Assessment of the total SBO accident progression after extended shutdown of VVER-1000.

This article discusses the results of analyses of an unmitigated total station blackout (SBO) accident at a VVER-1000 nuclear power plant in hot standby and in shutdown states, following an extended plant shutdown period. As part of the analyses, the isotopic composition of nuclear fuel and decay heat (DH) were estimated. Using these data as inputs, MELCOR simulations were conducted to model the plant's thermal-hydraulic response up to the point of severe nuclear fuel damage or temperature-induced failure of the reactor pressure vessel. The simulations provide the insights into the expected progression of accident under reduced DH conditions, which differs from the events sequence typically expected in full power SBO analyses. Though such conditions are atypical for normal plant operation, they can be realistically expected as a result of the military conflict. The findings provide valuable information for emergency response planning by defining the available time windows for implementing protective measures, a crucial input for planning. The required scope of these measures is being further evaluated in a subsequent study, focusing on the potential radiological consequences of the accident and will be detailed in a separate publication.