NAC459.95366. Analysis of worst-case scenarios.

     1. The owner or operator of a facility may use the guidelines set forth in the R.M.P. Guidance for Off-Site Consequence Analysis, which is adopted by reference pursuant to NAC 459.95528, to calculate any of the values required in this section.

     2. For each process, the owner or operator shall prepare:

     (a) One worst-case release scenario that is estimated to create the greatest distance in any direction to an endpoint resulting from an accidental release of a toxic highly hazardous substance under worst-case conditions as described in NAC 459.95364;

     (b) One worst-case release scenario that is estimated to create the greatest distance in any direction to an endpoint resulting from an accidental ignition or detonation of a flammable or explosive substance under worst-case release conditions as described in NAC 459.95364; and

     (c) Additional worst-case release scenarios for a facility if:

          (1) A worst-case release from another process at the facility potentially affects different public receptors than those affected by the worst-case release scenario prepared pursuant to paragraphs (a) and (b); or

          (2) A toxic or flammable highly hazardous substance is present in excess of the threshold quantity and was not considered as part of the worst-case release scenarios prepared pursuant to paragraphs (a) and (b).

     3. When preparing a worst-case release scenario:

     (a) For a highly hazardous substance, the owner or operator shall assume that the release quantity is the greater of:

          (1) For substances in a vessel, the greatest amount held in a single vessel, taking into account administrative controls that limit the maximum quantity.

          (2) For substances in pipes, the greatest amount in a pipe, taking into account administrative controls that limit the maximum quantity.

     (b) For an explosive, the owner or operator shall select the inventory that produces the greatest distance to an endpoint.

     4. The owner or operator shall model each substance as a toxic substance, a flammable substance or an explosive as described in NAC 459.9533. If a substance is not described as a toxic substance, a flammable substance or an explosive pursuant to NAC 459.9533, the owner or operator shall select the scenario providing the most significant impact on employees and the public.

     5. For toxic substances that are normally gases at ambient temperature and handled as a gas or as a liquid under pressure, the owner or operator shall:

     (a) Assume that the quantity in the vessel or pipe, as determined pursuant to subsection 3, is released as a gas over a period of 10 minutes;

     (b) Assume that the release rate, in pounds per minute, is the total quantity divided by 10, unless passive mitigation systems are in place; and

     (c) Calculate the impact of passive mitigation measures on the release rate using the R.M.P. Guidance for Off-Site Consequence Analysis, which is adopted by reference pursuant to NAC 459.95528.

     6. For gases handled as refrigerated liquids at ambient pressure, the owner or operator:

     (a) Shall assume that the substance is released as a gas in 10 minutes, if the released substance is not contained by passive mitigation systems or if the contained pool would have a depth of 1 cm (0.39 inch) or less; and

     (b) May assume that the quantity of the substance in the vessel or pipe, as determined pursuant to subsection 3, is spilled instantaneously to form a liquid pool, if the released substance is contained by passive mitigation systems in a pool with a depth greater than 1 cm (0.39 inch). The owner or operator shall calculate the volatilization rate at the boiling point of the substance and at the conditions set forth in subsections 7, 8 and 9.

     7. For toxic substances that are normally liquids at ambient temperature, the owner or operator shall assume that the quantity in the vessel or pipe, as determined pursuant to subsection 3, is spilled instantaneously to form a liquid pool. The owner or operator shall determine the surface area of the pool by assuming that the liquid spreads to 1 cm (0.39 inch) deep, unless passive mitigation systems are in place that serve to contain the spill and limit the surface area. If passive mitigation is in place, the owner or operator shall use the surface area of the contained liquid to calculate the volatilization rate. If the release would occur onto a surface that is not paved or smooth, the owner or operator may take into account the actual surface characteristics.

     8. When determining the volatilization rate for purposes of subsection 7, the owner or operator shall account for:

     (a) The highest daily maximum temperature occurring during the past 3 years;

     (b) The temperature of the substance in the vessel; and

     (c) If the liquid spilled is a mixture or solution, the concentration of the substance.

     9. For purposes of subsection 7, the owner or operator shall determine the rate of release to air from the volatilization rate of the liquid pool determined pursuant to subsection 8. The owner or operator may use the methodology set forth in the R.M.P. Guidance for Off-Site Consequence Analysis, which is adopted by reference pursuant to NAC 459.95528, or another publicly available technique that accounts for the modeling conditions and is recognized in the industry as a current practice. The owner or operator may use a proprietary model that accounts for the modeling conditions if the owner or operator allows the Division access to the model and describes to local emergency planners, upon request, the features of the model and any differences from publicly available models.

     10. The owner or operator shall assume that the quantity of the flammable substance determined pursuant to subsection 3 vaporizes resulting in a vapor cloud explosion. The owner or operator shall use a yield factor of 10 percent of the available energy released in the explosion to determine the distance to the explosion endpoint if the model used is based on TNT-equivalent methods.

     11. For explosives, the owner or operator shall employ methods for calculating overpressure based upon generally accepted practices.

     12. The owner or operator shall use the parameters defined in NAC 459.95364 to determine the distance to the endpoints. The owner or operator may use the methodology provided in the R.M.P. Guidance for Off-Site Consequence Analysis, which is adopted by reference pursuant to NAC 459.95528, or any commercially or publicly available technique for air dispersion modeling if the technique accounts for the modeling conditions and is recognized in the industry as a current practice. The owner or operator may use a proprietary model that accounts for the modeling conditions if the owner or operator allows the Division access to the model and describes to local emergency planners, upon request, the features of the model and any differences in the model from publicly available models.

     13. The owner or operator may consider passive mitigation systems for the worst-case release scenario analysis if the mitigation system is capable of withstanding the event that triggered the release and still function as intended.

     14. Notwithstanding the provisions of subsection 3, the owner or operator shall select as the worst-case scenario for a flammable substance, the worst-case scenario for a toxic highly hazardous substance or the worst-case scenario for an explosive, a scenario based on proximity to the boundary of the facility and smaller quantities of the substance handled at a higher process temperature or pressure if such a scenario would result in a greater distance to an endpoint beyond the facility boundary than the scenario provided pursuant to subsection 3.

     (Added to NAC by Environmental Comm’n by R121-98, eff. 5-27-99; A by R041-01, 10-25-2001; R137-04, 2-15-2005)