Srisht Pall Singh
Abstract: On 31st March 1993, a major fire broke out in NAPS unit 1 while it was generating 185 MWe power level. The fire had been triggered by sudden rupture at the roots of the 5th stage blades in the low pressure turbine leading to leakage of hydrogen from the generator cooling circuit. It led to deflagration in the turbine building which propagated along the cable ducts through several barriers to the control room and emergency control room. This fire disabled emergency power supply in about 7 minutes. Explosive sounds were experienced, and blue flames was observed in the turbine building by an operator. The ensuing fire propagated via power and control cables to the control room and emergency control room disabling all power supply sources, from class I to class IV types. This beyond-SBO situation lasted around 17 hours. It disabled all safety functions after the initial scram. Hence, the safety functions to actuate long term sub-criticality by poison injection, and residual heat removal were actuated manually, at great personal risk. The fire was brought under control in 90 minutes and was extinguished manually by use of portable fire pumps, in around 9 hours. This fire event ranked third after Chernobyl and Fukushima fire events in terms of severity. However, despite manifest signs of common cause failures and weak safety culture in the utility (NPCIL) and failure of all power sources from class I to class IV, this event was ranked at level 3 on INES scale, namely, a simple incident. The utility (NPCIL) had undertaken measures, including strengthened systems and procedures for fire control to address weaknesses that were dormant for over a decade in its NPPs. In this study, this event is reviewed and strengths and weaknesses of Indian NPP program and its regulatory control as they existed at the time of the event are described. Those strengths are weaknesses are also described. Suggestions are offered to weed out any dormant weaknesses.
Keywords: fire, explosion, beyond SBO, common cause failures, weak safety culture,