Calculated risks

QRA is fundamentally different from many other chemical engineering activities (e.g., chemistry, heat transfer, reaction kinetics) whose basic property data are theoretically deterministic. For example, the physical properties of a substance for a specific application can often be established experimentally. But some of the basic property data used to calculate risk estimates are probabilistic variables with no fixed values. Some of the key elements of risk, such as the statistically expected frequency of an accident and the statistically expected consequences of exposure to a toxic gas, must be determined using these probabilistic variables. QRA is an approach for estimating the risk of chemical operations using the probabilistic information. And it is a fundamentally different approach from those used in many other engineering activities because interpreting the results of a QRA requires an increased sensitivity to uncertainties that arise primarily from the probabilistic character of the data. 

The QRA project team can select the appropriate technical approach once you specify the study objectives, and together you can define the scope. A variety of modeling techniques and general data sources (discussed in Section 3.2) can be used to produce the desired results. Many computer programs are now available to aid in calculating risk estimates, and many automatically give more answers than you will need. The QRA team must take care to supply appropriate risk characteristics that satisfy your study objectives—and no more. 

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. 

Plants meeting all applicable NRC regulatory requirements have been fouiid to vary significantly in calculated risk and in terms of the key accident sequences. 

Another problem with risk estimates is that they are usually based on very conservati C assumptions. Thus, the aiuilyses may result in a calculation tliat presents too high a risk. Umicccsstiry equipment or procedures tnay have to be installcd/instituted at a facility to reduce tlie calculated risk. In an effort to better understand tlie significance of risk analyses, it is often helpful to place tlie estimated risks in perspective with otlicr risks. 

Contractor designs will be in the context of a competitive bidding situation and in-company checks of the design should cover not only design errors but also cases where calculated risks have been taken, which may not, however, be acceptable to the operating company. 

Multiple risk factors with a Framingham calculated risk (Fig. 9-6) greater than 20%... 

As discussed above, in a world with 4000 well-designed reactors, one would expect less than a 4% chance of a Chemobyl-scale reactor accident per century. If one estimates that such an accident might cause 20,000 eventual cancer deaths, the calculated risk over a century from reactor accidents would be of the order of 800 deaths. Reactors might do better or worse than this, but the anticipated scale of harm is in the ballpark of a thousand deaths per century - with large uncertainties in either direction,... 

SADA provides a full human health risk assessment module and associated databases. The risk models follow the USEPA s Risk Assessment Guidance for Superfund (RAGS) and can be customized to fit site-specific exposure conditions. It calculates risks based on the following exposure pathways ingestion, inhalation, dermal contact, food consumption, and also a combined exposure. 

IRAP-h View Interface for conducting a comprehensive multipathway human health risk assessment. It simultaneously calculates risk values for multiple chemicals, from multiple sources, at multiple exposure locations. IRAP-h view implements the US EPA - OSW Human Health Risk Assessment Protocol (HHRAP) US EPA [15]... 

It takes effort to work out how to make a component more generic, and the result might run slower when deployed. The investment will be repaid in savings every time the component is used in a design and every time maintenance must be done on only one component rather than many slightly different copies. But it is not always certain exactly when or how a component will be reused in the future. So, as with all investments, generalizing a component is a calculated risk. 

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