Thermalhydraulic Analyses for Optimization of Hydroaccumulators Parameters in VVER-440

Abstract

The passive ECCS system of VVER-440/213 according to the original Russian design consisted of 4 hydroaccumulators (HA) with nitrogen at pressure 6.0 MPa. In 90ties, the HA pressure in some VVER-440 was reduced to 3.5 MPa. This design modification was firstly done at NPP Dukovany (1998) in the Czech Republic and was based on TH analyses performed at UJV Rez. This pilot example was later followed by other VVER-440 NPP's, like Finish Loviisa, Hungarian Paks and Slovak Bohunice. Lastly the UJV Rez has prepared another step in optimization of HA parameters - increase of HA level and extension of allowed margins for HA pressure and level. The paper documents major steps in VVER-440 optimization and relevant thermal-hydraulic analyses including the latest work.