Safety assessments deterministic

The environmental concentration of a stressor, either measured or estimated, is compared with an effect concentration such as an LC50 (lethal concentration to kill 50% of individuals in a theoretical population in a set period of time) or no observed effect concentration (NOEC) [31, 32]. These are simple ratios of single exposure and effects values and may be used to express hazard or relative safety. This deterministic method uses point estimates to represent one or more factors in a risk assessment and treats them as if they were fixed and precisely known [33]. The calculation of HQs... 

Probabilistic safety assessments are state-of-the-art in the safety assessment of nuclear powerplants. They are a valuable supplement to safety assessments made on a deterministic basis, because a PSA allows a holistic representation of the influence of structures, systems, and components as well as of hmnan actions on the plant behavior. 

In the field of the safety assessment of nuclear power plants PSA are used as a supplement to the safety assessment made on a deterministic basis. With the aid of PSA a holistic representation of the influence of components, systems and structures as well as of human actions on the installation behaviour with respect to safety is given. Consequential the safety level can be quantitatively evaluated. 

The deterministic approach to the design of nuclear reactors was rapidly supplemented by the development of probabilistic studies, referred to as PSAs and also as PRA. The first study of this kind carried out in the United States was published in 1975 (Rasmussen report—USNRC 1975) and provided the first assessment of the potential risk of core damage for two power reactors. The accident in 1979 at the Three Mile Island plant generated renewed interest in this type of study. One of the recommendations made after the accident was that probabilistic analysis techniques should be used to supplement conventional safety assessment procedures for NPPs, and that probabilistic objectives should be developed in order to facilitate the determination of acceptable safety levels for nuclear facilities. 

Where a deterministic approach has been selected for the hazard evaluation or directly for the design basis specification, an estimation of the associated return period should be made, at least to allow a comparison with national standards for the design of industrial facilities. This value should then be assessed in the safety assessment phase, as recommended in Ret [9]. 

Deterministic design or safety assessment. For design or a safety assessment... 

Deterministic design or safety assessment. The equation, including a standard deviation of safety, should be used for deterministic design or safety assessment ... 

For deterministic design or safety assessment, Eq. (16.3) should be used with a coefficient of 2.2 instead of 2.6. Figure 16.8 shows measurements taken from the CLASH database and Eq. (16.3) with a mean prediction, the curve for deterministic design and 5% exceedance curves. 

For deterministic design or safety assessment, it is recommended to increase the average overtopping discharge by one standard deviation and use the value of 0.068. 

For probabilistic calculations or predictions of measurements or comparison with measurements, Ekj. (16.5) should be taken together with these stochastic coefficients or, for instance, 5% upper and lower exceedance curves. For deterministic design or safety assessment, a coefficient of 3.8 X 10 should be used in Eq. (16.5), instead of 2.7 x 10 . ... 

For deterministic design or safety assessment, Eq. (16.6) should be used with a coefficient 1.95 instead of 2.16. 

Probabilistic safety assessment can be seen as an extension of deterministic analyses by systematically considering accident sequences and events which go beyond the design basis of a nuclear power plant. PSA applications are evolving and are useful in complementing the deterministic approach as long as the PSA used has an adequate quality for the intended applications and die staff carrying out the probabilistic assessment include experienced personnel who know the capabilities and limitations of the PSA and of the plant design and operation. 

Therefore, there is a need for developing a comprehensive deterministic safety assessment approach, which should be able to consider the contributions of the various defence in depth provisions to the overall safety aim of defence in depth. 

The objective of the proposed screening approach is deterministic in nature and the approach can also be used for safety assessment of a plant without a PSA or with an incomplete PSA. A plant specific PSA, sufficiently broad in scope and with a sufficient level of detail, can be used to support the judgement on the adequacy of the defence in depth and of the logical structure of the defences. In addition, a good quality PSA facilitates a deep understanding of the interrelation between the various defences and supports prioritization of provisions according to their contribution to risk reduction. 

Other possible processes are not gradual but occur as random events and may have a disruptive effect on the repository and its environment. Processes such as seismic and tectonic phenomena which modify water flows could be important considerations for disposal in some geological formations, and future human activities such as drilling and mineral exploitation could have direct and indirect influences on some repositories. Disruptive processes could, in some situations, dominate the overall safety assessment of disposal. Usually they are examined using probabilistic techniques, although it is noted that in some countries seismic phenomena are treated in a deterministic manner. 

Until now, the commonly applied approach to the decision-making on nuclear facilities was based on Deterministic Safety Assessment (DSA), where a set of rules and requirements has been defined in order to ensure a high level of safety. This was done by applying the defense in depth principles and adequate criteria for safety margins (IAEA 2009). 

Adolfsson, Y. et al. 2012 (eds). In IDPSA-Integrated Deterministic-Probabilistic Safety Assessment, 2nd workshop, November 19-21,2012, KTH, Stockholm, Sweden. 

Probabilistic Safety Assessment (PSA) represents an important tool in both design and operation of NPPs (e.g., IAEA-TECDOC-1200 and SSR 2/1). In fact, the probabilistic tools extend traditional deterministic analyses providing a better understanding of accidental sequences characterized by combined failures. Additionally, in parallel to deterministic safety assessment, PSA helps identifying plant vulnerabilities and dependencies between systems, examining different design options and prioritizing improvement areas. 

In contrast, the process for judging changes in nuclear safety always requires at least a deterministic safety assessment to provide the evidence that the respective specifications and nuclear safety standards are fulfilled. Figure 2 describes how to determine the kind of participation of the authority and whether or not a probabilistic safety assessment is required. This is the reason why the method applied in the nuclear field does not contain qualitative criteria which are used for the consideration of already existing experience with the implementation of the change. 

This publication establishes design requirements for stractures, systems and components important to safety that must be met for safe operation of a nnclear power plant, and for preventing or mitigating the consequences of events that could jeopardize safety. It also establishes requirements for a comprehensive safety assessment, which is carried out in order to identify the potential hazards that may arise from the operation of the plant, under the various plant states (operational states and accident conditions). The safety assessment process includes the complementary techniqnes of deterministic safety analysis and probabilistic safety analysis. These analyses necessitate consideration of postulated initiating events (PIEs), which include mat r factors that, singly or in combination, may affect safety and which may ... 

Table 16.2 Summary and comparison of probabilistic risk assessment/probabilistic safety assessment and deterministic analysis approaches...

PRA, probabilistic risk assessment PSA, probabilistic safety assessment DA, deterministic analysis HRA, human relialnlity... 

The safety principle and safety system design are described in Sects. 6.2 and 6.3, respectively. Then, the deterministic approach to the Super LWR safety is described in Sects. 6.4-6.7 these describe safety analysis methods, selection and classification of abnormal events, the criteria for safety analyses, and the results of safety analyses. In addition, development of a transient subcharmel analysis code and its application to the flow decreasing events are described in Sect. 6.8. Based on the safety system design and the deterministic safety analyses, level-1 probabilistic safety assessment (PSA), which is also called level-1 probabilistic risk assessment (PRA), is presented. 

Both deterministic and probabilistic approaches are important and useful for clarifying the safety characteristics of new reactors concepts. In this section, simplified level-1 PSA (probabilistic safety assessment) of the Super LWR is introduced. Information from preliminary PSA studies on the SCWR [21, 33] and PSA documents for US LWRs [34-37] and for Japanese LWRs [38-41] is mainly referred to in this section. 

There are two possible approaches to estimating the human safe dose for chemicals that cause deterministic effects the use of safety and uncertainty factors and mathematical modeling. The former constitutes the traditional approach to dose-response assessment for chemicals that induce deterministic effects. Biologically-based mathematical modeling approaches that more realistically predict the responses to such chemicals, while newer and not used as widely, hold promise to provide better extrapolations of the dose-response relationship below the lowest dose tested. 

Although dose-response assessments for deterministic and stochastic effects are discussed separately in this Report, it should be appreciated that many of the concepts discussed in Section 3.2.1.2 for substances that cause deterministic effects apply to substances that cause stochastic effects as well. The processes of hazard identification, including identification of the critical response, and development of data on dose-response based on studies in humans or animals are common to both types of substances. Based on the dose-response data, a NOAEL or a LOAEL can be established based on the limited ability of any study to detect statistically significant increases in responses in exposed populations compared with controls, even though the dose-response relationship is assumed not to have a threshold. Because of the assumed form of the dose-response relationship, however, NOAEL or LOAEL is not normally used as a point of departure to establish safe levels of exposure to substances causing stochastic effects. This is in contrast to the common practice for substances causing deterministic effects of establishing safe levels of exposure, such as RfDs, based on NOAEL or LOAEL (or the benchmark dose) and the use of safety and uncertainty factors. 

In classifying waste, deterministic responses generally should be of concern only for hazardous chemicals (see Section 3.2.2.1). Therefore, the only important issue for risk assessment is the most appropriate approach to estimating thresholds for induction of responses in humans. The primary concern here is that consistent approaches should be used for all substances that induce deterministic effects. NCRP s recommendation that nominal thresholds in humans should be estimated using the benchmark dose method and a safety factor of 10 or 100, depending on whether the data were obtained in a study in humans or animals (see Section 6.1.2.1), is intended to provide consistency in estimating thresholds for all substances that cause deterministic effects. 

In many respects, the foundations and framework of the proposed risk-based hazardous waste classification system and the recommended approaches to implementation are intended to be neutral in regard to the degree of conservatism in protecting public health. With respect to calculations of risk or dose in the numerator of the risk index, important examples include (1) the recommendation that best estimates (MLEs) of probability coefficients for stochastic responses should be used for all substances that cause stochastic responses in classifying waste, rather than upper bounds (UCLs) as normally used in risk assessments for chemicals that induce stochastic effects, and (2) the recommended approach to estimating threshold doses of substances that induce deterministic effects in humans based on lower confidence limits of benchmark doses obtained from studies in humans or animals. Similarly, NCRP believes that the allowable (negligible or acceptable) risks or doses in the denominator of the risk index should be consistent with values used in health protection of the public in other routine exposure situations. NCRP does not believe that the allowable risks or doses assumed for purposes of waste classification should include margins of safety that are not applied in other situations. 

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