Abstract
Recent progress in the development of advanced nuclear power plants has shown the potential of smaller nuclear reactors with passive safety features. One of the major characteristics of the next generation nuclear power plants is passive decay heat removal. The present work concentrates primarly on this issue and investigates varios alternatives for passive decay heat removal with the goal of finding the maximum achievable power which can be removed using only natural heat transfer phenomena.
Three fundamental passive heat transfer mechanisms are identified and characterized with respect to their heat removal capabilities. Utilizing these mechanisms, the existing designs of advanced reactors are reviewed and their achievable power limits are assessed.
Next an investigation of varios alternatives of passive decay heat removal from the core to the ultimate heat sink through the various physical barriesers is performed. Three ultimate heat sinks - water, air, and the eather - are explored. Water appears to be the most efficient heat sink, while the heat transfer rates to earth are very limited. For LWRs, the vessel wall is found to be the most restricting thermal resistance to heat transfer.
Finally, the protential for new designs of Light Water Reactor (LWR) cores capable of dissipating decay heat energy under severe loss of coolant accidents is assessed. Introducing a solid matrix into an LWR. core considerably enhances the capability ot transmit the decay heat out of a fully uncovered core by conduction. Two design approaches are pursued - a solid matrix core in a pressure vessel and a system of solid matrix pressure tuves dispersed in a low pressure calandria. The latte approach yields much higher potenital with respect to achievable power output and also eliminates the choke pint on the heat transfer path posed by a thick pressure vessel wall.
