CHARMS is a mobile assay and measuring system for making laboratory quality measurements of drums, waste bags and other waste objects in the field. It consists of a trailer with a laboratory/office and a heavy-duty turntable. The detection system employs a high-resolution, high purity Germanium (HPGe) detector with a large crystal diameter to enhance gamma ray detection. The HPGe detector is collimated and shielded to reduce the effect of the radiation background and radioactivity from other objects. The shielded detector system is mounted within the laboratory/office with a direct view of objects placed on the turntable,with an integral load cell for weight measurement, has a capacity of 2 metric tonnes.
CHARMS employs the Far Field measurement method, viewing the entire waste object (drum, waste bag) being measured. Using the weight measured by the load cells in the turntable (rotation platform) an average density attenuation correction is performed. During each measurement the waste object (for example drum or waste bag) is rotated at a constant rotation speed to reduce the effect of inhomogeneity in the waste matrix.
The high efficiency (>50%) shielded and collimated HPGe detector with a large diameter crystal provides spectroscopic measurements with both excellent gamma ray energy resolution and sensitivity. Detection limits (MDA) have been determined based on field measurements with a realistic background dose-rate. Using a 5-minute measurement time, MDAs of 0.0035 Bq/g for Cs-137 and 0.0021 Bq/g for Co-60 have been determined. For measurements of Cs-137 and Co-60, and most other radionuclides of interest, CHARMS is able to sentence the waste into the various categories of Out-Of-Scope, Very Low Level Waste (VLLW), Low Activity LLW (LALLW) and LLW.
CHARMS is designed to operate on 110 VAC which may be supplied from 110 VAC mains or 240 VAC mains (through an isolation step-down transformer) or by on-board portable generator. Battery backup is provided as the CHARMS trailer is equipped with a 70 Wh backup battery (UPS) sufficient to keep the HPGe detector cooled down to operating temperature during non-operational times (power-cuts, nights and weekends). A safety circuit is provided incorporating emergency stop buttons to stop rotation of the turntable.