Nilesh Chokshi

Seismic responses and resistance of age degraded structures and components

Nuclear power plants, as with many other structures, can experience aging degradation on a continuing basis. In some cases, the environmental or operating conditions, which they are subjected to, may be more severe than those affecting non-nuclear structures. This degradation needs to be understood and adequately managed for the continued safe operation of nuclear power plants. To date a limited number of nuclear power plants in the US have specific monitoring and inspection programs for structures. The evaluation for seismic loading is particularly important because the degraded structures or components could be more vulnerable to the seismic loads. From a seismic analysis point of view, the aging or degradation may affect dynamic properties, structural response, resistance or capacity, failure modes, and locations of failure initiation. Further, the NRC license renewal rule requires in part that the current licensing basis (CLB) be maintained throughout the period of extended operation. To meet this requirement, an assessment and timely maintenance of the degraded structures and components becomes necessary. An international cooperative program is being considered to pull together resources to tackle the issue of seismic effects on aged/degraded structures and components, and to augment existing databases by using experience of nuclear power plants worldwide. Based on the insights and lessons learned from this program, technical bases could be established for each participating country, as needed, to develop guidelines to address the seismic capability of aged/degraded structures and components for continued and extended service. The extent of the program will depend on the results of a scoping or bounding study to be performed in the initial phase.

Preliminary perspectives gained from individual plant examination of external events (IPEEE) seismic and fire submittal review

As a result of the US Nuclear Regulatory Commission (USNRC) initiated Individual Plant Examination of External Events (IPEEE) program, every operating nuclear power reactor in the United States has performed an assessment of severe accident due to external events. This paper provides a summary of the preliminary insights gained through the review of 24 IPEEE submittals.

An update of preliminary perspectives gained from Individual Plant Examination of External Events (IPEEE) submittal reviews

Abstract As a result of the US Nuclear Regulatory Commission (USNRC) initiated Individual Plant Examination of External Events (IPEEE) program, virtually every operating commercial nuclear power reactor in the United States has performed an assessment of severe accident risk due to external events. The USNRC staff has received all 70 IPEEE submittals. This paper is based on the information available for those 41 plants for which at least preliminary Technical Evaluation Reports have been prepared by the review teams. The goal of the review is to ascertain whether the licensee’s IPEEE process is capable of identifying external events-induced severe accident vulnerabilities and cost-effective safety improvements to either eliminate or reduce the impact of these vulnerabilities. The review does not, however, attempt to validate or verify the results of the licensee’s IPEEE. The primary objective of this paper is to provide an update on the preliminary perspectives and insights gained from the IPEEE process. For most licensees the principal objectives of the IPEEE program have been met, and the program has had some impact on improving plant safety.

Nonlinear analysis techniques of block masonry walls in nuclear power plants

Abstract Concrete masonry walls have been used extensively in nuclear power plants as non-load bearing partitions serving as pipe supports, fire walls, radiation shielding barriers, and similar heavy construction separations. When subjected to earthquake loads, these walls should maintain their structural integrity. However, some of the walls do not meet design requirements based on working stress allowables. Consequently, utilities have used non-linear analysis techniques, such as the arching theory and the energy balance technique, to qualify such walls. This paper presents a critical review of the applicability of non-linear analysis techniques for both unreinforced and reinforced block masonry walls under seismic loading. These techniques are critically assessed in light of the performance of walls from limited available test data. It is concluded that additional test data are needed to justify the use of nonlinear analysis techniques to qualify block walls in nuclear power plants.