Coolants other than liquid metals are possible and previous sections of this book have dealt with reactor systems using supercritical steam and helium as coolants.

Although the U. S., British, French, and Soviet national fast breeder programs are based on sodium, gaseous coolants have not been ruled out. Indeed the gas-cooled system shows considerable breeding potential. Figure

4.2 shows comparative breeding ratios for different fuels and coolants.

Steam gives rise to a poorer breeding ratio, because it degrades the spectrum while low density helium, although a moderator, actually improves the spectrum in comparison to sodium.

In order to obtain adequate heat removal, a nonmetallic coolant must be used at high pressure: over 3000 psi for supercritical steam and of the order

+ See Dalle-Donne (4a) and Hummel and Okrent (4b).

of 1000 psi for the helium gas-cooled system. This high pressure then gives rise to problems associated with depressurization of the system.

Gas-cooled fast reactors are susceptible to water-flooding accidents, either from an external source or from the high-pressure steam generator. The resultant reactivity increase can be prohibited by the addition of resonance absorbers to the system to absorb the thermalized neutrons, but in practice this would probably be unneccessary because flooding-induced reactivity changes would be slow transients and could be engineered out of the system.

Despite the economic potential of gas-cooled fast reactors, the remainder of this volume will refer only to sodium-cooled systems, unless otherwise noted, because sodium is the chosen coolant for the first generation of fast — breeder power reactor plants around the world.