the mean Bacon factor and other properties averaged over the layer is not sufficient to characterize the layer quality, but that the property gradients present in the particle layers are decisive for the irradiation behavior Role-play. These previously neglected gradients can only be achieved through special control of the coating process Avoided under observance of certain boundary conditions. To make this problem understandable to make, the coating process must be discussed in more detail here will. "

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For coating, the particles are in a fluidized bed, which in the simplest case consists of a vertical, heated bed Consists of a tube with a conical bottom. The nozzle through which the necessary for whirling opens into the tip of the cone Carrier gas (argon, helium) and the coating gas {e.g. Methane, propylene, acetylene) are blown in. The hydrocarbon gas is heated when entering the bed and decomposes to carbon and hydrogen in several intermediate steps. The properties of the carbon deposited on the particles depend on the temperature and the amount of the offered Hydrocarbon gas from; the amount of gas offered results from the concentration and the total gas throughput or the flow rate. If these coating parameters are kept constant During the entire coating process, a certain growth rate is obtained at each time of coating and related specific layer properties.

It has now been recognized that there are two causes while

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a coating process changes in the growth rate and create layer properties:

1. The coating gas entering the fluidized bed decomposes as it flows through the particle bed, so that in the upper part of the bed the coating concentration is smaller than in the lower part. Parallel more or less strong temperature gradients occur in addition to the concentration gradient. Due to the different Different coating conditions present in the particle bed occur at these locations Also set different waxing speeds

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and layers with a wide variety of properties are deposited. The individual particles wander through thus, depending on the universal rolling speed, the individual coating zones and the total layer consists of one Sequence of thin individual layers with different properties.

2. These alternating fluctuations are superimposed by a uniform, one-way change in properties over the entire coating time. This change is due to the fact that the particle diameter and so that the surface to be coated grows continuously and thus the growth rate with a constant coating gas flow sinks. All other properties change depending on the growth rate the shift.

With a method developed by SGAE-Seibersdorf for measuring the optical anisotropy, the formation of property fluctuations over the layer thickness can be shown. Figure 1 shows the alternating fluctuations of the anisotropy with constant Growth rate., Figure 2 shows the superimposition of the alternating fluctuations for batch WM 381 and the unidirectional change in anisotropy with decreasing growth rate. Determine both property gradients In addition to the values of all layer properties averaged over the layer, the irradiation behavior of the particles.

For the production of gradient-free layers you can the following conditions are mentioned:

a) The wake-up speed must be used during the entire Coating time must be constant (see Figure 1),

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SAD ORIGINAL

b) the wake-up speed must be in all parts of the Particle bed must be the same size.

Compliance with condition 1 can be made easier, for example Achieve this way that either continuously removed particles and so the total particle surface is kept constant with the coating time, or the coating gas flow is continuously adapted to the growing particle surface. It arises for the latter Then a curve like the one shown in Figure 1.

The second condition requires that the growth rate is the same in all parts of the particle bed, so that alternating fluctuations (see Figure 1 and 2) can be avoided. This requirement can be translated into a tapered Only reach the bed against which a nozzle flows if the ratio of the working pipe cross-section to the nozzle cross-section ^. 150 is and at the same time the ratio of volume of the fluidized particles to the volume of the characteristic coating zone ^ S 1. The characteristic coating zone is defined here as a volume in which the concentration of coating gas is approximately constant. In this case corresponds to the volume of the entire particle bed of the characteristic coating zone (Figure 3).

With larger beds and correspondingly larger volumes fluidized particles can be used in different ways in order to avoid different coating zones will.

One possibility is to use a gas guide tube, which limits the coating to the characteristic zone will.

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Another possibility is to use an inflow base with multiple nozzles, in this case the characteristic coating zone is expanded.

For all of these geometries there are concentration-dependent limit growth rates that are not exceeded if gradient-free layers are to be achieved.

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Claims (2)

203RRAR '-7 - .. PATENT CLAIMS 1. Coated nuclear fuel particles and methods of manufacture. position thereof without unidirectional or alternating property gradients in the group consisting of pyrolytic carbon layers dad, urch gekennz: ei c line t, the amount of the Bescliichtungsgases offered per unit time and the coating surface so ηηύ the growth rate by withdrawing a portion of the particles from the fluidised bed during the coating or by increasing the coating gas flow or increasing the coating gas concentration is kept constant during the coating and that the growth rate is equal to the limit growth rate in the characteristic viewing zone. 2. The method according to claim 1, dajii [r ^ h_ £ ejce £ m ^ j. £ hnet that the correspondence of the total wake-up speed and the limit wake-up speed by using a gas pipe with a defined volume and staying in it defined particle quantities or through the use of inflow plates and / or multiple nozzles. Frankfurt / Main, July 21, 1970
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