Measurements to Elucidate the Mechanism of Thermal and Radiation Enhanced Diffusion of Cesium, Europium, and Strontium in Silicon Carbide.

Abstract

Containment of fission products (FP) within the TRISO fuel particle is critical to the success of the very high temperature reactor (VHTR). Over sixty years of experience developing and testing this fuel has yet to identify the mechanism by which several key fission products (cesium, europium, and strontium) escape through intact SiC at temperatures between 900C and 1,300C.

A novel diffusion couple was developed that was successful in making the first measurements of fission product diffusion in SiC. This design allows for the isolation of thermal diffusion and investigation of radiation enhanced diffusion using ion irradiation as a simulant for neutron radiation damage. The thermal and radiation enhanced diffusion of cesium, europium, and strontium were measured between 900C and 1,300C. The ion irradiation significantly enhanced the diffusion of all three fission products with enhancement factors ranging from 100x to 1E7x over thermal diffusion.

All three fission products exhibits mixed diffusion kinetics between 900C and 1,300C under purely thermal conditions, and between 900C and 1,100C under ion irradiation. This indicates that both bulk and grain boundary diffusion are active mechanisms for fission product release. A defect reaction model indicates that fission product diffusion can occur on both the silicon or carbon sub-lattices. Comparison of cesium diffusion with the literature suggests that the best quality TRISO fuel should exhibit minimal cesium release and that cesium release is a good indicator of TRISO fuel failure.