•A model was developed for use with FHRs and benchmarked with experimental data.•Model results match results of tritium diffusion experiments.•Corrosion simulations show reasonable agreement with ...molten salt loop experiments.•This is the only existing model of tritium transport and corrosion in FHRs.•Model enables proposing and evaluating tritium control options in FHRs.
The Fluoride Salt-Cooled High-Temperature Reactor (FHR) is a pebble bed nuclear reactor concept cooled by a liquid fluoride salt known as “flibe” (7LiF-BeF2). A model of TRITium Diffusion EvolutioN and Transport (TRIDENT) was developed for use with FHRs and benchmarked with experimental data. TRIDENT is the first model to integrate the effects of tritium production in the salt via neutron transmutation, with the effects of the chemical redox potential, tritium mass transfer, tritium diffusion through pipe walls, tritium uptake by graphite, selective chromium attack by tritium fluoride, and corrosion product mass transfer. While data from a forced-convection polythermal loop of molten salt containing tritium did not exist for comparison, TRIDENT calculations were compared to data from static salt diffusion tests in flibe and flinak (0.465LiF-0.115NaF-0.42KF) salts. In each case, TRIDENT matched the transient and steady-state behavior of these tritium diffusion experiments. The corrosion model in TRIDENT was compared against the natural convection flow-loop experiments at the Oak Ridge National Laboratory (ORNL) from the 1960s and early 1970s which used Molten Salt Reactor Experiment (MSRE) fuel-salt containing UF4. Despite the lack of data required by TRIDENT for modeling the loops, some reasonable results were obtained. The TRIDENT corrosion rates follow the experimentally observed dependence on the square root of the product of the chromium solid-state diffusion coefficient with time. Additionally the TRIDENT model predicts mass transfer of corrosion products from the hot to the cold leg (as was observed in the experiments with salts containing UF4). In a separate paper the results of TRIDENT simulations in a prototypical FHR are presented.
A graphite pebble lifting test platform was used to study the aerodynamic characteristics and collision behavior of graphite pebble in the lifting pipe. The impact strength of the pipe wall during ...pneumatic transport was measured by acceleration sensors and the number of collisions is counted by another visualization experiment. Meanwhile, hydrodynamic theory was combined with a dynamics model of the motion to predict the aerodynamic characteristics and trajectories of the pebble in the lifting pipe. The results show that the aerodynamic force on the graphite pebble increases with increasing inlet velocity and offset. Each graphite pebble in the pipeline follows an upward spiral trajectory due to the aerodynamic forces. The number of graphite pebble impacts decreases with increasing helium pressure due to the shorter residence time, which is in good agreement with experimental results. The present method can provide an efficient method for estimating the impact wear in real HTGR.
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•Vibration and visualization measurements are used to study the impacts of the pebble.•The aerodynamic force lifting the graphite pebble is simulated in a CFD model.•A dynamic model combined with CFD aerodynamic force is used to predict pebble impacts.
•Generation of a DAGMC calculation model of the HELIAS power reactor.•First neutronic analyses performed for the HELIAS power reactor.•Results for Neutron Wall Loading (NWL), Tritium Breeding Ratio ...and Neutron Flux.•Two different NWL approaches, results show agreement within statistical uncertainty.
A first neutronics analysis of the Helical-Axis Advanced Stellarator (HELIAS) power reactor is conducted in this work. It is based on Monte Carlo (MC) particle transport simulations with the Direct Accelerated Geometry Monte Carlo (DAGMC) approach which enables particle tracking directly on the CAD geometry. A suitable geometry model of the HELIAS reactor is developed, including a rough model of a breeder blanket based on the Helium Cooled Pebble Bed (HCPB) breeder blanket concept.
The resulting model allows to perform first neutronic calculations providing a 2D map of the neutron wall loading, a 3D distribution of the neutron flux, and a rough assessment of the tritium breeding capability. It is concluded that the applied methodology, making use of MC particle transport simulations based on the DAGMC approach, is suitable for performing nuclear analyses for the HELIAS power reactor.
•The burnup credit calculations for PBR spent fuel pebbles were performed using MCNP6.•The impact on the keff as a function of water density in the storage cask was investigated.•Three single effects ...and four compound effects were studied.•The mechanisms of all the single and compound effects were researched in depth.
This study attempts to investigate the influence due to the use of burnup credit in the criticality safety analysis for pebble-bed reactor (PBR) spent fuel pebbles. Recently, the development of PBR is very quick in China, thus the storage of spent fuel pebbles will become a significant issue in the foreseeable future. For the safe storage, the criticality safety analysis is of vital important. Furthermore, the utilization of burnup credit in the criticality safety analysis is essential due to the fact that it can result in a more compact design of storage system. In this study, all the calculations were performed using MCNP6 associated with the continuous energy neutron data library ENDF/B-VII. In addition, two geometrical models which represent the HTR-10 core and a proposed storage cask were adopted. The proposed storage cask model was established based on the real storage cask for HTR-10 spent fuel pebbles, but the capacity was increased in order to evaluate the impact of burnup credit. Moreover, the most conservative condition for this proposed cask occurs when the cask is full of water with a water density around 0.35 g/cm3. For burnup credit calculations, three operating parameters related to HTR-10 were investigated, including the usage of control rod, the fuel temperature, and the volume fraction of fuel pebbles in total pebbles. Additionally, these operating parameters were combined and classified as three single effects and four compound effects. The single and compound effects were defined as the influence on the effective multiplication factor (keff) due to simultaneous variations of one and multiple operating parameters, respectively. It is worth noting that in most of the compound effects, the reactivity deviation (or the change of keff, Δk) resulting from the compound effect was not a summation of Δk’s resulting from the associated single effects. This phenomena may affect the precise assessment to some extent. Finally, the mechanisms of both the single and compound effects were explored in depth from the analysis of the spectral distribution of fission. From this analysis, a harder spectral distribution of fission resulting from the single and compound effects corresponds a larger value of Δk.
Environmental impact statements (EISs) are based on science produced about specific project proposals, which results in a large body of grey literature. Spill risk estimates are part of that body of ...work. This is a critique the spill risk models cited in EISs for proposed drilling on the Alaskan Coastal Plain, the Pebble Mine transportation corridor, and Arctic offshore drilling, which are scored against published standards of best practices for ecological risk assessments. After a detailed peer review of Arctic offshore drilling risks, the findings and results from internal and external review processes of those reports are described. The amount of grey literature cited in recent EISs and how the alphanumeric ratings of draft EISs changed in 2017 are shown. Suggestions of how agencies, scientists, and peer reviewed journals can contribute to meaningful review of grey literature in regulatory science are offered.
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•The risks of substantial spills are often underestimated in the grey literature.•Three case studies show EIS risk assessments may fail to adhere to best practices.•At least one spill risk estimate may be an understatement by orders of magnitude.•Current review of EISs is often insufficient and can vary by administration.•Grey literature cited in EISs should withstand rigorous peer review.
Variations in 238U/235U and 234U/238U ratios were measured in uranium minerals from a spectrum of uranium deposit types, as well as diagenetic phosphates in uranium-rich basins and peraluminous ...rhyolites and associated autunite mineralisation from Macusani Meseta, Peru. Mean δ238U values of uranium minerals relative to NBL CRM 112-A are 0.02‰ for metasomatic deposits, 0.16‰ for intrusive, 0.18‰ for calcrete, 0.18‰ for volcanic, 0.29‰ for quartz-pebble conglomerate, 0.29‰ for sandstone-hosted, 0.44‰ for unconformity-type, and 0.56‰ for vein, with a total range in δ238U values from −0.30‰ to 1.52‰. Uranium mineralisation associated with igneous systems, including low-temperature calcretes that are sourced from U-rich minerals in igneous systems, have low δ238U values of ca. 0.1‰, near those of their igneous sources, whereas uranium minerals in basin-hosted deposits have higher and more variable values. High-grade unconformity-related deposits have δ238U values around 0.2‰, whereas lower grade unconformity-type deposits in the Athabasca, Kombolgie and Otish basins have higher δ238U values. The δ234U values for most samples are around 0‰, in secular equilibrium, but some samples have δ234U values much lower or higher than 0‰ associated with addition or removal of 234U during the past 2.5Ma.
These δ238U and δ234U values suggest that there are at least two different mechanisms responsible for 238U/235U and 234U/238U variations. The 234U/238U disequilibria ratios indicate recent fluid interaction with the uranium minerals and preferential migration of 234U. Fractionation between 235U and 238U is a result of nuclear-field effects with enrichment of 238U in the reduced insoluble species (mostly UO2) and 235U in oxidised mobile species as uranyl ion, UO22+, and its complexes. Therefore, isotopic fractionation effects should be reflected in 238U/235U ratios in uranium ore minerals formed either by reduction of uranium to UO2 or chemical precipitation in the form of U6+ minerals. The δ238U values of uranium ore minerals from a variety of deposits are controlled by the isotopic signature of the uranium source, the efficiency of uranium reduction in the case of UO2 systems, and the degree to which uranium was previously removed from the fluid, with less influence from temperature of ore formation and later alteration of the ore. Uranium isotopes are potentially superb tracers of redox in natural systems.
Neutronic feasibility study of U–Th–Pa based high burnup fuel for the pebble bed reactors has been carried out. An additional content of P231a in a mixed U233–T232h fuel pebble was optimized for ...simultaneously controlling long-term excess reactivity and attaining high fuel burnup. Because of a large thermal absorption cross section, P231a has a potential to absorb thermal neutrons and act as a burnable absorber in the early stage of burnup. In the latter stage, P231a is transmuted to U233 and act as a fertile fuel. Parametric study was performed for heavy metal (HM) amount, U233 enrichment and P231a content. The objective is to attain a target burnup greater than 100 GWd/T with a low uranium enrichment and a small content of P231a. Six fuel compositions with the HM amount of 9–18 g per pebble, the U233 enrichment of 8 and 10%, and the P231a content of 3.2–5.5% were selected for discussion and comparison. The burnup levels of about 130–170 GWd/t are achieved for the six fuel compositions. The results of temperature coefficient show that negative reactivity feedback during burnup with the increase of temperature is assured, which is one of the important safety parameters.
•Neutronic feasibility study of U–Th–Pa based fuel for pebble bed reactors has been conducted.•The fuel contains low enrichment of 233U and an additional 231Pa content within a few percent.•Target burnup of 130–170 GWd/T is achieved for selected fuel compositions.•Negative temperature coefficient is assured during burnup.
Characterizing the surface material of an asteroid is important for understanding its geology and for informing mission decisions, such as the selection of a sample site. Diurnal surface temperature ...amplitudes are directly related to the thermal properties of the materials on the surface. We describe a numerical model for studying the thermal conductivity of particulate regolith in vacuum. Heat diffusion and surface‐to‐surface radiation calculations are performed using the finite element (FE) method in three‐dimensional meshed geometries of randomly packed spherical particles. We validate the model for test cases where the total solid and radiative conductivity values of particulates with monodisperse particle size frequency distributions (SFDs) are determined at steady‐state thermal conditions. Then, we use the model to study the bulk radiative thermal conductivity of particulates with polydisperse, cumulative power law particle SFDs. We show that for each polydisperse particulate geometry tested, there is a corresponding monodisperse geometry with some effective particle diameter that has an identical radiative thermal conductivity. These effective diameters are found to correspond very well to the Sauter mean particle diameter, which is essentially the surface area‐weighted mean. Next, we show that the thermal conductivity of the particle material can have an important effect on the radiative component of the thermal conductivity of particulates, especially if the particle material conductivity is very low or the spheres are relatively large, owing to non‐isothermality in each particle. We provide an empirical correlation to predict the effects of non‐isothermality on radiative thermal conductivity in both monodisperse and polydisperse particulates.
Plain Language Summary
The thermal conductivity of asteroid regolith is related to the properties of the particulate assemblage (e.g., size distribution). Spacecraft missions that measure the surface temperature of asteroids, like OSIRIS‐REx at asteroid Bennu, can take advantage of this by relating the observed temperatures to the physical properties of the regolith. We present a 3D model for studying the thermal conductivity of regolith, where heat flow is simulated in randomly packed spheres. We found that for cases where the particle sizes are monodisperse, our model reproduces the thermal conductivity values predicted by simpler theoretical models. However, this is only true if the particles themselves are made of a material that itself has relatively high thermal conductivity, which may not be the case for the regolith on Bennu. We determined the values for a correction factor to account for these cases. Neglecting it could cause one to appreciably underestimate particle sizes on asteroid surfaces, which could pose a risk for sample collection. Finally, we found that regoliths with particle size mixtures can have radiative thermal conductivities that are identical to monodisperse regoliths. We found that the surface area‐weighted mean particle size of the mixed regoliths is representative of the bulk radiative thermal conductivity.
Key Points
A new finite element model for analyzing the solid and radiative conductivity of particulate regoliths is presented.
Non‐isothermality in particles with low material thermal conductivity or large sizes can markedly lower the radiative thermal conductivity
Particulate size mixtures are shown to have a radiative thermal conductivity that is equivalent to the Sauter mean particle diameter
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