PNNL

Abstract

Nondestructively determining the plutonium content in spent fuel assemblies continues to be a considerable challenge in the safeguarding of nuclear fuel cycles. Motivating needs for such measurements include quantifying material input at a reprocessing facility, determining the shipper-receiver difference, and recovering continuity of knowledge. A nondestructive assay (NDA) technology that could provide timely (tens of minutes), independent (no operator-declared information), and direct measurement of Pu mass that improves upon the uncertainty of today’s confirmatory methods would be a major step forward for spent fuel materials accountability. Lead slowing-down spectroscopy (LSDS) is one potential fuel assay technique; previous work has indicated promise of the method for specific fuel types and assay assumptions. In this work, the focus is the assay of pressurized water reactor assemblies over a wide burnup range, under the following assumptions: a) No a priori information (e.g. initial loading or burnup) is available about the assembly, and b) No pre-existing calibration generated from the measurement of many similar assemblies is available. The technical emphasis of this paper is the development of time-spectral analysis algorithms for LSDS that can extract as much isotopic information as possible from the complex, but content-rich assay signal. The key advancement described here is a mathematical relationship to account for self-shielding created by the fissile isotopes and the effects of neutron-absorbing fission and activation products. This formulation utilizes the known energy-dependent cross-sections from key isotopes, but leaves their mass as free variables. Multi-parameter regression analysis is used to directly calculate not only the mass of fissile isotopes in the fuel assembly (e.g. Pu-239, U-235 and Pu-241), but also the mass of key absorbing isotopes such as Pu-240 and U-238. Preliminary results, using a first-order self-shielding relationship, indicate that LSDS has the potential to directly measure total Pu with less than 5% average relative error, over a burnup range from 0 to 60 GWd/MTU. Assay of U-235 and U-238 is also possible, though with higher uncertainties. Deficiencies in the self-shielding model and methods to improve that formulation are described, along with application of the method to a wider range of fuel types and burnups.