An Overview of Quantitative Risk Assessment Methodology, and Worldwide Risk Tolerability Criteria

Quantitative Risk Analysis (QRA) as a technique for managing and understanding risks dates back to the 1970s, initially applied in the aerospace, electronics, and nuclear power industries. During the 1980s the technique was refined and applied to the chemical and petrochemical industries.

Risk-based legislation has been implemented by national governmental bodies, often with significant public input and/or scrutiny, indicating a high degree of societal endorsement of the values. There are at least three themes that are commonly used in the development of many generally accepted and recognized risk criteria: (1) a comprehensive risk management program must address both individual and societal risk; (2) risk criteria for the public must be lower, i.e. more conservative, than those for the workforce since the workforce risk is considered to be voluntary; and (3) with respect to individual risk, new facilities should be held to a higher level of risk performance than existing facilities. For new facilities many opportunities exist to apply new/advanced risk reduction technologies. In contrast, societal risk criteria are universally identical for new and existing situations; i.e., where a potential exists for major accident events affecting large numbers of people, most regulators have judged that older facilities must meet the same standards as newer facilities.
This paper outlines the main steps involved in conducting a QRA study, and the main inputs and outputs in such a study. It also discusses how closer examination of these results can enable risk analysts to better understand, manage, and mitigate risks at their facility. The paper goes on to evaluate various international risk criteria in use today, and evaluates their respective merits. It also provides suggestions for companies or countries considering implementing risk tolerability criteria.