DEVELOPMENT OF ADVANCED HTR FUEL-ELEMENTS

Spherical fuel elements with coated particles are the essential constituents of the integrity of a pebble-bed reactor, particularly with respect to fission product retention. While all gas-cooled reactors exhibit an extremely low primary circuit activity, recent developments have reduced the source term to virtually zero. This entails an enormous effort in the characterization of the fuel behaviour in manufacture, during irradiation, and in accident testing. With modern SiC-coated particles it can be shown that all radiologically relevant fission products are completely retained inside the silicon carbide layer of intact fuel particles. The dominant source term for fission product release will, therefore, be the small number of particles with defective coatings. Permanent damage to the fission product retaining SiC layer can only start in accidents involving temperatures significantly beyond 1600°C. © 1990.