The owner or operator of a stationary source covered by the risk management program regulations must conduct a worst-case release scenario analysis as part of the required hazard assessment (40 CFR §68.25). The worst-case release is defined as the release of the largest quantity of a regulated substance from a vessel or process line failure that results in the greatest distance to an endpoint (40 CFR §68.3). If a release from the process containing the largest quantity of a regulated substance would result in a shorter distance to an endpoint than a release from a smaller process, which scenario should be considered the worst-case release?

The worst-case release is the scenario that results in the greatest distance to an endpoint beyond the stationary source boundary (40 CFR §68.25(h)). EPA recognizes that there could be release scenarios in which a smaller process could generate a greater distance to an endpoint than a release from the largest vessel or pipeline (61 FR 31682; June 20, 1996). The regulatory language at 40 CFR §68.25(h) clarifies that a scenario involving a smaller quantity of regulated substance handled at a higher process temperature or pressure, as well as a scenario involving a smaller quantity located closer to the stationary source boundary may, in fact, result in the worst-case release.