227 The NRI Approach for Technical Specifications Evaluation (PSAM-0153) Available to Purchase

Nuclear Research Institute (NRI) in Rez near Prague, Czech Republic, maintains Living Probabilistic Safety Assessment (Living PSA) program for NPP Dukovany, a four unit NPP of VVER-440 type in the Czech Republic. This project has been established as a framework for all activities in NPP Dukovany that are related to risk assessment and support for risk-informed decision making. Continuously updated PSA model is extensively used for various PSA applications including Technical Specifications (TS) assessment. However, all performed TS evaluations using PSA model were done only for a limited set of risk measures using various available criteria since the Czech regulatory body (SUJB) has not issued any specific guideline or requirement for risk-informed assessment of TS as well as has not given any associated acceptance criteria. A comprehensive guideline for risk-informed TS evaluation based on agreed methodology and prescribed criteria appeared to be necessary.

The paper focuses on the process how NRI participates in development of applicable methodology for risk-informed evaluation of TS in the Czech Republic.

The PSA department in NRI reviewed available approaches to risk-informed TS evaluation. Trial calculations were performed to test features of the particular methods. Based on that, the methodology proposal for TS assessment of allowed outage time (AOT) changes or adequacy has been developed in NRI. It has been passed to SUJB for comments and approval.

The proposal adopted the three-tiered approach from US NRC RG 1.177. The key features of its application in NRI methodology are discussed. In addition to single-event AOT risk calculation, it recommends to assess acceptability of cumulative effect of TS changes as well as to determine adequacy of the safe state for LCO's. Great attention is paid to selection of risk configurations necessary to be evaluated. The methodology suggests using fixed criteria for conditional risk increase and average risk increase. Selection of appropriate numerical values is presented and discussed in the paper. Quantitative results of trial calculations to illustrate the presented approach are shown in the paper as well.