Fatality estimation risk assessment

Potential health and economic consequences of nuclear power plant accidents include early fatalities, early injuries, latent cancers, population doses, various health effects, and onsite and offsite costs. For such consequence measures, application of the preceding deftnition of risk becomes more complicated, because frequencies must be estimated for accidents with varying degrees of severity. For example, the frequency of transportation accidents involving 100 or more early fatalities is substantially lower than the frequency of transportation accidents involving only 1 fatality. In risk assessments, frequencies of accidents with all possible consequence levels are estimated. It is desirable to combine the risks associated with high, moderate, and low consequence accidents into an overall risk measure. For this purpose, the concept of actuarial or consequence-weighted risk is used. 

The reader should note tliat since many risk assessments have been conducted on the basis of fatal effects, there are also uncertainties on precisely what constitutes a fatal dose of thennal radiation, blast effect, or a toxic chemical. Where it is desired to estimate injuries as well as fatalities, tlie consequence calculation can be repeated using lower intensities of exposure leading to injury rather titan dcatli. In addition, if the adverse healtli effect (e.g. associated with a chemical release) is delayed, the cause may not be obvious. Tliis applies to both chronic and acute emissions and exposures. 

Quantitative risk assessment (QRA) The systematic development of numerical estimates of the expected frequency and consequence of potential accidents associated with a facility or an operation. Using consequence and probability analyses and other factors such as population density and expected weather conditions, QRA estimates the fatality rate for a given set of events. 

The use of MLEs of probability coefficients for radionuclides but UCLs for chemicals that induce stochastic responses is the most important issue that would need to be resolved to achieve a consistent approach to estimating risks for the purpose of waste classification. For some chemicals, the difference between MLE and UCL can be a factor of 100 or more. The difference between using fatalities or incidence as the measure of response is unlikely to be important. Use of the linearized, multistage model to extrapolate the dose-response relationship for chemicals that induce stochastic effects, as recommended by NCRP, should be reasonably consistent with estimates of the dose-response relationship for radionuclides, and this model has been used widely in estimating probability coefficients in chemical risk assessments. The difference in the number of organs or tissues that are taken into account, although it cannot be reconciled at the present time, should be unimportant. 

The hazard analysis of any industrial process impacts on risk assessment. Risk assessment involves the estimation of the frequency and consequences of a range of hazard scenarios and of individual and societal risk. The risk assessment process is shown in Figure 3.1. The risk criterion used in hazard analysis is the fatal accident rate (FAR). The FAR is defined as the number of fatalities per 108h exposure. The actual FAR in the U.K. was 3.5 in the chemical industry in 1975. No doubt the ideal FAR value should be zero, which is difficult to achieve in practice. 

It is emphasized that the results presented in this paper are preliminary and based on a combination of available information from risk assessments and expert judgment. Further detailed risk studies are necessary to come to a more detailed and accurate understanding of the level of fatality risks from floods throughout the Netherlands. The outcomes of the FLORIS project are expected by the year 2011. These outcomes will include estimates of the spatial distribution of individual risks. 

Scenario-based safety risk assessment, where the calculation estimates the frequency with which the hazardous scenario will lead to the calculated consequence (a certain number of fatalities within the total exposed population). The distinction between this calculation and an Individual Risk calculation is that this calculation does not focus on any specific individual but instead considers and aggregates the impact on the whole population. A single scenario-based risk assessment does not account for all the sources of harm to which an individual may be exposed in a given establishment. When scenario-based LOPA is carried out. Individual Risk should also be considered to ensure that Individual Risk limits are not exceeded. 

The techniques are based on probabilities which many people find difficult to understand. In essence, the fact that an accident happens does not mean that the risk assessment was incorrect. For example, if it is correctly calculated that there is a very low likelihood of a multiple fatality, the fact that the multiple fatality occurs does not necessarily mean that the estimate of likelihood was incorrect. Rather, it is the third of the factors listed above, i.e. the operation of chance. Whaf is required in the longer term are numerical techniques for risk assessment which identify the xmderlying level of risk and fhe exfenf to which risk control measures will reduce the risk. It will then be possible to predict the number of accidents and incidents that will occur by chance and this can be compared with the numbers of accidenfs and incidents that do occur. It may then also be possible, by examining the accident and incident data in more detail, to determine whether any problems are due to an xmderestimate of fhe underl5ung level of risk or a failure to select or implement appropriate risk control measures. These are discussed by Boyle. ... 

It has been estimated that at least a quarter of all fatal injuries at work involve failures in systems of work — the way things get done. A safe system of work is a formal procedure that results from a systematic examination of a task in order to identify all the hazards and assess the risks, and which identifies safe methods of work to ensure that the hazards are eliminated or the remaining risks are minimised. Where elements of risk remain, a safe system of work will be required. Some examples where safe systems of work will be part of the controls are ... 

Expected fatalities - per hour, per year, or per individual - of various exposed groups were estimated from post-data and then compared to assessments of the benefits accruing from these activities. Starr (1969) found that historical levels of risk acceptance increased in proportion to the cube root of the increase in benefits. Voluntary acceptance levels were about three orders of magnitude greater than involuntary acceptance levels. 

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