Estimation of Operator Instability Probability During Flood Event

During an extreme event such as seismic, flood, tsunami etc., an operator in Nuclear Power Plant (NPP) might have to perform certain actions in the field. In Probabilistic Safety Assessment (PSA) for external events such as flood, operator performance is modeled as human error probability (HEP) to perform certain actions. Operator instability probability while moving in flooded water can be correlated to time taken for action and hence, to HEP in performing action. Operators are trained on simulators to handle internal events in NPP, however for external events such training is not available on simulators. In the case of flood event, the plant personnel may have to move through water having finite depth and moving with some velocity. This water can also have debris and makes walking in it difficult and tiring. Also the illumination may be affected due to power failures. The literature on flood impact on locomotion instability for NPP operator is not available. Literature review on flood impacts on humans provides some data which may be applied for estimation of probability of instability. The data has a mix of subjects in test standing and walking. The studies in literature have been presented in the form of critical velocities and height of water that can cause human standing/walking difficult. This can be due to friction instability or moment instability. In this work, the empirical formulations on critical velocity- height relationship have been used along with uncertainty distributions for physical parameters to estimate the instability probability. The limitations in the number of subjects that were used in experiment in literature would have an impact on the deviation of mean value of the instability probability.