Accident considerations

Severe accidents vill be considered explicitly in the design of future NPPs. Based on formal criteria, appropriate severe accident sequences will be selected and formally addressed in the design as a separate category, with features that prevent or mitigate that sequence. 

Severe accidents will be considered on a best estimate basis. Part of this best-estimate approach includes the development of realistic source terms for each advanced design. 

The systems, structures, and components (SSCs) used for features that are added to the design to address severe accidents are of high quality, but safety-grade quality levels are not required. 

The containment design and its contribution to accident mitigation will be carefully considered and evaluated in the design process, with particular attention to those severe accidents addressed in the design. Mitigative features are designed and severe accident management is specified on the basis of representative severe accident conditions and correlated loads, to be identified. 

Accident management is treated comprehensively for future plants. Accident management augments design features to prevent degradation of an accident to severe accident conditions, and to mitigate accidents, if they occur. 

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In the case of a major accident you have all of the normal accident considerations and decisions, such as ... 

Beyond design basis and severe accident considerations... 

Three typical examples of the results of these calculations which are of interest in reactor accident considerations are shown in Fig. 7.24. The tendency of these results can be summarized as follows At pH 5, 100 °C and an initial I2 concentration of 10 g-atom/1, iodate formation proceeds very slowly, reaching the same concentration as I2 after about 1 day the equilibrium state of the reaction would only be established after about 100 days. This means that over a comparatively long period of time one has to deal with rather high fractions of the molecular species I2 and HOI. Raising pH to about 7 at the same temperatme results in a much faster IO3" formation, with the equilibrium state already being established after about 10 minutes. At lower temperatures, the reaction rates are correspondingly lower. In solutions with very low total iodine concentrations, the I2 fraction decreases very quickly at pH 7 however, HOI disproportionation proceeds rather slowly with the consequence that the equilibrium state of reaction (3) is attained only after several days. 

Fundamental design approaches Accident considerations Safety goals and decision processes Safety culture and human factors Miscellaneous issues. 

Methods and criteria for treating uncertainties, and the practical implementation of policies that require best estimate analysis for all severe accident considerations. 

Italian Approach to Severe Accidents" Consideration for Reactors of the Next Generation", by A. Ferreli, ANPA... 

Since the Three Mile Island accident, considerable effort has been devoted to the development of "symptom-based" procedures to replace (or at least significantly augment) the event-specific procedures. The basic premise underlying these symptom-based procedures is that there is a limited set of critical safety functions (CSFs), which, if successfully performed by either automatic plant response or manual action, result in a "safe" condition for the plant. The basic goal of the plant safety systems and the ultimate goal of operator actions is to ensure the performance of these critical safety functions. Symptom-based operating procedures relate critical safety function performance to specific plant/control room instruments. 

These environmental conditions should include the expected combinations of conditions for normal operation, during anticipated operational occurrences, and during and after design basis accidents. Consideration of severe accident conditions is not required in the equipment qualification programme. However, equipment credited for response to severe accidents should be shown, with reasonable confidence and to the extent possible, to function under anticipated severe accident conditions (Ref. [1], para. 5.46). 

Adequate isolation should be provided at the interfaces between the RCS and connecting systems operating at lower pressures to prevent the overpressure of such systems and possible loss of coolant accidents. Consideration should be given to the characteristics and importance of the isolation and its reliability targets. Isolation devices either should usually be closed or should close automatically on demand. The response time and speed of closure should be in accordance with the acceptance criteria defined for postulated initiating events (see Ref. [3] for guidance). 

It should, however, be considered that accident considerations as presented here allow only a definition of the measures to be taken into account around the nuclear facility. Emergency planning feasibility allows the analysis of aspects of the population distribution. Considerable experience and judgment is necessary to establish the acceptability as a whole of the population distribution around any nuclear facility. 

Exploration activities are potentially damaging to the environment. The cutting down of trees in preparation for an onshore seismic survey may result in severe soil erosion in years to come. Offshore, fragile ecological systems such as reefs can be permanently damaged by spills of crude or mud chemicals. Responsible companies will therefore carry out an Environmental Impact Assessment (EIA) prior to activity planning and draw up contingency plans should an accident occur. In Section 4.0 a more detailed description of health, safety and environmental considerations will be provided. 

A fatal accident and some other disasters, which were caused by small cracks, lead to a more strict consideration of the security of these steam drums. Parallel to these the economical pressure, due to the globalisation of the today s industry, lead to the increase of the pressure and the rotation speed of the paper production machines for a higher output of the production, which means, that all safety aspects from the design and the material will be exploited totally. On the other hand cast iron is also not a ductile and comfortable material, like the most steels for the pressure equipment. 

The capillary rise method is generally considered to be the most accurate means to measure 7, partly because the theory has been worked out with considerable exactitude and partly because the experimental variables can be closely controlled. This is to some extent a historical accident, and other methods now rival or surpass the capillary rise one in value. 

Bolted joints are friendlier than quick-release couplings. The former are usually dismantled by a fitter after issue of a permit to work. One person prepares the equipment and another person opens it up the issue of the permit provides an opportunity to check that the correct precautions have been taken. In addition, if the joints are unbolted correctly, any trapped pressure is immediately apparent and the joint can be remade or the pressure allowed to blow off. In contrast, many accidents have occurred because operators opened up equipment which was under pressure, without independent consideration of the hazards, using quick-release couphngs. There are, however, designs of quick-release couphngs which give the operator a second chance. 

When reading an accident report, look for the things that are not said. For example, a gland leak on a liquefied flammable gas pump caught fire and caused considerable damage. The report drew atten-... 

Hendershot, D. C. 1987. Safety Considerations in the Design of Batch Processing Plants, in Proceedings of the International Symposium on the Prevention of Major Chemical Accidents, Center for Chemical Process Safety/AIChE, New York, NY. 

There are a variety of ways to express absolute QRA results. Absolute frequency results are estimates of the statistical likelihood of an accident occurring. Table 3 contains examples of typical statements of absolute frequency estimates. These estimates for complex system failures are usually synthesized using basic equipment failure and operator error data. Depending upon the availability, specificity, and quality of failure data, the estimates may have considerable statistical uncertainty (e.g., factors of 10 or more because of uncertainties in the input data alone). When reporting single-point estimates or best estimates of the expected frequency of rare events (i.e., events not expected to occur within the operating life of a plant), analysts sometimes provide a measure of the sensitivity of the results arising from data uncertainties. 

Sometimes the expected consequences of an accident alone may provide you with sufficient information for decision-making purposes. Conventionally, the form of these estimates will be dictated by the purpose (concern) of the study (safety, economics, etc.). Absolute consequence estimates are best estimates of the impacts of an accident and, like frequency estimates, may have considerable uncertainty. Table 4 contains examples of typical consequence estimates obtained from QRA. These examples point to the difficulty in comparing various safety and economic results on a common basis—there is no common denominator. 

If sufficient experience does not exist, you should consider whether the consequence potential (Step 4) or the expected frequency of accidents (Step 5) is great. Consideration of consequence potential should include personnel exposure, public demographics, equipment density, and so forth in relation to the intrinsic hazard posed by the material of concern. In Step 5 you may perceive that the expected frequency of accidents alone is important enough to justify a QRA. However, even though your company may not have much relevant experience with the activity of interest, if the consequence potential of these accidents is not great, you may conclude that the expected frequency of the potential accidents is low enough for you to make your decisions comfortably using qualitative information alone. 

Many sophisticated models and correlations have been developed for consequence analysis. Millions of dollars have been spent researching the effects of exposure to toxic materials on the health of animals the effects are extrapolated to predict effects on human health. A considerable empirical database exists on the effects of fires and explosions on structures and equipment. And large, sophisticated experiments are sometimes performed to validate computer algorithms for predicting the atmospheric dispersion of toxic materials. All of these resources can be used to help predict the consequences of accidents. But, you should only perform those consequence analysis steps needed to provide the information required for decision making. 

Dale, S. E. (1987). Cost Effective Design Considerations for Safer Chemical Plants. Proceedings of the International Symposium on Preventing Major Chemical Accidents, February 3-5, 1987, Washington, D. C., ed. J. L. Woodward, 3.79-3.99. New York American Institute of Chemical Engineers. 

Selection of a PrHA methodology requires consideration of many factors including the availability of process information such as experience with the process, changes that have taken place, reliability, aging, maintenance, etc. If it is a new process, less reliance can be placed on experience and greater reliance must be placed on the analysis of possible accidents and accidents in similar or related processes. Size, complexity and hazard severity influences the dunce ot ihe most appropriate PrHA methodology. 

Systematic consideration of human error is neglected because of the belief that computerization of processes will make the human unnecessary. Experience shows numerous accidents in computer controlled plants. Human involvement in critical areas of maintenance and plant modification, continues even in the most automated processes. 

Potential accident scenarios and flood locations were identified from plant drawings and tlic RHR system fault tree that identifies the equipment and support needed for RHR system operation. The equipment location was correlated with flood areas with consideration for plant features which may impede or divert the flow. The flood scenarios identify the effect on systems required to prevent core damage. Quantification accounts for the rate of rise of the flood relative to the critical level in each specific plant area. The time available for any recovery action is calculated from tiic volume and the flow rate. 

IPE results genrally are consistent with the results of previous NRC and industry risk studies in indicating that the CDF is often determined by many accident sequence combinations, rather than a failure. The largest contributors to CDF vary among the plants (e.g., LOCAs dominate some IPE while station blackout [SBO] dominates others). Support systems, whose design varies considerably with plant, are important to all plants because they can cause multiple front-line system failures. This may account for much of the variability in the IPE results. 

Risk-based information provides a foundation for regulation of severe accidents. Early PRAs, with large uncertainties, indicated risk that was above or below the Safety Goals depending on containment performance. Consequently the NRC developed an Integration Plan for Closure of Severe Accident Issues (SECY-88-47) with six main elements to this plan 1) individual plant examinations (IPE), 2) containment performance improvements, 3) improved plant oper itions, 4) severe accident research, 5) external event considerations, and 6) accident management. 

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