4.1 Heat Ratings and Coolants

The fast reactor core is typically much smaller than a thermal reactor system with the same heat output, since there is no requirement for a moderating material. Much higher heat ratings result from the size of the core, and the problem of removing heat from the system becomes a major design problem.

In order to provide more heat transfer surface for the heat removal pro­cess, the fuel is subdivided more than it is in a thermal system. Pins will have characteristically about half the diameter of the thermal reactor. Figure 4.1 illustrates the difference in size of fuel pin lattice and core be­tween a 1000 MWe PWR and a 1000 MWe LMFBR (7).

The fast reactor is very small with 10 times the volumetric PWR heat rating. Thus an extremely efficient coolant is required for heat removal from the very close lattice that is inherent in the fast reactor.