Quantifying the Plutonium Mass in Spent Fuel by Combining Prompt Neutron, Neutron Densitometry and Passive Gamma Measurements

The Next Generation Safeguards Initiative (NGSI) is sponsoring a multi-laboratory/university collaboration to determine the elemental Pu mass in spent fuel assemblies with nondestructive assay (NDA). The first phase of this research effort focused on simulating the expected performance of a wide variety of nondestructive assay techniques. This paper will present preliminary work on the second research phase of the NGSI effort which involves integrating a few instruments together. The three NDA techniques being integrated are Californium Interrogation with Prompt Neutron (CIPN), Self-Interrogation Neutron Resonance Densitometry (SINRD) and Passive Gamma (PG). The CIPN technique involves measuring the neutron intensity from an assembly for two setups. The first setup is a simple passive measurement. The second setup involves bringing a 252Cf neutron source to the side of the assembly opposite the detector. The count rate in the detectors goes up when the 252Cf source is present since the interrogating neutrons induce fission in the assembly. The change in the signal between these two setups is proportional to the fissile material in the assembly, which is primarily comprised of 235U, 239Pu and 241Pu. The SINRD technique uses several fission chambers, some covered with absorbing cadmium and gadolinium filters and some without these filters. By combining the count rates from these tubes, the relative change in the neutron intensity in a part of the neutron energy spectrum that is particularly indicative of the presence of 235U or 239Pu is measured. With this energy-specific information SINRD can be calibrated to measure the 235U content, and potentially the content of other isotopes. The PG techniques can quantify burnup. The burnup provides an estimation of the relative concentration of five main Pu isotopes. This current research will quantify the elemental Pu mass in an assembly by combining the CIPN results of Hu et al.[1] with the SINRD results of LaFleur et al.[2] and the PG results produced by Fensin et al. [3] with a library of 64 assemblies in water each with different burnup, cooling-time and initial enrichment. This work is part of a larger effort sponsored by the NGSI to develop an integrated instrument, comprised of individual NDA techniques with complementary features, that is fully capable of determining Pu mass in spent fuel assemblies.