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3.10.6 Example of statistical data processing: Small areas of elevated activity

The guidance for the development of release criteria (or Derived Concentration Guideline Level – DCGL) is presented in Section 3.3.6, and will use in general exposure pathway models which in turn assume a relatively uniform distribution of contamination. While this represents an ideal situation, small areas of elevated activity are a concern at many sites.

EURSSEM [USNRC-2002] addresses the concern for small areas of elevated activity by using a simple comparison to an investigation level as an alternative to statistical methods. Using the elevated measurement comparison (EMC), which represents a conservative approach, requires that every measurement needs to be below the specified action level. The investigation level for this comparison is called the DCGLEMC, which is the DCGLW modified to account for the smaller area by an area factor.

This area factor correction (discussed in Section 3.5) is considered to be a defensible modification because the exposure assumptions (e.g., exposure time and duration) are the same as those used to develop the DCGLW. In the case of multiple areas of elevated activity in a survey unit, a posting plot (discussed in Section 3.10.8.3) or similar representation of the distribution of activity in the survey unit can be used to determine any pattern in the location of these areas.

If elevated levels of residual radioactivity are found in an isolated area, in addition to residual radioactivity distributed relatively uniformly across the survey unit, the unity rule (see Section 3.3.6.3) can be used to ensure that the total dose or risk meets the release criterion. If there is more than one of these areas, a separate term should be included in the calculation for each area of elevated activity. As an alternative to the unity rule, the dose or risk due to the actual residual radioactivity distribution can be calculated if there is an appropriate exposure pathway model available. Note that these considerations generally only apply to Class 1 survey units, since areas of elevated activity should not be present in Class 2 or Class 3 survey units.