Evaluation of VVER-1000/V446 ECCS passive part operability at severe accident conditions using RELAP5/SCDAP3.4 code

In this paper, VVER-1000/V446 nuclear reactor is modeled using RELAP5/SCDAP3.4 code in order to investigate its behavior during severe accident conditions in a LBLOCA scenario along with station blackout (SBO). The analysis is performed in two stages related to availability and unavailability of emergency core cooling systems (ECCS) passive part (i.e. Hydro Accumulators KWU tanks). The initial core inventory is calculated using MCNPX 2.7 code and the primary secondary systems are modeled in RELAP5/SCDAP3.4 code. The reactor core consists of 163 hexahedral fuel assemblies is divided into five groups based on average power peaking factors and modeled in SCDAP code. For each group a corresponding channel is considered in RELAP5 code plus a bypass channel. To investigate the phenomena of slumped core into the lower plenum, couple code with related geometric mesh is applied. Results of first stage analyze (availability of ECCS) shows that the operators have ~3hr before core degradation and results of second stage analyze (unavalability of ECCS) demonstrates that this time decreases to ~15min. The results prsent that 350 and 210 kg hydrogen in vessel is producted for availability and unavalability of ECCS, respectively. Also, the calculation of first stage analyze shows that beginning of oxidation, release of FPs, creep rupture of vessel with conservative calculation and creep rupture with best estimat calculation occur at times 9850, 10900, 17700 and 31000 seconds, respectively. These events for the second stage occur at 580, 660, 4100 and 9700 seconds, respectively.