PNNL

Abstract

Increased continuous data collection using automated data loggers and autonomous radiological survey devices or vehicles has introduced a need for corresponding guidance and statistical techniques for data that are collected without surveyor vigilance. This report presents a method for calculating the a priori scan minimum detectable concentrations (MDCs) for surveys performed without vigilance similar to methods described in the Multi-Agency Radiation Survey and Site Investigation Manual, NUREG-1507, and NUREG/CR-6364. A priori scan MDCs are calculated during survey planning to ensure that survey parameters (e.g., scanning speed, scanning altitude, detector geometry) will lead to collecting data in which potentially contaminated areas can be detected when data are processed after the survey, within acceptable statistical error probabilities. The a priori scan MDC calculations detailed in NUREG-1507 assume surveyor vigilance (i.e., pausing or stopping to investigate further when audio click data from a ratemeter indicates potential areas of concern). MARSSIM explains that ratemeters are the most common recording or display device used for portable radiation measurement systems; providing a display that represents the number of events occurring over a period (e.g., counts per minute [cpm]). Whereas the number of events can be accumulated over a period using a digital scaling device, resulting in information about the total number of events that occurred over a fixed period, ratemeter displays vary over time to provide short-term averages. (NRC 2020) This report expands upon NUREG-1507 to provide a priori scan MDC calculations assuming surveys will be completed without vigilance based on audio click data, providing an incremental advance in the a priori scan MDC methodology by moving from a with-vigilance to a without-vigilance surveying paradigm. A case study is provided to demonstrate the a priori scan MDC calculation. It is known, however, that there are differences between audio click used data in surveying with vigilance and data collected by the ratemeter used when surveys are conducted without vigilance. The magnitude of differences between the audio and ratemeter data, and their subsequent impacts on scan MDC calculations is yet unknown, to our knowledge. Future research and development will be required to assess such differences and determine the efficacy and applicability of the method developed in this report when applied to logged ratemeter data rather than audio data. Varying background radiation levels have also posed challenges for scanning surveys. When surveys are conducted with vigilance, implicit data processing takes place in real time while surveyors notice gradual changes in audible background. When surveys are conducted without vigilance, new methods are required for the purposes of analyzing continuously collected survey data. The lag-k method is well-suited to a varying background. It performs hypothesis testing by treating survey data as a linear time series and assessing differences between k consecutively collected data points before and after each observation to account for potentially changing background. Simulation studies show that the lag-k method is superior to traditional approaches in terms of meeting acceptable error probabilities when optimal values of k are selected and contamination is localized (as opposed to wide-spread). Several case studies are provided to demonstrate the lag-k method.