Tuesday, December 11, 2007

Top Screen Questions: Toxic Release Chemicals of Interest

The Top Screen on-line questions are divided up into different groups corresponding to the different types of chemicals and the hazards associated with them. While the instruction manual for the Top Screen has not yet been completed or issued, a copy of the CSAT Top-Screen Questions is available on-line. The first chemical group in the Top Screen is the Toxic Release Chemicals. These are toxic chemicals that if released on site would present a serious off-site consequence in the event of a terrorist attack.


The first portion of this area of the Top Screen is identifying which chemicals the facility has (or has had in the last 60 days) on site at or above the STQ. The chemicals are listed alphabetically by chemical name. For each chemical there is a CAS number listed to aid in identification of the chemical of interest. There is a minimum concentration that must be equaled or exceeded for the chemical to be counted (only the actual amount of the chemical in a mixture not the entire mixture) in the STQ calculations. Finally there are check boxes to indicate if the listed chemical was present at or above the STQ. The default value is “No”, so the only chemicals that need to actually be marked are those that meet or exceed the STQ.


At the end of this check off list are two questions with appropriate response boxes. The first is the same as the one found at the end of each group of chemicals on the Top Screen; essentially have you marked every chemical that should have been marked; responses are yes or no, with the “No” box the default value. Even if no Toxic Release Chemicals of Interest were present on the facility at or above the STQ within the last 60 days a yes response is required to this question to signify that list has been evaluated. The second question deals with the topography surrounding the facility. If the area is open with few obstructions (buildings, etc) then the “Rural” box will be marked, otherwise the “Urban” box will be selected.


The next area of the Toxic Release COI section of the Top Screen deals with the downwind hazard distance for toxic chemicals on site. This data is again organized as a list of the Toxic Release chemicals by name with the CAS number, Minimum Concentration and STQ listed for each chemical. There are then two blocks for each listed chemical.


The first block is the “Total On-Site Quantity” box where the maximum inventory in the last sixty days is recorded. If there are on-site storage tanks (including movable tanks like rail cars) the maximum inventory includes what could have been held in those tanks if they were full (unless there are administrative controls that limit the amount kept on hand, in which case that maximum amount is used). If the chemical is part of a mixture, only the actual amount in the mixture will be included. The total amount is rounded to two significant digits (7625 lbs is rounded to 7600 lbs for example). This is the quantity that will be used in subsequent calculations for that chemical.


The second block is the “Distance of Concern” block and is calculated with an Environmental Protection Agency program (RMP*Comp) that can be downloaded for free from the EPA. This is the downwind distance that there can be expected to be a dangerous concentration of the chemical in the event of a successful terrorist attack. There are three differences in the way this program is used for the Top Screen as opposed to the EPA RMP calculations that this program was developed for. For Top Screen use:


  1. Always use the “Worst Case” scenario, and
  2. The “Total On-Site Quantity” is used for the Quantity Released, and
  3. Passive mitigation measures are not to be taken into account in the calculation.


<PCLASS=MSONORMAL style="MARGIN: 0in 0in 0pt">When reporting the results from the RMP*Comp program in the Top Screen any results of less than 0.1 mile will be reported as 0.1 mile and results in excess of 25 miles will be reported as 25 miles.


The last area of the Toxic Release COI section of the Top Screen deals with the largest amount of those chemicals stored in the Area of Highest Quantity (AHQ). This is used to distinguish between the threat from chemicals stored in a concentrated area as opposed to more dispersed chemicals (dispersed chemicals are a harder target for terrorists to successfully attack). To determine where the AHQ is a circle with a radius of 170 feet is drawn that will encompass the largest amount of storage for that chemical.


The total amount of chemical stored within that circle (maximum capacity for storage tanks within that circle unless they are volume limited by administrative controls) is reported in the “Quantity in AHQ” block. That quantity is then used to calculate the “Distance of Concern” for the AHQ. The RMP*Comp is used for that calculation in the same way that the Total On-Site Quantity distance of concern was calculated.

Every chemical that had a “Yes” block marked in the first area of the Toxic Release COI (signifying present at or above the STQ) must have an amount and distance entered in the second (Total Quantity) and third area (AHQ) of this portion of the Top Screen. It should be noted that due to the way capacity is calculated that the Total Quantity may be larger than the amount used to calculate if the STQ has been met or exceeded.

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