As part of the Physics Experiment 1 (PE1) subsurface experiment series, a novel radioxenon injection system was developed to transfer radioactive xenon gas from a shipping cask to the experiment location through a sealed section of tunnel. The system consists of a combination of manual and pneumatic valves and is controlled and logged via a single board computer. The effectiveness of the injection system was demonstrated in the leadup to the PE1-A experiment in October 2023, where 55 GBq of 127Xe was safely and successfully injected into the experiment chamber. This process involved a combination of manual and remote management of pressures and flow rates to minimize radiation exposure and ensure the integrity of the experiment. The results underscore the system's viability for future subsurface transport experiments to better understand radioxenon migration mechanisms following underground nuclear explosions.

Underground explosions cause rock damage affecting fluid and gas transport properties. Understanding this damage is crucial for modeling underground nuclear explosions, but field-scale quantification is challenging. A series of chemical explosive experiments were conducted at Blue Canyon Dome in Socorro, NM, employing several novel measurement and sensing techniques. This study focuses on measurements of radon-222 gas concentration and its evolution post-explosions. Pre-explosion, concentrations showed significant spatial variation, while post-explosion concentrations generally decreased but varied by location. Results indicate that radon concentration alone is insufficient to characterize subsurface rock damage and that single measurements might not represent an average background.

This study assesses the potential impacts of two proposed novel medical isotope production facilities (MIPFs) by Niowave, Inc. and SHINE Technologies, LLC on the International Monitoring System (IMS) noble gas monitoring regime. By integrating publicly available data into software models, we simulated the release, decay, transport, and detection of noble gases from these facilities to IMS stations. The results of these simulations indicate that for releases from small, novel MIPFs like these, release magnitudes and facility location are likely to have a far more significant impact on IMS detections than differences in the facilities’ nuclear processes.

Fracture-released radiogenic noble gas nuclides are used to identify locations and constrain the volume of new fracture creation during subsurface detonations. Real-time, in situ noble gases and reactive gases were monitored using a field-deployed mass spectrometer and automated sampling system in a multilevel borehole array. Released gases were measured after two different detonations having distinct energy, pressure, and gas volume characteristics. Explosive-derived gases (N2O, CO2) and excess radiogenic 4He and 40Ar above atmospheric background are used to identify locations of gas transport and new fracture creation after each detonation. Fracture-released radiogenic 4He is used to constrain the volume of newly created fractures with a model of helium release from fracturing. Explosive by-product gas was observed in multiple locations both near and distal to the shot locations for both detonations. Radiogenic 4He and 40Ar release from rock damage was observed in locations near the detonation after the second, more powerful detonation. Observed 4He response is consistent with a model of diffusive release from newly created fractures. Volume of new fractures estimated from the 4He release ranges from 1 to 5 m2 with apertures ranging from 0.1 to 1 mu ${\upmu }$m. Our results provide evidence that radiogenic noble gases released during fracture creation can be identified at the field scale in real time and used to identify timing and location of fracture creation during deformation events. This technique could be useful in subsurface science and engineering problems where the location and amount of newly created rock fracturing is of interest including fault rupture, mine safety, subsurface detonation monitoring and reservoir stimulation.

This paper details the experimental development of a Cryogenic Tracer Irradiation Facility (CTIF) at the University of Texas at Austin (UT Austin). UT Austin produces gaseous radiotracers with in-core facilities, but an ex-core facility is possible using a helium compressor to freeze the target. A sample of natural xenon was irradiated using this method. The neutron flux was calculated to be 5.13 ∗ 1011 ± 3.94

Radionuclide monitoring for nuclear explosions includes measuring radioactive aerosol and noble gas concentrations in the atmosphere. The International Monitoring System (IMS) of the Comprehensive Nuclear Test-Ban Treaty has made such measurements for decades, revealing much about how atmospheric radioactivity impacts the sensitivity of the network. For example, civilian emissions of radioiodine make a substantial regional impact, but a minor global impact, while civilian radioxenon emissions create major regional and complex global impacts. The impacts are strongly influenced by the minimum releases anticipated to be interesting. The original design of the IMS anticipated relatively large releases, and the current IMS network substantially meets or exceeds the sensitivity needed to detect those levels. Much lower signal levels can be motivated from historical tests. Using a release that corresponds roughly to a one-ton equivalent of fission in the atmosphere rather than the design level of one-kiloton equivalent, the network detection probabilities for 140Ba and 131I are quite good (~ 75%) and for 133Xe is still considerable (~ 45%). Using measured and simulated background concentrations, various possible desired signal levels, and an innovative anomaly threshold, maps of sensitivity and a station ranking are developed for IMS radionuclide stations. These provide a strong motivation for additional experimentation to learn about sources and the potential plusses of new technology.

Rock damage from underground nuclear explosions (UNEs) has a strong influence on sub-surface gas movement and on seismic waveform characteristics, both of which are used to detect UNEs. Although advanced numerical simulation capabilities exist to predict rock damage patterns and corresponding detection signals, those predictions are dependent on (generally) unknown properties of the host rock. For example, the effects of in-situ mechanical heterogeneities on the explosively generated damage/fractures that provide gas flow pathways to the surface are not well understood, due largely to the difficulty in accessing and characterizing the near-source region. In this paper we demonstrate the emerging use of electrical resistivity tomography (ERT) for imaging rock damage and gas flow patterns resulting from two relatively small-scale underground chemical explosions. Pre-explosion ERT and crosshole seismic imaging revealed a natural fracture zone within the test bed. Post-explosion imaging revealed that the damage zone was non-symmetric and was focused primarily within the pre-existing fracture zone, located 10 m above the first explosion and 5 m above the second explosion. Time-lapse ERT imaging of heated air injected into the detonation borehole revealed the primary gas flow paths to be within the upper margin of the same primary damage zone. These results point to the utility of ERT imaging for understanding rock damage and gas flow patterns under experimental conditions, and to the importance of understanding the effects of geologic heterogeneity on UNE detection signals, particularly gas surface breakthrough times.

We present an overview of a small-scale tracer migration experiment that was carried out in July 2018 at the Nevada National Security Site. This experiment involved the injection of 133Xe into the bottom of a shallow borehole with multiple sampling intervals. Sampling was then conducted in the injection borehole and in a second borehole located 17 m from the injection site. A simple system for measurement of the 133Xe activity in whole air was utilized onsite. Though many samples were well below MDC, cross-hole tracer transport was observed. Along with experimental results, additional insights gained from numerical modeling are presented.

Molten Salt Reactors (MSRs) are one of six Generation IV reactor designs currently under development around the world. Because of the unique operating conditions of MSRs, which include molten fuel and the continuous removal of gaseous fission products during operation, work was performed to model the production of activation and fission products and analyze the potential impact of emissions on the International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Simulations were performed to predict the production of IMS-relevant radionuclides in four MSR designs operating under two scenarios: (1) a sealed reactor with releases only during operational shutdown, and (2) continuous reprocessing or sparging of the fuel salt. From these production estimates the radioxenon and radioiodine signatures were extracted and compared to three current reactor designs (Boiling Water Reactor, Pressurized Water Reactor, High-Power Channel-Type Reactor). In cases where continuous reprocessing of the fuel salt occurred, both the radioxenon and radioiodine signatures were nearly indistinguishable from a nuclear explosion. Estimates were also made of the potential emission rate of radioxenon for three reactor designs and it was found that MSRs have the potential to emit radioxenon isotopes at a rate of 1015−8×1016 Bq/d for 133Xe, which may adversely affect nuclear explosion monitoring, if no abatement is used. An assessment was made of activation products using a candidate fuel salt (FLiBe) mixed with corrosion products for the Thorium Molten Salt Reactor (TMSR-LF1).

Measurement of radioactive gas seepage from an underground nuclear explosion is one of the primary methods to confirm whether an event was nuclear in nature. Radioactive noble gas indicators that are commonly targeted by such measurements (e.g. 133Xe, 37Ar) have half-lives of 35 days or less. Argon-39, an activation product similar to 37Ar, is produced by the interaction between neutrons and potassium in the surrounding geology and has a half-life of 269 years. Measurements taken at three sites near three historic underground nuclear test locations at the Nevada National Security Site have all shown highly elevated levels of 39Ar in soil gas decades after the test events. Elevated levels of 39Ar were also detected in atmospheric air collected near two of these sites, and outside the entrance of the one tunnel site. These measurements demonstrate that 39Ar has the potential to be a long-term signature of an underground nuclear event which can be reliably detected at the surface or in the shallow subsurface. This radionuclide detection of an underground nuclear event decades after the event takes place is in contrast to the commonly held assumption that detecting underground nuclear events via radionuclides at the surface needs to be done in a matter of months. Depending upon what further studies show about the robustness of this signature in a variety of geological settings, it may in fact be easy to detect underground nuclear events at the surface for a very long time post-detonation.

Molten salt reactors (MSRs) are gaining support as many countries look for ways to increase power generation and replace aging nuclear energy production facilities. MSRs have inherently safe designs, are scalable in size, can burn transuranic wastes from traditional solid fuel nuclear reactors, can store excess heat in thermal reservoirs for water desalination, and can be used to produce medical isotopes as part of the real-time liquid-fuel recycling process. The ability to remove 135Xe in real time from the fuel improves the power production in an MSR because 135Xe is the most significant neutron-absorbing isotope generated by nuclear fission. Xenon-135, and other radioactive gases, are removed by sparging the fuel with an inert gas while the liquid fuel is recirculated from the reactor inner core through the heat exchangers. Without effective abatement technologies, large amounts of radioactive gas could be released during the sparging process. This work examines the potential impact of radioxenon releases on samplers used by the International Monitoring System (IMS) to detect nuclear explosions. Atmospheric transport simulations from seven hypothetical MSRs on different continents were used to evaluate the holdup time needed before release of radioxenon so IMS samplers would register few detections. Abatement technologies that retain radioxenon isotopes for at least 120 d before their release will be needed to mitigate the impacts from a molten salt breeder reactor used to replace a nuclear power plant. A holdup time of about 150 d is needed to reduce emissions to the average level of current nuclear power plants.

The emanation of radionuclides has been highlighted as a known source of uncertainty in the estimation of radionuclide source signatures from underground nuclear tests and other nuclear activities, particularly in the case of activation products. A system was developed at Pacific Northwest National Laboratory to quantify the emanation of argon from two powders and five rock types ranging in particle size from powder to small rocks. Samples were neutron irradiated and the percent emanation of 37Ar was measured to range between 0.3 and 25%. Measurements were also made of the 39Ar emanation for four of these materials and was found to be consistently lower than that of 37Ar.

While radioisotopes of noble gases are known to be indicators of underground nuclear explosions (UNE), McIntyre et al. (2017) was the first to report the presence of 39Ar in shallow soil gas in association with a decades old UNE. While this finding hinted at the potential application of 39Ar to be used as an indicator of a UNE, doing so would also require an understanding of the natural concentrations of 39Ar present in soil gas. Without knowing the expected range and variability of naturally occurring concentrations of 39Ar, it is difficult to determine what measured concentrations would be indicative of an elevated concentration. This paper presents results from 16 soil gas samples and three atmospheric air samples collected from various locations across the western United States. Shallow soil gas samples were collected into self-contained underwater breathing apparatus (SCUBA) tanks using a custom-built soil gas sampling system and then processed and analyzed for 39Ar. The measured concentrations of 39Ar varied from atmospheric air concentrations to about 3.5 times atmospheric air concentrations (58 mBq/m3). The results presented here represent the first measurements of natural background 39Ar concentrations in shallow soil gas. This data will be necessary if 39Ar is to be used as an indicator of UNE.