Investigation of severe accident phenomena for LBLOCA along with SBO scenario in VVER-1000/V446 nuclear reactor using RELAP5/SCDAP3.4 code

Severe accidents can cause significant damage to reactor fuel resulting in more or less complete core meltdown that imposes serious consequences such as possible release of radioactive materials from the fuel, substantial hydrogen production, the total core uncover, core heating up, melting of the fuel and its further relocation to the bottom of the reactor vessel and also creep rupture of vessel. Herein, the large-break loss-of-coolant accident (LBLOCA) beside station-black-out (SBO) scenario is analyzed using the RELAP5/SCDAP3.4 severe accident code for VVER-1000/V446 nuclear reactor. The analyses are carried out without considering the operator’s actions on the accident management. A model of Bushehr nuclear power plant (BNPP) which is a VVER-1000/V466 pressurized water reactor is chosen as the reference plant. Analysis of the scenario shows the heat up of the core and increasing the temperature of the reactor core up to 3000K which increases the steam in the core and consequently initiates the oxidation of the core and production of hydrogen at about 9850 seconds. Analysis of the scenario by the code indicates that around 350kg hydrogen with the maximum production rate of 1.05 kg/s is produced. The behavior of volatile FPs (I, Cs and Te) and fission gases (Xe and Kr) in terms of total release and release rate into the coolant is also presented. For the initial core inventory of fission products (FPs) calculated by MCNPX code, the results show releasing harmful levels of radionuclides into the reactor vessel in the order of 10kg. Also Calculation of analyze was showed that beginning of oxidation, release of FPs , creep rupture of vessel with conservative calculation and creep rupture with best estimat calculation respectively at times 9850, 10900 , 17700 and 31000 second after beginning of transient condition was occurred. The analysis results demonstrate that the user has about from 2/5 hours before fuel rod cladding rupture and entrance of radioactive material through cooling fluid and consequently the start of oxidation and hydrogen production and about 5 hours before core slumping and reactor vessel creep rupture (conservative estimate) and based on the best estimate about 8 hours before reactor vessel creep rupture to show quick reaction.