Delayed Neutron Detection with an Integrated Differential Die-Away and a Delayed Neutron Instrument

The Next Generation Safeguards Initiative (NGSI) of the U.S. Department of Energy (DOE) has funded a multi- laboratory/university collaboration to quantify the plutonium (Pu) mass in, and to detect the diversion of pins from, spent nuclear fuel assemblies. The first half of this NGSI effort is focused on quantifying the capability of a range of nondestructive assay (NDA) techniques with Monte Carlo modeling and the second half involves measuring spent fuel. The focus of this paper is on the performance of the detection of delayed neutrons. A delayed neutron counting instrument using 36 fission chambers and a 14 MeV pulsed DT neutron generator was studied previously as part of the NGSI effort. The present paper will quantify the capability of a new delayed neutron instrument using an array of four 3He gas filled proportional counters to detect the delayed neutrons and a DT generator as the source of interrogating neutrons. This new design was selected in order to minimize the fission of 238U, to use a more realistic neutron generator design in the model, to reduce the cost and to facilitate the integration of a delayed neutron with a differential die-away instrument. Since this paper will focus on delayed neutron detection, the goal is to quantify the signal from 235U, 239Pu and 241Pu, which are the isotopes present in spent fuel that respond most significantly to a slow neutron interrogation. This report will quantify the capability of this new delayed neutron design to measure the combined mass of 235U, 239Pu and 241Pu for 16 of the 64 assemblies of the NGSI Spent Fuel Library in water.