Applicability of a mobile in situ gamma detector with different scan speeds for evaluating hotspots in decommissioning sites

Abstract

In this work, scanning is performed in relation to the scan speed for evaluating the applicability of a mobile in situ gamma detector at a decommissioning site, based on the Multi-Agency Radiation Survey and Site Investigation Manual, NUREG-1575. Direct measurements and scanning are conducted under high- and low-concentration hotspot conditions. The high-concentration hotspot fails to meet the release criterion, whereas the low-concentration hotspot complies with the release criterion, despite exceeding DCGLW, and satisfies the elevated measurement comparison test. The maximum and average dose rates measured via scanning in both hotspot conditions are distinguishable from the background dose rate, thus enabling the identification of potential hotspots. In the low-concentration hotspot, photopeak counts appear during real-time scanning; the scanning efficiency exhibited inconsistent trends with respect to the scan speed and hotspot conditions. This study verifies that, at scan speeds less than 18 km/h, the mobile in-situ gamma detector can be applied to identify the hotspots at decommissioning sites and further suggests that direct measurements should also be performed for the quantification of contamination.