Techniques employed for detection of hot particles in the marine environment

Abstract

During the decommissioning of the Maine Yankee nuclear plant, several methods weredeveloped and employed to survey for hot particles in the marine environmentsurrounding the site. The methods used and the sensitivities achieved in the search forenvironmentally dispersed particles during the various decommissioning activitiesperformed are described in detail. Surveys were performed on dry soil, exposed marinesediment and submerged marine sediment. Survey techniques ranged from the use ofthe basic NaI detector coupled to a count rate meter to an intrinsic germaniumdetector deployed in a submarine housing coupled to a multi-channel analyser.The initial surveys consisted of collecting samples of marine sediment, spreading them out over a1 m2 surface in a thin layer, and scanning the deposited sediment by hand using a 5 cm by 5 cmNaI detector coupled to a standard count rate meter. This technique was later replaced bywalkover scans with the 5 cm by 5 cm NaI detector moved in a serpentine pattern overthe sediment surface. By coupling the detector to a ‘smart meter’, an alarm setpoint could be used to alert the surveyor to the presence of a particle within theinstrument’s field of view. A similar technique, with the detector mounted in awatertight housing secured to the end of a pole, was also employed to scan underwaterlocations. The most sensitive method developed for performing underwater surveyswas the use of the intrinsic germanium detector placed in a submarine housing.Detailed descriptions of the methods employed and the results obtained are presented. Thiswork demonstrates that there are several approaches to surveying for discreteparticles in the marine environment and the relative merits of each are considered.