Abstract
In this work, Probabilistic Safety Assessment (PSA) is used to evaluate Allowed Outage Times (AOT) and Surveillance Test Intervals (STI) extensions for three Angra 1 nuclear power plant safety systems. The interest in such an analysis lies on the fact that PSA comprises a risk-based tool for safety evaluation and has been increasingly applied to support both the regulatory and the operational decision-making processes. Regarding Angra 1, among other applications, PSA is meant to be an additional method that can be used by the utility to justify Technical Specification relaxation to the Brazilian regulatory body. The risk measure used in this work is the Core Damage Frequency, obtained from the Angra 1 Level 1 PSA study. AOT and STI extensions are evaluated for the Safety Injection, Service Water and Auxiliary Feedwater Systems using the SAPHIRE code. In order to compensate for the risk increase caused by the extensions, compensatory measures as (1) test of redundant train prior to entering maintenance and (2) staggered test strategy are proposed. Results have shown that the proposed AOT extensions are acceptable for two of the systems with the implementation of compensatory measures whereas STI extensions are acceptable for all three systems.
Highlights
Technical Specifications (TS) such as limiting conditions of operation, which include system/component Allowed Outage Times (AOT) and Surveillance Test Intervals (STI), have been established based only on deterministic analysis [1, 2] and engineering judgment [2]
The results indicate, most of the times, the need to introduce compensatory measures to bring the risk within the appropriate acceptance criterion
It should be noticed that difficulties in obtaining Angra 1 specific data for pump unavailability due to Preventive Maintenance (PM) and Corrective Maintenance (CM), led to the adoption of their failure rates as the frequencies for the calculation of the average yearly risk associated with the AOT extensions
Summary
Technical Specifications (TS) such as limiting conditions of operation, which include system/component AOT and STI, have been established based only on deterministic analysis [1, 2] and engineering judgment [2]. The SIS is a two-train standby system; the SWS is a two-train system but with three pumps (one of which is a swing) where one pump is in service during normal operation and the other two are in standby mode, and the AFWS is a standby system with two motor-operated pumps plus a turbine-driven pump as diversity. They were chosen to cover the types of typical safety systems of a Westinghouse two-loop PWR design. The development of Angra 1 PSA resulted in an average estimation for the Core Damage Frequency (CDF) value of 4.015E − 05 per reactor-year, originated from internal events and including the external event flood, typically Level 1 PSAs evaluate the core
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