Double wall tanks and "passive mitigation"
Performance of the hazard assessment required under 40 CFR Part 68, Subpart B, includes analysis of both worst-case release scenarios (40 CFR §68.25) and alternative release scenarios (40 CFR §68.28). In each of these analyses, passive mitigation systems may be taken into consideration. If a tank has a "double wall," does that qualify as "passive mitigation?"
No. Passive mitigation is defined at 40 CFR §68.3 as "equipment, devices, or technologies that function without human, mechanical, or other energy input." Passive mitigation systems include building enclosures, dikes, and containment walls (61 FR 31683; June 20, 1996), but a double wall of a tank is not considered to be passive mitigation. In general, passive mitigation serves to minimize potential adverse effects after the loss of containment, whereas a precaution such as a double wall of a tank serves to prevent the loss of containment in the first place.