Station black out conditions (i.e. loss of off-site power, with simultaneous failure of all diesel generators) over a period of several hours are a very unlikely, but can develop severe consequences. Previous analysis has shown that a well planned emergency operating procedure, utilizing just equipment powered by batteries, can considerably extend the grace period, and mitigate severe accident progression. The current paper presents an analysis with Relap5 and Cathare2 system codes of the heat transfer capabilities of horizontal and vertical steam generators during the progression of a station black out transient. A long term station black out transient is characterized by high pressure in the primary side, and removal of the decay heat via natural circulation by the inventory present in the steam generators at the time of the initiating event. Secondary side mass inventory boils down in a certain period of time, depending on the reactor type. Subject of the presented analysis are the dependence of the heat transfer on the level of the steam generator, the different behaviour of horizontal and vertical steam generators in this respect, and the consequences for main primary side parameters such as average coolant temperature and primary pressure. The analysis shows that in natural circulation regime only few centimetres of the U-tubes are needed to cool the primary fluid to the temperature of the secondary side. Vertical steam generators are cooling the primary fluid down to the temperature of the secondary side until the level of the steam generator stays above the U-tubes cooling length. Horizontal steam generators on the other hand show an increasing number of U-tubes completely uncovered with decreasing level. As a result the primary side coolant temperature at the SG outlet is continuously increasing with decreasing SG level. The consequence is a different primary side pressure and temperature trend during a SBO transient in NPP with horizontal and vertical steam generators.
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