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SAFETY ASSESSMENT OF INDIAN NUCLEAR POWER PLANTS AND LESSONS LEARNED POST FUKUSHIMA-2011

L.R. Bishnoi

Head, Siting & Structural Engineering Division Atomic Energy Regulatory Board, INDIA

Introduction Fukushima-2011 is the third severe accident in the history of civilian nuclear power plants. It has revealed yet another facet of NPP safety – impact of external natural events. Last decade witnessed external natural events affecting many NPPs. These were perhaps precursors to the Fukushima-2011 and wake up calls to be more serious about the impact of external events on NPP safety. This paper presents a brief overview of the nuclear power program in India and its safety evaluation in light of the Fukushima accidents as well as lessons learned for future guidance. Indian Nuclear Power Program Two government companies are the NPP utilities in India; NPCIL for thermal reactors (PHWRs & PWRs) and BHAVINI for FBRs. NPCIL operates twenty NPP units; two BWRs and eighteen PHWRs. The BWRs started operation in 1969. First four PHWRs of 220 MW were added during 1970s and 1980s. Nineteen nineties saw standardization of PHWR design with state of the art safety features/analyses and further evolution continued towards enhanced safety features. Remaining fourteen units (twelve 220 MW and two 540 MW) belong to the standardized and improvised designs, connected to the grid progressively during last two decades. The old units (viz. 2-BWRs and 4-PHWRs) had undergone extensive safety assessments and upgrades during the last decade. AERB was created in 1983 to formally regulate safety in nuclear and radiation facilities in India. Over the years AERB has evolved a robust procedure for safety review and issue of regulatory consents. All operating plants undergo a periodic safety review by AERB. Major safety regulations of AERB are issued in the form of safety codes and guides. Safety Evaluation Procedures Soon after the Fukushima incidents of March 11, 2011, NPCIL undertook review of its all operating NPPs to evaluate adequacy of existing capabilities to deal with extreme external events and their consequential effects including assessment of margins available beyond design basis for these events. Four task forces were constituted for specific review of operating NPPs of different types. Plant walk downs were conducted to inspect all important provisions required to withstand flood and fire events. The final reports of the task forces were put up on NPCIL web site. AERB constituted a committee to review capabilities of Indian NPPs and adequacy of provisions available to ensure safety in case of external natural events both within and beyond design basis. AERB also conducted special inspections to assess individual plant status and preparedness to deal with the situations arising out of natural disasters

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including SBO and multiple failures. The AERB committee constituted specialist working groups for detailed review of specific areas of concern vis-à-vis external natural events, i.e. adequacy of AERB regulations for handling external events and possible quantification of extreme events for margin assessment, safety of electrical, control and instrumentation systems, handling prolonged SBO and loss of designated UHS, safety of irradiated fuel in storage facilities at NPP sites, and severe accident management provisions and guidelines. The final report of the committee has been put up on AERB web site.

Results of Evaluation and Further Course of Action Safety evaluation of Indian NPPs by AERB and NPCIL committees reconfirmed inherent strengths in design, practices and regulation. The NPPs in India can withstand currently defined design basis external events. All units of standardized design have margins for beyond design basis external events and their consequential events like prolonged SBO and loss of normal heat sink. Necessary upgrades have been made/recommended for older units. Notwithstanding all the design provisions and mitigating measures, severe accident management has been considered and the guidelines to handle severe accident are being implemented. It is also recommended to create a robust emergency facility at each NPP site. The review also confirmed that the safety requirements given in AERB regulations are sufficiently conservative. However AERB will appropriately incorporate the lessons learnt from the Fukushima in its regulations. AERB’s safety review committee for operating plants took steps for review and follow-up of safety enhancement measures at all operating NPPs based on the inputs from the AERB committee and its specialist working groups, self assessment by NPCIL and special inspections conducted by AERB. Utility reports on safety substantiation and/or enhancement for plants under construction in light of the Fukushima accidents are being reviewed and followed up in project specific safety review committees of AERB. Lessons Learned for Future Guidance The Fukushima accidents and a few recent natural events of magnitudes beyond design basis values of the affected NPPs have revealed strengths as well as weaknesses of NPPs to face natural events. Natural calamities are characterized by more surprises in less prone regions. Considering human limitations in accommodating the entire space-time spread of natural phenomena in the scientific models and their validation, beyond design basis events are to be accepted as a reality as also the occurrence of severe accidents. Hence some of the important points to be considered for future guidance are; concerted efforts to understand and sharpen tools for investigating a site to minimize uncertainties with respect to natural vulnerabilities, margin assessment to handle beyond design basis events should form part of plant safety analysis as an element of defense-in-depth that may account inherent margins of the design process and build-up additional margins during design process, if required, to ensure a minimum level of margin, developing and maintaining SAMGs to handle accidents initiated by beyond design basis external events including common cause failures at multiunit sites leading to degraded plant and site infrastructure, and conducting PSAs for external events to identify and prevent/mitigate cliff edge effects.