When Is Heavy Gas Modeling Necessary

1. When Is Heavy Gas Modeling Necessary? Wayne DavisURS Safety Mgmt Solutionswayne.davis@wsms.com803.502.9789

2. Background � EM Letter on 1189 April 2009, memo from DOE-EM (Owendoff) on implementation of DOE-STD-1189 Chemical Hazards Issue: STD-1189 discusses ?/Q for chemical releases to onsite (100m) receptor; but doesn�t provide a value. Several approaches to estimate dispersion for collocated workers: use ?/Q in STD-1189 or derive ?/Q using EM Interim Guidance (95th % met with 3 cm surface roughness). EM Policy: use ?/Q in STD-1189 for 100m rad dose for chemical releases for all events, except high wind events or heavy gas... For heavy gas events, dispersion will be determined using DOE Toolbox code with heavy gas model such as ALOHA...

3. Background � Questions from the field How do I implement EM Policy? How can I tell if a gas is a heavy gas? Can I calculate a heavy gas X/Q? My response: Issue not well understood by those providing direction. Irrespective of gas density, unlikely we should model it as heavy gas. Dense gas effects dominate only in immediate vicinity. By the time a plume reaches a downwind distance of 100 m, it should likely be modeled as Gaussian.

4. When Does Each Model Apply? Heavy Gas Gravity is primary dispersal mechanism Gaussian Atmospheric turbulence is primary dispersal mechanism It may be non-conservative (and inaccurate) to model release of gas that�s heavier than air as a heavy gas.

5. Heavy Gas Release Initially behaves differently from neutrally buoyant. Sinks, because heavier than air. As it moves downwind, gravity causes spread; some vapor can even travel upwind. Farther downwind, cloud becomes more dilute; behaves like neutrally buoyant gas. When concentration < 1% (10,000 ppm) For small releases, likely occurs in first few meters For large releases, may happen further downwind

6. Comparison of Gaussian & Heavy Gas Threat Zones (Typical) When atmosphere is most unstable (A & B), heavy gas threat zones are longer than Gaussian. Under neutral conditions (C & D), heavy gas & Gaussian threat zones will be similar in length. When atmosphere is most stable (E & F), Gaussian threat zones are longer than heavy gas.

7. Test 1 � Chlorine Release PAC-2 (AEGL-2) is 2 ppm. Conservative assumptions (F & 1 m/s, 3 cm surface roughness) 1st allowed ALOHA to choose model ? heavy gas 2nd forced Gaussian Comparison of results (attached): Heavy Gas: Max distance to PAC-2 = 563 m Gaussian: Max distance to PAC-2 = 747 m Concentration at this distance is only 2 ppm - it would certainly not behave as heavy gas at this low concentration.