Risk assessment scenarios

The Hazard Consequence Reports documenting the identified accident scenarios are used as the starting point in the risk assessment. Scenarios with the same degree of consequence severity are grouped together in tree structures called consequence trees. Each tree uses Boolean logic to trace a top consequence to its basic initiating events and the related hazardous conditions. For each consequence tree, the minimal cut set equation is determined. 

The next part of the procedure involves risk assessment. This includes a deterrnination of the accident probabiUty and the consequence of the accident and is done for each of the scenarios identified in the previous step. The probabiUty is deterrnined using a number of statistical models generally used to represent failures. The consequence is deterrnined using mostiy fundamentally based models, called source models, to describe how material is ejected from process equipment. These source models are coupled with a suitable dispersion model and/or an explosion model to estimate the area affected and predict the damage. The consequence is thus determined. 

An important part of hazard analysis and risk assessment is the identification of the scenario, or design basis by which hazards result in accidents. Hazards are constandy present in any chemical faciUty. It is the scenario, or sequence of initiating and propagating events, which makes the hazard result in an accident. Many accidents have been the result of an improper identification of the scenario. 

The Modeling Engine in THERdbASE has the following model groups 1) Population Distributions, 2) Location/Activity Patterns, 3) Food Consumption Patterns, 4) Agent Releases Characteristics, 5) Microenvironment Agent Concentrations, 6) Macroenvironment Agent Concentrations, 7) Exposure Patterns and Scenarios, 8) Dose Patterns, and 9) Risk Assessment. 

Figure 1 shows part of a solvent phase polypropylene plant. The plant consists of three process lines, denoted A, B, and C. During a risk assessment review, a scenario was identified that involved a release of reactor contents from a location near the west end of the A line. Estimates are needed of the blast overpressures that would occur if the resulting cloud of vapor, mist, and power ignites. 

PECs have been calculated for various scenarios (production, formulation, and use) as a means to conduct a risk assessment. 

The degree of confidence in the final estimation of risk depends on variability, uncertainty, and assumptions identified in all previous steps. The nature of the information available for risk characterization and the associated uncertainties can vary widely, and no single approach is suitable for all hazard and exposure scenarios. In cases in which risk characterization is concluded before human exposure occurs, for example, with food additives that require prior approval, both hazard identification and hazard characterization are largely dependent on animal experiments. And exposure is a theoretical estimate based on predicted uses or residue levels. In contrast, in cases of prior human exposure, hazard identification and hazard characterization may be based on studies in humans and exposure assessment can be based on real-life, actual intake measurements. The influence of estimates and assumptions can be evaluated by using sensitivity and uncertainty analyses. - Risk assessment procedures differ in a range of possible options from relatively unso-... 

Grower et al. (1989) formulated a set of criteria for risk assessment (see Fig. 5.4-56) that are related to scenarios like that described above ... 

The first step in a process plant building risk assessment is to identify specific accident scenarios that endanger building occupants. As discussed in Chapter 2 and illustrated in Table 2.1, accident scenarios are sequences of events that lead to an outcome of concern. The specific outcomes of concern are those involving explosions or fires that could impact buildings in process plants. 

The risk assessment process begins by identifying specific accident scenarios that apply to the facility under review. Steps include ... 

Risk assessment pertains to characterization of the probability of adverse health effects occurring as a result of human exposure. Recent trends in risk assessment have encouraged the use of realistic exposure scenarios, the totality of available data, and the uncertainty in the data, as well as their quality, in arriving at a best estimate of the risk to exposed populations. The use of "worst case" and even other single point values is an extremely conservative approach and does not offer realistic characterization of risk. Even the use of arithmetic mean values obtained under maximum use conditions may be considered to be conservative and not descriptive of the range of exposures experienced by workers. Use of the entirety of data is more scientific and statistically defensible and would provide a distribution of plausible values. 

Exposure assessments have become an essential element of contemporary risk assessment (NAS/NRC, 1983). The primary purpose of exposure assessment is to qualitatively and/or quantitatively determine exposure and absorbed dose associated with a particular use practice or human activity. Contemporary exposure assessors and risk managers place a high premium on accurate data obtained by monitoring chemical exposure scenarios and critical human activities or work tasks. 

In general, plant-protection products are biocidal active substances and are therefore by nature toxic to target organisms. At least some of them are also toxic to humans therefore, the safe use of plant-protection products presupposes, among other things, an evaluation of worker exposure during re-entry, an adequate risk assessment on the basis of the various practical scenarios in agriculture and horticulture, and, if necessary, specific instructions for worker protection on the product label. 

It is advisable, then, in a tiered approach to concentrate first on crops and activities (scenarios) that are considered to be relevant with respect to the expected level of exposure and to exclude those not relevant. Second, whether or not the toxicological properties of the product may lead to general restrictions on re-entry should be investigated. If both the likelihood of reentry and the hazard due to the toxicity of the compound cannot generally be neglected, a risk assessment over several steps should be carried out. The assessment may be based on surrogate data and "worst-case" assumptions at first and then refined, if necessary. One possible approach to a tiered evaluation procedure is presented in Figure 1. 

On the other hand, in part II of this volume, a set of case studies are introduced. The application of the selected methodologies inside each one of the foresaid disciplines (e.g., risk assessment, life cycle assessment) to specific cases and countries is presented here. The results of such application are discussed as well as their reliability. Toxicological studies in Italy, risk assessment of electronic waste in China, or disposal of bearing lamps in India are some examples of selected scenarios. 

As it has been shown in this chapter knowing the concentrations of chemicals in the environment is a key aspect in order to carry out meaningful hazard and risk assessment studies. Predicting concentrations of chemicals can serve as a quick and robust way to produce an acceptable screening level assessment however if further precision is desired, the complexity of real environmental scenarios can make it a cumbersome and unaffordable task. Models improvement requires not only refining their computation algorithms but also and more important, implementing new inputs and processes in order to better describe real scenarios. 

For human health risk assessment, it is necessary to elaborate realistic scenarios. Knowledge of real scenarios where the contaminant is emitted to the environment will help to obtain information about the fate and transport of the contaminant once emitted to the environment and the route of exposure for the human beings living in this scenario of concern. There are different types of exposure, i.e., direct, indirect (as is the case of food contaminated by the air, water, or soil contaminated by the emission), occupational exposure, and consumer goods coming from outside the scenario of concern. Depending on the objective of the study, it will be necessary to consider in the exposure assessment one or more types of exposure. 

In each context, it is essential to define suitable parameters of the exposure scenario for a realistic risk assessment ... 

The exposure assessment in EUSES aims at reasonable worst-case, i.e., the exposure scenario was the worst scenario without being unrealistic and as much as possible using mean, median, or typical parameter values. If the outcome of the reasonable worst-case risk characterization indicates that the substance is not of concern, the risk assessment for that substance can be stopped with regard to the scenario considered. 

Based on these failure modes, a detailed risk assessment of most probable or most severe hydrogen accident scenarios has been conducted in some studies [66] including fuel tank... 

Fire risk assessment is made in order to determine the overall value of decreasing fire hazard in a particular scenario. The level of fire risk that is acceptable for a situation is, normally, a societal, and not a technical, decision. Therefore, fire hazard assessments are generally more common than fire risk assessments. The NFPA Research Foundation has undertaken a project to develop a methodology for fire risk assessment. It has done this by studying four cases in detail upholstered furniture in residential environments, wire and cable in concealed spaces in hotels and motels, floor coverings in offices and wall coverings in restaurants. 

The first question represents hazard identification. The last three questions are associated with risk assessment, considered in detail in chapter 11. Risk assessment includes a determination of the events that can produce an accident, the probability of those events, and the consequences. The consequences could include human injury or loss of life, damage to the environment, or loss of production and capital equipment. Question 2 is frequently called scenario identification. 

Figure 10-1 illustrates the normal procedure for using hazards identification and risk assessment. After a description of the process is available, the hazards are identified. The various scenarios by which an accident can occur are then determined. This is followed by a concurrent... 

Risk assessments vary widely in scope and application. Some look at single risks in a range of exposure scenarios such as the IPCS Environmental Health Criteria Document series, others are site-specific and look at the range of risks posed by an installation. 

Figure 23-1 shows the hazards identification and risk assessment procedure. The procedure begins with a complete description of the process. This includes detailed PFD and P I diagrams, complete specifications on all equipment, maintenance records, operating procedures, and so forth. A hazard identification procedure is then selected (see Haz-ard Analysis subsection) to identify the hazards and their nature. This is followed by identification of all potential event sequences and potential incidents (scenarios) that can result in loss of control of energy or material. Next is an evaluation of both the consequences and the probability. The consequences are estimated by using source models (to describe the... 

The principal application of PBPK models is in the prediction of the target tissue dose of the toxic parent chemical or its reactive metabolite. Use of the target tissue dose of the toxic moiety of a chemical in risk assessment calculations provides a better basis of relating to the observed toxic effects than the external or exposure concentration of the parent chemical. Because PBPK models facilitate the prediction of target tissue dose for various exposure scenarios, routes, doses, and species, they can help reduce the uncertainty associated with the conventional extrapolation approaches. Direct application of modeling includes... 

The goal of assessing risk is to build on the knowledge of chemical reactivity hazards, to understand how the hazard properties may lead to loss scenarios in the facility context, and to determine whether existing safeguards are adequate. Therefore, the assessment of risk can be performed at any stage of facility design, development, operation, or alteration. Of course, the more that is known about the facility and its equipment and operation, the more detailed the risk assessment can be. Methods used to determine chemical reaction risks are varied, as are their objectives and data requirements. 

By contrast, the nature of certain accident scenarios could prove to be quite sensitive to some design parameters. It should not be ruled out during the risk assessment phase, especially during detailed design, that discoveries during consequence analysis could lead to the revision of the design basis of the facility or some equipment or components. 

Transfer waivers are justifiable, in the very specific situations where an inter-laboratory trial is not justified based on an appropriate risk assessment (ICH Q9, 2005), the following examples reflecting scenarios that are typically encountered ... 

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