Effectiveness of Flooding Heat Transfer by Emergency Core Cooling Systems for Boiling Water Reactor

The effectiveness of emergency core cooling under loss-of-coolant accident conditions has been one of the most important safety issues that have interested the regulatory authorities in their review of applications for the construction of light-water-cooled nuclear power plants. Devices for flooding, to be adopted in conjunction with spray cooling, are included in emergency core cooling systems of Boiling Water Reactor (BWR) plants. The effectiveness of flooding heat transfer was tested with a full scale BWR fuel bundle simulator over the range of conditions expected in loss-of-coolant accidents. The results of the test prove the adequacy of the BWR flooding technique: (1) Core flooding effectively terminates core heat-up transients predicted to occur immediately following a loss-of-coolant accident. The post-accident peak fuel cladding temperature strongly depends on the fuel cladding temperature at the time of flooding initiation. Above 50mm/sec, differences in flooding rate are found to have no effect on the post-accident peak fuel cladding temperature. (2) Core decay heat is effectively removed for long periods of time after a loss-of-coolant accident when the core is flooded. Continued long term core cooling capability of BWR-ECCS is thus demonstrated. It should be noted that the water level required to cool the core decreases with increasing core power.