If a bubble of sodium vapor is surrounded by sodium liquid at the same pressure, thus providing a condensing medium, the bubble will then collapse. Initially the rate of collapse is slow and the rate of condensation keeps pace with it; however as the collapse rate increases the rate of condensation de­creases as the heat transfer area is also decreasing. Thus the rate of con­densation no longer matches the rate of collapse, so the internal bubble pressure will increase very rapidly indeed. The bubble then grows once more and collapses again and finally is fully condensed after several collapse and rebound cycles (6). The whole collapse and rebound cycle can occur in 10 msec and peak pressures of up to 8000 atm have been calculated for a 10-cm bubble. Such collapse pressures have been observed experimentally, although the energy possible in the pressure pulse is very limited. In the case cited it is about 4000 J. However this effect has not been observed in any reactor installation, although the phenomenon should be accounted for in accident analysis.