1132 C shouldn't be an infeasibly low temperature when the reactor is only expected to heat the coolant to 300-350 C. Meanwhile thorium metal is 1750 C. Or use

1132 C shouldn't be an infeasibly low temperature when the reactor is only expected to heat the coolant to 300-350 C. Meanwhile thorium metal is 1750 C. Or use uranium carbide. High melting point with still decent thermal conductivity.