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2.4.2 Exposure
Off-site doses are evaluated for the most exposed individual (MEI). This is a hypothetical
individual located at the closest point on the site boundary (or at off-site distance of maximum
air concentration for elevated releases).
2.4.3 Meteorological Dispersion
Site-specific 5% weather conditions (i.e., stability class and wind speed more unfavorable
than 95% of the expected weather) without regard to wind direction, defined by at least 1 yr of
weather data, should be used for diffusive transport to downwind receptors. Alternatively, site-
specific climatological studies using actual measurements of diffusion/dilution characteristics
under representative meteorological conditions can be used as a basis for determining site-
specific dilution factors (χ/Qs) (see NOAA 1989 for example). These weather conditions should
be determined using the anticipated release height of the accident cloud (e.g., ground level or
elevated). For evaporating chemicals, a range of stability class/wind speed combinations should
be examined due to the chemical-specific effects of these parameters on source emission rates
and downwind dispersion. A dense gas model may need to be used for evaluation of impacts at
near-field receptor distances if the chemical/air mixture density at the source exceeds the ambi-
ent air density by 50%. Dense gas effects are usually insignificant at far-field receptor distances.
2.5 Consequence Thresholds for PAGs and Emergency Response Planning Guidelines
As stated in DOE-STD-6002-96, the Fusion Radiological Release Requirement for off-
normal events at fusion facilities is that no events result in a public exposure greater than
10 mSv (1 rem). If the projected early dose to the surrounding population can be shown to be
less than 10 mSv, then no public protective action planning would be necessary.
In determining whether a given event at a fusion facility will require protective action, best
estimate meteorology and system operation are assumed. All estimates of the site-specific
transport coefficients/Q) are based on at least 1 yr of meteorological data. Best-estimate
meteorology can be used to determine public exposures in three ways:
a. Use the annual average windspeed and the highest-frequency stability conditions in
determining the χ/Q at the site boundary.
b. Calculate the hourly χ/Q for meteorological conditions throughout the year. Select the
50 percentile χ/Q to determine off-site transport.
c. Using meteorological data for at least 1 yr, use a Monte Carlo technique to select
random starting times for the off-normal event. Average the public exposure due to
each of the transients to obtain the best-estimate off-site doses.
Because of differences among the mean, median, and mode of the χ/Q distributions
through the year, the preferred method is "c" above. Further guidance in the application of best
estimate off-site dose calculation can be found in NUREG-0654/FEMA-REP-1.

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