Release Scenarios and Consequences

Chapter 2—Overview of Release Scenarios and Postrelease Mitigation This chapter provides an overview of the different types of releases that can occur, the consequences of the discharges, and a general description of available postrelease mitigation measures. 

The ha2ard assessment is to iaclude identification of a worst-case scenario and other more likely scenarios for release of a regulated substance, and analy2e the off-site consequences of such releases. The release and consequence assessment is to iaclude the rate, duration, and quantity of the release, the distances for exposure or damage (usiag atmospheric, called "F" stabiUty and a 1.5-m/s wiad, and most-often-occurriag conditions), populations that could be exposed, and environmental damage that could be expected. 

In any case, like frequency analysis, examining the uncertainties and sensitivities of the results to changes in boundary conditions and assumptions provides greater perspective. The level of effort required for a consequence analysis will be a function of the number of different accident scenarios being analyzed the number of effects the accident sequence produces and the detail with which the release, dispersion, and effects on the targets of interest is estimated. The cost of the consequence analysis can typically be 25% to 50% of the total cost of a large QRA. 

Consequence Phase 3 Develop Detailed Quantitative Estimate of the impacts of the Accident Scenarios. Sometimes an accident scenario is not understood enough to make risk-based decisions without having a more quantitative estimation of the effects. Quantitative consequence analysis will vary according to the hazards of interest (e.g., toxic, flammable, or reactive materials), specific accident scenarios (e.g., releases, runaway reactions, fires, or explosions), and consequence type of interest (e.g., onsite impacts, offsite impacts, environmental releases). The general technique is to model release rates/quantities, dispersion of released materials, fires, and explosions, and then estimate the effects of these events on employees, the public, the facility, neighboring facilities, and the environment. 

It can simulate a wide variety of release scenarios but is particularly well suited to assessing health consequence impacts and risk. 

An off-site consequence analysis that evaluates specific potential release scenarios, including worst-case and alternative scenarios... 

In the scenario for the controlled landfill site the treatment of effluent from the site by sewage treatment and the incineration of the sludge are taken into account. An additional scenario is made for an uncontrolled landfill site, assuming DEHP emissions. However, in an uncontrolled landfill site not only DEHP will emit from the site but also other toxic releases like heavy metals. So the results presented for the uncontrolled landfill site are an underestimation. For a more realistic assessment of impacts related to the uncontrolled landfill of PVC, additional estimates are necessary for the emissions of (toxic) releases. As a consequence, the impact assessment score for human and aquatic ecotoxicity for the uncontrolled landfill site will increase. The relative contribution of DEHP to these scores will decrease because also other emissions which are in the present assumptions are now lacking, like heavy metals, will contribute to the score. 

Hazard assessment is a consequence analysis for a range of potential hazardous chemical releases, including the history of such releases at the facility. The releases must include the worst-case scenario and the more likely but significant accident release scenarios. A risk matrix can be used to characterize the worst-case and more likely scenarios. 

The EPA requires the following consequence analyses (1) A single worst-case release scenario is analyzed for all covered flammable materials on the site, and only one flammable substance is analyzed for other more likely scenarios and (2) a single worst-case release scenario is analyzed for all toxic substances on the site, and more likely releases are analyzed for each toxic substance covered by the rule. 

Dispersion model calculations are normally used to estimate downwind concentrations these concentrations are the basis for determining the consequences resulting from toxicity, fires, and/or explosions. For those not interested in using dispersion models, the standard includes lookup tables for all the listed substances to help a facility determine the impact distances for specific release scenarios. 

Hazard assessment. A hazard assessment is required to assess the potential effects of an accidental (or intentional) release of a covered chemical/material. This RMP element generally includes performing an off-site consequence analysis (OCA) and the compilation of a five-year accident history. The OCA must include analysis of a least one worst-case scenario. It must also include one alternative release scenario for the flammables class as a whole also each covered toxic substance must have an alternative release scenario. USEPA has summarized some simplified consequence modeling... 

Fire hazard analysis (FHA) is the process to determine the size, severity, and duration of a scenario and its impact on personnel, equipment, operations, and the environment. Chapter 5 provided details of performing an FHA. The following paragraphs provide an overview of the FHA process. For example, one scenario could be a seal failure where the material being released is ignited and afire results. In assessing consequences, several questions must be considered ... 

Consequence modeling can be used to evaluate the impact of post-release mitigation measures and determine the relative effectiveness of techniques, or combinations of techniques. Release scenarios are described in Chapter 2, and mitigation techniques are presented in Chapters 3 through 6. 

First we will look at a release scenario that is unmitigated, and then at the modification of a scenario to include a postrelease mitigation technique. The effect of the postrelease mitigation technique will be evaluated by applying the consequence modeling techniques described above. It is important to note... 

Additional tests such as the addition of nucleophilic scavengers (e.g., thiols such as dithiothreitol or j8-mercaptoethanol) can provide further evidence for the presence of a free, reactive electrophilic species. The scavengers should quench all of the free reactive species, thereby protecting the enzyme from inhibition. Unfortunately, this method cannot exclude the possibility that a nucleophilic thiol may even attack the bound reactive species at the active site of the enzyme (which would also give rise to protection from inactivation). However, the use of a bulky thiol, such as reduced glutathione, should limit that possibility. An alternative scenario occurs wherein the released reactive species returns and reacts faster with an active-site nucleophile than with the added thiol. Clearly this is a complex problem and, consequently,it is advisable to use several different tests to avoid misleading conclusions. 

Using the consequence and likelihood categories, risk matrix, and risk evaluation criteria, the team reviewed three release scenarios (small, medium, and large) for the segments identified for each of the chemical movements. The result of the semi-quantitative risk estimation for this facility s hazardous material transportation operation is detailed in Table 4.12. From this results table, the following are determined ... 

It is necessary to perform the review with an open mind set to identify the risks and consequences. A major accident would generally occur from one of these three categories -a fire or an explosion or a toxic release. These three major hazard categories are further subdivided in Table 3.1 that could be helpful in developing relevant scenarios. 

Each of these features provides a safety benefit, however not all of these features are necessary to mitigate dose consequences to acceptable levels. Only those features that are relied upon to function or actuate to prevent or mitigate uncontrolled releases of radioactive materials are so identified. Analyses accomplished to evaluate the consequences of release of radiological materials, described in Section 3.4, identify those SSC s that are part of the primary success path in each scenario. The SSC s so identified are associated with a significant mitigation of radiological releases in abnormal and accident scenarios and therefore perform a defense in depth Safety Function. 

Potential doses at the 3000-m. boundary have been evaluated using methodology described in Section 3.4.1. These techniques have been used to calculate the potential dose consequences resulting from the release of the inventory described in Table 3.4-1, under various multiple target scenarios and with various degrees of mitigation. These scenarios and associated probabilities derived in Appendix 3E are summarized in Table 3.4-2. 

The source term potentially available for release consists of the cold trap noble gas inventory, the residual contamination in each SCB, the halogen inventory built up on the in-box filters, the residual process liquid source term, and the inventory in a maximally irradiated target. The specific mix of each of these inventories in each DBE scenario varies, and is dependent on the specific release scenario under consideration. The dose consequence at the 3000-meter exclusion area boundary for each of these potential Inventories is summarized in Table 3E.7-1. 

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