Probabilistic and deterministic analysis of BDBA in RBMK-1500

Lithuanian Energy Institute, Laboratory of Nuclear Installation Safety, Breslaujos g. 3, LT-44403 Kaunas, e-mail uspuras@mail.lei.lt By now, design basis accidents for the RBMK-1500 have been rather thoroughly investigated. Analyses helped to develop and implement a number of safety-ensuring modifications. The further plant safety enhancement requires developing the emergency procedures that would enable to manage beyond design basis accidents by preventing the core damage or mitigating consequences of severe accidents. To develop the emergency guidelines, it is necessary to collect information on initiating events leading to beyond design basis accidents, phenomena taking place during such accidents and possibilities of accident management. Collection of such information would require a lot of deterministic calculations. However, results of probabilistic safety assessment could be used to identify the most important scenarios. The paper presents results of the Ignalina NPP Level 1 and Level 2 probabilistic safety assessments and their use for compiling a list of beyond design basis accidents. The paper describes the most important phenomena for the management of severe accidents in RBMK-type reactors. The paper also presents the physical processes that occur in an overheated reactor core and discusses its cooling capabilities. The discussion also continues on the processes that occur in the reactor and primary circuit surrounding compartments, and recommendations on their management are presented.