Numerous types of high temperature ceramic fuel elements for the Pebble Bed Reactor are being evaluated. Specimens are 1-1/2 in diameter uranium graphite spheres with external coatings such as silicon carbide or pyrolytically deposited high density graphite and feul particle coatings such as alumina. Low diffusion product leakage rates at high temperatures have been observed for some of these coatings. High level irradiation damage to either the silicon carbide coating or the coating-graphite bond.

The basic fuel element consisted of a uniform dispersion of fuel in a 1 1/2 inch diameter graphite sphere. Ceramic coatings for the retention of fission products were studied. It was found-that molecularly deposited'' ceramics such as alumina, siliconized silicon carbide, and pyrolytic carbon were excellent barriers to fission product leakage. The most advantageous location for ceramic coatings was found to be on the individual fuel particles, where the coating was subject to smaller forces and where a larger thickness-todiameter ratio could be used than if the coating were on the surface of the graphite sphere. Fuel elements were irradiated to burnups ranging up to about 6 at.% U235. In all specimens containing a uniform dispersion of fuel, the graphite spheres were found to retain their structural properties after irradiation. Data are given on fuel particle coatings of A12O3, pyrolytic carbon, and metals: surface coatings of siliconized silicon carbide, pyrolytic carbon, and metal carbides; properties of and the effects of irradiation on graphite spheres; the use of natural graphite in preparing a high-density matrix material; graphite fueling by thorium nitrate infiltration; subsurface metal and metal carbide coatings for graphite; and an in-pile loop program on the behavior of fission products in a recycle helium stream. (auth).

An exploratory program on subsurface coatin8s for graphite fuel elements is summarized. A number of coatings with various melting points which could be located beneath the surface of a fueled graphite sphere were investigated. Of the materials with lower melting points. nickel and a special glass compound appeared to form continuous coatings when a hot-pressing technique was employed. Several materials with high melting points. such as Ti, Cr, and MoSi2, showed some promise, even though present equipment limitations prevented these specimens from being hot-pressed at the melting point of the coating. (W.L.H.).