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Technical Questions |
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Other Questions/Comments
QuestionsThe DCGL (Derived Concentration Guidance Level)What is a Derived Concentration Guideline Level (DCGL)? The MDC (Minimum Detectable Concentration)What is the Minimum Detectable Concentration (MDC)? The LBGR (Lower Bound of the Grey Region)
Double Sampling
Size of the Survey Unit
AnswersThe DCGL (Derived Concentration Guideline
Level)
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Chapter 2: Overview of the Radiation Survey and Site Investigation Process (Section 2.2) |
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Methods for deriving DCGLs are outside the scope of MARSSIM. Consult the appropriate regulatory agency personnel or documents for methods used to develop DCGL values.
Appendix L: Regional Radiation Program Managers
The answer depends upon a number of factors. In the most general terms, with all other factors being equal, the number of measurements varies inversely with the DCGL.
In reality, a change in the DCGL reflects a change in the assumptions used to translate dose or risk into concentration. This could affect the survey design in several ways. For example, changing the area of radioactivity in the exposure pathway model would change the size of survey units specified in the survey design. In another example, changes in the depth of radioactivity assumed by the model would change the sample collection procedures and scan sensitivity required for the final status survey design. In these cases it is difficult to predict what exact affect the DCGL will have on the survey design.
Controlling the number of measurements is the key to efficient surveys, which make the best use of limited resources. MARSSIM allows you to examine the factors that drive the number of measurements required:
- DCGL
- radionuclide concentration variability in the survey unit and background
- tolerable decision error rates
- identifying elevated areas.
Once you identify the reason the survey design recommends a specific number of measurements, you can determine what, if any, changes to the survey design are appropriate.
Chapter 5:Survey Planning and Design
The MDC is the net concentration that has a specified chance of being detected. It is an estimate of the detection capability of a measuring protocol and is calculated before measurements are taken.
The detection limit is the lowest net response level, in counts, that you expect to be see with a fixed level of certainty, customarily 95%. The MDC is the detection limit expressed as an activity concentration. If the activity concentration in a sample is equal to the MDC, then there is a 95% chance that radioactive material in the sample will be detected.
You can calculate the MDC for an instrument by considering the background counts during a typical measurement, total detection efficiency, conversion factors, and the probe area.
Variability in the calculated MDC reflects natural variability in the detection efficiency and conversion factors. This variability may or may not be significant. For the MDC to be applicable, the sample or field measurement conditions must match the conditions under which the background was measured.
The concept of the scan MDC is especially important in MARSSIM. A scan MDC is an MDC calculated for an instrument that takes continuous or "scanning" measurements. A scanning instrument can often take more measurements for less cost and in less time than a non-scanning instrument. However, it is important that the scan MDC obtained by the scanning procedures used will actually detect the required DCGL in the field.
If the scan MDC is adequate to detect the DCGLEMC, it can be used to greatly reduce or eliminate the possibility of missing an area of elevated concentration.
Missing an elevated measurement area may cause release of a survey unit that exceeds the dose criteria.
The LBGR (Lower Bound of the Grey Region) is a concentration. It is less than the DCGLW and is chosen to be easily distinguishable from the DCGLW. You apply a statistical test to the data to determine whether the true concentration in the survey unit is above the DCGLW or below the LBGR. You are more likely to make an inaccurate decision if the true concentration of the survey unit is between the LBGR and the DCGLW. Thus, it is called the "grey region" because in that case, the decision is usually neither "black" nor "white." See the figure below for a graphic depiction of the LBGR.
An Example Decision Rule for the Final Status Survey
You need the LBGR to calculate the number of data points, N, which is used to test whether the survey unit concentration is less than the DCGLW. You must set a value for the LBGR to calculate the shift, (shift = DCGLW - LBGR =
). The shift is then used to calculate the relative shift,
, which is an intermediate step necessary to calculate N.
Choosing the LBGR is part of an iterative process used to determine the number of data points, N, needed. You can set the LBGR at the median residual radioactivity concentration believed to be remaining in the survey unit. If the true, but unknown, concentration in the survey unit is in the grey region, you will have difficulty in determining if the survey unit concentration is less than the DCGLW. You should take into account the following when choosing the LBGR:
- variation of the concentrations in the survey unit (determined from prior surveys of the survey unit)
- variation in the measurements due to instrumentation at the candidate values for the LBGR
- possibility a survey unit could fail even though its average concentration is, in fact, less that the DCGLW
- costs of measurements at the sensitivities needed to measure the candidate values for the LBGR
You can optimize the trade-offs between increased instrument sensitivity, costs, and the number of data points needed by setting the LBGR so that you get a relative shift greater than one and less than three.
The relative shift expresses the width of the grey region, or shift,
When no other information is available, MARSSIM suggests a default value for the LBGR equal to ˝ the DCGLW. Then you can begin the iterative process described in How do I choose the LBGR?
You should choose a preliminary value of the LBGR before remediation and a final value of the LBGR as you complete the Final Status Survey design. There are a number of factors that determine the final value of the LBGR. In some cases, you will want to remediate to concentrations sufficiently below the DCGLW, so you can demonstrate the survey unit meets the release criterion.
Double sampling is taking a second set of samples in a one-stage survey, because the retrospective power of the test did not meet design objectives. At the same time, double sampling causes the Type I error rate to exceed the rate specified for the one-stage survey.
Before the initial round of sampling takes place, DQOs should mention any allowances for double sampling and be approved by the appropriate regulators. During the DQO process, double sampling could be considered as an option to setting the Type I error rates.
Double Sampling. Draft NRC NUREG-1757, vol. 2, Appendix C addresses the provision of collecting additional samples if the initial FSS sample size fails to demonstrate compliance with the release criterion. Page C-6 states “…double sampling should not be used as a substitute for adequate planning. If it is to be allowed, this should be agreed upon with NRC staff as part of the DQO process.” Please refer to the NUREG for more details. Additionally, Dr. Carl Gogolak has prepared a paper titled "Use of Two-Stage or Double Sampling in Final Status Decommissioning Surveys" that provides further detail on this topic.
When the retrospective power of a set of samples is below the required design objectives, double sampling can raise it. Insufficient retrospective power can occur for a number of reasons, most commonly because:
- The spatial variability in residual radioactivity concentrations is larger than anticipated.
- Samples were lost, did not pass analytical QA/QC, or were otherwise unavailable for inclusion in the analysis.
MARSSIM discourages double sampling. The DQO process, which MARSSIM uses, explicitly sets objectives for both the Type II error rate and the retrospective power during the design process. Adequate initial sampling to achieve the desired power makes decisions based on the data more objective and defensible. A better solution to the issue of double sampling is to plan for data collection in two stages, and design the final status survey accordingly.
If your regulator allows double sampling, it should be decided upon during the DQO process. You should both agree upon the number of samples allowed in the second set of samples, because the Type I error rate could be as much as double the error rate for a single set of samples.
Class 2 or Class 3 survey units are not appropriate for double sampling. Concentrations in these survey units should not exceed the DCGLW, and should always pass with the first set of samples. The need for a second set of samples in Class 2 or Class 3 survey units raises the issue of misclassification. Double sampling is also generally not appropriate for Class 1 survey units having confirmed areas of elevated activity, or "hot spots."
The survey unit sizes in MARSSIM are not intended to be prescriptive. However, MARSSIM does offer one possible set of survey unit sizes, primarily as an example. For Class 1 survey units, MARSSIM suggests survey unit sizes of 100 m2 for structures and 2,000 m2 for land areas. For Class 2 survey units, MARSSIM suggests survey unit sizes between 100 and 1,000 m2 for structures, and between 2,000 and 10,000 m2 for land areas. However, these survey unit sizes are not intended to be prescriptive. For Class 3 survey units, MARSSIM does not suggest a limit for either structure or land areas. Section 4.6 of MARSSIM recommends limiting survey units based on classification, exposure pathway modeling assumptions, and site-specific conditions.
Chapter 4: Preliminary Survey Considerations (Section 4.6)
Yes. The survey unit size should correspond to the model assumptions used to establish the DCGLW for the survey unit. As always, it is important to document the rationale for the assumptions and to consult with your regulator.
If the DCGLW was derived by environmental pathway modeling, then as the survey unit size changes in the model, the DCGLW may change too. Several factors affect this relationship, including the radionuclides of concern, the potential exposure pathways, and the uncertainties inherent in the model. Therefore, when considering changing the survey unit size, it is best to work with an experienced environmental pathway modeler to fully account for all the complexities inherent at your site. These changes may or may not affect the survey design. These changes may or may not affect the survey design. It is important to talk with your regulator to determine if the resulting changes have significant impact on the DCGLW previously agreed upon with your regulator.
In general, when you use a survey unit size smaller than the modeled survey unit size, it is possible that you could take more measurements than necessary using the same the DCGLW. However, if the size is not significantly smaller, it will usually be simpler to make the number of measurements calculated, rather than re-deriving a new DCGLW with new modeling. If the survey unit is larger than what was modeled, then you should divide the survey unit into sizes that conform to the model. Alternatively, you may re-calculate the DCGLW to conform with the larger survey unit size by inserting the larger survey unit size into the model. In either case, approval from the regulator is recommended before making changes to survey unit sizes.
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