DE2036545B2 - COATED NUCLEAR FUEL PARTICLES AND THE METHOD OF MANUFACTURING THEM - Google Patents

Description

Coating or by increasing the coating gas flow is kept constant during the coating and that the growth rate is equal to the limit growth rate in the characteristic coating zone.

To this problem underlying the invention To make it understandable, the coating process must be discussed in more detail here.

For coating, the particles are in a fluidized bed, which in the simplest case consists of a vertical, heated pipe with a conical bottom. The nozzle opens into the tip of the cone, through which the carrier gas (argon, helium) and the coating gas (eg methane, propylene, acetylene) required for whirling are injected. The hydrocarbon gas is heated when it enters 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 ao on the amount of the hydrocarbon gas available; 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 and, associated with this, certain layer properties are obtained at each time of coating.

It has now been recognized that there are two causes for changes during a coating process induce in growth speed and layer properties:

1. The coating gas entering the fluidized bed decomposes as it breaks the particle bed flows through, so that in the upper part of the bed the coating concentration is smaller is than in the lower part. More or less occur parallel to the concentration gradient strong temperature gradients. Due to the at different points of the particle bed The different coating conditions that exist at these points also differ Waxing speeds up and there are layers with the most varied of properties deposited. The individual particles migrate through depending on the rate of circulation the individual coating zones and the overall layer consists of a series of thin n of individual layers of different properties.

2. These alternating fluctuations are superimposed by a uniform, one-way change in the Properties over the entire coating time. This change is due to that the particle diameter and thus the surface to be coated is constantly growing and as a result, the growth rate decreases with a constant flow of coating gas. Dependent on all other properties of the layer change as a result of the growth rate.

With a method for measuring the optical anisotropy developed by SGAE-Seibersdorf the formation of property fluctuations over the layer thickness can be shown.

Fig. 1 shows the alternating fluctuations using the coating batch DO 357 K as an example the anisotropy at constant growth rate;

Fig. 2 shows the superposition of the alternating fluctuations and the unidirectional change in anisotropy with decreasing growth rate. Both property gradients determine not only the values of all layer properties averaged over the layer, but also the irradiation behavior of the particles.

The following conditions can be specified for the production of gradient-free layers:

a) The wake up speed must be during the be constant over the entire coating time (see

Fig. 1) _{,, ..., · D} · „-τ- ·

b) the up-speed must be in all parts

len of the particle bed must be the same.

Compliance with condition 1 can be z. B. achieve in a simple manner that either Particles are continuously removed and so the entire particle surface with the coating time is kept constant, or the coating gas flow continuously of the growing particles - surface is adapted. In the latter case, the result is a curve as shown in Fig. 1 is shown.

The second condition requires that the wake-up speed is the same in all parts of the particle bed, so that alternating fluctuations (see Fig. 1 and 2) can be avoided. This requirement can be expressed in a conically tapering BeU, that is flown against by a nozzle can only be achieved if the ratio of working tube cross-sons to the nozzle cross-section is <150 and at the same time the ratio of the volume of the fluidized particles to the volume of the characteristic coating zone is <1. The characteristic coating zone is defined here as a volume in which the concentration of coating gas is approximate is constant. In this case the volume of the entire particle bed corresponds to the characteristic one Treatment zone (A b b. 3).

In the case of larger beds and correspondingly larger volumes of fluidized particles, for the purpose of Avoidance of different coating zones different paths are drilled.

One possibility is to use a gas duct, which causes the coating to appear the characteristic zone is delimited.

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

There are concentration-dependent geometries for each of these geometries Limit growth rates that must not be exceeded if gradient-free Layers are to be achieved.

1 sheet of drawings

Claims (3)

overcome. So are ζ. B. Carbide particles with two claims: layers, oxide particles with three layers made of pyro- lytic carbon and oxide 1. Nuclear fuel parts which are coated with pyrolytic carbon particles with five layers of pyrolytic carbon alone or in combination with silicon carbide and silicon carbide (B ic k erd ι c κ e, bid, characterized by Ranson, Vivante, DPR 139, iy63).
It was recognized next that pyrocarbons do not break up unidirectional material layers under neutron bombardment even without or alternating gradients of the optical interaction with the inner part of the particle anisotropy and the other properties, namely then if they are strongly anisotropic. are. Model calculations have shown that 2. Process for the production of coated internal stresses occur in such layers, Nuclear fuel particles according to claim 1, which lead to breakage (literature e.g. J. W. Prasa characterized in that the amount of per dos and J. L. Scott, Nuclear Applications, VoI. 2 Time unit and coating surface offered 15 [1966], p. 402). The degree of anisotropy of the layers of a growth rate by removing a batch of particles is determined by the Bacon anise part of the particles from the fluidized bed during tropiefaktor indicated that on the layers the coating or by increasing the amount of graphite platelets is measured, which is the respective sealing gas flow during the coating 20 draws the particle charge during the coating? - is kept constant and that the wax-ups were mixed. The anisotropy factor increases mi '. speed equal to the limit growth of increasing anisotropy and is for isotropic layer speed in the characteristic range 1.0. In radiation tests there has now been a stratification zone is. shows that a low Bacon anisotropy factor - 3. The method according to claim 2, characterized in that the correspondence of Ge dig, but not sufficient, ie one finds z. B. together with the growth rate and limit of particle types with a Bacon factor <1.05 growth rate through the use of one and other identical properties, both gas ducts with a defined volume and those with a neutron dose of 1.2 · 10 ^s - and defined particles residing therein still intact, as well as those whose layers are torn or due to the use of a gas (literature: irradiation experiment in the DFR guide tubes with a defined volume and Dounreay). The defined particle searches resulting therefrom led to the present invention, quantities or by using a cone, which is based on the knowledge that the specification of the graphite soil, the number of boreholes 35 mean Bacon factor and others about the Layer of different thicknesses for the injection of averaged properties to characterize the carrier gas as well as single or multiple nozzles layer quality is not sufficient, but that for the coating gas contains is achieved. Irradiation behavior in the particle layers existing property gradients a decisive 40 play a decisive role. It is in itself from the reports of the Germans Ceramic Society (1966) from P. Koss Sections 2.2 and 3 on pages 242 to 244 Fuel assemblies for high-temperature reactors or Fig. 6 and 10 known to use sintered plates as gas containing the fuel usually in the form of 45 feed. In the special case, they lead to coated particles. For this purpose, spherical coating of nuclear fuel particles in the case of moderate fuel particles from uranium and thorium batch sizes over 2 kg of unsatisfactory oxide or carbide is produced, with layers of results. It has been found that precisely pyrolytic carbon alone or in combination with defined flow conditions are enveloped for the coating with silicon carbide. The diameter and carrier gas are necessary and that these such particles are between 100 and 1000 μ. cannot be achieved with porous plates.
The coating is usually carried out in vortex In the reference by JL Kaae (1969) in beds from the thermal decomposition of coal - "Journal of Nuclear Materials" are only those Hydrogen. These layers have the task of calculating the radial temperature gradients the fuel and the transport that occurs during the burn-up in the particle core in the reactor Fission products in the individual particles, the fission heat standing through the layers hold back yourself. This gives rise to the form; they are negligibly small. Through the changes that these layers both the thermal and mechanical loads and combustion survive element production as well as during the as a result of structural changes during irradiation Burning does not suffer any damage. 60 mechanical stresses occur in the layers It has been shown that especially the dose that is calculated in a manner similar to that in fast neutrons has a considerable influence on the literature already mentioned,
which has layers of pyrocarbon; the Pyrokoh- According to the invention, the production of the fuel shrinks, the fuel core expands from stratified fuel particles in that the and stresses occur, which lead to the formation of cracks in the amount of per unit time and surface of the coating Lead shift. Attempts have been made to use coating gas offered by the surface and thus bring the growth speed of successive individual layers by peeling off one different properties this difficulty to part of the particles from the fluidized bed during the