[en] Under prompt burst conditions the work potential of expanding fuel vapor is a sensitive function of the ability of fission product gases to cause fuel dispersal milliseconds before dispersal would occur from fuel vapor pressure alone. This relation is of potentially significant importance and merits investigation. A fuel disruption experiment at Sandia Laboratories exhibited rapid disruption probably below but near the fuel melt temperature. The fuel was being heated at 130K/msec when vigorous dispersal occurred in the hottest regions near the fuel melting point. Fuel vapor could not have been responsible. Dispersal is attributed to rapid release of fission product gases under the prompt burst conditions. Further tests are planned to explore this important phenomenon

[en] The statistical concepts of tolerance and confidence are reviewed, with particular reference to the presentation of quantitative probability based arguments as applied in safety analysis. The increasing difficulty in performing a meaningful analysis when sample data are used to represent component populations is demonstrated by a model calculation. (author)

[en] The study Use of System Reliability Analysis for Enhancing Plant Operational Safety and Productivity is a major ongoing research and development endeavor within the Risk Assessment Program of the Nuclear Power Division at Electric Power Research Institute (EPRI). It will develop a practical software tool that combined the key features of plant information management systems with applications of system reliability techniques and modern computer technology to assist plant personnel and engineering staff in performing their functions more effectively and accurately. This software is called Reliability Assessment Program with In-Plant Data (RAPID). It can help to resolve many technical difficulties of maintaining an up-to-date probabilistic risk assessment (PRA). More importantly, it aids operators in complying with technical specifications, minimizing violations, enhancing availability, and reducing the number of unplanned scrams. This paper provides an overview of the RAPID activity

[en] Common cause failures (CCFs) have been a continuing concern in nuclear plant operation; first, because they do occasionally occur, and second, because probabilistic risk assessment studies indicate that CCFs are potentially significant risk contributors due to the success of defenses against multiple, independent failures. A comprehensive Electric Power Research Institute, (EPRI) study of CCFs has (a) developed a well-defined classification system for analyzing fault events, and (b) studied licensee event report data for the auxiliary feedwater system (AFWS) and reactor protection system (RPS) using this classification system and analysis of root cause, failure modes, detection methods, corrective actions, and defensive measures. In this work, the confusion generally encountered with the definition of CCF has been resolved by directing attention to branched events - i.e., any event in which a root or component cause resulted in concurrent, multiple unavailabilities (either physical failure or functional unavailability). The current emphasis in the EPRI project is on defensive measures against branched events. This paper discusses some preliminary project results

[en] A fuel disruption experiment at Sandia Laboratories exhibited rapid disruption probably below but near the fuel melt temperature. The fuel was being heated at 130K/msec when vigorous dispersal occurred in the hottest regions near the fuel melting point. Fuel vapor could not have been responsible. Dispersal is attributed to rapid release of fission product gases under the prompt burst conditions. Further tests are planned to explore this important phenomenon. 13 refs

[en] A diagrammatic technique for writing down the integral equations of reliability theory is presented. The way in which time enters the equations is discussed. A simple modification allows the moments of the distributions of interesting quantities to be calculated. (Auth.)

[en] The graphical approach is applied to the calculation of both the moments and the density of the downtime distribution for a single repairable component. Following a discussion of the influence of different time structures the extension to series connected systems is discussed. (Auth.)

[en] The Electric Power Research Institute sponsored the review and comparison of five PRA studies: Arkansas Nuclear One - Unit 1, Big Rock Point, Grand Gulf, Limerick, and Zion - Unit 1. The review has been conducted in two phases. The Phase I review may be characterized as a qualitative look into many aspects of a PRA study. The Phase II review was performed to quantify the extent that differences in analytical techniques or key assumptions in these areas affect the differences in study results. In each of the PRA studies reviewed, the general descriptions of analytical approaches and descriptions of the analyses of event tree, fault tree and human interaction analyses that affected the dominant core damage sequences were reviewed. When these descriptions aroused interest because of seeming inconsistencies within the study or with other studies, they were pursued in some depth. The approaches or assumptions were contrasted to similar elements from other studies, and sensitivity analyses were performed in many cases to test the significance of results to the analytical models or assumptions. Inferences were drawn from the results regarding significance of the item to plant-specific results and, where possible, were generalized to other PRAs. This paper describes the results of the review of system dependencies and human interactions

[en] Recently, increased attention has been given to understanding the role of humans in the safe operation of nuclear power plants. By virtue of the ability to combine equipment reliability with human reliability probabilistic risk assessment (PRA) technology was deemed capable of providing significant insights about the contributions of human interations in accident scenarios. EPRI recognized the need to strengthen the methodology for incorporating human interactions into PRAs as one element of their broad research program to improve the credibility of PRAs. This research project lead to the development and detailed description of SHARP (Systematic Human Application Reliability Procedure) in EPRI NP-3583. The objective of this paper is to illustrate the SHARP framework. This should help PRA analysts state more clearly their assumptions and approach no matter which human reliability assessment technique is used. SHARP includes a structure of seven analysis steps which can be formally or informally performed during PRAs. The seven steps are termed definition, screening, breakdown, representation, impact assessment, quantification, and documentation

[en] A risk standard is proposed that relates the frequency of occurrence of single events to the consequences of the events. Maximum consequences and risk aversion are used to give the cumulative risk curve a shape similar to the results of a risk assessment and to bound the expectation of deaths. Societal costs in terms of deaths are used to fix the parameters of the model together with an approximate comparison with individual risks. The proposed standard is compared with some practical applications of risk assessment to nuclear reactor systems