With the assistance of the nuclear regulatory authority of the United States of America, U.S. Nuclear Regulatory Commission (U.S. NRC), a training workshop to improve knowledge on modeling of severe accident sequences to be considered in Level 2 Probabilistic Safety Assessment (PSA)* was held in Rockville in the U.S. NRC Professional Development Center. Anastasiia Ilina, Researcher of the NPP Thermohydraulic and Probabilistic Safety Analysis Department of the State Scientific and Technical Center for Nuclear and Radiation Safety (SSTC NRS), participated in the workshop.
The participants were presented with information on the evaluation of processes occurring with the fuel in the reactor core in the case of an incident/accident: chemical reactions (importance of fuel enrichment consideration), corium formation (geometry of fuel melting in fuel assemblies), corium pathways (melting of in-vessel internals) and release of radioactive material beyond the primary side. Besides, they were provided with the data on processes occurring directly in the containment (containment rooms), in particular, information on the assessment of the ability to withstand the release of radioactive materials into the environment for various NPP designs.
Within the training course, the participants considered the peculiarities of different reactor designs: Boiling Water Reactors (BWRs) – Mark I, Mark IІ, Mark IІІ and Pressurized Water Reactors (PWRs) – Large Dry, Subatmospheric, Ice Condensers, AP1000. The workshop also covered issues of using MELCOR, RELAP5 and MAAP computer codes for modeling of processes occurring in the containment during severe accidents.
The lecturers presented information on the consideration of phenomena in the probabilistic model, which occur in the reactor and containment during severe accidents: examples of grouping unit damage conditions, modeling of containment event trees considering the differences in the design, focus on the importance to consider specific top events and their specific order in containment event trees.
At the end of the workshop, the participants passed the exam, where they demonstrated the level of processing the information received.
Gained knowledge will be used to provide the SNRIU with technical support during state reviews of nuclear and radiation safety of Level 2 PSA (NPP documents on safety justification) and documents on the assessment of reactor pressure vessel resistance to brittle fracture (NPP documents on long-term operation). In addition, the information obtained will be used for the activities within SSTC NRS international projects (Multi-Unit PSA, development of PSA methodological guidelines, etc.).
*Level 2 PSA covers the analysis of accident sequences that cause core damage and/or can lead to release of radioactive substances beyond the containment.