Identification and Assessment of Material Models for Age-Related Degradation of Structures and Passive Components in Nuclear Power Plants

Abstract

When performing seismic safety assessments of nuclear power plants (NPPs), the potential effects of age-related degradation on structures, systems, and components (SSCs) should be considered. To address the issue of aging degradation, the Korea Atomic Energy Research Institute (KAERI) has embarked on a five-year research project to develop a realistic seismic risk evaluation system which will include the consideration of aging of structures and components in NPPs. Three specific areas that are included in the KAERI research project, related to seismic probabilistic risk assessment (PRA), are probabilistic seismic hazard analysis, seismic fragility analysis including the effects of aging, and a plant seismic risk analysis.To support the development of seismic capability evaluation technology for degraded structures and components, KAERI entered into a collaboration agreement with Brookhaven National Laboratory (BNL) in 2007. The collaborative research effort is intended to continue over a five year period with the goal of developing seismic fragility analysis methods that consider the potential effects of age-related degradation of SSCs, and using these results as input to seismic PRAs. In the Year 1 scope of work BNL collected and reviewed degradation occurrences in US NPPs and identified important aging characteristics needed for the seismic capability evaluations that will be performed in the subsequent evaluations in the years that follow. This information is presented in the Annual Report for the Year 1 Task, identified as BNL Report-81741-2008 and also designated as KAERI/RR-2931/2008. The report presents results of the statistical and trending analysis of this data and compares the results to prior aging studies. In addition, the report provides a description of U.S. current regulatory requirements, regulatory guidance documents, generic communications, industry standards and guidance, and past research related to aging degradation of SSCs.This report describes the research effort performed by BNL for the Year 2 scope of work. This research focused on methods that could be used to represent the long-term behavior of materials used at NPPs. To achieve this BNL reviewed time-dependent models which can approximate the degradation effects of the key materials used in the construction of structures and passive components determined to be of interest in the Year 1 effort. The intent was to review the degradation models that would cover the most common time-dependent changes in material properties for concrete and steel components.

A Procedure for Determination of Degradation Acceptance Criteria for Structures and Passive Components in Nuclear Power Plants

“…For degradation scenario C, a hybrid degradation model was developed based on available anchorage test results for cracked concrete and regression models based on measured data at Korean NPPs. The resultant models are summarized below to facilitate the development of DAC in terms of time in this section; more details about the background of these models can be found in Section 3 as well as the Year 3 and Year 4 annual reports [Nie et al, 2009[Nie et al, , 2011. In which time t is in years, the crack width W in mm, and the tensile capacity T in kips.…”

“…The material degradation model for the stainless steel tank shell used for the seismic fragility analysis of the CST is the mechanochemical model for stress corrosion cracking (SCC), which is one of the three time-dependent material degradation models for stainless steel that are documented in the Year 2 annual report [Nie et al, 2009]. It can be seen from Figure 3-1 that as there are no signs of significant degradation in the CST shell, the consideration of tank shell degradation in the seismic fragility analysis of the CST was for the purpose of demonstration.…”

“…The protecting stainless steel cover as shown in Figure 3-1 may not be leak-tight. A power model for steel corrosion was chosen from the Year 2 annual report [Nie et al, 2009] to model the degradation of the anchor bolts. This model had been used by Mori [2005] to predict reliability-based service life, indicating its suitability for fragility analysis.…”

“…The five year collaboration program consists of five tasks: (1) identification and characterization of degradation occurrences in US NPPs and important aging characteristics needed for the seismic capability evaluations [Nie et al 2008], (2) identification and assessment of degradation models for the long-term behavior of dominant materials that are determined to be risk significant to NPPs [Nie et al 2009], (3) seismic fragility analysis methodology and its application to a condensate storage tank with three degradation scenarios , (4) additional seismic fragility analysis of the condensate storage tank to consider non-perfect correlations among degradation scenarios [Nie et al 2011], and (5) development of a procedure for determination of degradation acceptance criteria for structures and passive components (SPCs) in nuclear power plants, which is the topic of this report.…”

“…The effect of agerelated degradations of SPCs on their seismic fragility was investigated in this collaboration program through the integration of the fragility calculation and the degradation effect, which may be expressed in terms of observed degradation levels or time using proper time-dependent degradation models [Nie et al 2009[Nie et al , 2011. The seismic fragility is the conditional probability of failure of an SPC for given levels of seismic demand, often conveniently described in terms of peak ground acceleration (PGA).…”