Braden Goddard

Predicting Concrete Roadway Contribution to Gamma-Ray Background in Radiation Portal Monitor Systems

Abstract The collapse of the Soviet Union ushered in an era of interest in the security of the radiological and nuclear material holdings of the Russian Federation and other countries of the Former Soviet Union. Additionally, the increasing sophistication of international criminal and terrorist organizations highlighted the need to secure these materials and prevent them from being smuggled from their point of origin and across international boundaries. To combat the growing threat of radiological and nuclear smuggling, radiation portal monitors (RPMs) are deployed at ports of entry (POEs) around the world to passively detect gamma and neutron radiation signatures from cargo and pedestrian traffic. In some locations, RPMs are reporting abnormally high gamma-ray background count rates, a situation that has been attributed, in part, to the building materials surrounding the RPMs. The primary objective of this work was to determine the impact of different types of concrete on the gamma-ray background readings in a particular RPM. Secondary objectives include developing an adaptable model to estimate the gamma-ray background contribution from any composition of concrete in any RPM configuration and determining the elemental composition of different concrete samples through neutron activation analysis (NAA) techniques. The specific activities of 40K and isotopes from the 238U and 232Th decay series were determined with a high-purity germanium detector and computer-generated calibration files. Through NAA, 34 elemental compositions were determined for six concrete samples from three different parent slabs. The total weight percentages determined were 84% to 100% of the total mass of the samples. The Monte Carlo N-Particle (MCNP) transport code was used to simulate the RPM response to the different concrete slabs. The MCNP model was validated by comparing actual and simulated detector responses to 137Cs check sources of varying strengths. For all validation cases, the MCNP estimates were 6% to 16% less than the value obtained from the actual RPM data. This work shows that it is possible to estimate the gamma-ray response of an RPM to the underlying concrete roadway. Knowing the amount of this contribution will allow RPM customers to choose suitable foundation materials before installation and accurately set alarm thresholds. This could ultimately increase the ability of RPMs to detect radiation at POEs, thereby increasing the probability of a seizure of smuggled radiological and nuclear materials.

Quantitative NDA Measurements of Multiactinide Oxide Fuels

Abstract As new reprocessing techniques and fuel forms are developed, the ability of inspection agencies and facility operators to measure powders containing several actinides becomes increasingly necessary. Neutrons emitted from induced and spontaneous fission of different nuclides are very similar, making it difficult to measure these powders with nondestructive assay techniques. To measure the powders, a neutron multiplicity technique based on first-principle methods was developed to exploit isotope-specific nuclear properties, such as energy-dependent fission cross sections and neutron-induced fission multiplicity. This technique was tested through measurements using an epithermal neutron multiplicity counter with two different interrogation (α,n) sources and varying plutonium materials. To complement these measurements, extensive Monte Carlo N-Particle eXtended (MCNPX) simulations were performed for each measured sample, as well as samples that were not available to measure. The primary application of this first-principle technique is the measurement of materials containing uranium, neptunium, plutonium, and americium. This technique still has several challenges that need to be overcome, the largest of these being the ability to produce results with acceptably small uncertainties.

IAEA “significant quantity” values: time for a closer look?

ABSTRACT The International Atomic Energy Agency uses a concept of “significant quantity” to establish accounting limits and to determine how much nuclear material can be unaccounted for before the construction of a nuclear device becomes possible. Under the current SQ values, some materials could potentially be under-regulated, while others have disproportionately tight accounting practices, thus interfering with optimal safeguards efficacy. This article compares the SQ values of various materials and reviews the categories adopted by a variety of national regulators to show similarities and differences of approaches. The article concludes by offering possible solutions to increase safeguards efficacy and reduce costs, while acknowledging potential drawbacks. All findings and conclusions are based on open-source publications and publicly available information.