An overview of the main improvements in updated version 2.1 of MELCOR computer code related to more representative mathematical modeling of complex thermohydraulic severe accident processes of core degradation, transfer of molten fragments to the bottom of the reactor, heating and failure of the bottom of the reactor pressure vessel is presented. The elements of WWER-1000 NPP computer model for the MELCOR 1.8.5 (control volumes, thermal structures and structures of the reactor core) that are reproduced for a reactor with the primary side, the secondary side and the containment are described. The changes implemented in WWER-1000 NPP model for MELCOR 1.8.5 to convert it to MELCOR 2.1 version that are mainly related to more detailed modeling of the reactor core and reactor pressure vessel bottom are provided. The paper presents the results of comparative analysis of severe accident scenario of total station blackout at WWER-1000 NPP with MELCOR 1.8.5 and 2.1. The comparison demonstrates good agreement between the main parameters’ results (pressure and temperature in hydraulic elements of the primary, secondary sides and the containment, temperature of core elements, the mass of the generated non-condensed gases and their concentration in the containment) obtained with these code versions for severe accident in-vessel phase. The identified differences in the time of core structures degradation and reactor vessel bottom failure are insignificantly affected by the behavior of the parameters in the primary side and the containment in the in-vessel phase of the severe accident and are related to more detailed modelling of the reactor core and bottom part of the reactor in MELCOR 2.1.
2. WENRA Safety Reference Levels for Existing Reactors. (2014). Update in relation to lessons learned from TEPCO Fukushima Daiichi accident. Report, 13.
3. Council Directive 2014/87/EURATOM of 8 July 2014 amending Directive 2009/71/Euratom establishing a Community framework for the nuclear safety of nuclear installations.
4. MELCOR Computer Code Manuals. (2000). Vol. 2, Rev. 2. Reference Manuals. Version 1.8.5. NUREG/CR-6119, SAND2000-2417/2. Albuquerque, Sandia National Laboratories, 306.
5. MELCOR Computer Code Manuals. (2005). Vol. 2, Rev. 2. Reference Manuals. Version 1.8.6. NUREG/CR-6119, Vol. 2, Rev. 3, SAND 2005-5713. Albuquerque, Sandia National Laboratories, 776.
6. Kotsuba, O., Vorobyov, Yu., Zhabin, O., Gumenyuk, D. (2016). Analysis of severe accidents in spent fuel pool of Fukushima Daiichi NPP using MELCOR 1.8.6 computer code. Nuclear and Radiation Safety, 4(72), (ISSN 2073-6231), 13 – 20.
7. MELCOR Computer Code Manuals. (2015). Reference Manuals. Vol.2. Version 2.1.6840, SAND2015-6692 R. Albuquerque, Sandia National Laboratories, 807.
8. AT6-10.300.OD1. (2010). WWER-1000/320 model for simulation of severe accidents (MELCOR code). SSTC NRS, 590.
9. BOA No. 257586, Task order No. 17. (2017). Support in mastering the computer codes obtained within the 2014 CAMP and CSARP agreements. Development of MELCOR model for WWER‑1000/320. (Stage No. 3c). SSTC NRS, 125.