Year
2008
Abstract
The Box Neutron Assay System is the neutron component of the Integrated Crate Interrogation System (ICIS) developed for use at the Savannah River Site (SRS) to assay transuranic (TRU) waste. The BNAS is a large volume passive neutron correlation counter fitted with multiplicity electronics, and a multi-position add-a-source (AAS) capability. It was designed and factory calibrated to assay large containers up to and including the Standard Large Box. Radioactive waste can often be heterogeneous in matrix distribution and may also exhibit a non-uniform distribution of radionuclides. As it is not practicable to calibrate for all possible measurement scenarios, the formulation of a complete and justifiable uncertainty budget is extremely important in order to set confidence limits on the assay value for the diversity of unknown items. This paper describes the major contributing factors to the Total Measurement Uncertainty (TMU) in the assay of 240Pueff using the BNAS. Some of these factors are controllable and quantifiable in a straightforward way; others are less amenable to statistical evaluation and can only be estimated by different means. Since the TMU for each measurement is different due to variations in matrix, radioactive content, and measurement conditions, even the algorithms invoked to generate TMUs must be evaluated for efficacy on a case by case basis. In this paper TMU values are presented for various matrices and two container types that span the range of containers to be assayed in the BNAS, and the application of the TMU algorithms for large containers is discussed.