[en] The objective of the study is to estimate the probability of failure of the battery system of a nuclear power plant due to loss of ventilation in the battery room during ac blackout. This is part of the more general problem of the interaction between non-safety systems and safety systems. The emergency battery system and the associated ventilation system are a pair of interactive systems, the former being a safety system and the latter a non-safety system. Described are: (1) the emergency battery room; (2) the heat sources in the room following ac blackout; (3) the overall problem of insufficient power in the plant, the frequency of loss of ac power, and the time-dependent probability of nonrestoration of ac power; (4) the probabilistic approach to the problem; and (5) the thermal models used to evaluate the temperature rise in the battery room. A series of tables show the failure probabilities at several times and for different values of the standard deviations of the parameters of importance