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
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.