Risks assumption

Irrespective of the risk, assumptions and decisions will have to be made because of uncertainty. Implications of attempting to characterize all variability and uncertainty in the risk assessment need to be considered. For example, exaggerating uncertainties can obscure the scientific basis of risk management decisions, leaving the impression that the decision has been arbitrary in nature (NRC 1989). The purpose of the uncertainty factor together with the type of assessment (e.g., deterministic or probabilistic, protective or best estimate) must be clearly communicated. Uncertainty factors can be described in 3 categories ... 

Assumption of risk means that the injured party has voluntarily expressed in advance or implied that he or she was aware of the risks involved and agreed to take his or her chances of being injured. The assumption of risk may be by express agreement, implied agreement, or simple awareness (knowledge) of an obvious risk. Assumption of risk is often included in the statutory or comparative negligence provision in many states. An example of this concept could include a citizen refusing to evacuate as a hurricane approaches the community, even when local officials mandate an evacuation from the area. 

No activity is free of assumptions, and every assumption introduces risk. Assumptions are often made for good reasons, such as when information necessary for a better-informed decision is unavailable. In such cases the assumptions are known and should be recorded. Assumptions that are initially valid may become invalid with time (and often do) and they should be monitored. Many assumptions are implicit, particularly when... 

The time shares of installation and troubleshooting are not listed, because these shares are relatively unimportant for the risk assumption caused by control failures on the other hand they represent accidents focuses caused by other reasons. 

Hierarchy of controls Without an understanding of a job s hazards and associated risk, assumption are made on selecting the hazard control level potentially leaving a high risk stiU uncontrolled. 

Statistical Criteria. Sensitivity analysis does not consider the probabiUty of various levels of uncertainty or the risk involved (28). In order to treat probabiUty, statistical measures are employed to characterize the probabiUty distributions. Because most distributions in profitabiUty analysis are not accurately known, the common assumption is that normal distributions are adequate. The distribution of a quantity then can be characterized by two parameters, the expected value and the variance. These usually have to be estimated from meager data. 

If both frequency and consequence values are calculated and reported on an absolute basis, then they may be reported graphically in combination with one another (Chapter 3), or simply as the product of frequency and consequence. Table 5 contains some examples of typical risk estimates (frequency and consequence products). Based on absolute risk estimates, you can decide whether the risk of a specific activity exceeds your threshold of risk tolerance (risk goal). If so, analysts can estimate the reduction in risk, given that certain improvements are made, assumptions changed, or operating circumstances eliminated. 

Adequate support from the facility staff is absolutely essential. The facility staff must help the analysis team gather pertinent documents (e.g., PSilDs, procedures, software descriptions, material inventories, meteorological data, population data) and must describe current operating and maintenance practices. The facility staff must then critique the logic model(s) and calculation(s) to ensure that the assumptions are correct and that the results seem reasonable. The facility staff should also be involved in developing any recommendations to reduce risk so they will fully understand the rationale behind all proposed improvements and can help ensure that the proposed improvements are feasible. Table 12 summarizes the types of facility resources and personnel needed for a typical QRA. 

Another way to evaluate risks is to calculate the sensitivity of the total risk estimates to changes in assumptions, frequencies, or consequences. Risk analysts tend to be conservative in their assumptions and calculations, and the cumulative effect of this conservatism may be a substantial overestimation of risk. For example, always assuming that short-term exposure to chemical concentrations above some threshold limit value will cause serious injury may severely skew the calculated risks of health effects. If you do not understand the sensitivity of the risk results to this conservative assumption, you may misallocate your loss prevention resources or misinform your company or the public about the actual risk. 

If you do not listen to people, you cannot expect them to listen to you. Communication is a two-way activity. Do not make assumptions about what people know, think, or want done about risks. Take the time to find out what people are thinking. Often, people are more concerned about issues such as trust, credibility, competence, control, voluntariness, fairness, and compassion than about mortality statistics and the details of QRA. Use techniques such as interviews, focus groups, and surveys to gauge what people are thinking. 

The analysis on whether to buy or lease should be done very carefully. The assumption of risk, impact on the balance sheet, tax consequences, etc., must be studied. 

Cross-comparing the risks of various activities is difficult because of the lack of a common basis of comparison, however Cohen and Lee, 1979 provide such a comparison on the basis of loss of life expectancy. Solomon and Abraham, 1979 used an index of harm in a study of 6 occupational harms - three radiological and three nonradiological to bracket high and low estimates of radiological effects. The index of harm consists of a weighting factor for parametric study the lost time in an industry and the worker population at risk. The conclusions were that the data are too imprecise for firm conclusions but it is possible for a radiation worker under pessimistic health effects assumptions to have as high index of harm as the other industries compared. 

The relationships between the importance measures is based on the assumption that the systems are not reconfigured in response to a component outage. If this is done, the basic definition of the importance measure is still valid but there is not such a simple relationship. Disregarding this complication, some interpretations of the importances may be made. The Bimbaum Importance is the risk that results when the i-th system has failed (i.e., it is the A, term in Equation 2.8-9). Inspection Importance and RRWI are the risk due to accident sequences containing the i-th system. Fussell- Vesely Importance is similar except it is divided by the risk so may be interpreted as the fraction of the total risk that is in the sequences contains the Q-th system. The Risk Achievement Worth Ratio (RAWR) is the ratio of the risk with system 1 failed to the total risk and is necessarily greater than one. The Risk Achievement Worth Increment (RAWI) is the incremental risk increase if system 1 fails and the Risk Reduction Worth Ratio (RRWR) is the fraction by which the risk is reduced if system 1 were infallible. 

Cutsets are the least compact representation of a complex plant, they may be so numerous that they are unmanageable which obscures significant risk contributors. To address this hydra-like expansion, cutsets may be truncated according to order, probability, or risk. Truncation by order is an approximation to truncation by probability as if each component has about the same probability of failure (a very gross assumption). Truncation by order and by probability are featured in most codes that calculate cutsets. A better truncation method is by risk, as provided in ALLCUTS in as much as a low probability cutoff may delete a high consequence, significant risk contributor. Truncation by risk is difficult because the consequence of a sequence may not be known when the... 

Assumptions used in analyzing the output of the method and how final probability v.ilucs tor ose in tlie overall risk assessment were determined, mid... 

It is unclear whether previously published fire risk analyses have adequately ircaicd dependent failures and systems interaetions. Examples of either experienced or postulated system interactions that have been missed include unrelated systems that share common locations and the attendant spatially related physical interactions arising from fire. Incomplete enumeration of causes of failure and cavalier assumptions of independence can lead to underestimation of accident l rci uencies by many orders of magnitude,... 

The next level of presentation is a technical summary that gives details of the risks including the system s importance measures systems, effects of data changes, and assumptions that are critical to the conclusions. It details the conduct of the analysis - especially the treatment of controversial points. The last level of presentation includes all of the details including a roadmap to the analysis so a peer can trace the calculations and repeat them for verification. 

The purpose of a scoping analysis is to determine, under worst case assumptions, if there is a risk that can cause injury, death or financial impact to the public, workers, company, or environment. The PSA begins by identifying the hazards, their physical and chemical properties, the confinement, conditions and distance for transport to a target, estimating the effects on the target, and comparing these effects with accepted criteria. 

Plant-specific features and modeling assumptions affecting risk, and Use of IPEs for risk-based regulation. 

Core damage and containment performance was assessed for accident sequences, component failure, human error, and containment failure modes relative to the design and operational characteristics of the various reactor and containment types. The IPEs were compared to standards for quality probabilistic risk assessment. Methods, data, boundary conditions, and assumptions are considered to understand the differences and similarities observed. 

The Pickering A Risk Assessment (PARA) (Ontario Hydro, 1995) is also a level 3 PSA for 1 of the 4 units at Pickering. A difference between PARA and DPSE is that sequences beyond the design basis were modeled using the MAAP-CANDU codes with best estimate assumptions. Other parts of the analysis used licensing-type conservative assumptions. 

For the models described, the usual assumption for air nodes in regard to the room air distribution is still valid. This means that each air node represents a volume of perfectly mixed air. Thus, the same limitations as for thermal and airflow models apply Local air temperatures and air velocities as well as local contaminant concentrations can he neither considered nor determined. This also means that thermal comfort evaluations in terms of draft risk cannot be performed. 

Most human or environmental healtli hazards can be evaluated by dissecting tlie analysis into four parts liazard identification, dose-response assessment or hazard assessment, exposure assessment, and risk characterization. For some perceived healtli liazards, tlie risk assessment might stop with tlie first step, liazard identification, if no adverse effect is identified or if an agency elects to take regulatory action witliout furtlier analysis. Regarding liazard identification, a hazard is defined as a toxic agent or a set of conditions that luis the potential to cause adverse effects to hmnan health or tlie environment. Healtli hazard identification involves an evaluation of various forms of information in order to identify the different liaz.ards. Dose-response or toxicity assessment is required in an overall assessment responses/cffects can vary widely since all chemicals and contaminants vary in their capacity to cause adverse effects. This step frequently requires that assumptions be made to relate... 

Much of the attention focused on e.xposure assessment has come recently. This is because many of the risk assessments done in tlie past used too many conseix ative assumptions, wliich caused an ovcrcstimation of the actual exposure. Without exposures there are no risks. To experience adverse effects, one must first come into contact with the toxic agent(s). Exposures to chemicals can be via inlialation of air (brcatliing), ingestion of water and food (eating and drinking), or absorption Uu ough the skin. These arc all pathways to the human body. 

There me two major types of risk ina. imuin individual risk and population risk. Maximum risk is defined e.xacUy as it implies, Uiat is the ma.ximum risk to an individual person. Tliis person is considered to have a 70-year lifetime of exposure to a process or a chemical. Population risk is Uie risk to a population. It is expressed as a certain number of deaths per Uiousand or per million people. For example, a fatal annual risk of 2 x 10 refers to 2 deatlis per year for every million individuals. These risks are based on very conser ative assumptions, llich may yield too high a risk. 

Toxicity alucs for carcinogenic effects also can be c.xprcsscd in terms of risk per unit concentration of the substance in the medium where human contact occurs. These measures, called unit risks, are calculated by dividing the slope factor by 70 kg and multiplying by the inhalation rate (20 m /day) or the water consumption rate (2 L/day), respecti ely, for risk associated with unit concentration in air or water. Where an absorption fraction less than 1.0 has been applied in deriving the slope factor, an additional conversion factor is necessary in the calculation of unit risk so that the unit risk will be on an administered dose basis. The standardized duration assumption for unit risks is understood to be continuous lifetime c.xposure. Hence, when there is no absorption conversion required ... 

Certain assumptions are usually made about an "average" person s attributes for risk assessments applied to large groups of individuals. List the standard values usually assigned to represent the "average" values for the following. 

More attention lias been recently focused on exposure assessment. Tliis is because many of the risk assessments performed in tlie past used too many and overly conservative assumptions. This in turn, caused an overestimation of the actual exposure. 

If there are specific data germane to the assumption of dose-additivity (e g., if two compounds arc present at the same site and it is known that the combination is five times more toxic than the sum of the toxicitics for the two compounds), then tire development of the hazard index should be modified accordingly. The reader can refer to the EPA (1986b) mi.xiure guidelines for discussion of a hazjird index equation that incorporates quantitative interaction data. If data on chemical interactions are available, but arc not adequate to support a quantitative assessment, note the information in the assumptions being documented for the risk assessment. 

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