Failure Of A Priori Scan MDCs To Predict Field Performance
J. Shonka, R. Burns, R. Burmeister, D. Debord, M. Marcial, J. Kelley
Often, licensees complete a final status survey and submit license termination documentation only to fail an independent verification survey when previously unknown residual activity is found. One cause of such failures is inadequate scan surveys that occur when a priori calculations of minimum detectable concentration (MDC) are improperly equated with expected field performance. The a priori MDC for scan surveys, also called the "scan MDC", has been studied by many groups. These previous evaluations have treated variables such as efficiency and scan rate as constants or normally distributed variables with known uncertainties. There have only been a few attempts to performance test scan MDCs in the field. All of these tests have been of short duration, and have not addressed fatigue on the part of the surveyor. We have measured many of these variables and have found significant differences from the commonly employed assumptions concerning both their magnitude and uncertainty. Measurements of scan speed for nine users were made under ideal conditions with a wheel encoder attached to a commercially available floor monitor. Measurements of efficiency under field conditions were also made. These data were used to propagate the detection limit for scan surveys via Latin Hypercube sampling. Differences between the true scan MDC and the calculated a priori scan MDC show that the confidence at which a given amount of radioactivity can be determined is greatly overstated in virtually all field measurements made today.
This abstract was presented at the 35th Annual Midyear Meeting, "Decommissioning and Environmental Restoration", Instrumentation Session, 2/17/2002 - 2/20/2002, held in Orlando, FL.