Consideration of NPP Systems Structures and Components Ageing Effects in Probabilistic Safety Assessments

The Thesis presents the results of research studies on the use of PSA for evaluation of SSC aging effect to the overall plant safety carried out in the frame of EC JRC Network activities on Incorporating Ageing Effects into Probabilistic Safety Assessment (Aging PSA). The basic concern to use the PSA for aging evaluation is coming from the requirement to accomplish the safety goals during the whole lifecycle of the nuclear installation (including the extended lifetime). In probabilistic terms, INSAG-12 specifies a safety goal as follow : The target for existing nuclear power plants consistent with the technical safety objective is a frequency of occurrence of severe core damage that is below about 10¿4 events per plant operating year. Severe accident management and mitigation measures could reduce by a factor of at least ten the probability of large off-site releases requiring short term off-site response.¿ So, for the units which approach to the end of initial design lifetime and especially for those which planned to extend the lifetime, it has to be demonstrated that the plant safety level, at least, will be remain in accordance with this target till the end of operation. Additional motivation to use the PSA for aging assessment proceeds from the significant limitations of deterministic approach and needs to prioritize the Aging Management or Life Extension actions to maintain established safety goals. Objectives of the Thesis are - to show what PSA tasks have to be addressed and performed to take into account aging effects and to demonstrate their impact to the overall plant safety, - define the new approaches needed and priority for their development and demonstration. PSA could be used as a tool for safety assessments of aged NPPs, in order to do that the the following steps have to be done : - review of PSA scope and assumptions, - identify components susceptible for aging by using risk importancy, analyzis of aging trends and qualitative analysis of ageing effects, - elaborate age-dependent reliability models, - introduce them into PSA (consider, where necessary, test and maintenance effectiveness and periodicity), - perform time dependent quantifications and sensitivity analysis. The study presents an approach and detailed procedures how to do that with some examples of application. The Thesis was conducted in the frame of EC JRC Ageing PSA Network activities under the Collaboration Agreement with Obninsk State University for Nuclear Engineering (IATE).

RODIONOV Andriy; 

2009-08-25

JRC48668