Conceptual Site Model - Argonne National Laboratory

WVDP Phase 1 CSAP11.2 Detector TechnologiesThe Phase 1 DP identified 18 ROIs. Some of the 18 are readily detectable by gross activitygamma surveys, and others are not. Two radionuclides are known to have relatively widespreadenvironmental impacts on-site: Cs-137 and Sr-90. Cs-137 is readily detectable at its CG levels,but Sr-90 is not. There are areas of the WVDP premises where one or the other of theseradionuclides exists in isolation. Examples include the northern border of the premises wheresurface contamination is likely only Cs-137 and its progeny (associated with the Cs-137 prongthat extends to the north), and the North Plateau Groundwater Plume extending underneath WMA4, where subsurface contamination is likely only Sr-90 and its progeny. There are other areas ofthe WVDP premises where these two radionuclides are likely both present in varying ratios. Inthese instances they may also be commingled with one or more of the other 16 ROIs.Field survey instruments for soil contamination are generally limited to those that can detectphotons (gamma ray and X-ray), given the uneven terrain and conditions encountered in the field.This is in contrast to survey instruments that can be used for buildings, many of which allow forthe detection of alpha and beta contamination as well as photon emissions.Four ROIs have no or minimal photon emissions, making them impractical to detect with fieldscanning instruments. The other 14 ROIs are potentially detectable by a field NaI detector.Table 5 provides estimates of minimum detectable concentrations (MDCs) for the 14 ROIs thatare potentially detectable by a NaI detector, along with the type of NaI detector expected toprovide the lowest MDC.As indicated by Table 5, a thin-crystal NaI detector, known as a FIDLER, provides the bestperformance for the greatest number of ROIs. Consequently, all GWSs conducted as part of theCSAP will make use of a FIDLER.In the case of Cs-137, MDC performance is better with a 2 in. × 2 in. NaI detector. The FIDLERis relatively less sensitive to the high-energy Cs-137 photons; however, a FIDLER is moresensitive to the low-energy Compton scattering associated with Cs-137 soil contamination.Consequently, the expectation is that a FIDLER will also provide a sufficiently low MDC for Cs-137. To test this assumption, early in the CSAP data collection process 2 in. × 2 in. NaI detectorRev. 1 81