No abstract

An overview of the known aging mechanisms is given in NUREG-6157 and NUREG 1801. These mechanisms are corrosion, corrosion assisted fatigue, embrittlement, mechanical wear and fretting, time dependent relaxation, low cycle fatigue and high cycle fatigue. A literature search was conducted and further analysis performed of these mechanisms. Large and small bore piping (including buried piping), valve bodies, pump casings, tanks, heat exchangers, accumulators and HVAC ducting were considered in this evaluation. Additional monitoring for aging effects on seismic resistance was mainly limited to small bore piping. A screening criterion for small bore piping is proposed for the two mechanisms (erosion/corrosion and low cycle fatigue) that will select a sample of the most critical locations for baseline examination. The sampling techniques are considered adequate due to the large design margin inherent in small bore piping. This paper will demonstrate that the two mechanisms, which need to be managed during life extension due to their significant effect on passive functions, are erosion/corrosion and low cycle fatigue.

The nuclear steam supply system (NSSS) design for the new generation PWR and BWR reactor designs and advanced high temperature gas reactor design consider different onsite emergency power sources required to bring the nuclear plant to a safe shutdown condition following a design basis event concurrent with a loss of offsite power. This paper will discuss the various onsite power generation sources, i.e., diesel, gas and battery based on existing operating experience and information available for existing plants. It will discuss the merits and demerits of each of these sources as related to maintenance and reliability. The paper will compare the three sources and attempt to determine which design would be more cost effective for the power plants.

Since the United States Nuclear Regulatory Commission (USNRC) published its landmark “Reactor Safety Study — An Assessment of Accident Risks in U. S. Commercial Nuclear Power Plants” in late 1975, commercial nuclear power industry, encouraged by the USNRC, have since then been applying Probabilistic Risk Assessment (PRAs) in their nuclear power units in areas of in-service testing, in-service inspection, quality assurance, technical specifications, maintenance, etc. To guide and regulate the industry in use of PRAs, Regulatory Guides and Standards have been written and are being revised continuously by the USNRC, American Society of Mechanical Engineers (ASME) and American Nuclear Society (ANS). The current use of PRA takes credit for single failure criterion based on applicability of codes and standards. The proposed new USNRC regulation 10 CFR Part 53 applicable for all reactor technologies is silent on the applicability of current standards endorsed by the regulatory body. The impact of the proposed new rule to both new and the current application needs to be studied. This paper will review the application of the various guidance documents for their use in commercial nuclear power plants with emphasis on the new generation nuclear power plants.

One of the requirements for license renewal for US nuclear plants stated in the United States Nuclear Regulatory Commission (USNRC) regulations in the License Renewal Rule (LRR) 10CFR Part 54 (Ref. 1) is the identification and updating of Time-Limited Aging Analyses (TLAA). During the design phase for a plant, certain assumptions about the length of time the plant would be operated were made and incorporated into design calculations for several of the plant’s systems, structures and components (SSCs). Examples of TLAAs are analyses of metal fatigue, environmental qualification (EQ) of electric equipment, etc. For a renewed license, these calculations have to be reviewed to verify that they remain valid for the period of extended operation. However, the LRR does allow TLAA-associated aging effects to be managed by an aging management program. This paper discusses the USNRC regulatory requirements for TLAAs and the industry’s response for addressing the TLAAs. It also discusses the issues regarding the generic set of TLAAs that have been identified by the NRC in NUREG-1801 (Ref. 2), and how these have been addressed by all the plants that have received their renewed license. The paper also identifies certain plant specific TLAAs.