Preparation of coated (th, u)c2-particles by reaction, sintering and coating in a fluidized bed

JOURNAL

OF NUCLEAR

PREPARATION

MATERIALS

OF COATED

29 (1969)

0

NORTH-HOLLAND

(Th, U)Ca-PARTICLES

COATING T. GijRGENYI, NUKEM,

126-128.

BY

IN A FLUIDIZED H. HUSCHKA

Nuldear-Chemie

Received

18

urd

PUBLISHING

REACTION,

CO., AMSTERDAM

SINTERING

AND

BED

and W. Metallurgic,

POPP CfmbH

August 1968

containers and finally taken to the fluidized bed for coating. This process requires much equipment. There are two furnaces necessary : one for the reaction and the sintering and an other one for coating.

In high temperature gas cooled reactors coated particle fuel is used. The kernel material is usually melted or sintered (U, Th)Cs, or the kernel diameter is sintered (U,Th)Os; between 200 ,um and 1000 pm. The coating is about 100 ,um to 200 ,um thick and it consists usually of 2 or more layers of different types of pyrocarbon with or without a Sic interlayer. The purpose of the coating is to prevent fission product escape. It is deposited on the kernels in a fluidized bed by thermal decomposition of hydrocarbons and silanes respectively. Up to now it is not yet completely clear which type of fuel kernels-oxide or carbide will finally be used in HTGR’s. Therefore, it is of interest to work out suitable production methods for all types of kernels.

For handling the (Th, U)Cz-kernels a gloveboxline with inert gas atmosphere is needed because the carbide hydrolyses very rapidly in air. Further by sintering of loose particles a sintered cake is obtained which has to be broken down to particles subsequently. By this operation kernels with sharp edges may be formed, or twins may be left. By embedding the particles in graphite flour this difficulty can be overcome but the process is complicated by mixing the green kernels from the graphite powder and separating the sintered kernels from the graphite

Particles with sintered carbide kernels have usually been prepared along the following line: - Green kernels consisting of a mixture of UOs,

afterwards. Further, the costs are increased by the price of the graphite flour and finally some uranium adheres to the graphit’e powder and

ThOs and C are prepared by granulation of the powders or by dividing a sol or emulsion containing Th, U and C into droplets and solidifying them, e.g. by precipitation or gelation and drying in a following step. - While heating the green kernels in vacuum the oxide reacts at about 1700 “C with the carbon and carbide particles are obtained. The temperature is then raised to about 2000 “C and the particles are sintered. During the heat treatment the kernels are either loose in a graphite crucible or embedded in graphite flour. - The kernels are sieved and filled into gastight

is lost. The process can be simplified considerably by doing the conversion of oxide to carbide, the sintering and the coating in one run in the same fluidized bed working along this line: Green kernels are inserted into the lluidized bed. While they are fluidized they are heated up and converted into carbide. A thin layer of pyrolytic carbon is then deposited on the particles to prevent them from sticking together during the subsequent heating. Coating is then done in the usual way. In our experiments we used green kernels prepared in the sol-gel-process. Fig. 1 shows 126

PREPARATION

OF COATED

(Th,

U)Cz-PARTICLES

127

Fig. 1. Conversion of (Th,U)O@-particles to (U, Th) Ca-kernels and sintermg the kernels in a fluidized bed. a. Temperature-time program; b. The decreaseof total carbon during the reaction (Th, U)Oa +4C + (Th, U)Ca+2 CO. The increase at 3 h is due to a thin pyrocarbon layer applied at that time; o. The increasein combined carbon duringthe reaction; d. The diameter shrinkage during the reaction and sintering.

the change of temperature, total carbon, combined carbon and shrinkage with time. In this experiment

the reaction

Fig. 2. Sintered (Th, U)Cs-particles coated with pyrolytic carbon. Kerneldiameter: 400 ,am. Coating thickness: 135 pm.

(U, Th)Oa + 4C + (U, Th)Cs + 2C0 takes place between 1.5 h and 2.5 h. The total carbon content of the particles decreases during this period due to the formation of CO. At 3 h the total carbon content is increased by applying a thin carbon layer. The content of combined carbon increases during the reaction. From the amount of combined carbon formed it can be concluded that at 2.5 h the reaction is practically completed. Shrinkage of the kernels occurs mainly during the reaction. In fig. 2 particles fabricated in this process are shown. During the operation some kernels - about lo/00 - were broken and the fragments were coated. By

sieving the broken particles could be separated. Formation of twins was not observed. The shape of the kernels is as good as of kernels sintered in a graphite bed. A problem of the process described is the abrasion of powder from the kernels before they have been sintered properly. By choosing suitable fluidisation conditions or by applying coatings in a very early stage of the process this difficulty can certainly be overcome. If coatings are deposited before the kernels have reached their final size a gap is formed between the kernel and the pyrocarbon layer when the kernel shrinks during subsequent

128

T.

GijRGENYI

carbon deposition at higher temperature. A gap of this kind may improve irradiation behaviour. Coated

particles

with oxide

kernels

can be

prepared in a similar way starting from green kernels without carbon. Thus, by reacting,

sintering

and coating

in

ET

AL.

a fluidized bed the process for the preparation of coated particles with sintered (Th, U) CBkernels is simplified and a considerable reduction of equipment handling

is achieved.

the sintering

phere is not needed.

A gloveboxline

for

kernels in inert atmos-