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At least 1,099 records · Page 61

Update to the Performance Assessment for the Savannah River Site Saltstone Disposal Facility - 20124

In 2019, Savannah River Remediation developed a revision to the performance assessment (PA) on behalf of the U.S. Department of Energy (DOE) Savannah River Operations Office (SR) for the near-surface disposal of low-level waste at the Savannah River Site (SRS) Saltstone Disposal Facility (SDF). Soluble waste from SRS Tank Farms undergoes salt processing to remove cesium and other high activity constituents. The low-activity decontaminated salt solution (DSS) is then immobilized by mixing it into a cementitious waste form known as saltstone. After mixing, the saltstone is poured into leak-tight concrete vaults, known as saltstone disposal units (SDUs), where the waste form cures. By the time of facility closure, the SDF is expected to consist of 15 SDUs with a combined capacity of 1.06 E+09 L (280 Mgal) of cured saltstone. The facility operates under a Disposal Authorization Statement (DAS) from DOE and a permit from the South Carolina Department of Health and Environmental Control (SCDHEC). Since the start of operations in 1990, the SDF has received almost 6.7 E+07 L (18 Mgal) of DSS, resulting in the safe disposal of 2.7 E+16 Bq (7.3 E+05 Ci) of activity. Due to the radioactive decay of short-lived contaminants, the total remaining activity in the disposed waste is estimated to be approximately 1.4 E+16 Bq (3.9 E+05 Ci), as of September 2018. The Disposal Authorization Statement requires a demonstration that the system of engineered and natural features of the disposal facility will limit releases from the facility and be protective of human health and the environment for at least the next 1,000 years. The long-term performance of the facility was evaluated under the requirements of the DoE's Radioactive Waste Management Manual (US DOE Manual 435.1-1). Simulations were performed to demonstrate that the disposal facility would meet performance objectives specified in the manual. The evaluation was based on numerical models that simulate the releases of contaminants from the saltstone waste form. Contaminants were transported through groundwater and air pathways to points of assessment to evaluate compliance. In addition, the potential consequences of an inadvertent human intrusion (IHI) were also evaluated. In accordance with the guidance and recommendations in US DoE's technical standard for DAS, deterministic and probabilistic analyses were performed to demonstrate the SDF system, which includes the engineered cover system, the SDUs, the waste form, and the natural features of the site, provides a reasonable expectation that saltstone disposal will meet performance objectives. Post-closure doses to future members of the public (MOP) were evaluated; dose estimates from the air pathway were well below the 1.0 E-04 Sv/yr (10 mrem/yr) performance objective within 1,000 years, and the calculated doses to the MOP from all exposure pathways, including the groundwater and air pathways, were well below the 2.5 E-04 Sv/yr (25 mrem/yr) performance objective within 1,000 years. Doses following an assumed intrusion event within the facility boundaries were well below the IHI performance objectives, where the acute IHI dose was below the acute IHI dose performance objective of 5.0 E-03 Sv (500 mrem) and the chronic IHI dose was below the chronic IHI dose performance objective of 1.0 E-03 Sv/yr (100 mrem/yr) within 1,000 years. (author)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

Countdown to perovskite space launch: Guidelines to performing relevant radiation-hardness experiments

Perovskite photovoltaics (PVs) are under intensive development for promise in terrestrial energy production. Soon, the community will find out how much of that promise may become reality. Perovskites also open new opportunities for lower cost space power. However, radiation tolerance of space environments requires appropriate analysis of relevant devices irradiated under representative radiation conditions. We present guidelines designed to rigorously test the radiation tolerance of perovskite PVs. We review radiation conditions in common orbits, calculate nonionizing and ionizing energy losses (NIEL and IEL) for perovskites, and prioritize proton radiation for effective nuclear interactions. Low-energy protons (0.05-0.15 MeV) create a representative uniform damage profile, whereas higher energy protons (commonly used in ground-based evaluation) require significantly higher fluence to accumulate the equivalent displacement damage dose due to lower scattering probability. Furthermore, high-energy protons may "heal" devices through increased electronic ionization. These procedural guidelines differ from those used to test conventional semiconductors.

14 SOLAR ENERGY↗

Modeling of surface-state induced inter-electrode isolation of n -on- p devices in mixed-field and γ -irradiation environments

Position sensitive n-on-p silicon sensors will be utilized in the tracker and in the High Granularity Calorimeter (HGCAL) of the Compact Muon Solenoid (CMS) experiment at High Luminosity Large Hadron Collider (HL-LHC). The detrimental effect of the radiation-induced accumulation of positive net oxide charge on position resolution in n-on-p sensors has typically been countered by the application of isolation implants like p-stop or p-spray between n + -electrodes. In addition to the positively charged layer inside the oxide and close to the Si/SiO 2 -interface, surface damage introduced by ionizing radiation in SiO 2 -passivated silicon particle detectors includes the accumulation of trapped-oxide-charge and interface traps. A previous study of either n/γ (mixed field)- or γ-irradiated Metal-Oxide-Semiconductor (MOS) capacitors showed evidence of substantially higher introduction rates of acceptor- and donor-type deep interface traps (N it,acc/don ) in mixed-field environment. Here, in this work, an inter-pad and -strip resistance (or resistivity (ρ int )) simulation study of n-on-p sensors with and without p-stop isolation implants was conducted for both irradiation types. Higher levels of ρint showed correlation to higher densities of deep N it,acc/don , with the inter-pad isolation performance of the mixed-field irradiated sensors becoming independent of the presence of p-stop implant between the n + - electrodes up to about 100 kGy. The low introduction rates of deep N it,acc/don in γ-irradiated sensors resulted in high sensitivity of ρ int to the presence and peak doping of p-stop above the lowest dose of about 7 kGy in the study. As a consequence of the advantageous influence of radiation-induced accumulation of deep N it on the inter-electrode isolation, position sensitive n-on-p sensors without isolation implants may be considered for future HEP-experiments where the radiation is largely due to hadrons.

Akchurin, N. [Texas Tech Univ., Lubbock, TX (Unite↗

Global perspectives of the bulk electronic structure of URu 2 Si 2 from angle-resolved photoemission

Previous high-resolution angle-resolved photoemission (ARPES) studies of URu 2 Si 2 have characterized the temperature-dependent behavior of narrow-band states close to the Fermi level ( E F ) at low photon energies near the zone center, with an emphasis on electronic reconstruction due to Brillouin zone folding. A substantial challenge to a proper description is that these states interact with other hole-band states that are generally absent from bulk-sensitive soft x-ray ARPES measurements. Here we provide a more global k -space context for the presence of such states and their relation to the bulk Fermi surface (FS) topology using synchrotron-based wide-angle and photon energy-dependent ARPES mapping of the electronic structure using photon energies intermediate between the low-energy regime and the high-energy soft x-ray regime. Small-spot spatial dependence, f -resonant photoemission, Si 2 p core-levels, x-ray polarization, surface-dosing modification, and theoretical surface slab calculations are employed to assist identification of bulk versus surface state character of the E F -crossing bands and their relation to specific U- or Si-terminations of the cleaved surface. The bulk FS topology is critically compared to density functional theory (DFT) and to dynamical mean field theory calculations. In addition to clarifying some aspects of the previously measured high symmetry Γ, Z and X points, incommensurate 0.6 a * nested Fermi-edge states located along Z – N – Z are found to be distinctly different from the DFT FS prediction. The temperature evolution of these states above T HO , combined with a more detailed theoretical investigation of this region, suggests a key role of the N -point in the hidden order transition.

36 MATERIALS SCIENCE↗

Probing soft X-ray induced photoreduction of a model Mn-complex at cryogenic conditions

Soft X-ray absorption spectroscopy of first row transition elements at their respective L -edges provides important information about the oxidation and spin states of the metal centers. However, the associated sample damage in radiation-sensitive samples substantially alters the electronic and chemical structures of redox-active metal centers. Here, we measure the soft X-ray spectrum of the model Mn III (acac) 3 complex containing a redox-active Mn III metal center in an octahedral environment with a superconducting transition-edge sensor detector. To reduce the secondary damage resulting primarily from the diffusion of radicals and electrons, the spectra are collected at 30 K and 80 K on solid samples. Starting from the first scan, we detect the contribution of X-ray induced sample damage leading to a change in the Mn II intensity. However, at low temperatures, particularly at 30 K, we do not observe a gradual increase in the radiation damage with successive scans with the X-ray beam at the same spot. At our estimated dose of 90 kGy, we find 62% of Mn III (acac) 3 is still intact at 30 K. However, at room temperature, we see a gradual increase in radiation damage with increasing numbers of scans at the same spot, which is consistent with the possibility of increased diffusion rates of secondary radicals and electrons as noted in other studies.

L-edge spectroscopy↗

Preliminary results from the heavy ions in space experiment

The Heavy Ions In Space (HIIS) experiment is intended to provide a deep survey of intensely ionizing particles in low earth orbit. Intensely ionizing particles produce single event effects in microelectronic components and are now recognized as the principal cause of spacecraft anomalies. These particles also make an important contribution to radiation doses. Besides the practical applications, HIIS data will be used to study several important scientific questions. By measuring heavy ions that stop in HIIS, the anomalous component of cosmic rays can be studied, evidence can be searched for of heavy ions trapped in the earth's magnetic field and by comparing HISS data with observations from outside the magnetosphere, the mean ionic charge state can be determined of solar energetic particles. HIIS will also record relativistic ultraheavy galactic cosmic rays. Their elemental composition will be measured from tin to uranium. Results are presented on the elemental resolution for stopping heavy ions and relativistic heavy ions.

Adams, J. H., Jr.↗

Absorbed dose measurements and predictions on LDEF

The overall radiation environment of LDEF was determined in part through the use of thermoluminescent detectors (TLDs). The results given are from four experiments (Biostack, Fiber Optics Data Link, Seeds in Space, and Linear Energy Transfer Spectrum Measurement) and represent a large fraction of existing absorbed dose data. The TLDs (located on leading and trailing edges) were under various shielding depths. The measured absorbed doses were found to reflect both directional dependence of incident trapped protons and shielding. Radiation dosages at both locations and under various shielding depths are given. The effect of the trapped proton anisotropy was such that the western side of LDEF received about 3 times the dose of the eastern side at shielding depths of approx. 1 gm/sq cm. These data are being used to evaluate the accuracy of models currently in use for predicting ionizing radiation environments for low Earth orbit missions.

Frank, A. L.↗

Development of a Neutron Spectrometer to Assess Biological Radiation Damage Behind Spacecraft Materials

Astronauts who spend months and years traveling long distances in spacecraft and working on other planets will be subjected to high energy radiation of galactic and solar origin without the protection of the Earth's thick (one writer has called it buff) atmosphere and magnetic field. The lack of natural protection will allow high energy cosmic ray particles and solar protons to crash directly into relatively thin spacecraft walls and planetary atmospheres producing energetic secondary particles in these collisions. A substantial fraction of these secondaries will be neutrons that carry no electric charge and, consequently, are difficult to detect. At sea level on Earth the remaining neutrons are the result of many generations (approximately 10) of collisions, have very low energies (scientists call them thermal neutrons), and do not penetrate deeply into the human body. They do contribute to the natural background radiation seen by humans on Earth, but much of the dose is only at the surface or skin of the body. In the International Space Station or on the surface of Mars, the secondary neutrons will be the result of only one or two generations of interaction due to the thinner (about a factor of 20 compared to the Earth's atmosphere) walls or atmosphere, have considerably more energy and penetrate deeply into the human body. In addition, neutrons are substantially moderated by hydrogenous material such as water. A significant fraction of the water exists in the astronaut's body. Therefore, the neutron can not only penetrate more deeply into the body, but also be stopped there and deposit all or most of its radiation dose in organs such as the liver, spleen, kidney, etc. We hypothesize that the risk of serious cancers will be increased for the exposed humans. The portable, real time neutron spectrometer being developed by our team will monitor the environment inside spacecraft structures and on planetary surfaces. Activities supported by this grant will evaluate the neutron environment inside several candidate spacecraft materials at accelerator facilities. These experiments will enable engineers to choose the structure materials that minimize the production of secondary neutrons. With the information that the neutron energy spectrometer produces, scientists and doctors will be able to assess the increased risk of cancer and develop countermeasures. The instrument itself will include an alarm system to warn astronauts when high radiation fluxes are occurring so that they can seek shelter immediately.

Maurer, R. H.↗

Cellular Response to Proton Irradiation: A Simulation Study with TOPAS-nBio

The cellular response to ionizing radiation continues to be of significant research interest in cancer radiotherapy, and DNA is recognized as the critical target for most of the biologic effects of radiation. Incident particles can cause initial DNA damages through physical and chemical interactions within a short time scale. Initial DNA damages can undergo repair via different pathways available at different stages of the cell cycle. The misrepair of DNA damage results in genomic rearrangement and causes mutations and chromosome aberrations, which are drivers of cell death. This work presents an integrated study of simulating cell response after proton irradiation with energies of 0.5–500 MeV (LET of 60–0.2 keV/µm). A model of a whole nucleus with fractal DNA geometry was implemented in TOPAS-nBio for initial DNA damage simulations. The default physics and chemistry models in TOPAS-nBio were used to describe interactions of primary particles, secondary particles, and radiolysis products within the nucleus. The initial DNA double-strand break (DSB) yield was found to increase from 6.5 DSB/Gy/Gbp at low-linear energy transfer (LET) of 0.2 keV/µm to 21.2 DSB/Gy/Gbp at high LET of 60 keV/µm. A mechanistic repair model was applied to predict the characteristics of DNA damage repair and dose response of chromosome aberrations. It was found that more than 95% of the DSBs are repaired within the first 24 h and the misrepaired DSB fraction increases rapidly with LET and reaches 15.8% at 60 keV/µm with an estimated chromosome aberration detection threshold of 3 Mbp. The dicentric and acentric fragment yields and the dose response of micronuclei formation after proton irradiation were calculated and compared with experimental results.

62 RADIOLOGY AND NUCLEAR MEDICINE↗

Activation of Thermal Insulation and Contamination Prevention

The goals of this paper are to enhance the understanding of the potential activation and contamination hazards from the use of insulating materials in the Molten Chloride Reactor Experiment and to foster safer work environments through informed decision-making and strategic planning. It was written to resolve a gating comment from the core heating system preliminary design review. The largest dose expected from the considered insulation materials is about 0.1 mrem per year from Pyrogel. Although Cerablanket stood out among the insulation materials for encouraging ALARA practices, it has a higher thermal conductivity. At low power levels, focusing on reducing the generation of dust from ERD activities may be more effective for reducing the potential spread of contamination than changing the insulating material type. However, regardless of the material chosen, PPE and safety conscious procedural practices can help reduce the production and spread of activated dust––thus minimizing the radiological hazard to workers.

21 - SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLAN↗

Charged particle LET-spectra measurements aboard LDEF

The linear energy transfer (LET) spectra of charged particles was measured in the 5 to 250 keV/micron (water) interval with CR-39 and in the 250 to 1000 keV/micron (water) interval with polycarbonate plastic nuclear track detectors (PNTDs) under different shielding depths in the P0006 experiment. The optimal processing conditions were determined for both PNTDs in relation to the relatively high track densities due to the long term exposure in space. The total track density was measured over the selected samples, and tracks in coincidence on the facing surfaces of two detector sheets were selected for measuring at the same position on each sheet. The Short Range (SR) and Galactic Cosmic Ray (GCR) components were measured separately. The integral dose and dose rate spectra of charged particles are also given. The high LET portion of the LET spectra was measured with high statistical accuracy. This is a unique result of this experiment due to the low flux of this type of particle under typical shielding conditions.

Csige, I.↗

A polymer, random walk model for the size-distribution of large DNA fragments after high linear energy transfer radiation

DNA double-strand breaks (DSBs) produced by densely ionizing radiation are not located randomly in the genome: recent data indicate DSB clustering along chromosomes. Stochastic DSB clustering at large scales, from > 100 Mbp down to < 0.01 Mbp, is modeled using computer simulations and analytic equations. A random-walk, coarse-grained polymer model for chromatin is combined with a simple track structure model in Monte Carlo software called DNAbreak and is applied to data on alpha-particle irradiation of V-79 cells. The chromatin model neglects molecular details but systematically incorporates an increase in average spatial separation between two DNA loci as the number of base-pairs between the loci increases. Fragment-size distributions obtained using DNAbreak match data on large fragments about as well as distributions previously obtained with a less mechanistic approach. Dose-response relations, linear at small doses of high linear energy transfer (LET) radiation, are obtained. They are found to be non-linear when the dose becomes so large that there is a significant probability of overlapping or close juxtaposition, along one chromosome, for different DSB clusters from different tracks. The non-linearity is more evident for large fragments than for small. The DNAbreak results furnish an example of the RLC (randomly located clusters) analytic formalism, which generalizes the broken-stick fragment-size distribution of the random-breakage model that is often applied to low-LET data.

DNA/radiation effects↗

Crack Growth Rate and Fracture Toughness Tests on Irradiated Ex-Plant Materials

The performance of structural materials is critical for the safe and economic operation of light water reactors. Exposed to neutron irradiation during service, the reactor core internal materials can undergo significant microstructural and microchemical changes, leading to irradiation hardening and embrittlement. To ensure the structural integrity and functionality of nuclear reactor components during long-term operation, material degradation and damage mechanisms must be understood and managed adequately. In this work, irradiated materials harvested from the decommissioned Zorita reactor were studied for their cracking susceptibility and fracture resistance as a function of irradiation dose up to 47 displacement per atom (dpa). The material is a Type 304 stainless steel sectioned from the baffle plates of this pressurized water reactor with 38 years of service. Crack growth rate and fracture toughness J-resistance (J-R) curve tests were performed in low-corrosion-potential environments at ~315°C. All samples behaved similarly under cyclic loading, and no deteriorated corrosion-fatigue behavior was observed in the test environments. Under constant stress intensity factors, most samples did not show elevated crack growth rates, suggesting an adequate stress corrosion cracking resistance in the test environments. However, an unstable cracking behavior was observed in a 47-dpa sample, resulting in significantly higher crack growth rates than expected at high stress intensity factors. Crack instability was also observed in a 0.06-dpa sample but did not lead to a sustained high crack growth rate. The impact of neutron irradiation was more evident in the fracture toughness J-R curve tests. As the dose increased, the J-R curve declined considerably and became very shallow at high doses. A fully intergranular fracture morphology was also observed among the high-dose samples ruptured in an air atmosphere at room temperature. This brittle fracture mode in the absence of high temperature water environment confirmed a high degree of embrittlement of this material resulting from its service exposure to neutron irradiation.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗

Double ChemFET for the In-Line Monitoring of Silver Dosing in Potable Water Systems

With NASA Advanced Exploration Systems (AES) Life Support Systems (LSS) baselining ionic silver (Ag+) as the biocide of choice, development in silver monitoring technologies becomes necessary to monitor and control Ag+ release. Ion-selctive field effect transistors (ISFETs) and chemically-sensitive field effect transistors (ChemFETs) are capable of selective detection of Ag+ with reversible and low detection limit responses. However, one major hurdle to ChemFET technologies is the requirement of a reference electrode, which can leak its fill solution into the potable water system and is prone to drift requiring frequent calibration. To circumvent this issue, we are developing a double chemically-sensitive field effect transistor (dChemFET). The dChemFET uses a second ChemFET to selectively monitor the counter ion and serves in place of the reference electrode. We report our work in developing the dChemFET, starting with the development of the Ag+ and NO3- ion selective memberanes (ISMs) to be used in the ChemFETs. The Ag+ ISM has a detection range of 50-105 ppb . The counter ion ISM, NO3- ISM, has shown a detection range of 400-105 ppb. NO3- ISMs are limited in detecting at lower ppb levels required for the dChemFET application on the potable water system. In response to this issue, we also report our studies in using the Ag+ ISM to measure deionized (DI) water as a reference measurement to be implemented in-line before the Ag+ is dosed.

Lance Dean Delzeit↗

C10-Benzoate Esters of Anhydrotetracycline Inhibit Tetracycline Destructases and Recover Tetracycline Antibacterial Activity

Tetracyclines (TCs) are an important class of antibiotics threatened by enzymatic inactivation. These tetracycline-inactivating enzymes, also known as tetracycline destructases (TDases), are a subfamily of class A flavin monooxygenases (FMOs) that catalyze hydroxyl group transfer and oxygen insertion (Baeyer–Villiger type) reactions on TC substrate scaffolds. Semisynthetic modification of TCs (e.g., tigecycline, omadacycline, eravacycline, and sarecycline) has proven effective in evading certain resistance mechanisms, such as ribosomal protection and efflux, but does not protect against TDase-mediated resistance. Here, we report the design, synthesis, and evaluation of a new series of 22 semisynthetic TDase inhibitors that explore D-ring substitution of anhydrotetracycline (aTC) including 14 C10-benzoate ester and eight C9-benzamides. Overall, the C10-benzoate esters displayed enhanced bioactivity and water solubility compared to the corresponding C9-benzamides featuring the same heterocyclic aryl side chains. The C10-benzoate ester derivatives of aTC were prepared in a high-yield one-step synthesis without the need for protecting groups. The C10-esters are water-soluble, stable toward hydrolysis, and display dose-dependent rescue of tetracycline antibiotic activity in E. coli expressing two types of tetracycline destructases, represented by TetX7 (Type 1) and Tet50 (Type 2). The best inhibitors recovered tetracycline antibiotic activity at concentrations as low as 2 μM, producing synergistic scores <0.5 in the fractional inhibitory concentration index (FICI) against TDase-expressing strains of E. coli and clinical P. aeruginosa. The C10-benzoate ester derivatives of aTC reported here are promising new leads for the development of tetracycline drug combination therapies to overcome TDase-mediated antibiotic resistance.

60 APPLIED LIFE SCIENCES↗

A New UV Spot Test for Quality Control

Here, we present a fast, low-cost ultraviolet (UV) spot test to screen photovoltaic modules for UV-induced degradation (UVID). The measurement is based on a fiber-coupled arc lamp that delivers a highly accelerated local UV dose-approximately five years of equivalent field exposure in less than a day. Spot-exposed areas are evaluated using electroluminescence imaging. We test a “known bad” product, which was observed to experience UVID in the field, and a “known good” product which did not exhibit UVID in the field. Results indicate that the method may be useful as a quick and relatively inexpensive quality control screening tool for customers and manufacturers.

14 SOLAR ENERGY↗

Radiation Damage Mitigation in FeCrAl Alloy at Sub-Recrystallization Temperatures

Traditional defect recovery methods rely on high-temperature annealing, often exceeding 750 °C for FeCrAl. In this study, we introduce electron wind force (EWF)-assisted annealing as an alternative approach to mitigate irradiation-induced defects at significantly lower temperatures. FeCrAl samples irradiated with 5 MeV Zr 2+ ions at a dose of 10 14 cm −2 were annealed using EWF at 250 °C for 60 s. We demonstrate a remarkable transformation in the irradiated microstructure, where significant increases in kernel average misorientation (KAM) and low-angle grain boundaries (LAGBs) typically indicate heightened defect density; the use of EWF annealing reversed these effects. X-ray diffraction (XRD) confirmed these findings, showing substantial reductions in full width at half maximum (FWHM) values and a realignment of peak positions toward their original states, indicative of stress and defect recovery. To compare the effectiveness of EWF, we also conducted traditional thermal annealing at 250 °C for 7 h, which proved less effective in defect recovery as evidenced by less pronounced improvements in XRD FWHM values.

FeCrAl alloys↗

Effect of combined heat and radiation on microbial destruction

A series of experiments at several levels of relative humidity and radiation dose rates was carried out using spores of Bacillus subtilis var. niger to evaluate the effect of heat alone, radiation alone, and a combination of heat and radiation. Combined heat and radiation treatment of microorganisms yields a destruction rate greater than the additive rates of the independent agents. The synergistic mechanism shows a proportional dependency on radiation dose rate, an Arrhenius dependence on temperature, and a dependency on relative humidity. Maximum synergism occurs under conditions where heat and radiation individually destroy microorganisms at approximately equal rates. Larger synergistic advantage is possible at low relative humidities rather than at high relative humidities.

Fisher, D. A.↗