ABSTRACT
Safe operation of NPP (Nuclear Power Plants) depends on the Technical Specifications (TS), so that TS are part of the Licensee Basis (LB) to operate a NPP, which were established taking into account mainly deterministic criteria. A number of problems have been identified connected to TS that can jeopardize plant safety. The development of PRA (Probabilistic Risk Assessment) and its application since the early 80’s to analyze TS changes has brought the opportunity to review TS consistency from a risk viewpoint, i.e. addressing the impact of the changes on plant safety on the basis of the risk information provided by the PRA, with particular attention to the role of the STI (Surveillance Test Intervals) included within the SR (Surveillance Requirements), and of the AOT (Allowed Outage Times) included within the LCO (Limiting Conditions for Operation),
Nowadays, regulatory bodies are encouraging the use or PRA where practical, consistent with the state-of-the-art, to support a risk-informed regulatory framework. The US Nuclear Regulatory Commission (NRC) issued RG 1.174 (2002), which is a key element in this framework to support risk informed decisions on changes to LB, which particularizes to analyze TS changes in RG 1.177 (1998). They present five principles of the risk-informed decision-making to be used for making decisions regarding plant-specific changes. The fourth principle states “if the proposes change increases risk, the increase should be small. This principle is the one we are concerned with in this paper. Risk and risk increases are quantified using PRA, i.e. using the CDF (Core Damage Frequency)
derived from a level I PRA and the LERF (Large Early Release Frequency) derived from a level II PRA. Small changes are defined using the CDF and LERF acceptance guidelines.
