Combat induced multiple main steam line break at VVER-1000/320

While nuclear power plants are designed to withstand various internal and external events, they are not designed to endure combat between opposing military forces on-site. The effects of military conflicts are not taken into account for the safety case and during licencing. However, as observed during the 2022 Russia–Ukraine war, nuclear power plants hold strategic importance, and warring parties often seek to gain military control over them, even though they are covered by Article 56 of the Additional Protocol I to the Geneva Conventions. Therefore, the aim of this paper is to analyse the results of an exemplary scenario — an artillery induced multiple steam line break at a VVER-1000 reactor in a war/combat zone. We assumed that the attacker’s intention was to gain control of the nuclear power plant, but that they are met with armed resistance. We further assumed that during combat on site the reactor building was inadvertently hit at the location where all main steam- and feedwater lines pass to the turbine hall. We have analysed scenarios where 3 or all 4 steam lines are ruptured in the non-isolable part and the feedwater supply is interrupted in the effected loops. Furthermore. we conducted calculations where the operators would abandon the control room, and calculations where we credited operator actions. The simulations were calculated with MELCOR 1.8.6 severe accident analysis code and RELAP5-3D/4.6.2 thermal hydraulic system code. Our analysis has shown that without operator measures, reactor pressure vessel and containment failure in case of a 4 time main steam line break may occur as early as 6 h into the transient, as the automatic safety systems cannot control the accident. At the same time our Relap5-3D and Melcor analysis in good agreement came to the result that the assumed operator actions, primary system feed and bleed, are effective if at least one train of the high pressure injection system is available. Furthermore, only one steam generator, if isolated and supplied with emergency feed water, can prevent accident progression to core damage. Our results show that scenarios which are considered highly unlikely, as a multiple main steam line break, have to be considered differently in case of combat scenarios on-site and that the risks from nuclear power plants in war zones are not covered by the traditional safety case. • NPP safety analyses exclude initiating events from combat actions on plant sites. • The present analysis shows that combat on-site may lead to radionuclide releases. • NPP safety could improve by addressing accidental combat damage in safety analyses.