Optimization of a Shield for a Heat-Pipe-Cooled Fast Reactor Designed as a Nuclear Electric Space Power Plant.

Abstract

'A reactor shield optimization procedure based on the ASOP shield optimization computer code and the DOT radiation transport code was used to determine a minimum-weight shield for a small fast reactor designed for a space nuclear electric power plant. The reactor, cylindrical in shape, is fueled with uranium nitride and cooled by potassium circulating through a matrix of stainless steel heat pipes embedded in the core; the design power is 450 kWt. The surrounding shield is typically asymmetric, having the overall shape of a truncated 90-deg cone whose thick base is positioned between one end of the reactor and the crew compartment. The heat pipes emerge from the opposite end of the reactor, penetrating through the apex of the shield. The dose constraints are 3 mrem/hr at all lOO-ft radii falling within the shadow cast by the base of the cone and 300 mrem/hr at all other 100-ft radii. The optimized shield consists of alternate layers of tungsten and lithium hydride, the thick bottom section extending out to a radius of 112 cm and the tapered side decreasing to a radius of 89 cm. The top heat-pipe shield region consists of a 59-cm-thick inner layer of a stainless-steel-Bi+C mixture and a 30.5-cm-thick outer layer of a BeC-Bi+C mixture. The total shield weight is 25,589 lb. A partially optimized shield having a 45-deg cone angle and a higher dose constraint for positions outside the cone shadow (100 rem/hr) has a total weight of 14,708 lb. These shield weights include an allocation for 3.5 vol% of stainless steel structure in the LiH regions.

Multi-Megawatt Power System Trade Study

“…For liquid metal cooled reactors, shield masses were estimated using ALKASYS-PC logic, which is based on Carlson (1985), Engle (1971), and Robinson (1996). For gas-cooled reactors, shield masses were scaled from the Enabler NERVA Derivative design.…”