A Common Approach to Risk Analysis Methodology

Risk analysis has been used extensively to inform decisions throughout government and industry for many years. Many methodologies have been developed to perform these analyses, resulting in differences in terminology and approach that make it difficult to compare the results of an analysis in one field to that in another. In particular, many approaches result only in a risk ranking within a narrow area or field of interest, so the results cannot be compared to rankings in other areas or fields. However, dealing with terrorist threats requires prioritizing the allocation of homeland defense resources across a broad spectrum of possible targets. Therefore, a common approach is needed to allow comparison of risks. This presentation summarizes an approach that will allow the results of risk analyses based on using current methodologies to be expressed in a common format with common terminology to facilitate resource allocation decisions.

Terrorism Risk Models: The Need for Common Measures

From the hot dog vendor located near a national monument to a multinational corporation with operations and significant assets around the world, we are all faced with decisions about addressing terrorism. Since the events of 9/11, enormous amounts of financial and intellectual capital have been invested to develop security responses to potential terrorism threats. Beyond specific, focused initiatives — most visibly increased airport security — a plethora of risk models have been (or are being) developed. Ostensibly, these models attempt to address the basic risk proposition: Risk = Frequency * Severity.

Guidelines on Environmental Fatigue Evaluation for LWR Component

Japanese utilities and vendors have taken environmental effects on fatigue (EF) into consideration in the plant life management (PLM) activity of operating plants for several years. In Sep. 2000 MITI notified the utilities to adopt “The Guidelines for Evaluating Fatigue Initiation Life Reduction in LWR Environment (MITI guidelines)” for PLM evaluation of operating plants [1]. In April 2001, the study started to establish detailed procedures for EF evaluation and the committee was organized for developing detailed guidelines at Thermal and Nuclear Power Engineering Society (TENPES). The evaluation guidelines were completed and published as TENPES guidelines [2]. These guidelines proposed several practical options to apply fatigue life reduction factor for environmental effects (Fen) on actual operating plant fatigue evaluation.

Probabilistic Analysis of 60-Year Environmental Fatigue Effects for Reactor Components

In NUREG/CR-6674, a probabilistic fracture mechanics analysis was conducted to assess the effects of light water reactor environmental effects on the probability of fatigue initiation and subsequent crack growth leading to leakage and possible core damage. The results were based on stresses for typical locations in BWR and PWR reactors as determined from an analysis reported in NUREG/CR-6260. Although environmental effects were shown to have an insignificant effect on core damage frequency, the study concluded that there could be a significant increase in probability of leakage. A detailed review of the methodology and input conditions used in NUREG/CR-6674 has been completed, including use of an altered probabilistic fatigue curve with more representative high-cycle stress variance and consideration of results from more recent environmental fatigue testing. This revised analysis indicates that environmental effects on the probability of leakage and core damage frequency in an extended nuclear plant operating period are significantly less than previously reported in NUREG/CR-6674. This paper summarizes the analysis performed and the results obtained.

Consideration of Environmental Fatigue in the ASME Code for Carbon and Low-Alloy Steel Components

The potential impact of reactor water environment on reducing the fatigue life of light water reactor (LWR) piping components has been an area of extensive research. While available data suggest a reduction in fatigue life when laboratory samples are tested under simulated reactor water environments, reconciliation of this data with plant operating experience, plant-specific operating conditions, and established ASME Code design processes is necessary before a conclusion can be reached regarding the need for explicit consideration of reactor water environment in component integrity evaluations. U.S. nuclear industry efforts to better understand this issue and ascertain the impact, if any, on existing ASME Code guidance have been performed through the EPRI Materials Reliability Program (MRP). Based on the MRP activities completed to date there is no need for explicit incorporation of reactor water environmental effects for carbon and low-alloy steel components in the ASME Code. This paper summarizes ongoing MRP activities and presents the technical arguments for resolution of the environmental fatigue issue for carbon and low-alloy steel locations.

Pipe Line Security: Design of Pipe to Resist Blast Type Loads

Pipe line security is divided into two categories; pipe line security associated with above ground piping and buried piping. With respect to above ground piping security, the pipe’s ability to resist blast loads which are dynamic and impulsive in nature is required. In this evaluation it is assumed to be dealing with a cratering charges located some distance (feet or meters) from the pipe line. The use of breaching charges which typically consist of plastic explosives in direct contact with the pipe wall or shaped charges which are designed to locally cut or penetrate the pipe wall are not in the scope of this discussion.

Section III and Section XI: Integration in the Design Specification and Design Report

One of the activities of power plant owner’s and equipment providers today is upgrading and/or repair/replacement of original equipment. Plant life extension, and fatigue evaluation are also activities that are being performed routinely. These activities come under the rules and guidance of the ASME Boiler and Pressure Vessel Code, Section XI, whereas, the original equipment generally came under the rules of Section III.

Differences Between EN 13445 and Other International Standards

The harmonized standard EN 13445 [1] provides the means by which a manufacturer can claim presumption of conformity to the essential safety requirements of the Pressure Equipment Directive. In addition, it aims to bring together the best available European pressure vessel technology in a single unified document. Use of this standard however, has still to be established in an industry which has been slow to accept change, especially when other well-known codes remain in use. This paper highlights the main technical differences between EN 13445-3 and other international standards (principally PD5500 [2] and ASME VIII [3]) and considers the implications for industry, research organisations and the various code-writing bodies.

Development of a Water Environment Fatigue Design Curve for Austenitic Stainless Steels

Technical support is provided for a fatigue curve that could potentially be incorporated into Section III of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code. This fatigue curve conservatively accounts for the effects of light water reactor environments on the fatigue behavior of austenitic stainless steels. This paper presents the data, statistical methods, and basis for the design factors appropriate for Code applications. A discussion of the assumptions and methods used in design curve development is presented.

Evaluation of Fatigue Damage in LWR Water With and Without Threshold and Moderation Factor

During the past twenty years, the fatigue initiation life of LWR structural materials, carbon, low alloy and stainless steels has been shown to decrease remarkably in the simulated LWR (light water reactor) coolant environments. Several models for evaluating the effects of environment on fatigue life reduction have been developed based on published environmental fatigue data. Initially, based on Japanese fatigue data, Higuchi and Iida proposed a model for evaluating such effects quantitatively for carbon and low alloy steels in 1991. Thereafter, Chopra et al. proposed other models for carbon, low alloy and stainless steels by adding American fatigue data in 1993. Mehta developed a new model which features the threshold concept and moderation factor in Chopra’s model in 1995. All these models have undergone various revisions. In Japan, the MITI (Ministry of International Trade and Industry) guideline on environmental fatigue life reduction for carbon, low alloy and stainless steels was issued in September 2000, for evaluating of aged light water reactor power plants. The MITI guideline provide equations for calculations applicable only to stainless steel in PWR water and consequently Higuchi et al. proposed in 2002 a revised model for stainless steel which incorporates new equations for evaluation of environmental fatigue reduction in BWR water. The paper compares the latest versions of these models and discusses the conservativeness of the models by comparison of the models with available test data.