The Xcounts algorithm for calculating air concentrations of radioactive xenon isotopes (Eslinger et al., 2023) has been extended to estimate 127Xe in addition to 131mXe, 133mXe, 133Xe, and 135Xe. The algorithm was applied to 119 samples collected with a SAUNA QB system (Ringbom et al., 2023) during a two-month atmospheric tracer release experiment. The algorithm identified two samples with 127Xe present from a single 1.5 h release about 3.5 km upwind of the sampler and no false detections of 127Xe were observed in the other samples.

A nuclear explosion screening exercise in 2023 (Maurer et al., 2023) found challenges with discerning anomalous radioxenon activity concentrations relative to elevated background concentrations. Research has continued into methods to detect anomalous radioxenon concentrations by comparing samples to estimates of atmospheric radioxenon background concentrations caused by releases at nuclear reactors or medical isotope production facilities. A new approach estimates the sample concentrations using time-varying radioxenon release rates obtained using optimization techniques that constrain the facility release rates to plausible amounts based on historical data or facility knowledge. The purpose of the optimization is to determine whether any combination of plausible release rates from emitting facilities can explain a series of radioxenon measurements at one or more sampling stations. A case study uses radioxenon data collected at three locations in western Europe for a month in 2021 and considers releases from 77 locations. Fewer samples are identified as being anomalous using a simplistic flagging rule than from an application of the current International Monitoring System (IMS) activity concentration-level rule.

Numerous algorithms have been developed to determine the source characteristics for an atmospheric radionuclide release, e.g., (Bieringer et al., 2017). This study compares three models that have been applied to the data collected by the International Monitoring System operated by the Comprehensive Nuclear-Test-Ban Treaty Organization Preparatory Commission to estimate source event parameters. Each model uses a different approach to estimate the parameters. A deterministic model uses a possible source region (PSR) approach (Ringbom et al., 2014) that is based on the correlation between predicted and measured sample values. A model (now called BAYEST) developed at Pacific Northwest National Laboratory uses a Bayesian formulation (Eslinger et al., 2019, 2020; Eslinger and Schrom, 2016). The FREAR model uses a different Bayesian formulation (De Meutter and Hoffman, 2020; De Meutter et al., 2021a, 2021b). The performance of the three source-location models is evaluated with 100 synthetic release cases for the single xenon isotope, 133Xe. The release cases resulted in detections in a fictitious network with 120 noble gas samplers. All three source-location models use the same sampling data. The two Bayesian models yield more accurate location estimates than the deterministic PSR model, with FREAR having slightly better location performance than BAYEST. Samplers with collection periods of 3, 6, 8, 12, and 24-h were used. Results from BAYEST show that location accuracy improves with each reduction in sample collection length. The BAYEST model is slightly better for estimating the start time of the release. The PSR model has about the same spread in start times as the FREAR model, but the PSR results have a better average start time. The Bayesian source-location algorithms give more accurate results than the PSR approach, and provide release magnitude estimates, while the base PSR model does not estimate the release magnitude. This investigation demonstrates that a reasonably dense sampling grid will sometimes yield poor location and time estimates regardless of the model. The poor estimates generally coincide with cases where there is a much larger distance between the release point and the first detecting sampler than the average sampler spacing.

The purpose of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) is to establish a legally binding ban on nuclear weapon test explosions or any other nuclear explosions. The Preparatory Commission for the CTBT Organization (CTBTO PrepCom) is developing the International Monitoring System (IMS) that includes a global network of 80 stations to monitor for airborne radionuclides upon entry into force of the CTBT. All 80 radionuclide stations will monitor for particulate radionuclides and at least half of the stations will monitor for radioxenon. The airborne radionuclide monitoring is an important verification technology both for the detection of a radionuclide release and in the determination of whether the release event originates from a nuclear explosion as opposed to an industrial use of nuclear materials. Nuclear power plants and many medical isotope production facilities release radioxenon into the atmosphere. Low levels of a few particulate isotopes, such as iodine, may also be released. Detections of multiple isotopes are useful for screening the radionuclide samples for relevance to the Treaty. This paper examines the anticipated joint detections in the IMS of noble gas and particulate isotopes from underground nuclear explosions where breaches in the underground containment vents from low levels to up to 1% of the radionuclide inventory of the resulting fission products to the atmosphere. Detection probabilities are based on 844 simulated release events spaced out at 17 release locations and one year in time. Six different release (venting) scenarios, including two fractionated scenarios, were analyzed. When ranked by detection probability, 11 particulate isotopes and one noble gas isotope (133Xe) appear in the top 20 isotopes for all six release scenarios. Using the 11 particulate isotopes and the one noble gas isotope, the IMS has nearly the same detection probability as when 45 particulate and 4 noble gas isotopes are used. Thus, a limited list of relevant radionuclides may be sufficient for treaty verification purposes. The probability that at least one particulate and at least one radioxenon isotope would be detected in the IMS from the release events ranged from 0.15 to 0.86 depending on the release scenario.

Many countries are considering nuclear power as a means of reducing greenhouse gas emissions, and the IAEA (IAEA, 2022) has forecasted nuclear power growth rates up to 224% of the 2021 level by 2050. Nuclear power plants release trace quantities of radioxenon, an inert gas that is also monitored because it is released during nuclear explosive tests. To better understand how nuclear energy growth (and resulting Xe emissions) could affect a global nonproliferation architecture, we modeled daily releases of radioxenon isotopes used for nuclear explosion detection in the International Monitoring System (IMS) that is part of the Comprehensive Nuclear Test-Ban Treaty: 131mXe, 133Xe, 133mXe, and 135Xe to examine the change in the number of potential radioxenon detections as compared to the 2021 detection levels. If a 40-station IMS network is used, the potential detections of 133Xe in 2050 would range from 82% for the low-power scenario to 195% for the high-power scenario, compared to the detections in 2021. If an 80-station IMS network is used, the potential detections of 133Xe in 2050 would range from 83% of the 2021 detection rate for the low-power scenario to 209% for the high-power scenario. Essentially no detections of 131mXe and 133mXe are expected. The high growth scenario could lead to a 2.5-fold increase in 135Xe detections, but the total number of detections is still small (on the order of 1 detection per day in the entire network). The higher releases do not pose a health issue, but better automated methods to discriminate between radioactive xenon released from industrial sources and nuclear explosions will be needed to offset the higher workload for people who perform the monitoring.

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.

Three unusual radioactive isotopes of xenon-Xe-125, Xe-127, and (129)mXe-have been observed during testing of a new generation radioxenon measurement system at the manufacturing facility in Knoxville, Tennessee. These are possibly the first detections of these isotopes in environmental samples collected by automated radioxenon systems. Unfortunately, the new isotopes detected by the Xenon International sampler can interfere with quantification of the radioactive xenon isotopes used to monitor for nuclear explosions.Xenon International sampling data collected during February through September 2020 were combined with an atmospheric transport model to identify the possible release location. A source-location analyses using sample counts dominated by Xe-125 strongly supports the conclusion that the release point is near (within 20 km) the sampler location. Wind patterns are not consistent with releases coming from more distant nuclear power plants. The High Flux Isotope Reactor (HFIR) and the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory are located in the region of most likely source locations.The source-location analysis cannot rule out either facility as a release location, and some of the samples may contain a combination of releases from both facilities. The source-location results using Xe-125 are not unexpected because Klingberg et al. (2013) previously published the production rate of radioactive xenon isotopes from neutron activation of stable xenon in the air at the HFIR. Up to 10(12) Bq of Xe-125 could be produced per operational day and other xenon isotopes would be produced in lesser quantities.

Objectives/scope: StimuFrac (US Patents 9,873,828 B2 and 9,447,315 B2), a CO2-reactive polymer aqueous so-lution [polyallylamine (PAA) 1wt% in water] combined with CO2, can potentially be used as a less water-intensive fracturing fluid for enhanced geothermal systems (EGS). Our previous results show that in hot dry rock (HDR), PAA/CO2 fracturing fluids outperformed other fluids such as water, CO2, and CO2/water in generating large fractures with less fluid consumed. The objective of this work is to investigate the effect of initial water saturation on the performance of StimuFrac by conducting hydraulic fracturing tests with 1/2 foot cubic rock samples held under representative EGS stress/temperature conditions and by using cyclic injection strategies (under constant injection rate). The resulting fracture hydraulic conductivities, breakdown pressures, and vol-umes of fluids required are compared.Methods/procedures/process: To simulate geothermal reservoir conditions, in all tests, the rock sample was held under triaxial confinement and at 200 degrees C, and different volumes of water were initially injected into the rock sample before any fracturing processes were initiated. For the single-cycle PAA (or water) alternating CO2 (PAG or WAG) injection fracturing experiments, one complete cycle consisted of two steps: (1) injecting a PAA slug (or water slug) followed by (2) injecting CO2 to initiate and propagate the fracture. For experiments involving multiple injection cycles, the CO2 injection pressure is increased until it peaks and begins to decline (indicating fracture initiation at this moment), and then continued being injected for another 30 s to propagate the fracture. Then, these two-step cycles [injection of PAA (or water) followed by CO2 injection (up to 2-4 mL/min)] are repeated. Applications/significance/novelty: The results of this study suggest that water saturation significantly affects the fracturing fluid transmission into the rock pore space, thus affecting fracture initiation and propagation. In this study, fracturing tests via a single injection cycle or multiple injection cycles were performed. When the rock samples are split in half following testing, it is evident that fracture propagation is considerably restricted under high water saturation conditions (where a three-day initial water injection was conducted) in comparison to stimulation experiments carried out in hot and dry rock. The fractures propagate less than 1/3 of the distance from the wellbore to the outer rock surface, and in some cases, no fracture is generated. This may be caused by leak-off dominating the fracturing process and the fluid injection rate is insufficient to overcome leak-off, even under high injection rate conditions. Additionally, CO2 could be leaking off into the wellbore annulus and this may be making it more difficult to generate sufficiently high-pressure gradients away from the near-wellbore region. Under low water saturation conditions (dry rock or after 1-day initial water injection), PAA/CO2 consistently generated significantly larger fractures compared with the other fluids. CO2 generated large frac-tures only in the hot dry rock and only when using high injection rates, though data variability is high.

The use of surfactants for foam generation and gas-mobility control to improve oil recovery of CO2 flooding processes has been extensively studied. This review covers the most important aspects regarding the design of CO2-foams for mobility control in carbonate reservoirs, including recent advances in the formulation of novel surfactants, analytical techniques to determine key properties such as adsorption and thermal stability, and bench-scale visualization systems of foam flow. Several key concepts regarding foam transport in porous media are reviewed such as the minimum pressure gradient, the effect of the partition coefficient for CO2-soluble surfactants, and the effect of oil on foam stability and rheology. This work also discusses the most recent advances of two major foam modeling methods, the semi-empirical STARS foam model and the population balance model, along with recent CO2 foam pilot tests and the elements governing success in foam deployment and oil recovery.

The objective of this work is to study the performance of 1wt% StimuFrac fluid [polyallylamine (PAA) in water] in 1/2-foot size rock samples under thermal and fluid phase conditions representative of enhanced geothermal systems (EGS) and reveal the mechanisms governing the fracturing process. Four fracturing fluids including water, CO2, CO2 with water, and CO2 with aqueous PAA were used. For all the water "only" fracturing tests, the conductivity of the rock fractured is quite low (less than 2 mu m(3) based on radial flow assumption), regardless of the constant flow rate or discrete pressure increments injection approach. For all three CO2-based fracturing fluids, granite was fractured at higher breakdown pressures, higher transient flow rates, and generated higher-conductivity fractures as compared to water. When partially saturating the rock sample with 1wt% PAA aqueous solution followed by fracturing with CO2, the volume expansion and viscosity increase triggered by CO2-induced cross-linking PAA leads to a faster pressure increase than CO2 in water saturated or initially dry. This faster pressurization rate is caused by a decrease in relative permeability of CO2 in the presence of the high-viscosity cross-linked fluid compared to the un-crosslinked CO2/water. It was also found that CO2 as a fracturing fluid can generate high fracture conductivity only when injected at very high flow rates in the presence of water or in hot dry rock (HDR). However, the conductivity of CO2 fracturing in HDR is highly variable. CO2 injected in the presence of 1wt% aqueous PAA generates fractures with the highest conductivity independently of injection flow rates and using 1/6 of the mass of CO2 as compared to CO2 fracturing in HDR. The results of this study suggest CO2/PAA is the best performing stimulation fluid under the studied P/T conditions. CO2/PAA offers the following three additional advantages over waterless CO2, and CO2/water fracturing fluids: i) it requires significantly lower volumes of CO2 due to the reduced leak off; ii) large fractures can be generated reproducibly at both low and high CO2 injection flow rate, and iii) the reversible (previously reported) viscosity increase could be beneficial to transport proppants when they become available for EGS.

Objectives/Scope: StimuFrac, a CO2-reactive polymer aqueous solution [polyallylamine (PAA) 1wt% in water], can be used as a less water-intensive fracturing fluid for enhanced geothermal systems (EGS) based on laboratory-scale investigations. Fracturing tests from inch scale samples indicate that StimuFrac in the presence of CO2 could generate fractures with high conductivity at lower breakdown pressures compared with water, CO2, and a combination of water/CO2 fracturing processes. The objective of this work is to study the performance of patented StimuFrac fluid in 1⁄2 foot side rock samples under representative EGS pressure/temperature conditions and reveal the mechanisms governing the fracturing process assisted with multiphase flow numerical simulations using Subsurface Transport Over Multiple Phases (STOMP). Methods/Procedures/Process: StimuFrac was evaluated using a high-temperature true-triaxial fracturing apparatus and 1⁄2 foot side granite cubic samples. Three representative fracturing fluids including water, CO2, CO2 with water were used as control. The fluid transport was simulated with STOMP based on a home-built model which is designed with feedback from the experimental setup and conditions. Results/Observations/Conclusions: For all the water “only” fracturing tests, the conductivity of the rock fractured is quite low (less than 2 μm based on radial flow assumption). All three CO2-based fracturing fluids, i.e., CO2 injected in hot dry rock (HDR), CO2 injected in rock partially saturated with water, and CO2 injected in rock partially saturated with aqueous PAA (1wt%), fractured granite at higher breakdown pressures, high transient flow rates, and generated higher-conductivity fractures as compared to water. In addition, faster pressurization rates with CO2-based fracturing fluids are found to be associated with higher fracture conductivities. When partially saturating the rock sample with 1wt% PAA aqueous solution followed by fracturing with CO2, the volume expansion and viscosity increase triggered by CO2-induced cross-linking of PAA leads to a faster pressure increase than CO2/water and dry CO2. This faster pressurization rate is possibly caused by (1) decrease in relative permeability of CO2 compared to that for the uncrosslinked CO2/water system, and (2) a decreased leakoff due to the increase in viscosity of PAA. It was also found that CO2 as a fracturing fluid injected in HDR can generate high fracture conductivity only when injected at very high flow rates (10 mL/min). However, the conductivity of CO2 fracturing in HDR is highly variable while CO2 injected in rock partially saturated PAA consistently generates large fractures with significantly lower variability in conductivity values. In addition, CO2/PAA fracturing fluid system generates fractures with the highest conductivity independently of injection flow rates and using 1/6 of the mass of CO2 as compared to CO2 injected in HDR. The fractured rock samples show that all three CO2-based fracturing fluids, CO2 injected in HDR, CO2 injected in rock partially saturated with water, and CO2 injected in rock partially saturated with aqueous PAA, can generate larger fracture planes than water. From the simulation pressure distribution map of water fracturing, we observed a plateau (water vapor pressure) of the pressure distribution of water in the radial direction of rock which hinders the fracture propagation. STOMP modeling results show a higher pressurization length for PAA/CO2 than other fluids, which may contribute to the consistently higher conductive fractures generated. Applications/Significance/Novelty: The results of this study suggest CO2/PAA as the best performing stimulation fluid under the studied geothermal P/T conditions. CO2/PAA offers the following three additional advantages over waterless CO2, and CO2/water fracturing fluids: 1) it requires significantly lower volumes of CO2 due to the reduced leak off, 2) large fractures can be generated reproducibly and independently of CO2 injection flow rate, and 3) the reversible viscosity (Shao et al. 2015) increase is beneficial to transport proppants when they become available for enhanced geothermal systems.

This report documents a risk assessment of CO2 containment loss and induced shear failure due to geologic carbon storage at the Wabash CarbonSAFE site. The operator, Wabash Valley Resources, has proposed adapting the onsite integrated gasification combined cycle (IGCC) facilities to produce hydrogen and injecting the byproduct CO2 stream into the subsurface Potosi dolomite formation. The purpose of this study is to assess (1) CO2 sequestration performance relative to the CarbonSAFE goals of storing 50 Mt over 30 years, (2) the risk of containment loss due to leakage along a wellbore and into an overlying aquifer, and (3) the state of stress and risk of reactivating existing fractures. This study relied upon the initial site characterization work performed by the Illinois State Geologic Survey (ISGS) along with analogue data collected from other carbon sequestration projects in the region.