Mechanical and thermophysical properties of materials in HTGR fuel bundles

Fuel bundles for a high-temperature gas-cooled reactor (HTGR) are subjected to high temperatures (to 3300 K), peak neutron fluxes to 1015-1016 sec-1·cm2, and static and dynamic mechanical and thermal loads. The high thermal stresses (up to 15 MW/m2) and the thermochemical reaction of the fuel-bundle materials with the working fluid (hydrogen) make it necessary to develop and use materials with a unique combination of properties. The most important requirements for HTGR fuel-bundle materials can be formulated as follows: The melt and sublimation temperatures of the materials must be higher than the maximum operating temperature. Materials with a high neutron capture cross section cannot be used. The material thermal strength must be sufficient to withstand high thermal fluxes and heating rates to 1000 degrees/sec. It must also be sufficient maintain the hydraulic resistance of the working fluid if separate parts of the fuel-bundle fail. The material strength in the elastic-brittle temperature range must be sufficient to withstand static and vibration loads. In the viscous-plastic operating temperature range the materials must have the required strength and creep resistance to prevent a dangerous change in the geometry of the fuel-bundle components. The evaporation of chemical components of materials which make up the structure and fuel and the interaction of the materials with the working fluid should not lead to an impermissible change in the nuclear physics characteristics of the HTGR nor should they lower the thermal strength or thermal physics characteristics of the components. Radiation damage of the materials and the structural changes over the whole temperature range should not lower the operational reliability of the fuel-bundle components