Nevada Test Research Articles

Locating Shallow Seismic Sources With Waves Scattered by Surface Topography: Validation of the Method at the Nevada Test Site

AbstractAccurate and robust source location is fundamental in seismology. Previously, we proposed a new full waveform location method using waves scattered by topography near the source, and we demonstrated its potential in obtaining accurate source location using synthetic data. In the work presented here, we validate this method with observed seismic data from the 1993 Non‐Proliferation Experiment, a chemical explosion with a precisely known location in a region with moderate topography at the Nevada Test Site. We select the first arrivals (Pn/Pg) and their immediate codas to determine the source location and compare our solution with the known location. We use a collocated‐grid finite‐difference method to calculate the strain Green's tensor in a grid‐search volume containing the source and obtain three‐component synthetic waveforms at 12 broadband seismic stations at regional distances using source‐receiver reciprocity. We assess the solution using a least squares misfit between the observed and synthetic waveforms. When scattered coda waves are incorporated, the best solution is within a few hundreds of meters of the exact source location, and the estimated uncertainty of the solution is reduced compared to the waveform‐based solution using only the P waves. The solution is robust to the choice of the frequency content and to the addition of random velocity heterogeneity. We conclude that the full waveform source location method is effective for real seismic data, though more validation tests like this one are needed to further understand its efficacy for source‐station geometry, roughness of topography, signal quality, and other factors.

Hydrothermal Alteration of Nuclear Melt Glass, Colloid Formation, and Plutonium Mobilization at the Nevada National Security Site, U.S.A.

Approximately 2.8 t of plutonium (Pu) has been deposited in the Nevada National Security Site (NNSS) subsurface as a result of underground nuclear testing. Most of this Pu is sequestered in nuclear melt glass. However, Pu migration has been observed and attributed to colloid facilitated transport. To identify the mechanisms controlling Pu mobilization, long-term (∼3 year) laboratory nuclear melt glass alteration experiments were performed at 25 to 200 °C to mimic hydrothermal conditions in the vicinity of underground nuclear tests. The clay and zeolite colloids produced in these experiments are similar to those identified in NNSS groundwater. At 200 °C, maximum Pu and colloid concentrations of 30 Bq/L and 150 mg/L, respectively, were observed. However, much lower Pu and colloid concentrations were observed at 25 and 80 °C. These data suggest that Pu concentrations above the drinking water Maximum Contaminant Levels (0.56 Bq/L) may exist during early hydrothermal conditions in the vicinity of underground nuclear tests. However, formation of colloid-associated Pu will tend to decrease with time as nuclear test cavity temperatures decrease. Furthermore, median colloid concentrations in NNSS groundwater (1.8 mg/L) suggest that the high colloid and Pu concentrations observed in our 140 and 200 °C experiments are unlikely to persist in downgradient NNSS groundwater. While our experiments did not span all groundwater and nuclear melt glass conditions that may be present at the NNSS, our results are consistent with the documented low Pu concentrations in NNSS groundwater.

Exploiting fission chain reaction dynamics to image fissile materials

Abstract Radiation imaging is one potential method to verify nuclear weapons dismantlement. We present a method to discriminate between non-multiplying and multiplying neutron sources using a neutron coded aperture imaging system. This method applies time-correlated pulse-height (TCPH) analysis to identify neutrons in fission chain reactions and recreate images using only these events. This analysis was applied to measurements of weapons-grade plutonium (WGPu) metal and 252Cf performed at the Nevada National Security Site (NNSS) Device Assembly Facility in July 2015. The results demonstrate it is possible to eliminate the non-fissile 252Cf source from the image while preserving the fissile WGPu source. TCPH analysis was also applied to additional scenes in which the WGPu and 252Cf sources were measured individually. The results of these separate measurements further demonstrate the ability to remove the non-fissile 252Cf source and retain the fissile WGPu source. Simulations performed using MCNPX-PoliMi indicate that in a one hour measurement, hollow WGPu spheres are retained at a 1 σ level for neutron multiplications M ≃ 2 . 7 and above. This work has potential application to dismantlement verification where it may be valuable to verify that all neutron sources in an image are multiplying.

Solid state streak camera prototype performance testing and future upgrades

Streak Cameras are an essential diagnostic tool used in shock physics and high energy density physics experiments. Such experiments require well calibrated temporally resolving diagnostics for studying events that occur on the nanosecond to microsecond time scales. The Nevada National Security Site (NNSS) and Sandia National Laboratories (SNL) have built a 42-channel solid state streak camera (SSSC) prototype as a proof of concept for use as a streak camera replacement. This work is part of an ongoing project to develop the technology to a level competitive with analog streak cameras. The device concept, results from electronic and laser testing, along with recent improvements to increase the device's dynamic range will be discussed in this manuscript.

Coriolis force-assisted inertial confinement fusion

AbstractA fundamental problem for the realization of laser fusion through the implosion of a spherical target is Kidder's E−1/6 law, where E is the energy needed for ignition, proportional to the 6th power of the ratio R/R0, where R0 and R are the initial and final implosion radii, respectively. This law implies that the ignition energy is very sensitive to the ratio R0/R, or vice versa, the ratio R0/R is very insensitive to the energy input, with R0/R limited by the Rayleigh–Taylor instability. According to still classified data of the Centurion–Halite experiment at the Nevada Test Site, ignition would require an energy of ${\rm E}\simeq 50\,{\rm MJ}$, 25 times larger than the 2 MJ laser at the National Ignition Facility (NIF) reported in the New York Times. This means that even a tenfold increase from 2 to 20 MJ would only decrease the R/R0 ratio by an insignificant factor of 10−1/6 ≃ 0.7. To overcome this problem, it is proposed that the spherical target is replaced with a hollowed-out, rapidly rotating, cm-size ferromagnetic target, accelerated by a rotating traveling magnetic wave to a rotational velocity of ~1 km/s, at the limit of its tensile strength. In a rotating reference system, the general theory of relativity predicts the occurrence of negative gravitational field masses in the center of rotation, with their source located in the Coriolis force field. The density of this negative gravitational field mass can be larger than the magnitude of the positive mass density of a neutron star. The repulsive gravitational force causes the centrifugal force. For a magnetized plasma placed in the rapidly spinning, hollowed-out target chamber, this repulsive force can be balanced by the magnetic force generated by thermomagnetic currents of the Nernst effect. Such a configuration does not suffer from the Rayleigh–Taylor instability, but becomes a small magnetohydrodynamic generator, amplifying the magnetic field to values about equal to those of the Nernst effect, axially confining the plasma. By placing the spinning target in the center of a lithium vortex, the fusion neutrons absorbed in the vortex can breed tritium, and at the same time remove heat from the target chamber to sustain the Nernst effect. A hot spot is thereby produced in the target chamber, which launches a thermonuclear burn wave into a cylindrical deuterium–tritium configuration. With the stability of a rapidly rotating target greatly increased, and the range of 10 MeV electrons in the wall of the cm-size ferromagnetic target, an intense 10 MeV relativistic electron beam drawn from a 10 MJ Marx generator should be sufficient to implode the target for thermonuclear ignition.

In the Shadow of the Mushroom Cloud: Nuclear Testing, Radioactive Fallout, and Damage to U.S. Agriculture, 1945 to 1970

In the 1950s the United States conducted scores of atmospheric nuclear tests at the Nevada Test Site. This article studies the effects of radioactive fallout from nuclear tests on agriculture in regions hundreds of miles from the NTS. While research has shown that this radioactive material posed a health risk near the NTS, little is known about the direct economic effects nuclear testing may have had. I find that fallout from nuclear tests adversely affected U.S. agricultural production, and this result suggests that nuclear testing had a much broader economic and environmental impact than previously thought.

Numerical Evaluation of Unsaturated‐Zone Flow and Transport Pathways at Rainier Mesa, Nevada

Core Ideas Permeability contrasts in layered rocks can result in perched water and lateral flow. Diversion of water above the testing horizon slows the downward transport of radionuclides. Permeable rocks within saturated rocks of lower permeability can remain unsaturated. Sixty‐one underground nuclear tests were conducted at Rainier Mesa at the Nevada National Security Site between 1957 and 1992. The mesa includes the highest‐elevation areas at the Nevada National Security Site and experiences some of the highest estimated infiltration rates. Perched water is ubiquitous in many of the tunnels used for testing. The presence of sloping layers with strongly varying degrees of fracturing and overall permeability suggests that some infiltrating water may be diverted laterally along higher‐permeability layers, thereby reducing overall percolation flux and radionuclide transport through the low‐permeability testing horizons but also potentially transporting radionuclides laterally away from the test locations. A high‐resolution, two‐dimensional, cross‐sectional model was created to simulate water and particle movement along potential lateral flow paths and to investigate flow and transport paths within and surrounding the perched water bodies. The model results indicate that particles with starting locations within the low‐permeability perching layers will follow dominantly vertical trajectories through the perching horizons and reach the water table in the underlying carbonate aquifer only after many thousands of years. In contrast, particles starting near ground surface above the testing horizon are mostly diverted laterally before reaching the testing horizon, except where nearby faults through the perching layers are present. Where the rock above the perching horizon is fractured, particles originating near ground surface can move laterally with relatively high transport velocities. Transport calculations for 3H show that transport extent is limited by slow matrix flow, radioactive decay, and matrix diffusion in the fractured units.

Seismoacoustic Analysis of Chemical Explosions at the Nevada National Security Site

AbstractIn recent years, two sets of chemical explosions have been conducted at the Nevada National Security Site: six explosions from Phase I of the Source Physics Experiment and four explosions from the Forensics Surface Events. We have estimated the explosive yield and depth‐of‐burial/height‐of‐burst of both from the synthesis of seismic and low‐frequency acoustic data. Analysis of the seismic signal is performed using a waveform envelope technique. Using seismic data only, however, there is a trade‐off between the yield and depth or height of the explosion, which becomes especially acute for near‐surface events. This trade‐off between yield and depth/height can be broken by including complementary analysis of the acoustic signal using the acoustic waveform inversion method. The acoustic signal also has a strong trade‐off between a yield and depth/height, but the slope is opposite that of the seismic for near‐surface events. We use a likelihood function to combine the seismic and acoustic analysis and improve our estimates of yield and depth/height for these sets of explosions. The results are generally consistent with the known parameters, but can be improved upon by higher‐frequency seismic analysis and improved acoustic impulse scaling for large‐scaled depths‐of‐burial. These events serve as prime examples of data fusion from multiple data streams. The challenge will be to apply the method to events at longer regional distances where the seismic signals will include phases traveling in the upper mantle, and the acoustic signals will be recorded as infrasound signals, which are subject to atmospheric variability.

Evaluating the Importance of Barometric Pumping for Subsurface Gas Transport Near an Underground Nuclear Test Site

Core Ideas Gas transport modeling can estimate ground surface breakthrough after an underground nuclear explosion. Comparison tracer‐injection experimental and model results reveals parameter sensitivity. Barometric pumping may explain gas breakthrough following an underground nuclear explosion. Amplitude and period of barometric pressure signal are key in controlling breakthrough. An underground nuclear explosion (UNE) generates and distributes radioactive gases that can be transported to the ground surface though preexisting and explosion‐induced fractures over timescales of hours to months. If detected, the presence of short‐lived radionuclides in gas is evidence of a recent UNE. Numerical modeling can provide estimates of surface arrival times that can help inform gas monitoring strategies at suspected foreign test sites. Efforts are underway at historic US UNE sites to better understand subsurface gas‐transport processes following a UNE by geologic characterization of the near‐field damage structures, field‐scale tracer experiments, and subsurface air pressure monitoring. The development of numerical models using historical and experimental datasets from former UNE sites can improve predictions by testing conceptual models, highlighting key processes, and constraining parameter ranges. The models developed in this study represent the U20az site at the Nevada National Security Site where the Barnwell device was expended in December 1989. A two‐phase (water and air), dual‐permeability flow and transport model of the U20az site was built to investigate gas transport processes under recent experimental conditions and following the Barnwell nuclear event. Results indicate that the model can explain both the lack of arrival of radioactive gas tracers in a 2013 field experiment as well as the observed arrival of radioactive gases following the 1989 Barnwell event using barometric pressure records from the respective periods, even when additional advective processes associated with the Barnwell detonation are ignored. The results demonstrate that the character of the barometric records may be a key factor in explaining the differences in transport behavior.

Review: The Nevada Nuclear Test Site, Las Vegas and Mercury, Nevada

Review: The Nevada Nuclear Test Site, Las Vegas and Mercury, Nevada

Triggered Seismicity after North Korea’s 3 September 2017 Nuclear Test

Research Article| September 12, 2018 Triggered Seismicity after North Korea’s 3 September 2017 Nuclear Test Jiayuan Yao; Jiayuan Yao aLaboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China, kakayao@mail.ustc.edu.cndongzhi@mail.ustc.edu.cnluzhou@mail.ustc.edu.cn Search for other works by this author on: GSW Google Scholar Dongdong Tian; Dongdong Tian aLaboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China, kakayao@mail.ustc.edu.cndongzhi@mail.ustc.edu.cnluzhou@mail.ustc.edu.cn Search for other works by this author on: GSW Google Scholar Zhou Lu; Zhou Lu aLaboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China, kakayao@mail.ustc.edu.cndongzhi@mail.ustc.edu.cnluzhou@mail.ustc.edu.cn Search for other works by this author on: GSW Google Scholar Li Sun; Li Sun bChina Earthquake Networks Center, Beijing 100045, China, sunli@seis.ac.cn Search for other works by this author on: GSW Google Scholar Lianxing Wen Lianxing Wen cDepartment of Geosciences, State University of New York at Stony Brook, Stony Brook, New York 11794 U.S.A., lianxing.wen@stonybrook.edudAlso at Laboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China. Search for other works by this author on: GSW Google Scholar Author and Article Information Jiayuan Yao aLaboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China, kakayao@mail.ustc.edu.cndongzhi@mail.ustc.edu.cnluzhou@mail.ustc.edu.cn Dongdong Tian aLaboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China, kakayao@mail.ustc.edu.cndongzhi@mail.ustc.edu.cnluzhou@mail.ustc.edu.cn Zhou Lu aLaboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China, kakayao@mail.ustc.edu.cndongzhi@mail.ustc.edu.cnluzhou@mail.ustc.edu.cn Li Sun bChina Earthquake Networks Center, Beijing 100045, China, sunli@seis.ac.cn Lianxing Wen cDepartment of Geosciences, State University of New York at Stony Brook, Stony Brook, New York 11794 U.S.A., lianxing.wen@stonybrook.edudAlso at Laboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China. Publisher: Seismological Society of America First Online: 12 Sep 2018 Online Issn: 1938-2057 Print Issn: 0895-0695 © Seismological Society of America Seismological Research Letters (2018) 89 (6): 2085–2093. https://doi.org/10.1785/0220180135 Article history First Online: 12 Sep 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jiayuan Yao, Dongdong Tian, Zhou Lu, Li Sun, Lianxing Wen; Triggered Seismicity after North Korea’s 3 September 2017 Nuclear Test. Seismological Research Letters 2018;; 89 (6): 2085–2093. doi: https://doi.org/10.1785/0220180135 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search ABSTRACT The 3 September 2017 nuclear test conducted by the Democratic People’s Republic of Korea was followed by a series of small seismic events. In this study, we conduct a survey of small seismic events from 1 May 2017 to 25 April 2018 near North Korea’s nuclear test site. We report 88 seismic events, including the 2017 nuclear test, the collapse event 8.5 min later, and 86 earthquakes afterward. Among the 86 earthquakes, 11 are well constrained to be distributed along a near north–south direction within a region of 2 km, ∼8.4 km north of the test site. The null detection before the 2017 test and the intense seismicity after the test, along with the predicted normal tensile stress the 2017 test applied on the fault, provide strong evidence that the 2017 test triggered these earthquakes. Some characteristics (the distance to the nuclear test center, depth, and magnitude) of these triggered aftershocks fit the general characteristics of the aftershocks in the Nevada National Security Site and the Semipalatinsk test site after nuclear tests, but the aftershocks of North Korea’s 2017 test exhibit a different evolution lasting at least eight months with two bursts occurring on early December 2017 and early February 2018 and an otherwise general low occurrence rate, possibly caused by a different geological setting of the test site. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The effect of atmospheric variations on subsurface flow and transport

Yucca Mountain, Nevada, located on the border of the Nevada Test Site in Nye County, is under consideration as a prospective commercial nuclear waste repository for spent nuclear fuel and other high-level radioactive waste. This paper studies the effect that diurnal and seasonal temperature cycles and barometric pressure perturbations have on near-surface flow and transport conditions at Yucca Mountain, both without and with the repository in place. The potential near-surface temperature increase as a direct result of an emplaced repository is also examined.

“The most bombed nation on Earth”: Western Shoshone resistance to the Nevada National Security Site

ABSTRACTThis article explores a set of protests challenging U.S. occupation of the Nevada Nuclear Test Site as a means of understanding the deployment of bordering rhetorics in colonial expansion and indigenous resistance. The protests have used a variety of strategies that appropriate artifacts historically controlled by colonial powers such as passports and No Trespassing signs to assert their own sovereignty and demand a change to the material conditions of U.S. occupation of land recognized as belonging to the Western Shoshone in the Treaty of Ruby Valley. These protests offer a chance to complicate current scholarly understandings of decolonial protest. This article analyzes the verbal, visual, and performative elements of these protests and argues that indigenous citizenship and border protests can coopt and reappropriate traditionally hegemonic rhetorics as a means of challenging naturalized assumptions about nationhood, borders, and sovereignty.

SEISMIC ARRAYS “OZHERELYE” OF GEOPHYSICAL OBSERVATORY “BOROVOYE”

Susbsystem “Ozherelye” of peripheral points Zerenda, Vostochniy, Chkalovo of the large-base seismic array of Geophysical Observatory “Borovoye” has been studied. At the example of recording of underground nuclear explosions carried out at Nevada Test Site, local travel time curves and linear trends of the P wave travel time at the traces: “Pahute test site – peripheral points Zerenda, Vostochniy, Chkalovo” have been investigated.

Estimation of Full Moment Tensors, Including Uncertainties, for Nuclear Explosions, Volcanic Events, and Earthquakes

AbstractA seismic moment tensor is a 3 × 3 symmetric matrix that provides a compact representation of a seismic source. We develop an algorithm to estimate moment tensors and their uncertainties from observed seismic data. For a given event, the algorithm performs a grid search over the six‐dimensional space of moment tensors by generating synthetic waveforms for each moment tensor and then evaluating a misfit function between the observed and synthetic waveforms. “The” moment tensor M0 for the event is then the moment tensor with minimum misfit. To describe the uncertainty associated with M0, we first convert the misfit function to a probability function. The uncertainty, or rather the confidence, is then given by the “confidence curve” , where is the probability that the true moment tensor for the event lies within a certain neighborhood of M that has fractional volume V. The area under the confidence curve provides a single, abbreviated “confidence parameter” for M0. We apply the method to data from events in different regions and tectonic settings: 17 nuclear explosions and 12 earthquakes at the Nevada Test Site, 63 small (Mw<2.5) events at Uturuncu volcano in Bolivia, and 21 moderate (Mw>4) earthquakes in the southern Alaska subduction zone. Characterization of moment tensor uncertainties puts us in better position to discriminate among moment tensor source types and to assign physical processes to the events.

Point Spread Function Estimation in X-Ray Imaging with Partially Collapsed Gibbs Sampling

The point spread function (PSF) of a translation invariant imaging system is its impulse response, which cannot always be measured directly. This is the case in high-energy X-ray radiography, and the PSF must be estimated from images of calibration objects indirectly related to the impulse response. When the PSF is assumed to have radial symmetry, it can be estimated from an image of an opaque straight edge. We use a nonparametric Bayesian approach, where the prior probability density for the PSF is modeled as a Gaussian Markov random field and radial symmetry is incorporated in a novel way. Markov chain Monte Carlo posterior estimation is carried out by adapting a recently developed improvement to the Gibbs sampling algorithm, referred to as partially collapsed Gibbs sampling. Moreover, the algorithm we present is proven to satisfy invariance with respect to the target density. Finally, we demonstrate the efficacy of these methods on radiographic data obtained from a high-energy X-ray diagnostic system at the U. S. Department of Energy's Nevada National Security Site.

A Designer’s Approach for Estimation of Nuclear‐Air‐Blast‐Induced Ground Motion

A reliable estimate of free‐field ground displacement induced by nuclear‐air‐blast is required for design of underground strategic structures. A generalized pseudostatic formulation is proposed to estimate nuclear‐air‐blast‐induced ground displacement that takes into account nonlinear stress‐strain behaviour of geomaterials, stress‐dependent wave propagation velocity, and stress wave attenuation. This proposed formulation is utilized to develop a closed‐form solution for linearly decaying blast load applied on a layered ground medium with bilinear hysteretic behaviour. Parametric studies of closed‐form solution indicated that selection of appropriate constrained modulus consistent with the overpressure is necessary for an accurate estimation of peak ground displacement. Stress wave attenuation affects the computed displacement under low overpressure, and stress‐dependent wave velocity affects mainly the occurrence time of peak displacement and not its magnitude. Further, peak displacements are estimated using the proposed model as well as the UFC manual and compared against the field data of atmospheric nuclear test carried out at Nevada test site. It is found that the proposed model is in good agreement with field data, whereas the UFC manual significantly underestimates the peak ground displacements under higher overpressures.

Nocturnal Velvet Ants (Hymenoptera: Mutillidae) of Joshua Tree National Park, Riverside County, California with the description of three new species

The diversity of nocturnal velvet ants at Joshua Tree National Park (JTNP) is investigated along with seasonal activity. The diversity of velvet ants found at JTNP is compared to the diversity of velvet ants found at the Algodones Sand Dunes, Philip L. Boyd Deep Canyon Desert Research Center, Ash Meadows National Wildlife Refuge and the Nevada Test Site. Diagnoses and a key are provided for the velvet ants of JTNP. Forty-one species in six genera, based on 10,202 specimens collected from a transect of 14 sampling sites transitioning from the Mojave to the Sonoran deserts, are found to inhabit areas in and around the park. Odontophotopsis dalyi, O. odontoloxia and Photomorphus schoenwerthi are described as new species. Odontophotopsis (Odontophotopsis) ambigua Mickel (1983) is a junior synonym of O. delodonta Viereck (1904), new synonym. The females of too few of the species are known, so only the males are treated. A key to all of the species and illustrations for those species not previously illustrated are provided for the males.

An algorithm for continuum modeling of rocks with multiple embedded nonlinearly-compliant joints

We present a numerical method for modeling the mechanical effects of nonlinearly-compliant joints in elasto-plastic media. The method uses a series of strain-rate and stress update algorithms to determine joint closure, slip, and solid stress within computational cells containing multiple “embedded” joints. This work facilitates efficient modeling of nonlinear wave propagation in large spatial domains containing a large number of joints that affect bulk mechanical properties. We implement the method within the massively parallel Lagrangian code GEODYN-L and provide verification and examples. We highlight the ability of our algorithms to capture joint interactions and multiple weakness planes within individual computational cells, as well as its computational efficiency. We also discuss the motivation for developing the proposed technique: to simulate large-scale wave propagation during the Source Physics Experiments (SPE), a series of underground explosions conducted at the Nevada National Security Site (NNSS).

Space: The Final Frontier-Research Relevant to Mars.

A critically important gap in knowledge surrounds the health consequences of exposure to radiation received gradually over time. Much is known about the health effects of brief high-dose exposures, such as from the atomic bombings in Japan, but the concerns today focus on the frequent low-dose exposures received by members of the public, workers, and, as addressed in this paper, astronauts. Additional guidance is needed by the National Aeronautics and Space Administration (NASA) for planning long-term missions where the rate of radiation exposure is gradual over years and the cumulative amounts high. The direct study of low doses and low-dose rates is of immeasurable value in understanding the possible range of health effects from gradual exposures and in providing guidance for radiation protection, not only of workers and the public but also astronauts. The ongoing Million Person Study (MPS) is 10 times larger than the study of the Japanese atomic bomb survivors of 86,000 survivors with estimated doses. The number of workers with >100 mSv career dose is substantially greater. The large study size, broad range of doses, and long follow-up indicate substantial statistical ability to quantify the risk of exposures that are received gradually over time. The study consists of 360,000 U.S. Department of Energy workers from the Manhattan Project; 150,000 nuclear utility workers from the inception of the nuclear age; 115,000 atomic veterans who participated in above-ground atmospheric tests at the Nevada Test Site and the Bikini and Enewetak Atolls and Johnston Island in the Pacific Proving Grounds (PPG); 250,000 radiologists and medical workers; and 130,000 industrial radiographers. NASA uses an individual risk-based system for radiation protection in contrast to the system of dose limits for occupational exposures used by terrestrial-based organizations. The permissible career exposure limit set by NASA for each astronaut is a 3% risk of exposure-induced death (REID) from cancer at a 95% confidence level to account for uncertainties in risk projections. The large size of the MPS will reduce the uncertainty in the risk estimates, narrowing the 95% confidence interval, and thus allow more time in space for astronauts. Further differences between men and women in their response to radiation can be more fully examined, and non-cancer outcomes, such as neurological disorders and cardiovascular disease, can be evaluated in a way not hitherto possible.