Although 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 very unlikely, analysis of station black out transients and possible countermeasures are important. A well planned emergency operating procedure, utilizing just equipment powered by batteries, can extend the grace period considerably, and mitigate severe accident progression. Until the occurrence of core damage best estimate thermal hydraulic system codes like Relap5 or Cathare2 can give a good picture of the development of the transient. Heat transfer from the primary to the secondary side is one of the dominant phenomena during the first hours of a station black out transient. Primary side pressure stays high, and the decay heat is transferred by natural circulation to the secondary side. The steam generator inventory boils off, and the steam generator level decreases. Modelling of the steam generator therefore is of special importance. The current paper presents an analysis for VVER-1000 type steam generators (horizontal) with different nodalisation detail and different codes (Cathare2 and Relap5). The nodalisation approach has been verified by comparing calculated results to PSB-VVER steam generator experiments. The results show that codes like Relap5 and Cathare2, by assuming uniform bulk flow properties over the flow cross section and within volume units, introduce a distortion compared to the real physics in case station black out conditions. The reason lies in the specifics of the horizontal steam generator. While the heat transfer surface decreases continuously in the actual steam generator, the necessity of modelling the steam generator with nodes of finite volume creates a step-wise reduction of the heat transfer surface. The effects on the primary side temperature and pressure trends are not negligible, and decrease with increasing detail of modelling.
Effect of SG modeling on primary side pressure and temperature trend during long term station black out conditions in VVER using Relap5 and Cathare2 system
D'AURIA, FRANCESCO SAVERIO
2008-01-01
Abstract
Although 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 very unlikely, analysis of station black out transients and possible countermeasures are important. A well planned emergency operating procedure, utilizing just equipment powered by batteries, can extend the grace period considerably, and mitigate severe accident progression. Until the occurrence of core damage best estimate thermal hydraulic system codes like Relap5 or Cathare2 can give a good picture of the development of the transient. Heat transfer from the primary to the secondary side is one of the dominant phenomena during the first hours of a station black out transient. Primary side pressure stays high, and the decay heat is transferred by natural circulation to the secondary side. The steam generator inventory boils off, and the steam generator level decreases. Modelling of the steam generator therefore is of special importance. The current paper presents an analysis for VVER-1000 type steam generators (horizontal) with different nodalisation detail and different codes (Cathare2 and Relap5). The nodalisation approach has been verified by comparing calculated results to PSB-VVER steam generator experiments. The results show that codes like Relap5 and Cathare2, by assuming uniform bulk flow properties over the flow cross section and within volume units, introduce a distortion compared to the real physics in case station black out conditions. The reason lies in the specifics of the horizontal steam generator. While the heat transfer surface decreases continuously in the actual steam generator, the necessity of modelling the steam generator with nodes of finite volume creates a step-wise reduction of the heat transfer surface. The effects on the primary side temperature and pressure trends are not negligible, and decrease with increasing detail of modelling.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
