APPLICATION OF SMAP METHODOLOGY TO A10 POWER UP-RATE FOR ZION NPP SBO TRANSIENT

摘要

During the past years, nuclear power plants underwent some major changes in their design and operation mode to fulfill new objectives, such as power up-rate, life extension and/or increased fuel burn up. While fulfilling all the regulatory requirements, these changes - not necessarily and/or completely accounted for in the original design - can challenge the plant safety margins and induce a potential increase of the risk. In order to assess the impact of such modifications on the safety margins, the Committee on the Safety of Nuclear Installations (CSNI) approved in December 2003 a Safety Margins Action Plan (SMAP) and established an international Working Group aimed at developing a new methodology to address the problem, which has been successfully done [1].Then, the CSNI mandated a SM2A (Safety Margin, Assessment and Application) group to apply the SMAP methodology to a real case. A 10% power up-rate for Zion PWR has been selected as NPP modification and the increase of Core Damage Frequency (CDF) was selected as a metric for change in safety margin appraisal.A Probabilistic Safety Analysis - based (PSA) investigation phase, shared by the organizations participating in the group, aimed at selecting the event trees and the main sequences, which the change could potentially affect in a significant way. It was agreed that, once the selection done, calculations had to be run to quantify the effects.The reference PSA for Zion NPP used in the exercise was performed in the framework of NUREG 1150. For the SM2A exercise it was agreed that the risk measure is CDF. IRSN was in charge of the Loss Of offSite Power (LOSP) event trees. In this family of events it appeared that 2 sequences correspond to the conditions (a potential effect with a non negligible probability). These 2 selected sequences were Station Black Out (SBO) scenarios: SBO with loss of Auxiliary Feed Water (AFW) and SBO with a seal Loss Of Coolant Accident (LOCA).This paper describes the PSA based sequences selection and the preparation of the input desks for CATHARE code, presents the assumptions of modeling, and discusses the numerical results for those two scenarios. The Peak Cladding Temperature (PCT) being the safety variable of interest, it appears that it is sensitive to the power increase only in the first scenario (SBO-AFW). For this case, an uncertainty analysis has been carried out, too, in order to assess the increase of CDF. Its main findings are discussed in the paper.