Nuclear Resonance Transmission Analysis Using a Vehicle-Portable Source

Project Overview

New techniques will be needed to safeguard next-generation nuclear fuels for International Atomic Energy Agency (IAEA) treaty compliance, such as thorium-based fuels. Some problems include 1) new fuel isotopes exist without adequate cross-sections and standards (233U); 2) some next-generation spent fuel has ~ 1000x more gamma radiation (due to 232U), overwhelming current assay methods; and 3) next-generation fuel geometries (molten salt, pebbles) lack IAEA standards.

We proposed using a dense plasma focus (DPF) with an active neutron interrogation method—neutron resonance transmission analysis (NRTA)—to assay and safeguard fuel. These solve the above solutions because 1) neutron nuclear cross-sections already exist and can positively identify all new isotopes; 2) neutron detectors have low gamma-ray sensitivity and using a bright active neutron-interrogation source will complete the assay in 200 microsecond acquisition, greatly reducing background contribution, enabling in situ assay of hot fuel; and 3) new geometries are compatible with NRTA measurements because they are local, line-integrated, quantitative measurements.

Additionally, a DPF is much cheaper and smaller than a large laser, accelerator, or photofission source, but produces a neutron pulse much faster than comparable handheld neutron sources.

To establish feasibility for making this measurement, we established a full Monte-Carlo nuclear particle (MCNP) model of the MegaJOuLe Neutron Imaging Radiography (MJOLNIR) DPF at B391 to test targets and optimize detector setup. We then built parts of this setup to test the detector response.

Through a combination of MCNP simulations, prototype setups, and a few shots in a high-rad DPF environment, we have determined it is possible to use a DPF to make a quantitative NRTA measurement in a high-rad environment with a customized setup.

The major findings include using MCNP to find an optimal setup for the moderator (size, thickness, material), detector (distance, thickness, material), and shielding (size, thickness, type) to achieve a high signal/noise ratio for a high-rad environment. A prototype setup was built in the laboratory to test the construction method and materials with a blueprint for building a detector. Finally, we tested a few photomultiplier tube styles and scintillator types on MJOLNIR shots to determine the proper amplification gains and identify the need for time-gated detectors or saturation-proof detectors.

Mission Impact

We designed a new technique and system with the potential to assay highly radioactive nuclear fuel for nuclear accountancy. With further testing and development, this setup could help support nuclear-safeguards missions by providing a way to quantitatively measure and track nuclear fuel throughout the cycle. A future system could advance nonproliferation, counterterrorism, counter-proliferation, and emergency-response capabilities across the entire threat spectrum.

This setup also establishes a new nuclear-assay capability at LLNL and the NNSA complex to measure isotopic content of unknown samples, which could serve missions in emergency response as well as nuclear forensics.