Derek A. Haas

Methods for fabrication of multi-material or functionally graded ceramic composite architectures are of interest for numerous applications. However, conventional co-sintering of multi-material ceramic parts is a challenge because differences in the sintering behavior of the two materials leads to interfacial strain and, ultimately, component failure. Direct ink writing (DIW) is an extrusion-based additive manufacturing process that excels at multi-material printing because multiple extrusion nozzles can be installed on the same gantry system. Furthermore, the use of DIW as a method to fabricate multi-material ceramic green bodies offers an additional variable for controlling and potentially matching sintering kinetics in the slurry formulation used for two dissimilar feedstocks. In the work documented in this manuscript, we explored two strategies to successfully co-sinter multi-material ceramic oxides: slurry optimization to match sintering kinetics and material gradients to step from one material to another. This manuscript also quantifies the allowable mismatch that avoids part cracking in solid solution forming multi-material systems and discusses best strategies to reduce mismatch during co-sintering. Inks composed of gadolinium oxide (Gd 2 O 3 ) and zirconium oxide (ZrO 2 ), a surrogate for uranium oxide (UO 2 ), were thermally matched, which resulted in a sintering mismatch reduction of over 10%. It was found that ~1% mismatch is tolerable during debind cycles and that ~5% mismatch is manageable during sintering cycles after slurry formulations are optimized to match the sintering behavior. Use of continuous gradients is shown to reduce sintering mismatch, although geometric resolution may be lost due to solid solution formation.

Molten Salt Reactors (MSRs) promise significant advantages over traditional light water reactors, including enhanced safety and improved fuel efficiency. Among MSR designs, those using fueled fluoride salts have the most extensive operational history and are the focus of this study. In these systems, understanding and controlling salt chemistry is essential to reactor safety and performance. Challenges include preventing the release of volatile uranium species, avoiding unintended metallic uranium deposition, and managing fission product speciation as burnup progresses. These issues impact heat exchanger fouling, component corrosion, and potential criticality fluctuations, which define operational limits. Given the complexity of operating MSRs’ chemical and isotopic inventory, accurate simulation is challenging, particularly with limited experimental data. This work introduces a simulation surrogacy methodology to simplify the reactor system by grouping chemically similar species based on available data, balancing computational efficiency with fidelity. The approach integrates SCALE reactor physics framework for modeling fuel salt depletion and fission product generation with the Molten Salt Thermochemical Database-Thermochemical (MSTDB-TC) and Thermochimica for Gibbs energy minimization to predict species behavior. The methodology is demonstrated for a simple MSR design at key points in its lifetime, addressing operational challenges and safety concerns. Key data gaps, including radiological, neutronic, and quantitative limitations, are identified. Although focused on fueled fluoride salts, this methodology is adaptable to other reactor designs, providing a versatile framework for advancing MSR chemistry simulations. While fundamentally exploratory due to current database limitations, this methodology provides a systematic framework for advancing MSR chemistry modeling and identifying critical data needs.

The production of 37Ar gas through the neutron activation of 40Ca is important for underground nuclear explosion monitoring. Some of the activation occurs with thermal neutrons, but the thermal neutron cross section for the 40Ca(n, α)37Ar reaction is not well understood. There are disagreements between the Evaluated Nuclear Data File and the Japanese Evaluated Nuclear Data Library on the existence of the thermal cross section. An experiment was designed at The University of Texas to use alpha spectroscopy to measure the cold neutron cross section. The experiment was repeated five times and a weighted average cross section of 196.7 ± 47.2 mbarns was calculated.

Neutron activation analysis is not usually known used for environmental radioactivity measurements as compared to low-level passive gamma-ray counting. Texas local background samples were taken as part of a study to compare neutron activation versus passive counting. The benefits and drawbacks of each method are explored for soil samples which are in secular equilibrium, including gamma-ray self-attenuation, sample size and counting times.

<ns7:p> The nuclide <ns7:sup>225</ns7:sup> Ac is an alpha-emitting nuclide that can be used in targeted alpha therapy. The main production method for <ns7:sup>225</ns7:sup> Ac is the decay of <ns7:sup>233</ns7:sup> U produced by the neutron irradiation of <ns7:sup>232</ns7:sup> Th. Isotopically pure <ns7:sup>233</ns7:sup> U poses proliferation concerns, so production of <ns7:sup>225</ns7:sup> Ac in relevant quantities using this method requires denaturing (down-blending) of <ns7:sup>233</ns7:sup> U with natural or depleted uranium. Denaturing can be performed preemptively by directly mixing <ns7:sup>233</ns7:sup> Pa separated from a fuel or blanket material with an unirradiated uranium solution. This method minimizes the mass of highly enriched <ns7:sup>233</ns7:sup> U in the process stream at any time. </ns7:p>

On 18 October 2023 a 16.3-ton TNT equivalent chemical explosion was detonated underground at the Nevada National Security Site, generating a seismic event with a magnitude of 1.7 (Meyers et al., 2024). The associated seismic wavefield was measured on a Distributed Acoustic Sensing (DAS) array with slant range distances from 27 m – 1123 m. The first arriving phase traveled at an apparent velocity of about 2640 m s-1 from 27 m to 420 m slant range and about 2470 m s-1 from 505 m to 1123 m slant range according to the first arrival moveouts on the DAS data. The first arrival from the explosion temporarily saturated the cable from a slant range of 27 m – 186 m and 0.009 s to 0.084 s post detonation. From 186 m slant range to 420 m slant range, peak strain rates of 5.6 x 106 nm m-1 s-1 were observed for the first arrival phase. For the first arrival from 505 m slant range to 1123 m slant range, peak strain rates reduced to 8.0 x 104 nm m-1 s-1. A comparison of the scaled accelerations computed from DAS, the geophone pairs, and the measurements of co-located accelerometer pairs show common agreement at the scaled ranges of the single point sensors. This study adds to the body of work reporting near-source DAS observations of the seismic wavefields generated by underground chemical explosions. These results indicate that near-source DAS observations can refine interpretations of phase identification from single-point sensor observations. Phase identification could be one mechanism that contributes scatter to single point seismic measurements which would confound the performance of empirical relationships for small explosions. Removing that mechanism may therefore reduce interstation variability and increase empirical relationship performance for small explosions.

<ns5:p>This technical report aims to provide energy developers and policymakers with information and preliminary analyses on the potential for Texas coal plant sites to be repurposed for nuclear power. Investigation into coal-to-nuclear (C2N) has shown that constructing a nuclear reactor on the site of a retired coal plant has both economic and environmental benefits. The data presented includes operational details of the coal power plants, the presence of nearby hazards, geological and hydrological data, and population statistics. This information was gathered for 19 coal powered electricity generation sites in Texas. Thirteen of the sites assessed have no hazards or other factors that would disqualify them from hosting a Small Modular Reactor (SMR). Of these, 11 sites are also suitable for a Light Water Reactor (LWR). The smaller size and power output of SMRs makes these additional 2 sites possible, even near a population center. The remaining 6 coal plant sites would require more specific on-site analysis or potential adjustments to the reactor design to be considered for licensing. Information supporting these findings are presented here in detail and are intended to give interested parties easy access to relevant C2N conversion information. These results can be utilized to make informed decisions on this strategy for nuclear development and its benefits to grid stability and baseload power in Texas.</ns5:p>

In this analysis we examine the economic costs and benefits of installing dozens of small modular reactors at recently retired coal power plants in Texas to determine the viability of a grid or “program-level” approach to nuclear power plant planning in the United States. Previous studies have indicated that utilizing stranded infrastructure assets at retired coal power plants, known as the “coal-to-nuclear” transition, could greatly reduce the amount of time and capital required to build just a single commercial nuclear plant. A discounted cash flow analysis was created using data from regional electricity markets, coal-to-nuclear studies, and other industry sources to estimate the potential value of SMR projects. The analysis includes multiple scenarios to account for varying project sizes, changes in technology learning rates, and recently implemented energy tax credits. Results indicate that increasing the rate of learning has only a minimal lowering effect on the Levelized Cost of Electricity (LCOE), as both the learning rate and LCOE quickly plateau. The most significant cost reductions were enabled by tax credits and coal-to-nuclear cost enhancements that decreased the LCOE to a competitive range of $43–52/MWh. However, our work finds that program-level benefits will likely be the result of cost sharing and risk modularization rather than direct improvement in metrics like LCOE and net present value as those are still smaller in comparison. • Economic analysis to determine viability of program-level nuclear plant planning. • SMRs can be financially attractive replacements for recently retired coal power plants. • Cost savings from reusing coal power plant assets are especially significant. • Detailed cash flow analysis includes federal energy tax credits and learning rates. • Resulting LCOE and NPV indicate competitiveness with renewable energy generation sources.

Underground nuclear explosions release noble gases into the atmosphere that can be detected to support international monitoring efforts. Atmospheric transport models help predict the movement of these gases over long distances, but struggle to predict the movement in the atmosphere local to the release. A field experiment was designed to monitor the movement of 127Xe within a 5-km radius. Four gas samplers were deployed as part of this experiment to collect atmospheric samples at various distances from the release point. These samples were then analyzed in a near-field lab using a NaI(Tl) detector and in an off-site lab using gamma-gamma coincidence and beta-gamma coincidence counting.

<ns3:p>This technical report aims to provide energy developers and policymakers with information and preliminary analyses on the potential for Texas coal plant sites to be repurposed for nuclear power. Investigation into coal-to-nuclear (C2N) has shown that constructing a nuclear reactor on the site of a retired coal plant has both economic and environmental benefits. The data presented includes operational details of the coal power plants, the presence of nearby hazards, geological and hydrological data, and population statistics. This information was gathered for 19 coal powered electricity generation sites in Texas. Thirteen of the sites assessed have no hazards or other factors that would disqualify them from hosting a Small Modular Reactor (SMR). Of these, 11 sites are also suitable for a Light Water Reactor (LWR). The smaller size and power output of SMRs makes these additional 2 sites possible, even near a population center. The remaining 6 coal plant sites would require more specific on-site analysis or potential adjustments to the reactor design to be considered for licensing. These findings exhibit the potential for cost effective nuclear development to benefit grid stability and provide baseload power to Texas.</ns3:p>

Effectively employing these seven strategies will help ASCs maximize their profitability.

Abstract Current noble gas detection systems for nuclear explosion monitoring are based on the detection of four radioxenon isotopes—Xe-131m, -133, -133m and -135. The data provided by radioxenon detection could be enhanced by other radionuclide signatures such as Ar-37. Activation of Ca-40 in rock by neutrons produces Ar-37, and monitoring for this additional nuclide could help distinguish detections of nuclear explosions from background sources, such as medical isotope production. This work studies the capabilities of a hypothetical argon detection network. A 10 kt explosion was modeled using MCNP and SCALE to determine the inventory of Ar-37 created in a representative granite rock layer, assuming either 0.1, 1 or 10% of the total inventory was released. The Ar-37 inventory was combined with atmospheric transport data from HYSPLIT compiled in a previous study, along with the detection limits of standard Ar-37 detection systems, to determine how many hypothetical monitoring stations would detect Ar-37 from an explosion. This method was repeated for 365 HYSPLIT data sets to create a year’s worth of hypothetical explosions, releases, and detections. The study quantified the average number of detections per release, the number of stations detecting Ar-37, and the possibility of detecting Ar-37 in coincidence with xenon.

A series of multi-physics experiments, referred to as Physics Experiment 1 (PE1) is underway at the United States’ Nevada National Security Site (NNSS). The PE1 series includes detonations of three underground chemical explosions in P-tunnel, with fully coupled (PE1 A), partially decoupled (PE1 D_L), and fully decoupled (PE1 B) emplacements. Canisters with gas tracers are imbedded in the explosives, and the tracers are released when the canister is destroyed by the detonation. A dedicated electromagnetic (EM) experiment (EMX) generates well-characterized EM signals at an underground location near the chemical explosive experiments. A series of atmospheric experiments (METEX, REACT, and METREX) release smoke and radioactive tracers around Aqueduct Mesa to test gas transport in complex topography. Each of the chemical explosive experiments includes a network of sensors to record seismic, acoustic, and electromagnetic waves, measurement of atmospheric conditions, and air sample collection for measurement of tracer concentration. EMX records EM signals underground and on the surface of Aqueduct Mesa. METEX, REACT, and METREX include measurement of atmospheric condition, as well as tracking smoke releases. REACT and METREX add low-level radioactive gas tracers to the atmospheric releases.

<ns3:p> Background 37Ar, produced from the neutron activation of 40Ca in sub-surface rock and soil, is important for nuclear explosion monitoring. Most of the cross section measurements done for the 40Ca(n, <ns3:italic>α</ns3:italic> ) 37 Ar reaction are in the fast neutron region. A thermal cross section measurement was attempted in 1989, with the assumption that the 4-8 MeV neutrons would be completely thermalized with 8-cm of water. Methods The experimental setup was modeled using Monte Carlo N-Particle code to see if the assumptions for the neutron distribution were accurate. A model of a 3.0 MeV deuteron source was used to recreate the neutron source from the experiment. Additional modeling was done using SCALE to compare the predictions of 37Ar yields when using the measured cross section from 1989 compared to current nuclear data models. Results Modeling showed the neutron distribution was closer to 10% thermal and 90% fast as opposed to entirely thermalized. The additional modeling showed how the overestimation of the thermal cross section would affect the predicted yield of 37Ar. Conclusions Based on the modeling results, the assumption that all the 4-8 MeV neutrons are thermalized with 8-cm of water is not accurate. The high fraction of fast neutrons interacting with the Ca will result in an overestimation of the thermal cross section. </ns3:p>

Abstract In the early years of nuclear explosion monitoring, experts used downwind detections with meaningful ratios of radioactive species to identify an explosion. Today’s reality is sparse networks of radionuclide monitoring stations looking for weak signals. Analysts need to discriminate between industrial background radioactivity and nuclear explosion signals, even using the detection of one isotope. Aerosol and xenon measurements potentially related to nuclear tests in 2006 and 2013 announced by the Democratic People’s Republic of Korea and from worldwide civilian background radioactivity are considered when defining radionuclide detection anomalies to objectively guide the use of limited analyst resources and reduce the possibility of not detecting nuclear explosions.

The term nuclear forensics encompasses the detection and analysis of nuclear materials before they are used in a weapon, the analysis of radioactive debris following a nuclear event or the investigation of the pedigree of nuclear materials in non-proliferation. Nuclear forensics typically encompasses these three main broad areas: radiochemistry, chemical instrumentation and non-destructive techniques (typically gamma-ray spectrometry). Research reactors (10kW – 10MW) are a great resource to perform research and development in nuclear forensics as well as in the critical need of education. Research reactors' ability to produce isotopes in small or large amounts means that researchers have a unique source of material for nuclear forensics studies. A graduate course in nuclear forensics was developed as a companion to a research project with a grant through the US Department of Homeland Security. Besides the usual lectures on the basics of nuclear phenomena, a successful effort was made to include a significant amount of historical background. In addition, several guest lecturers from national laboratories and universities were added to the curriculum, resulting in a more meaningful and current course. Due to COVID-19 restrictions, all the laboratories were virtually delivered in both fully remote and hybrid modes across several semesters.

<ns3:p> Background 37Ar, produced from the neutron activation of 40Ca in sub-surface rock and soil, is important for nuclear explosion monitoring. Most of the cross section measurements done for the 40Ca(n, <ns3:italic>α</ns3:italic> ) 37 Ar reaction are in the fast neutron region. A thermal cross section measurement was attempted in 1989, with the assumption that the 4-8 MeV neutrons would be completely thermalized with 8-cm of water. Methods The experimental setup was modeled using Monte Carlo N-Particle code to see if the assumptions for the neutron distribution were accurate. A model of a 3.0 MeV deuteron source was used to recreate the neutron source from the experiment. Additional modeling was done using SCALE to compare the predictions of 37Ar yields when using the measured cross section from 1989 compared to current nuclear data models. Results Modeling showed the neutron distribution was closer to 10% thermal and 90% fast as opposed to entirely thermalized. The additional modeling showed how the overestimation of the thermal cross section would affect the predicted yield of 37Ar. Conclusions Based on the modeling results, the assumption that all the 4-8 MeV neutrons are thermalized with 8-cm of water is not accurate. The high fraction of fast neutrons interacting with the Ca will result in an overestimation of the thermal cross section. </ns3:p>

Abstract Current noble gas detection systems for nuclear explosion monitoring are based on the detection of four radioxenon isotopes – Xe-131m, -133, -133m and − 135. The data provided by radioxenon detection could be enhanced by other radionuclide signatures such as Ar-37. Activation of Ca-40 in rock by neutrons produces Ar-37, and monitoring for this additional nuclide could help distinguish detections of nuclear explosions from background sources, such as medical isotope production. This work studies the capabilities of a hypothetical argon detection network. A 10 kt explosion was modeled using MCNP and SCALE to determine the inventory of Ar-37 created in a representative granite rock layer, assuming either 0.1, 1, or 10% of the total inventory was released. The Ar-37 inventory was combined with atmospheric transport data from HYSPLIT compiled in a previous study, along with the detection limits of standard Ar-37 detection systems, to determine how many hypothetical monitoring stations would detect Ar-37 from an explosion. This method was repeated for 365 HYSPLIT data sets to create a year’s worth of hypothetical explosions, releases, and detections. The study quantified the average number of detections per release, the number of stations detecting Ar-37, and the possibility of detecting Ar-37 in coincidence with xenon.

This work reports yarn pull-out studies of commercially available Kevlar® KM2+ individual yarns coated with metallic layers (copper, aluminum, aluminum nitride and silver) via a directed vapor deposition process. The uncoated control and metal-coated Kevlar® yarns are hand-woven into fabric swatches for quasi-static pull-out experiments. To perform these experiments, a yarn pull-out fixture is custom-designed and fabricated to apply transverse pre-tension to the fabric. Three levels of transverse pre-tensions are studied at 100 N, 200 N, and 400 N. The results showed that both peak pull-out force and energy absorption during the pull-out process increase with increase in transverse pre-tension. All the metal-coated groups showed an approximately 200% increase in peak pull-out force and a 20% reduction in tenacity compared to uncoated control. Furthermore, all the metal-coated groups showed an increase in energy absorption, with aluminum-coated yarns showing the highest increase of 230% compared to control. These results suggest enhanced frictional interactions during yarn pull-out in metal-coated yarns compared to uncoated control as evidenced by the surface roughness profile of individual fibers and inter-yarn frictional calculations.