The presence of bubbles has been noted as providing valuable nucleation sites (6) at which coolant boiling might start, if the heat removal from the system were inadequate.

In pure sodium, superheats of more than 500°F have been observed before boiling commenced due to overheating (7). If such superheats were possible in reactor accident conditions, then Table 4.2 shows that the time

needed to void a channel completely would be much shorter and the excess channel pressures would be much greater. Thus, although accident condi­tions are to be avoided, if they can not be avoided then it would be prefer­able to have sodium boiling occur relatively quietly at little or no super­heating.

The radiation present in the reactor core is expected to limit practical superheats to less than 200°C, but it has been estimated (6) that one bubble of radius 10-3 cm in every 100 cm3 of sodium would prevent superheats of more than 10 or 20°C. Absorbed gas appears to have little or no effect, but some entrained small bubbles are likely to maintain conditions at the lower end of Table 4.2. Fortunately, in a reactor system there will always be one or two small bubbles.