Nevada Test Research Articles

Evaluation of several relevant fractionation processes as possible explanation for radioxenon isotopic activity ratios in samples taken near underground nuclear explosions in shafts and tunnels

Gas samples taken from two historic underground nuclear tests done in 1989 at the Nevada National Security Site (NNSS), formerly the Nevada Test Site (NTS), were examined to determine how xenon isotopes fractionate because of early-time cavity processes, transport through the rock, or dispersal through tunnels. Xenon isotopes are currently being used to distinguish civilian sources of xenon in the atmosphere from sources associated with underground nuclear explosions (UNEs). The two nuclear tests included (1) BARNWELL, a test conducted in a vertical shaft approximately 600 m below ground surface at Pahute Mesa, and (2) DISKO ELM, a horizontal line-of-sight test done in P-tunnel approximately 261 m below the surface of Aqueduct Mesa. Numerical flow and transport models developed for the two sites had mixed success when attempting to match the observed xenon isotope ratios. At the BARNWELL site, the simulated xenon isotope ratios were consistent with measurements from the chimney and ground surface, and appeared to have been affected primarily by fractionation during subsurface transport. At the DISKO ELM site, samples taken from two elevations in the chimney failed to show the degree of fractionation predicted by the models during transport, and did not show evidence for significant fractionation due to early-time condensation of refractory xenon-precursor radionuclides into the melt glass. Gas samples taken from the adjacent tunnels in the days following the test showed mixed evidence for early-time separation of xenon isotopes from their iodine precursors.

Measuring Gas Transport and Sorption in Large Intact Geologic Specimens via the Piezometric Method

Uptake of noble gases into heterogeneous geologic core samples was measured using a piezometric methodology. In addition to measuring accessible porosity—as with gas pycnometry—by monitoring the rate of pressure decay, this method can also be used to estimate the gas effective diffusivity in the sample. In contrast to previous applications of this method, where milligram quantities of fractured grains are characterized, here approximately kilogram core samples were left intact when tested. In doing so, a more representative sample of the heterogeneous field geology is provided. Additionally, alteration of the pore structure and connectivity during sample preparation is avoided. To scale the piezometric method from milligrams to kilograms, the system was designed to operate at medium vacuum (1 to 100 Pa) to restrict transport in pores less than approximately 60 µm to large Knudsen numbers. To test the system performance, two samples of interest were selected: a rhyolitic welded tuff from Blue Canyon Dome at the Energetic Materials Research and Testing Center and a zeolitized non-welded rhyolitic tuff from the Nevada National Security Site. Three noble gases were utilized in this test series; Argon and xenon as they are of direct interest to nuclear monitoring efforts and helium as it is a weakly adsorbing reference standard. Additionally, mercury intrusion porosimetry measurements were made on subsamples of the core to compare the observed porosity by the two methods and to discuss gas transport rates in the context of the measured pore distribution.

Characterization of Agfa Structurix series D4 and D3sc x-ray films in the 0.7-4.6 keV energy range.

X-ray films remain a key asset for high-resolution x-ray spectral imaging in high-energy-density experiments conducted at the National Ignition Facility (NIF). The soft x-ray Opacity Spectrometer (OpSpec) fielded at the NIF has an elliptically shaped crystal design that measures x rays in the 900-2100eV range and currently uses an image plate as the detecting medium. However, Agfa D4 and D3sc x-ray films' higher spatial resolution provides increased spectral resolution to the data over the IP-TR image plates, driving the desire for regular use of x-ray film as a detecting medium. The calibration of Agfa D4 x-ray film for use in the OpSpec is communicated here. These calibration efforts are vital to the accuracy of the NIF opacity measurements and are conducted in a previously un-studied x-ray energy range under a new film development protocol required by NIF. The absolute response of Agfa D4 x-ray film from 705 to 4620eV has been measured using the Nevada National Security Site Manson x-ray source. A broader range of energies was selected to compare results with previously published data. The measurements were taken using selected anodes, filters, and applied voltages to produce well-defined energy lines.

Heterogeneous multiphase flow properties of volcanic rocks and implications for noble gas transport from underground nuclear explosions

AbstractOf interest to the Underground Nuclear Explosion Signatures Experiment are patterns and timing of explosion‐generated noble gases that reach the land surface. The impact of potentially simultaneous flow of water and gas on noble gas transport in heterogeneous fractured rock is a current scientific knowledge gap. This article presents field and laboratory data to constrain and justify a triple continua conceptual model with multimodal multiphase fluid flow constitutive equations that represents host rock matrix, natural fractures, and induced fractures from past underground nuclear explosions (UNEs) at Aqueduct and Pahute Mesas, Nevada National Security Site, Nevada, USA. Capillary pressure from mercury intrusion and direct air–water measurements on volcanic tuff core samples exhibit extreme spatial heterogeneity (i.e., variation over multiple orders of magnitude). Petrographic observations indicate that heterogeneity derives from multimodal pore structures in ash‐flow tuff components and post‐depositional alteration processes. Comparisons of pre‐ and post‐UNE samples reveal different pore size distributions that are due in part to microfractures. Capillary pressure relationships require a multimodal van Genuchten (VG) constitutive model to best fit the data. Relative permeability estimations based on unimodal VG fits to capillary pressure can be different from those based on bimodal VG fits, implying the choice of unimodal vs. bimodal fits may greatly affect flow and transport predictions of noble gas signatures. The range in measured capillary pressure and predicted relative permeability curves for a given lithology and between lithologies highlights the need for future modeling to consider spatially distributed properties.

Preliminary Analysis of Source Physics Experiment Explosion‐Triggered Microseismicity Using the Back‐Projection Method

AbstractA series of four chemical explosions were detonated in a deep borehole within the Yucca Flat Dry Alluvium Geology (DAG) at the Nevada National Security Site between 2018 and 2019. The two larger chemical explosions of 50 tons (DAG‐2) and 10 tons (DAG‐4) TNT equivalent yield triggered energetic aftershock sequences numbering 1392 and 347 microearthquakes, respectively, within the first 10 days. No significant aftershock activity was observed for the two smaller 1‐ton explosions (DAG‐1 and DAG‐3). We used a back‐projection method based on travel‐time migration and stacking of signal‐to‐noise ratio traces to detect, associate and locate aftershocks from a subset of 22‐geophones within a larger 2 2 km seismic array surrounding the borehole. The aftershocks located within 300 m of the borehole and the depths were above the working points of 300 and 50 m depths of DAG‐2 and DAG‐4, respectively, ruling out triggering slip on geologic faults or disturbances beneath neighboring collapse craters. DAG‐2 and DAG‐4 aftershocks decayed at similar rates, with power‐law exponents of p = 1.48 and p = 1.49, respectively. These decay rates are comparable to aftershocks sequences triggered by earthquakes and historical nuclear explosions at Yucca Flat. A smooth power‐law aftershock decay within the first 10 days suggests a triggering mechanism from explosion generated stress relaxation due to the diffusion of high gas pressures in the cavity and radial fractures. A more random and episodic aftershock rate would be expected due to cavity collapse or falling rubble in chimney formation.

Investigating Vegetation Responses to Underground Nuclear Explosions Through Integrated Analyses

AbstractVegetation has the potential to respond to underground nuclear explosions, yet these links have not been fully explored. Given the lack of previously described signatures, the changes in vegetation are possibly subtle. The integration of multiple different data streams is potentially a useful approach to improve signal detection. Here, we investigate whether semi‐arid vegetation growth patterns responded to eight legacy underground nuclear tests at the Nevada National Security Site in southern Nevada, USA. We tested for spatial and temporal changes in vegetation cover, tree growth patterns, and tree leaf spectral properties using ground‐based measurements, including those from tree‐rings and hyperspectral surface vegetation reflectance, as well as space‐based measurements of Normalized Difference Vegetation Index (NDVI) from Landsat. Multiple data streams suggest a localized (<1.2 km) spatial pattern whereby tree growth is enhanced closer to the source of the underground test relative to sites further away. We also observed a more regional (>1.2–9 km) pattern whereby tree growth is suppressed coincident with a drought beginning 1 year before the 1989 tests, but continuing in the 5 years following the tests, which is anomalous relative to what is expected based on the response of tree growth to previous droughts. Quantification of the relative effects of the tests on vegetation remains a challenge due to the coincident drought and the potential for other disturbances to have impacted tree growth at this time, but the integration of these data reveals a more nuanced growth response than any other one data set indicates alone.

Imaging the Shallow Structure of the Yucca Flat at the Source Physics Experiment Phase II Site with Horizontal-to-Vertical Spectral Ratio Inversion and a Large-N Seismic Array

AbstractThe Source Physics Experiment (SPE) is a series of chemical explosions at the Nevada National Security Site (NNSS) with the goal of understanding seismic-wave generation and propagation of underground explosions. To understand explosion source physics, accurate geophysical models of the SPE site are needed. Here, we utilize a large-N seismic array deployed at the SPE phase II site to generate a shallow subsurface model of shear-wave velocity. The deployment consists of 500 geophones and covers an area of, approximately, 2.5×2 km. The array is located in the Yucca Flat in the northeast corner of the NNSS, Nye County, Nevada. Using ambient-noise recordings throughout the large-N seismic array, we calculate horizontal-to-vertical spectral ratios (HVSRs) across the array. We obtain 2D seismic images of shear-wave velocities across the SPE phase II site for the shallow structure of the basin. The results clearly image two significant seismic impedance interfaces at ∼150–500 and ∼350–600 m depth. The shallower interface relates to the contrast between Quaternary alluvium and Tertiary volcanic rocks. The deeper interface relates to the contrast between Tertiary volcanic rocks and the Paleozoic bedrock. The 2D subsurface models support and extend previous understanding of the structure of the SPE phase II site. This study shows that the HVSR method in conjunction with a large-N seismic array is a quick and effective method for investigating shallow structures.

A New Method for High Resolution Surface Change Detection: Data Collection and Validation of Measurements from UAS at the Nevada National Security Site, Nevada, USA

The use of uncrewed aerial systems (UAS) increases the opportunities for detecting surface changes in remote areas and in challenging terrain. Detecting surface topographic changes offers an important constraint for understanding earthquake damage, groundwater depletion, effects of mining, and other events. For these purposes, changes on the order of 5–10 cm are readily detected, but sometimes it is necessary to detect smaller changes. An example is the surface changes that result from underground explosions, which can be as small as 3 cm. Previous studies that described change detection methodologies were generally not aimed at detecting sub-5-cm changes. Additionally, studies focused on high-fidelity accuracy were either computationally modeled or did not fully provide the necessary examples to highlight the usability of these workflows. Detecting changes at this threshold may be critical in certain applications, such as global security research and monitoring for high-consequence natural hazards, including landslides. Here we provide a detailed description of the methodology we used to detect 2–3 cm changes in an important applied research setting—surface changes related to underground explosions. This methodology improves the accuracy of change detection data collection and analysis through the optimization of pre-field planning, surveying, flight operations, and post-processing the collected data, all of which are critical to obtaining the highest output data resolution possible. We applied this methodology to a field study location, collecting 1.4 Tb of images over the course of 30 flights, and location data for 239 ground control points (GCPs). We independently verified changes with orthoimagery, and found that structure-from-motion, software-reported root mean square errors (RMSEs) for both control and check points underestimated the actual error. We found that 3 cm changes are detectable with this methodology, thereby improving our knowledge of a rock’s response to underground explosions.

Travel time curves and isochron maps from the Borovoye digital archive for the Nevada and Semipalatinsk Nuclear Test Sites

The paper presents generalized parameters of digital seismograms of the Borovoye Geophysical Observatory (BRVK). The seismograms were obtained after reformatting the archive of the Institute of Geosphere Dynamics (IDG RAS). For this research, we consider looking at the most representative entries – underground nuclear explosions (UNE). The particular interest is the analysis of the P-wave travel times at the regional distances Δ≈5–7° and Δ≈89–91°. These deep paths cross the boundaries of the Earth crust, mantle and outer core. There are UNE digital records at the Semipalatinsk Test Site (ΔSTS ≈5.56–6.27°) and at the Nevada Test Site (ΔNTS≈89.77–90.15°). For this reason, we use UNEs from STS and NTS from 1967 to 1992. UNEs locations and magnitudes are taken from published open sources. The results for P-wave travel time curves on the BRVK – NTS and BRVK – STS paths and the isochron maps are obtained. The linear functions for travel time curves are defined by the linear regression. The resulting seismic travel time curves for P waves are compared to AK135. Because global one-dimensional seismic reference models are non-unique solution to regional seismic data, in terms of physical structure. The results help identify seismic wave velocities at the source-receiver travel paths and character of geologic media. Such physical models could significantly facilitate physical interpretation of seismic structures in the area of Central Asia (e.g. Tien Shan) and global.

A blocking scheme for dimension-robust Gibbs sampling in large-scale image deblurring

ABSTRACT Among the most significant challenges with using Markov chain Monte Carlo (MCMC) methods for sampling from the posterior distributions of Bayesian inverse problems is the rate at which the sampling becomes computationally intractable, as a function of the number of estimated parameters. In image deblurring, there are many MCMC algorithms in the literature, but few attempt reconstructions for images larger than pixels ( parameters). In quantitative X-ray radiography, used to diagnose dynamic materials experiments, the images can be much larger, leading to problems with millions of parameters. We address this issue and construct a Gibbs sampler via a blocking scheme that leads to a sparse and highly structured posterior precision matrix. The Gibbs sampler naturally exploits the special matrix structure during sampling, making it ‘dimension-robust’, so that its mixing properties are nearly independent of the image size, and generating one sample is computationally feasible. The dimension-robustness enables the characterization of posteriors for large-scale image deblurring problems on modest computational platforms. We demonstrate applicability of this approach by deblurring radiographs of size pixels ( parameters) taken at the Cygnus Dual Beam X-ray Radiography Facility at the U.S. Department of Energy's Nevada National Security Site.

Development of nuclear underground engineered test surrogates (NUGETS): preliminary composition study and production method

A method for formulation and production of a new variety of surrogate nuclear explosive melt debris (NEMD), Nuclear UnderGround Engineered Test Surrogates (NUGETS), based on detonation of a notional improvised nuclear device in an underground environment analogous to the Nevada National Security Site is presented. Extensive statistical analyses of precursory geochemical and geophysical characteristics are combined with an augmented surrogate debris cooling technique. Comparison of NUGETS formulation to those of other NEMD surrogates is reported. Application of NUGETS methodology to future studies in urban, underground, post-detonation technical nuclear forensic analysis is suggested.

Investigating the detection of underground nuclear explosions by radon displacement

A novel approach is proposed to detect underground nuclear explosions (UNEs) through the displacement of natural radon isotopes (222Rn and 220Rn). Following an explosion, it is hypothesized that the disturbance and pressurization of the sub-surface would facilitate the movement of radon from the depth of the UNE towards the surface resulting in increased soil gas activity. The resulting signal may be magnified by a factor of 2.0–4.9 by the decay of radon to its short-lived progeny. Increases in background activity may be useful for identifying locations to perform additional measurements, or as a detectable signal at monitoring stations. To validate this hypothesis, radon detection instrumentation was deployed at the Dry Alluvium Geology (DAG) site of the Source Physics Experiment (SPE) at the Nevada National Security Site (NNSS). Natural fluctuations in the soil gas activity due to barometric pumping, and the lower yield of the chemical explosions (1–50 t) made it difficult to confirm a displacement of radon from the explosions, and further study to validate the proposed hypothesis is recommended.

Using Dense Array Waveform Correlations to Build a Velocity Model with Stochastic Variability

ABSTRACTIn an ongoing effort to improve 3D seismic-wave propagation modeling for frequencies up to 10 Hz, we used cross correlations between vertical-component waveforms from an underground chemical explosion to estimate the statistical properties of small-scale velocity heterogeneities. The waveforms were recorded by a dense 2D seismic array deployed during the Source Physics Experiments for event number 5 (SPE-5) in a series of six underground chemical explosions, conducted at the Nevada National Security Site. The array consisted of 996 geophones with a 50–100 m grid spacing, deployed at the SPE site at the north end of the Yucca Flat basin. The SPE were conducted to investigate the generation and propagation of seismic and acoustic waves from underground explosions.Comparisons of decay rates of waveform cross correlations as function of interstation distance, computed for observed and synthetic seismograms from the SPE-5 chemical explosion, were used to constrain statistical properties of correlated stochastic velocity perturbations representing small-scale heterogeneities added to a geology-based velocity model of the Yucca Flat basin. Using comparisons between recorded and simulated waveform cross correlations, we were able to recover sets of statistical properties of small-scale velocity perturbations in the velocity model that produce the best-fit between the recorded and simulated ground motion. The stochastic velocity fluctuations in the velocity model that produced the smallest misfits have a horizontal correlation distance of between 400 and 800 m, a vertical correlation distance between 100 and 200 m, and a standard deviation of 10% from the nominal model velocity in the alluvium basin layers. They also have a horizontal correlation distance of 1000 m, a vertical correlation distance of 250 m, and a standard deviation of 6% in the underlying and consolidated sedimentary layers, up to a depth of 4 km.Comparisons between observed and simulated wavefields were used to assess the proposed small-scale heterogeneity enhancements to the Yucca Flat basin model. We found that adding a depth-resolved stochastic variability to the geology-based velocity model improves the overall performance of ground-motion simulations of an SPE-5 explosion in the modeled frequency range up to 10 Hz. The results may be applicable to other similar basins.

The Nevada Movement: A Model of Trans-Indigenous Antinuclear Solidarity

Corbin Harney, Western Shoshone elder and spiritual leader, rises in prayer. He lights a ceremonial pipe and upon inhaling offers it to Olzhas Suleimenov, Kazakh national poet and leader of the Nevada-Semipalatinsk movement, who smokes it in turn. After completing the Western Shoshone Pipe Ceremony, the two reach down into the earth, each pulling up a stone, which they then proceed—in accordance with Kazakh custom—to throw at the face of evil—in this case, the face of nuclear fallout. This face is everywhere at the Nevada Test Site, and yet, nowhere to be seen. Guidelines for direct action campaigns at the test site caution would-be activists to be afraid of it—to be afraid of the dust. Contaminated from decades of nuclear weapons testing, this dust kills—just one more thing the Western Shoshone share with the Kazakhs, who, nearly a year-and-a-half earlier and halfway across the globe, gathered at Semipalatinsk, the Soviet counterpart to the Nevada Test Site, to hurl their own stones at the face of this very same evil. In 1989, inspired by Western Shoshone attempts to end nuclear weapons testing on their ancestral homeland, the Kazakhs rose up to demand an immediate cessation of Soviet testing at Semipalatinsk. They not only named their nascent movement Nevada, but they also took as their logo a Kazakh nomad sharing a pipe with a Western Shoshone. Over the next two years, Western Shoshone and Nevada activists engaged in cultural and political exchanges that sent delegates to protest in each other’s respective homeland. Soviet officials have repeatedly credited the Nevada-Semipalatinsk movement in their decision to halt their nuclear weapons testing program. By August 1991 Semipalatinsk closed. And without a credible Soviet threat the United States halted its own nuclear weapons testing program the following year. This essay documents the origins of this historic trans-Indigenous activism, as well as the joint strategies, tactics, and discourses employed by both movements in their bid to end nuclear weapons testing in their respective homelands.

Using Carbon-14 of dissolved organic carbon to determine groundwater ages and travel times in aquifers with low organic carbon

This study used dissolved organic carbon (DOC) carbon-14 (14C) to determine groundwater ages that were then used to calculate groundwater travel times in southern Nevada aquifers that have low organic content. These travel times are compared with the more standard dissolved inorganic (DIC) 14C method for determining groundwater ages and travel times. Groundwater travel times for aquifers in southern Nevada using DOC 14C are thousands of years shorter than DIC 14C travel times for groundwater flow outside of recharge areas along four of the five flow paths evaluated for this study. The DOC 14C travel times range from 2,300 to 2,900 years (yrs) as compared to DIC 14C travel times that range from 8,200 to 22,000 yrs (uncorrected ages) and 4,700 to 19,000 yrs (corrected ages). The DOC 14C groundwater travel times in carbonate-rock and volcanic-rock aquifers of southern Nevada are similar to travel times determined from: (1) hydrogeologic data; (2) observations of rapid high-level tritium transport at the Nevada National Security Site; and (3) 550,000-yr δ18O and δ13C global climate records for calcite precipitated in Devils Hole, Nevada at the end of one of the flow paths. DOC 14C travel time calculations need to account for fewer processes than DIC 14C for aquifers that contain little organic matter and do not have redox reactions. DOC 14C travel-times can be calculated directly from the DOC 14C data in these aquifers without corrections if dissolution of organic carbon and sorption and matrix diffusion of DOC 14C onto or into the aquifer matrix is minimal. Laboratory experiments showed that little organic carbon was being leached from aquifer rocks, sorption of organic carbon ranged from 4.3% sorbed in carbonate rocks to 0.5% sorbed in volcanic rocks, and matrix diffusion coefficients were slower in lower porosity carbonate rocks (1.7 × 10−7 cm2/s) than in higher porosity volcanic rocks (2.9 × 10−7 cm2/s). The lack of dissolution of organic carbon in study area aquifers is also supported by the decrease in DOC along flow paths and the DOC composition of groundwater changing little as groundwater flows from recharge areas into the adjacent valleys. In contrast, DIC 14C groundwater travel-time calculations must be corrected for complex chemical reactions and physical processes, including mineral/gas dissolution, mineral/gas precipitation/exsolution, cation exchange, and carbon isotopic exchange that can significantly change the amount of DIC 14C in groundwater along flow paths by processes other than radioactive decay. DIC 14C can also be affected by sorption and matrix diffusion, but these processes may, or may not, be captured in geochemical modeling of precipitation/dissolution and carbon isotopic exchange reactions. Corrected DIC 14C ages, therefore, represent maximum ages.

Dense Seismic Array Study of a Legacy Underground Nuclear Test at the Nevada National Security Site

ABSTRACTThe complex postdetonation geologic structures that form after an underground nuclear explosion are hard to constrain because increased heterogeneity around the damage zone affects seismic waves that propagate through the explosion site. Generally, a vertical rubble-filled structure known as a chimney is formed after an underground nuclear explosion that is composed of debris that falls into the subsurface cavity generated by the explosion. Compared with chimneys that collapse fully, leaving a surface crater, partially collapsed chimneys can have remnant subsurface cavities left in place above collapsed rubble. The 1964 nuclear test HADDOCK, conducted at the Nevada test site (now the Nevada National Security Site), formed a partially collapsed chimney with no surface crater. Understanding the subsurface structure of these features has significant national security applications, such as aiding the study of suspected underground nuclear explosions under a treaty verification. In this study, we investigated the subsurface architecture of the HADDOCK legacy nuclear test using hybrid 2D–3D active source seismic reflection and refraction data. The seismic data were acquired using 275 survey shots from the Seismic Hammer (a 13,000 kg weight drop) and 65 survey shots from a smaller accelerated weight drop, both recorded by ∼1000 three-component 5 Hz geophones. First-arrival, P-wave tomographic modeling shows a low-velocity anomaly at ∼200 m depth, likely an air-filled cavity caused by partial collapse of the rock column into the temporary postdetonation cavity. A high-velocity anomaly between 20 and 60 m depth represents spall-related compaction of the shallow alluvium. Hints of low velocities are also present near the burial depth (∼364 m). The reflection seismic data show a prominent subhorizontal reflector at ∼300 m depth, a short-curved reflector at ∼200 m, and a high-amplitude reflector at ∼50 m depth. Comparisons of the reflection sections to synthetic data and borehole stratigraphy suggest that these features correspond to the alluvium–tuff contact, the partial collapse cavity, and the spalled layer, respectively.

Determination of fracture apertures via calibration of three-dimensional discrete-fracture-network models: application to Pahute Mesa, Nevada National Security Site, USA

The Pahute Mesa Corrective Action Unit on the Nevada National Security Site (USA) contains several fractured aquifers that can potentially provide high-permeability pathways for migration of radionuclides away from underground nuclear testing locations. Though geometric properties of fractures such as length and orientations, can generally be obtained by geophysical methods and borehole image analyses, their hydraulic properties (primarily influenced by their apertures) are often unknown or have a high degree of uncertainty. This study presents a novel approach to determine fracture apertures at a site by integrating numerical models of flow with data of certain geometrical fracture attributes and hydraulic response of the system because of long-term pumping. Discrete fracture network (DFN) models were used in this study to construct a three-dimensional (3-D) flow model for lava-flow aquifers of Western Pahute Mesa (WPM). Models built using a 3-D DFN code, dfnWorks, were calibrated to hydraulic drawdown observations recorded at the site of a forced-gradient experiment (BULLION FGE). Multiple conceptualizations of boundary conditions, fracture aperture distributions, and realizations of the fracture network were considered to simulate flow and migration of particles between an injection well and a pumping well. Calibrating the DFN flow models to hydraulic drawdown data constrained the ranges of aperture values and helped develop a realistic description of the properties of fractured rocks in WPM. The aperture values resulting from this study are expected to enhance understanding of radionuclide transport in WPM and support the development of large-scale flow and transport models.

PKIKPBC ON THE TRAVELPATH NEVADA – “MIRNYY” STATION (ANTARCTICA)

The Geophysical Service of the Russian Academy of Sciences (Obninsk, Moscow Region) made it possible to process seismograms of “Mirnyy” station in Antarctica. Seismograms for the period 1961–1991 were viewed, when underground nuclear explosions were conducted at the Nevada Test Site of the United States. To calculate and construct a local travel time curve and a linear trend of the PKIKPBC body wave travel time on the Nevada Test Site (NTS) – “Mirnyy” seismic station (MIR), 120 seismograms from Kirnos seismograph (SVKM) were selected for the period May 13, 1966 – October 18, 1991: 60 for each site Pahute and Yucca of the Nevada Test Site (NTS). The article presents local travel-time curves and linear trends of the PKIKPBC wave on the travelpath NTS – MIR. Increase in time travel with years as in travelpaths Nevada Test Site – Borovoye [14]. Dependency of arrival times of the PKIKPBC wave on mb magnitude was detected. The obtained data can be used for comparison of results of PKIKPBC wave study from the Nevada Test Site on other stations, for examples, at “Novolazarevskaya” Antarctic station.

Accounting for Unfissioned Plutonium from the Trinity Atomic Bomb Test.

The Trinity test device contained about 6 kg of plutonium as its fission source, resulting in a fission yield of 21 kT. However, only about 15% of the Pu actually underwent fission. The remaining unfissioned plutonium eventually was vaporized in the fireball and after cooling, was deposited downwind from the test site along with the various fission and activation products produced in the explosion. Using data from radiochemical analyses of soil samples collected postshot (most many years later), supplemented by model estimates of plutonium deposition density estimated from reported exposure rates at 12 h postshot, we have estimated the total activity and geographical distribution of the deposition density of this unfissioned plutonium in New Mexico. A majority (about 80%) of the unfissioned plutonium was deposited within the state of New Mexico, most in a relatively small area about 30-100 km downwind (the Chupadera Mesa area). For most of the state, the deposition density was a small fraction of the subsequent deposition density of Pu from Nevada Test Site tests (1951-1958) and later from global fallout from the large US and Russian thermonuclear tests (1952-1962). The fraction of the total unfissioned Pu that was deposited in New Mexico from Trinity was greater than the fraction of fission products deposited. Due to plutonium being highly refractory, a greater fraction of the Pu was incorporated into large particles that fell out closer to the test site as opposed to more volatile fission products (such as Cs and I) that tend to deposit on the surface of smaller particles that travel farther before depositing. The plutonium deposited as a result of the Trinity test was unlikely to have resulted in significant health risks to the downwind population.

Impulse response measurements of fast scintillator-based current mode detectors

Experimental measurements were performed to characterize the impulse response of seven different gamma ray detector prototypes being considered to be fielded as a current mode diagnostic for neutron-diagnosed subcritical experiments at the Nevada National Security Site (NNSS). For such experiments, the impulse response of the fielded detector is required to have a full-width half-maximum (FWHM) of approximately 5 ns or less and a low amplitude tail. Each of the detector prototypes evaluated in this work were selected based on the potential to meet this performance requirement. An impulse of bremsstrahlung x-rays created from 2 MeV electrons from the NNSS’s Transformational Diagnostics and Imaging, Los Alamos, linear accelerator was measured with each detector. The measured impulse response was evaluated for each detector configuration. A comparison of these impulse responses revealed a relationship between fast timing performance and scintillator material, geometry, surface reflectivity conditions, and photomultiplier tube (PMT) selection. The detector configuration yielding an impulse response with the shortest FWHM of 3.82 ± 0.11 ns and possessing a low amplitude tail was a 5-inch (12.7-cm) diameter, 5-inch (12.7-cm) height cylindrical EJ-399-17-VI liquid scintillator in a black-painted housing coupled to an Adit D798B 5-inch (12.7-cm) PMT by means of a 0.079-inch (0.2-cm) EJ-560 optical coupling pad.