AN OVERVIEW OF THE INTEGRATED CRATE INTERROGATION SYSTEM (ICIS) FOR USE AT THE SAVANNAH RIVER SITE

The Integrated Crate Interrogation System (ICIS) was developed for use at the Savannah River Site to assay transuranic waste in large containers. The system comprises a Box Segmented Gamma Scanner (BSGS) providing high resolution gamma spectroscopy, and a Box Neutron Assay System (BNAS) providing passive neutron counting capability. The multi-modality approach is taken where the assay results from the gamma and neutron systems are combined to complement each other in satisfying Waste Isolation Pilot Plant (WIPP) criteria. This paper gives an overview of the system that has been built, factory calibrated, and delivered to the site. The BSGS is similar to a standard Canberra Segmented Gamma Box Counter, but with the addition of a transmission option for ascertaining density and rudimentary fill-height information. This supplements the Multi-Curve approach based on efficiency in energy and density. The BSGS has a moving trolley which travels on rails through a passive emission counting station using large BEGe detectors, and a transmission counting station using NaI detectors in conjunction with high- and low-beam transmission stages. The assay result provides information on the Pu, Am and/or U isotopic ratios using standard isotopic analysis codes, quantitative measurement of Pu and U for low and medium density matrices, and direct measurement of other gamma emitters in the waste that are not identified in the isotopic measurement. It also provides basic positional information to improve the accuracy of both the gamma and the neutron measurement. The BNAS is a passive neutron coincidence counter intended for use with multiplicity counting of items of various geometries. The counter utilizes 320 3 He filled proportional counters arranged in a 4π geometry about the assay cavity and divided into 80 counting channels. The assay result provides the 240 Pu-effective mass with which the gamma isotopic measurement is combined to provide reliable reporting of Pu and other isotopes. It also provides the total alpha activity that must be reported as part of the characterization requirements for WIPP (or bounds on the alpha activity based on the chemical form of the alpha-emitter). The neutron assay also complements the gamma results where matrix effects, self shielding or self absorption effects, and the presence of high-Z material, severely limits accuracy. Matrix moderator corrections for the neutron assay are based on a multi-position Add-A-Source correction augmented by a Modified Hydrogen Correction. The data acquisition also incorporates a Local Coincidence Veto circuit.