Refueling outages at the Kozloduy Nuclear Power Plant (KNPP) Units 5 and 6 are used to perform technical service (preventive maintenance) activities, as well as inspection and testing activities. The refueling outages typically require in excess of 90 days. Following implementation of the Modernization Program at Units 5 and 6 in 2007, the refueling outage lengths were reduced to 60 days. To further shorten the outage lengths, a risk-informed (RI) approach was used to reduce the impact of technical service activities on refueling outage length.The approach used consisted of a four step process: 1) identification of systems to address, 2) technical service assessment and alternate strategy identification, 3) risk assessment, and 4) deterministic evaluation. Systems were selected based on their safety significance and impact on outage length. Primary candidates were systems with a high impact on outage length and low safety significance, and secondary candidates were systems with a high impact on outage length and high safety significance. The safety significance was determined using an expert panel with input from the KNPP Probabilistic Safety Assessment (PSA) model, deterministic factors, and operation and maintenance considerations. Seven safety systems were selected and included fluid, AC power, and ventilation systems.The technical service assessment used a reliability-centered maintenance (RCM) approach that included detailed data analysis and fault tree assessments, and review of current technical service activities against component failures that have occurred. This led to identification of critical components of the systems, a qualitative assessment of the effectiveness of current technical service activities, and identification of alternate technical service approaches to reduce outage length. It was concluded that the current technical service activities are appropriate and the most effective method to reduce outage duration is to move some technical service activities currently done during the refueling outage to power operation.The risk assessment evaluated the impact on plant risk of moving activities for the systems of interest to power operation. Since this will require a change to the KNPP Technical Specifications, the three-tiered RI approach defined in the U.S. Nuclear Regulatory Commission (NRC) Regulatory Guides 1.174 and 1.177 was followed. The impacts of the proposed changes on core damage frequency (CDF) and large early release frequency (LERF) were determined, as was the incremental conditional core damage probability and incremental conditional large early release probability. Following the Regulatory Guides, the impacts on defense-in-depth and safety margins were assessed and Tier 2 requirements identified. The preliminary results, currently under review, indicate that the proposed changes will have a small impact on risk and result in reduced outage length.
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