Safety status of HTGR structural ceramics

Abstract

The objective of this study is to assess the safety status of structural ceramics proposed for use in an HTGR. In this report the mechanical and physical properties of the proposed structural ceramics are reviewed, and the current design and material specifications are discussed. Potential safety problem areas are identified. Analytical models for creep deformation, devitrification, compatibility, fatigue, and thermal insulation stability are based on published work in the literature. The predictions made are generally based upon extrapolations to HTGR conditions. The data indicate that creep deformation in silicon nitride and alumina ceramics is acceptable if the purity and composition are properly controlled. Devitrification and creep in fused silica used in the core support structure and the thermal barrier system could be excessive. Thermodynamic data indicate that silicon nitride will be incompatible with the graphite seat used in the design. Under normal reactor operating conditions, or during start-up and shutdown, static fatigue and thermal shock effects in all three members of the core support structural ceramics could cause reductions in safety factors.