ADVANCED NEUTRON IMAGING TECHNIQUE (ANIT) WITH DIFFICULT ASSAYS

The Passive Active Neutron Waste Assay System (PANWAS) has been in operation for 3 years measuring 200 l and 400 l waste drums [1, 2,3] at the Nucleco facility in Casaccia, IT. The waste drum streams consist of uranium and plutonium in highly heterogeneous matrices and non-uniform Special Nuclear Material (SNM) distributions within 200 l and 400 l drums as well as 400 l drums containing several compacted 200 l drums. The PANWAS deploys a measurement technique [4] using passive and active neutron imaging to correct for the non-uniform SNM distribution using a course volume element correction factor based on a volume weighted average scheme. The Neutron Imaging Technique (NIT) algorithm [4] can underestimate the required correction factor under conditions of azimuthal symmetry (point source on axis). For real waste drums, the position of the SNM may also be slightly displaced from the calibration volume element positions used to form the reference maps and this can lead to an incorrect (less accurate) NIT correction factor in highly hydrogenous matrix waste streams. In addition, the NIT algorithm is designed for single point source SNM distributions and can provide a misleading result when multiple point source SNM distributions are encountered. To support and guide the PANWAS results, the 200 l drum waste database is augmented with results from the Segmented Gamma Scanner (SGS) which provides information on the vertical distribution of both the matrix and SNM. In addition, the facility operates a radiography instrument which provides insight on the degree of matrix inhomogeneity and its composition. Exploiting the augmented information from the SGS and radiography instruments, on real waste drums from difficult assays, the objective of this paper is to demonstrate the feasibility of a second generation version of NIT called, Advanced Neutron Imaging Technique (ANIT) that addresses the limitations of the current implementation of the NIT.