Index > 3 Characterisation of radioactively contaminated sites >

3.5.3 Determining data points for small areas of elevated activity

The statistical tests described above (also see Section 3.10 and Appendix B) evaluate whether or not the residual radioactivity in an area exceeds the DCGLW for contamination conditions that are approximately uniform across the survey unit. In addition, there should be a reasonable level of assurance that any small areas of elevated residual radioactivity that could be significant relative to the DCGLEMC are not missed during the final status survey. The statistical tests introduced in the previous sections may not successfully detect small areas of elevated contamination. Instead, systematic measurements and sampling, in conjunction with surface scanning, are used to obtain adequate assurance that small areas of elevated radioactivity will still satisfy the release criterion or the DCGLEMC. The procedure is applicable for all radio-nuclides, regardless of whether or not they are present in background, and is implemented for survey units classified as Class 1.

The number of survey data points needed for the statistical tests discussed in the above alineas dealing with ‘Determining numbers of data points for areas with and without contaminant present in background’ is identified (the appropriate section depends on whether the contaminant is present in background or not). These data points are then positioned throughout the survey unit by first randomly selecting a start point and establishing a systematic pattern. This systematic sampling grid may be either triangular or square. The triangular grid is generally more efficient for locating small areas of elevated activity. Appendix E includes a brief discussion on the efficiency of triangular and square grids for locating areas of elevated activity.

The number of calculated survey locations, n, is used to determine the grid spacing, L, of the systematic sampling pattern (see Section 3.5.1). The grid area that is bounded by these survey locations is given by A = 0.866 × L2 for a triangular grid and A = L2 for a square grid. The risk of not sampling a circular area – equal to A – of elevated activity by use of a random-start grid pattern is illustrated in Figure B.5 in Appendix B.

One method for determining values for the DCGLEMC is to modify the DCGLW using a correction factor that accounts for the difference in area and the resulting change in dose or risk. The area factor is the magnitude by which the concentration within the small area of elevated activity can exceed DCGLW while maintaining compliance with the release criterion. The area factor is determined based on specific regulatory agency guidance.

Table 3.25 and Table 3.26 [USNRC-2002], [EPA-1996c], [USNRC-1999] provide examples of area factors generated using exposure pathway models. For each radionuclide, all exposure pathways were calculated assuming a concentration of 37 Bq/kg for outdoor and assuming a concentration of 37 Bq/m2. The EURSSEM user should consult with the responsible regulatory agency for guidance on acceptable techniques to determine area factors.

The minimum detectable concentration (MDC) of the scan procedure – needed to detect an area of elevated activity at the limit determined by the area factor – is calculated as follows:

Scan MDC (required) = (DCGLW) × (Area Factor) ……………………………… (3-18)

The actual MDC’s of scanning techniques are then determined for the available instrumentation (see Section 3.3.7.5 and Section 3.3.7.6). The actual MDC of the selected scanning technique is compared to the required scan MDC. If the actual scan MDC is less than the required scan MDC, no additional sampling points are necessary for assessment of small areas of elevated activity. In other words, the scanning technique exhibits adequate sensitivity to detect small areas of elevated activity.
.

Nuclide Area Factor
1 m2 3 m2 10 m2 30 m2 100 m2 300 m2 1000 m2 3000 m2 10000 m2
241Am 208.7 139.7 96.3 44.2 13.4 4.4 1.3 1.0 1.0
60Co 9.8 4.4 2.1 1.5 1.2 1.1 1.1 1.0 1.0
137Cs 11.0 5.0 2.4 1.7 1.4 1.3 1.1 1.1 1.0
63Ni 1175.2 463.7 154.8 54.2 16.6 5.6 1.7 1.5 1.0
226Ra 54.8 21.3 7.8 3.2 1.1 1.1 1.0 1.0 1.0
232Th 12.5 6.2 3.2 2.3 1.8 1.5 1.1 1.0 1.0
238U 30.6 18.3 11.1 8.4 6.7 4.4 1.3 1.0 1.0

Table 3.38 Illustrative examples of outdoor area dose factors. The values listed in Table 3.38 are for illustrative purposes only. Consult regulatory guidance to determine area factors to be used for compliance demonstration.
.

Nuclide Area Factor
1 m2 4 m2 9 m2 16 m2 25 m2 36 m2
241Am 36.0 9.0 4.0 2.2 1.4 1.0
60Co 9.2 3.1 1.9 1.4 1.2 1.0
137Cs 9.4 3.2 1.9 1.4 1.2 1.0
63Ni 36.0 9.0 4.0 2.3 1.4 1.0
226Ra 18.1 5.5 2.9 1.9 1.3 1.0
232Th 36.0 9.0 4.0 2.2 1.4 1.0
238U 35.7 9.0 4.0 2.2 1.4 1.0

Table 3.39 Illustrative examples of indoor area dose factors. The values listed in Table 3.39 are for illustrative purposes only. Consult regulatory guidance to determine area factors to be used for compliance demonstration.
.

If the actual scan MDC is greater than the required scan MDC (i.e., the available scan sensitivity is not sufficient to detect small areas of elevated activity), then it is necessary to calculate the area factor that corresponds to the actual scan MDC:

Area Factor = scan MDC (actual) / DCGL ………………………………………. (3-19)

The size of the area of elevated activity (in m2) that corresponds to this area factor is then obtained from specific regulatory agency guidance, and may be similar to those illustrated Table 3.38 or Table 3.39. The data needs for assessing small areas of elevated activity can then be determined by dividing the area of elevated activity acceptable to the regulatory agency into the survey unit area. For example, if the area of elevated activity is 100 m2 (from Table 3.38) and the survey unit area is 2,000 m2, then the calculated number of survey locations is 20. The calculated number of survey locations, nEA, is used to determine a revised spacing, L, of the systematic pattern (refer to Section 3.5.1). Specifically, the spacing, L, of the pattern (when driven by the areas of elevated activity) is given by:

L = √(A / (0.866 nEA)) for a triangular grid …………………………………. (3-20)

L = √(A / nEA) for a square grid ……………………………………………. (3-21)

where A is the area of the survey unit. Grid spacings should generally be rounded down to the nearest distance that can be conveniently measured in the field.

If the number of data points required to identify areas of elevated activity (nEA) is greater than the number of data points calculated using Equation 3-16 (N/2) or Equation 3-17 (N), L should be calculated using Equation 3-20 or Equation 3-21. This value of L is then used to determine the measurement locations as described in Section 3.5.1. If nEA is smaller than N/2 or N, L is calculated using Equation 3-22 or Equation 3-23 as described in Section 3.5.1. The statistical tests are performed using this larger number of data points. If residual radioactivity is found in an isolated area of elevated activity – in addition to residual radioactivity distributed relatively uniformly across the survey unit – the unity rule (described in Section 3.3.6.3) can be used to ensure that the total dose or risk does not exceed the release criterion (see Section 3.10.8.8). If there is more than one elevated area, a separate term should be included for each. 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 apply only to Class 1 survey units, since areas of elevated activity should not exist in Class 2 or Class 3 survey units.

When the detection limit of the scanning technique is very large relative to the DCGLEMC, the number of measurements estimated to demonstrate compliance using the statistical tests may become unreasonably large. In this situation perform an evaluation of the survey objectives and considerations. These considerations may include the survey design and measurement methodology, exposure pathway modeling assumptions and parameter values used to determine the DCGLs, historical site assessment conclusions concerning source terms and radio-nuclide distributions, and the results of scoping and characterization surveys. In most cases the result of this evaluation is not expected to justify an unreasonably large number of measurements.

Example 3.16: Determining data points for small areas of elevated activity; Class 1 area potentially contaminated with 60Co (example 1)

A Class 1 land area survey unit of 1,500 m2 is potentially contaminated with 60Co. The DCGLW value for 60Co is 110 Bq/kg and the scan sensitivity for this radio-nuclide has been determined to be 150 Bq/kg. Calculations indicate the number of data points needed for statistical testing is 27. The distance between measurement locations for this number of data points and the given land area is 8 m. The area encompassed by a triangular sampling pattern of 8 m is approximately 55.4 m2. From Table 3.38 an area factor of about 1.4 is determined by interpolation. The acceptable concentration in a 55.4 m2 area is therefore 160 Bq/kg (1.4 × 110 Bq/kg). Since the scan sensitivity of the procedure to be used is less than the DCGLW times the area factor, no additional data points are needed to demonstrate compliance with the elevated measurement comparison criteria.

Example 3.17: Determining data points for small areas of elevated activity; Class 1 area contaminated with 60Co (example 2)

A Class 1 land area survey unit of 1500 m2 is potentially contaminated with 60Co. The DCGL for 60Co is 110 Bq/kg. In contrast to Example 1, the scan sensitivity for this radio-nuclide has been determined to be 170 Bq/kg. Calculations indicate the number of data points needed for statistical testing is 15. The distance between measurement locations for this number of data points and land area is 10 m. The area encompassed by a triangular sampling pattern of 10 m is approximately 86.6 m2. From Table 3.38 an area factor of about 1.3 is determined by interpolation. The acceptable concentration in a 86.6 m2 area is therefore 140 Bq/kg (1.3 × 110 Bq/kg). Since the scan sensitivity of the procedure to be used is greater than the DCGLW times the area factor, the data points obtained for the statistical testing may not be sufficient to demonstrate compliance using the elevated measurement comparison. The area multiplier for elevated activity that would have to be achieved is 1.5 (170/110 Bq/kg). This is equivalent to an area of 30 m2 (Table 3.38) which would be obtained with a spacing of about 6 m. A triangular pattern of 6 m spacing includes 50 data points, so 50 measurements should be performed in the survey unit.