Risk-based approach towards design evaluation and re-assessment of shutdown safety margin

Abstract

Traditional approach for design, operation and regulation of the nuclear plants is deterministic in nature where the principle of defense in depth governs incorporation of multiple barriers and levels of protection. Single failure criteria, redundancy, diversity, fail safe criteria, quality assurance, etc., form the major cornerstone of this approach for safety demonstration. Traditional approach, which is conservative and prescriptive in nature, has served well for high level of safety since the inception of nuclear industry. However, with the accumulation of operating experiences, insights obtained from the accidents in the nuclear plants and growth of the probabilistic risk assessment methods, there is an increasing trend to implement a risk-informed approach in the nuclear plants. Over conservatism built in design, operation and regulation can be addressed using quantitative insights on risk and uncertainty such that the system availability or performance can be enhanced without compromising the safety. This paper proposes an integrated risk-based approach wherein probabilistic risk assessment framework has been used along with the deterministic insights to give credit to the available design provision in the nuclear plants. The objective is to use the margins available with the existing design to demonstrate safety of the plant by using risk framework along with the improved understanding of uncertainty. The model and procedure, involved in the risk-based approach, have been demonstrated through a case study performed on an Indian research reactor.