Effect of Class-IV power supply failure frequency on Core Damage Frequency (CDF)

In India, grid disturbance is a major cause of plant transients in nuclear power plants, and thus having an impact on plant safety. With better regulation and load management, the frequency of grid disturbance has come down substantially with time. Nevertheless the plant transients initiated by Class-IV power supply failure are experienced regularly. Further the incapability of emergency diesel generators´ to restore the power supply following Class-IV failure raises several safety issues. Similarly, the maintenance down time of diesel generators either planned or unplanned contributes to the system unavailability. Realizing the significance of Class-IV power supply for nuclear power plants, due weightage is given to the event sequence progression initiated due to Class-IV failure by incorporating a dedicated event tree while estimating Core Damage Frequency (CDF) in Probabilistic Safety Assessment (PSA) Level-I. At MAPS, in the recently carried out PSA Level-I analysis, Class-IV failure was observed to be the 6^th most significant contributor to the CDF. On a closure scrutiny it was observed that the Class-IV failure frequencies were high till nineties due to poor grid conditions which came down subsequently through the concerted efforts of grid authorities. In this paper, we have presented a parametric study to assess the sensitivity of CDF to Class-IV failure by using the PSA software package RISKSPECTRUM with the observed Class-IV related failure data for the entire operating year of MAPS station. It has been observed that with time the contribution of Class-IV power supply failure to CDF has got reduced. The Class-IV power supply failure frequency has hardly any effect on CDF, inferring that any external cause like this has no significant impact on CDF.