Patent Number: 
Section: abstract

In a nuclear reactor core, each of a plurality of pressure tubes contains fuel elements spaced apart to permit coolant to flow through spaces between adjacent fuel elements. Each fuel element comprises fuel pellets in cladding, e.g., sapphire, having a melting temperature of at least 1900° C. and does not form significant hydrogen if exposed to high temperature steam. Each pressure tube has an internal insulator sleeve, e.g., fused silica, that has relatively low thermal conductivity over a range of normal operating temperatures and relatively high thermal radiation transmission at temperatures higher than said normal operating temperature range. When coolant is absent from said spaces, the insulator sleeve transmits to the pressure tube at least about 10%, but preferably more than about 40% of thermal radiation from the fuel for conduction through the pressure tube to the moderator and fuel temperature remains within safe limits after the reactor is shut down.