Alex C. Misner

High-Energy Delayed Gamma Spectroscopy for Spent Nuclear Fuel Assay

High-accuracy, direct, nondestructive measurement of fissile and fissionable isotopes in spent fuel, particularly the Pu isotopes, is a well-documented, but still unmet challenge in international safeguards. As nuclear fuel cycles propagate around the globe, the need for improved materials accountancy techniques for irradiated light-water reactor fuel will increase. This modeling study investigates the use of delayed gamma rays from fission-product nuclei to directly measure the relative concentrations of 235U, 239Pu, and 241Pu in spent fuel assemblies. The method is based on the unique distribution of fission-product nuclei produced from fission in each of these fissile isotopes. Fission is stimulated in the assembly with a pulse-capable source of interrogating neutrons. The measured distributions of the short-lived fission products from the unknown sample are then fit with a linear combination of the known fission-product yield curves from pure 235U, 239Pu, and 241Pu to determine the original proportions of these fissile isotopes. Modeling approaches for the intense gamma-ray background promulgated by the long-lived fission-product inventory and for the high-energy gamma-ray signatures emitted by short-lived fission products from induced fission are described. Benchmarking measurements are presented and compare favorably with the results of these models. Results for the simulated assay of simplified individual fuel rods ranging from fresh to 60-GWd/MTU burnup demonstrate the utility of the modeling methods for viability studies, although additional work is needed to more realistically assess the potential of High-Energy Delayed Gamma Spectroscopy (HEDGS).

Signatures and Methods for the Automated Nondestructive Assay of

International Atomic Energy Agency (IAEA) inspectors currently perform periodic inspections at uranium enrichment plants to verify UF6 cylinder enrichment declarations. Measurements are typically performed with handheld high-resolution sensors on a sampling of cylinders taken to be representative of the facilitys entire cylinder inventory. These measurements are time-consuming, expensive, and assay only a small fraction of the total cylinder volume. An automated nondestructive assay system capable of providing enrichment measurements over the full volume of the cylinder could improve upon current verification practices in terms of manpower and assay accuracy. The 185-keV emission from U-235 is utilized in todays cylinder measurements, but augmenting this “traditional” signature with more-penetrating “non-traditional” signatures could help to achieve full-volume assay in an automated system. This paper describes the study of non-traditional signatures that include neutrons produced by F-19 (α, n) reactions (spawned primarily from U-234 alpha emission) and the high-energy gamma rays (extending up to 8 MeV) induced by those neutrons when they interact in the cylinder wall and nearby materials. The potential of these non-traditional signatures and assay methods for automated cylinder verification is explored using field measurements on a small population of cylinders ranging from 2.0% to 5% in U-235 enrichment. The standard deviation of the non-traditional high-energy gamma-ray assay approach was 4.7% relative to the declared cylinder enrichments; the standard deviation of the traditional enrichment meter approach using a well-collimated high-resolution spectrometer was 4.3%. The prospect of using the non-traditional high-energy gamma-ray signature in concert with the traditional 185-keV signature to reduce the uncertainty of automated cylinder assay is discussed.

{\rm UF}_{6}

It is well known that radon is present in relatively high concentrations below the surface of the Earth due to natural decay of uranium and thorium. However, less information is available on the background levels of other isotopes such as 133Xe and 131mXe produced via spontaneous fission of either manmade or naturally occurring elements. The background concentrations of radioxenon in the subsurface are important to understand because these isotopes potentially can be used to confirm violations of the comprehensive nuclear-test-ban treaty during an on-site inspection. Recently, Pacific Northwest National Laboratory measured radioxenon concentrations from the subsurface at the Nevada Nuclear Security Site (NNSS—formerly known as the Nevada Test Site) to determine whether xenon isotope background levels could be detected from spontaneous fission of naturally occurring uranium or legacy 240Pu as a result of historic nuclear testing. In this paper, we discuss the results of those measurements and review the sources of xenon background that must be taken into account during OSI noble gas measurements.

Cylinders at Uranium Enrichment Plants

Future arms control treaties may push nuclear weapons limits to unprecedented low levels and may entail precise counting of warheads as well as distinguishing between strategic and tactical nuclear weapons. Such advances will require assessment of form and function to confidently verify the presence or absence of nuclear warheads and/or their components. Imaging with penetrating radiation can provide such an assessment and could thus play a unique role in inspection scenarios. Yet many imaging capabilities have been viewed as too intrusive from the perspective of revealing weapon design details, and the potential for the release of sensitive information poses challenges in verification settings. A widely held perception is that verification through radiography requires images of sufficient quality that an expert (e.g., a trained inspector or an image-matching algorithm) can verify the presence or absence of components of a device. The concept of information barriers (IBs) has been established to prevent access to relevant weapon-design information by inspectors (or algorithms), and has, to date, limited the usefulness of radiographic inspection. The challenge of this project is to demonstrate that radiographic information can be used behind an IB to improve the capabilities of treaty-verification weapons-inspection systems.

Underground sources of radioactive noble gas

Pacific Northwest National Laboratory (PNNL) is developing the concept of an automated UF6 cylinder verification station that would be located at key measurement points to positively identify each cylinder, measure its mass and enrichment, store the collected data in a secure database, and maintain continuity of knowledge on measured cylinders until the arrival of International Atomic Energy Agency (IAEA) inspectors. At the center of this unattended system is a hybrid enrichment assay technique that combines the traditional enrichment-meter method (based on the 186 keV peak from 235U) with non-traditional neutron-induced high-energy gamma-ray signatures (spawned primarily by 234U alpha emissions and 19F(alpha, neutron) reactions). Previous work by PNNL provided proof-of-principle for the non-traditional signatures to support accurate, full-volume interrogation of the cylinder enrichment, thereby reducing the systematic uncertainties in enrichment assay due to UF6 heterogeneity and providing greater sensitivity to material substitution scenarios. The work described here builds on that preliminary evaluation of the non-traditional signatures, but focuses on a prototype field system utilizing NaI(Tl) and LaBr3(Ce) spectrometers, and enrichment analysis algorithms that integrate the traditional and non-traditional signatures. Results for the assay of Type-30B cylinders ranging from 0.2 to 4.95 wt% 235U, at an AREVA fuel fabrication plant in Richland, WA, are described for the following enrichment analysis methods: 1) traditional enrichment meter signature (186 keV peak) as calculated using a square-wave convolute (SWC) algorithm; 2) non-traditional high-energy gamma-ray signature that provides neutron detection without neutron detectors and 3) hybrid algorithm that merges the traditional and non-traditional signatures. Uncertainties for each method, relative to the declared enrichment for each cylinder, are calculated and compared to the uncertainties from an attended HPGe verification station at AREVA, and the IAEA’s uncertainty target values for feed, tail and product cylinders. A summary of the major findings from the field measurements and subsequent analysis follows: • Traditional enrichment-meter assay using specially collimated NaI spectrometers and a Square-Wave-Convolute algorithm can achieve uncertainties comparable to HPGe and LaBr for product, natural and depleted cylinders. • Non-traditional signatures measured using NaI spectrometers enable interrogation of the entire cylinder volume and accurate measurement of absolute 235U mass in product, natural and depleted cylinders. • A hybrid enrichment assay method can achieve lower uncertainties than either the traditional or non-traditional methods acting independently because there is a low degree of correlation in the systematic errors of the two individual methods (wall thickness variation and 234U/235U variation, respectively). This work has indicated that the hybrid NDA method has the potential to serve as the foundation for an unattended cylinder verification station. When compared to today’s handheld cylinder-verification approach, such a station would have the following advantages: 1) improved enrichment assay accuracy for product, tail and feed cylinders; 2) full-volume assay of absolute 235U mass; 3) assay of minor isotopes (234U and 232U) important to verification of feedstock origin; single instrumentation design for both Type 30B and Type 48 cylinders; and 4) substantial reduction in the inspector manpower associated with cylinder verification.

Low-Intrusion Techniques and Sensitive Information Management for Warhead Counting and Verification: FY2011 Annual Report

International Atomic Energy Agency (IAEA) inspectors currently perform periodic inspections at uranium enrichment plants to verify UF6 cylinder enrichment declarations. Measurements are typically performed with handheld high-resolution sensors on a sampling of cylinders taken to be representative of the facilitys entire product-cylinder inventory. Pacific Northwest National Laboratory (PNNL) is developing a concept to automate the verification of enrichment plant cylinders to enable 100 percent product-cylinder verification and potentially, mass-balance calculations on the facility as a whole (by also measuring feed and tails cylinders). The Integrated Cylinder Verification System (ICVS) could be located at key measurement points to positively identify each cylinder, measure its mass and enrichment, store the collected data in a secure database, and maintain continuity of knowledge on measured cylinders until IAEA inspector arrival. The three main objectives of this FY09 project are summarized here and described in more detail in the report: (1) Develop a preliminary design for a prototype NDA system, (2) Refine PNNLs MCNP models of the NDA system, and (3) Procure and test key pulse-processing components. Progress against these tasks to date, and next steps, are discussed.