Analysis of High-Pressure extended SBO accident in VVER-1000: A sensitivity study on lower plenum structures modeling and core region nodalization in MELCOR

• Published in: Nuclear engineering and design 2023-12, Vol.414, p.112584, Article 112584
• Main Authors: Nabavi, A., Pirouzmand, A., Hadad, K.
• Format: Article
• Language: English
• ISSN: 0029-5493; 1872-759X
• DOI: 10.1016/j.nucengdes.2023.112584

•Modeling of lower plenum structures in a VVER-1000 type reactor during a High Pressure Extended SBO with MELCOR•Investigation of Important safety parameters such as in-vessel hydrogen production.•Sixteen different simulations to investigate the sensitivity of the safety parameters.•MELCOR models showed the uncertatinty in timing of the accident, the amount of hydrogen produced and total radioactive material release up to 24 h after the accident. The lower plenum structures of VVER-1000 reactors exhibit a unique shape. This paper aims to investigate the sensitivity of various important parameters related to safety using different models in the MELCOR 1.8.6 code's COR package. The study focuses on modeling the lower plenum structures and the core region using different CVH package nodalizations during a High Pressure Extended Station Black-Out accident. At first, MELCOR calculated parameters are investigated during the course of a severe accident for a basic model of the lower plenum structures and core region nodalization. Different important parameters from a safety point of view are also examined. These include the total amount of in-vessel hydrogen production, the lower head breach time and elevation, the synchronicity between lower head breach and debris ejection, the total amount of hydrogen produced during molten corium concrete interaction, the total amount of ejected debris to the cavity, and the total amount of radioactive material release. Then, by considering sixteen different simulations corresponding to four lower plenum structures modeling and four core region nodalizations, the sensitivity of the mentioned parameters is investigated. The obtained results show a sensitivity of about 12.7% for the timing of the accident (equivalent to), 103% for the hydrogen produced inside the vessel (equivalent to 417.9 to 847.8 kg), 143% for the hydrogen produced outside the vessel (equivalent to 445.7 to 1081.4 kg), 47% for the total hydrogen produced (equivalent to 1207.5 to 1780.7 kg), and 76% for total radioactive material release (equivalent to 625.311 to 1101.892 kg) up to 24 h after the accident.