Risk and Reliability Assessment

Systems Analysis of Criticality Safety, R. J. Hall, E. D. Clayton (BNW) 

As the nuclear ftiel recycle industry grows, the probability of a criticality accident will tend to grow proportionately unless current criticality safety control is improved. (Accident Probability Probability per Plant times Number of Plants.) Optimum control of criticality safety requires an overview of the entire system. The relationships between the criticality safety philosophy, the criticality data, the use of the data, and various human Victors must be considered. Application of the foult tree analysis (FTA) methodology will permit identification of the important control foctors and their relationships, indication of what is required to reduce the accident probability per plant, and systematic evaluation of our current criticality safety position. FTA is not new but the application to criticality safety presented in this 

In fuel recycle, criticality safety is controlled by a set of criticality cmitrol requirements (CCR). Current programs, in general, are set up with the goal that at least two independent concurrent violations of CCR are required before criticality can occur. However, one must always guard against the possibility of a criticality occurring with one, or no, violation of CCR. The partially devel q ed left-hand branch of the tree illustrates a way 

The right-hand branch of the tree illustrates that even teclmhsdly correct OCR could contribute to the probability of criticality occurring if they were poorly selected 

The above examples illustrate some advantages of FTA applied to criticality safety programs. A top-level fiiult tree model for determining the most probable types of criticality sirfety limit violations (e.g., geometry, concentration, volume, Spacing limit), which will contribute to a future accidental critic ity, will also be presented. 

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The book contains, in alphabetical order, failure rates, event rates and probabilities, and descriptive information which has been collected since 1970 in the course of doing risk and reliability assessments. Twenty appendices contain results of surveys on bursting discs, pipes, valves, relief valves, pump failures and information on human error, international fire losses, and blast effects. 

In this study detailed fault trees with probability and failure rate calculations were generated for the events (1) Fatality due to Explosion, Fire, Toxic Release or Asphyxiation at the Process Development Unit (PDU) Coal Gasification Process and (2) Loss of Availability of the PDU. The fault trees for the PDU were synthesized by Design Sciences, Inc., and then subjected to multiple reviews by Combustion Engineering. The steps involved in hazard identification and evaluation, fault tree generation, probability assessment, and design alteration are presented in the main body of this report. The fault trees, cut sets, failure rate data and unavailability calculations are included as attachments to this report. Although both safety and reliability trees have been constructed for the PDU, the verification and analysis of these trees were not completed as a result of the curtailment of the demonstration plant project. Certain items not completed for the PDU risk and reliability assessment are listed. 

The central question here is whether the established methods of risk and reliability assessment developed for comphcated systems can be applied to complex systems as well. 

In the last three decades, stimulated by public reaction and health and safety legislation, the use of risk and reliability assessment methods has spread from the higher risk industries to an even wider range of applications. The Reactor Safety Study undertaken by the U.S.A (U.S Nuclear Regulatory Commission (1975)) and the Canvey studies performed by the UK Health Safety Executive (HSE (1978, 1981a,b)) resulted from a desire to demonstrate safety to a doubtful public. Both these studies made considerable use of quantitative methods, for assessing the likelihood of failures and for determining consequence models. 

Reliability Data Collection and Use in Risk and Availability Assessment Varied 72 papers, several of which contain some data Wide variety of systems and components 48. 

Reliability Data Collection and Use in Risk and Availability Assessment... 

Assessment of Accident Risk and Reliability of Nuclear Safety Measures... 

Statistical survival models are a well known approach for modeling patient survival statistics (Kaplan and Meier, 1958). In recent years, statistical survival models have been applied in risk and reliability. The Kaplan Meier statistical survival model is usually applied with reference to time. In AUV operation management the science manager interest is to assess the chances of AUV survival with range. A simple modification to the Kaplan-Meier approach is necessary in order to capture this requirement (Brito, et al., 2008). 

Perez Castaneda, G.A., Aubry, XF. Brinzei, N. 2011. Stochastic hybrid automata model for dynamic reliability assessment. Proceedings of the Institution of Mechanical Engineers Part O Journal of Risk and Reliability 225(1) 2 -A1. 

Paltrinieri N., Hauge S., Dionisio M., Nelson W.R., 2013, Towards a dynamic risk and barrier assessment in an lO context. In Steenbergen R.D.J.M., van Gelder P.H.A.J.M., Miraglia S., Vrouwenvelder A.C.W.M (eds.), Safety, Reliability and Risk Analysis Beyond the Horizon, CRC Press, Boca Raton, FL. 

Braband, J. Schabe, 2013. Assessment of national reference values for railway safety a statistical treatment, Journal of Risk and Reliability, 227(4) 405-410. 

In effect, workers are treated as being components in a system, just like equipment items. Hence the THERP analysis can be integrated into probabilistic risk assessment (PRA) analyses—particularly fault and event trees—topics that are discussed in depth in Process Risk and Reliability Management. A THERP analysis is most effective when the tasks are routine and proceduralized, and when the persons involved are not under stress. 

Next, we review the eosts of quality that typieally exist in a manufaeturing business, and how these are related to the way produets fail in serviee. The remainder of the ehapter diseusses the important elements of risk assessment as a basis for design. This puts in eontext the work on designing for quality and reliability, whieh are the main topies of the book. 

In order to quantify the sometimes intangible elements of variability associated with the product design and the safety aspects in service requires an understanding of risk . The assessment of risk in terms of general engineering practice will be discussed next. This will lead to a better understanding of designing for quality and reliability, which is the main focus of the book. 

When performing human reliability assessment in CPQRA, a qualitative analysis to specify the various ways in which human error can occur in the situation of interest is necessary as the first stage of the procedure. A comprehensive and systematic method is essential for this. If, for example, an error with critical consequences for the system is not identified, then the analysis may produce a spurious impression that the level of risk is acceptably low. Errors with less serious consequences, but with greater likelihood of occurrence, may also not be considered if the modeling approach is inadequate. In the usual approach to human reliability assessment, there is little assistance for the analyst with regard to searching for potential errors. Often, only omissions of actions in proceduralized task steps are considered. 

In this phase of the risk assessment, the validity and reliability of conclusions and advice to risk managers depend on the quality, reliability, and relevance of available exposure data. Therefore it is necessary to (1) critically review the facts from food composition tables and the reasons for differences reported by and within countries, (2) consider the way foods are categorized and thus made comparable (or not) in food consumption surveys, and (3) explore how to refine assessments as more information becomes available. ... 

In conclusion, a reliable assessment of the potential risk of UV filters to the ecosystems is, therefore, not yet possible, due to the scarcity of data on environmental concentrations and to the few species used to identify biological effects. 

Island/Thurrock Area, HMSO, London, 1978. Rasmussen, Reactor Safety Study An Assessment of Accident Risk in U. S. Commercial Nuclear Power Plants, WASH-1400 NUREG 75/014, Washington, D.C., 1975. Rijnmond Public Authority, A Risk Analysis of 6 Potentially Hazardous Industrial Objects in the Rijnmond Area—A Pilot Study, D. Reidel, Boston, 1982. Considine, The Assessment of Individual and Societal Risks, SRD Report R-310, Safety and Reliability Directorate, UKAEA, Warrington, 1984. Baybutt, Uncertainty in Risk Analysis, Conference on Mathematics in Major Accident Risk Assessment, University of Oxford, U.K., 1986. 

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