Compatibility of Tin with Graphite as a Gap-Filler in a Prismatic VHTR

In the Nuclear Hydrogen Development and Demonstration (NHDD) project, two types of VHTRs (Very High Temperature Reactors), prismatic or pebble bed, are under investigation as the nuclear heat source for a hydrogen production. In general, the targeted coolant outlet temperature of the VHTR is 950∼1000 .deg. C and the maximum allowable fuel temperature is 1250 .deg. C during a normal operation. In the case of the prismatic reactor (PMR), conventional fuel designs result in a small margin in the maximum fuel temperature. This is one of the biggest demerits of the prismatic type. In this study, a technique for lowering the maximum fuel temperature is suggested. The PMR fuel assembly is comprised of many coolant holes and fuel channels. Cylindrical fuel compacts are stacked inside the fuel channel. Consequently, there should be a physical gap between the fuel compact and graphite block, which is filled with He gas in the conventional design. The heat transfer coefficient of the He gap is very poor, and this increases the fuel temperature substantially. In the proposed design measurement, the gap is filled with a liquid metal, tin (Sn) which has a very high thermal conductivity. The effects of tin in the gap on the fuel temperature and the core reactivity are quantitatively discussed and the material compatibility is experimentally studied