Atmospheric Dispersion - doe-std-3009-94_cn3

DOE-STD -3009-94

Appendix A

designation, and DOE's safety management programs, address onsite safety.

However, an annual assessment of any changes in the site boundary and

potent ial effects on safety SSC classification should be performed in

association with the required annual update of the DSA for a facility.

Privatization and site turnover initiatives may affect these determinations.

ATMOSPHERIC DISPERSION. The 95th percentile of the distribution of

doses to the MOI, accounting for variations in distance to the site boundary as

a function of direction, is the comparison point for assessment against the EG.

The method used should be consistent with the statistical treatment of

calculated X/Q values described in regulatory position 3 of NRC Regulatory

Guide 1.145 for the evaluation of consequences along the exclusion area

boundary. The determination of distance to the site boundary should be made

in accordance with the procedure outline in position 1.2 of Regulatory Guide

1.145. NRC Regulatory Guide 1.23 describes acceptable means of generating

the meteorological data upon which dispersion is based. Accident

phenomenology may be modeled assuming straight- line Gaussian dispersion

characteristics, applying meteorological data representing a 1- hour average for

the duration of the accident. Accident duration is defined in terms of plume

passage at the location of dose calculation, for a period not to exceed 8 hours.

Prolonged effects, such as resuspension, need not be modeled. The accident

progression should not be defined so that the MOI is not substantially exposed

(i.e., using a release rate that is specifically intended to expose the MOI to only

a small fraction of the total material released, or defining time and windspeed

so that the plume has not reached the MOI). The exposure period begins from

the time the plume reaches the MOI.

For ground releases, the calculated dose equates to the centerline dose at the

site boundary. For elevated, thermally buoyant, or jet releases, plume

touchdown may occur beyond the site boundary. As noted in the discussion of

receptor location, these cases should locate the dose calculation at the point of

maximum dose beyond the site boundary, which is typically at the point of

plume touchdown.

Accidents with unique dispersion characteristics, such as explosions, may be

modeled using phenomenon-specific codes more accurately representing the

release conditions. Discussion should be provided justifying the

appropriateness of the model to the specific situation. For accident phenomena

defined by weather extremes, actual meteorological conditions associated with

the phenomena may be used for comparison to the EG.

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