Determining Plutonium Mass in Spent Fuel Assemblies with Nondestructive Assay


Dr. Stephen J. Tobin
Los Alamos National Laboratory

Reducing the chance that nuclear weapons will be used in the future is a challenge. One facet to addressing this challenge is reducing the probability that the commercial nuclear fuel cycle can be misused - this is the primary task of the field of nuclear safeguards. Stated more specifically – the purpose of safeguards is to detect the diversions of significant quantities of nuclear materials and to deter such diversions by the risk of early detection. Since “detection” is central to the safeguards purpose, detection technology is crucial to the safeguard mission. This talk will give a brief overview of the DOE’s Next Generation Safeguards Initiative (NGSI) to detect the mass of plutonium in spent fuel. The majority of the talk will focus on the various signatures (photon and neutron) being measured from spent (used) nuclear fuel.

The NGSI research effort is nominally a 5 year effort. The first half of this effort is primarily focused on Monte Carlo modeling to quantify the capability of a range of nondestructive assay techniques. Some of the techniques being researched include the following: Delayed Gamma, Delayed Neutrons, Differential Die-Away, Differential Die-Away Self-Interrogation, Lead Slowing Down Spectrometer, Neutron Multiplicity, Nuclear Resonance Fluorescence, Passive Prompt Gamma, Passive Neutron Albedo Reactivity, Self-integration Neutron Resonance Densitometry, Total Neutron, X-Ray Fluorescence, 252Cf Interrogation with Prompt Neutron Detection. Since none of these techniques, in isolation, is capable of quantifying the elemental mass of plutonium, the goal of this first phase is to integrate a few techniques into a system, or a few systems, capable of quantifying elemental plutonium mass. The second half of the NGSI effort involves fabricating a few of the most promising systems and quantifying how well they work in the field.

Dr. Tobin has been working in the area of nuclear safeguards research and development since 2001.  He has worked primarily in nondestructive assay using photons and neutron signatures. Before working in safeguards, Dr. Tobin worked for 10 years in magnetic confinement fusion.  He has a B.S. degree in Physics from Xavier University and M.S. and Ph.D. degrees in Nuclear Engineering from The University of Michigan. Dr. Tobin currently works at the Los Alamos National Laboratory in Nuclear Nonproliferation Division.