Search

Molten salt systems have become of growing interest in the energy industry due to a wide range of applications (concentrated solar power systems, energy storage, Generation IV fission reactors, and high magnetic field fusion reactors). Because of the high temperature that characterizes such materials, radiative heat transfer (RHT) may become a nonnegligible heat transfer mechanism in molten salt components. In this paper, an investigation of FLiBe RHT has been conducted, with a focus on Affordable, Robust, Compact (ARC)–class fusion reactors, a preconceptual design proposed by Commonwealth Fusion Systems and the Plasma Science and Fusion Center. This class of reactors largely employs FLiBe molten salt due to its thermal and neutronic properties. The reactor is characterized by high temperatures, and its 0.5-m-thick liquid immersion blanket is a component where RHT contribution to the temperature distribution is yet to be evaluated. Therefore, this study is the first work that quantifies the contribution of RHT in ARC-class reactor FLiBe systems. FLiBe optical property spectral-banding assessment is carried out, and the impact of RHT in FLiBe systems is assessed in operational ARC-class scenarios through computational fluid dynamics models by taking advantage of COMSOL® Multiphysics. Heat transfer, thermal-dependent properties, and buoyancy effects are considered in a comparison between scenarios with and without RHT modeling. The flow field in the tank is unaffected by RHT effects, even when considering buoyancy effects. The external layer of the vacuum vessel shows an average decrease in the temperature of 5.4 K and an average decrease in temperature on the surface in contact with the FLiBe tank of 8.1 K. Results indicate that for ARC-class reactors, RHT phenomena are negligible (<1% increase in heat transfer) in operational conditions. [ABSTRACT FROM AUTHOR]

Molten salts such as 2LiF-BeF2 (FLiBe) have been proposed as coolants for advanced nuclear fission and fusion reactors. Critical to the design, licensing and operation of these reactors is characterization and understanding of the chemical behavior and mass transport of activation and fission products, corrosion products, and other solutes in the coolant. Electrochemical techniques are a powerful suite of tools for probing these phenomena. The design of an experimental cell for molten salt electrochemistry is described herein. As a demonstration of this design, details of the experimental methods used to conduct electrochemical experiments with molten FLiBe with addition of LiH are provided. Decommissioning of the cell is considered from the point of view of decontamination and waste generated. Main features of the cell include:•Suitable for operation up to 800 °C; suitable for operation inside and outside of a glovebox.•Enables sweep gas, gas sampling and analysis; enables addition of solid and liquid materials during operation.•Supports a variety of electrode materials and arrangements.

The design of advanced nuclear reactors [Generation IV (Gen IV)] involves an array of challenging fluid-flow issues that affect its safety and performance. Given that Gen IV designs have improved passive safety features, the downcomer plays a crucial role in loss-of-power scenarios. Fluid-flow behavior in the downcomer can involve forced to mixed to natural convection, and characterizing the heat transfer for these changing regimes is a daunting challenge. The creation of a high-resolution heat transfer numerical database can potentially support the development of precise and affordable reduced-resolution heat transfer models. These models can be designed based on a multiscale hierarchy developed as part of the recently U.S. Department of Energy–funded Center of Excellence for Thermal Fluids Applications in Nuclear Energy, which can help address industrial-driven issues associated with the heat transfer behavior of advanced reactors. In this paper, the downcomer is simplified to heated parallel plates, and high Prandtl number fluid (FLiBe) is considered for all simulations. The calculations are performed for a wide range of Richardson numbers from 0 to 400 at two different FLiBe Prandtl numbers (12 and 24), which result in 40 simulated cases in total. Time-averaged and time series statistics, as well as Nusselt number correlations, are investigated to illuminate mixed convection behavior. The calculated database will be instrumental in understanding flow behavior in the downcomer. Ultimately, we aim to evaluate existing heat transfer correlations, and some modifications are proposed. [ABSTRACT FROM AUTHOR]

Abstract Developing heat transfer correlations for buoyancy-driven flows and mixed convection is challenging, especially if the fluid’s Prandtl (Pr) number is not close to 1. For advanced nuclear reactor (Generation IV) designs, the downcomer plays a crucial role in normal operation and loss-of-power scenarios. The fluid-flow behavior in the downcomer can involve forced, mixed, or natural convection. Characterizing the heat transfer for these changing regimes is a serious challenge, especially in the heat transfer deterioration region. In this paper, the downcomer is simplified to heated parallel plates. The high–Pr number fluid FLiBe (a mixture of lithium fluoride and beryllium fluoride) is considered for all simulations. Direct numerical simulations using the graphics processing unit–based spectral element code NekRS are performed for a wide range of the Richardson number, from 0 to 400, at two different FLiBe Pr numbers (12 and 24). This results in an unprecedented 74 cases in total. Each case’s Nusselt number is calculated to evaluate existing heat transfer correlations.Moreover, we propose several new modifications for cases without satisfactory choice. As a result, several novel mixed-convection heat transfer correlations have been built for high–Pr number fluids. The correlations are expressed as a function of the buoyancy number, covering several mixed-convection regimes. The Pr number effect on the Nusselt number behavior is also analyzed in detail. We also propose a novel method to evaluate the heat transfer deterioration region. Modified Reynolds-Gnielinski forced-convection correlations are defined for the laminarization region, and a free-convection correlation is used for the natural-convection-dominated region. These correlations can describe well the trend in the heat transfer–deficient region. [ABSTRACT FROM AUTHOR]

Predicting the distribution and release of tritium remains a technical challenge for advanced nuclear reactors with molten Flibe (2LiF-BeF2) salt coolants. Tritium transport models, which are currently used to forecast release behavior, are limited by uncertainty in Flibe-related tritium transport properties and by a lack of relevant benchmark experiments to test input parameters and solution methods. A new test facility has been developed at the Massachusetts Institute of Technology Research Reactor (MITR) to irradiate a molten Flibe target in an ex-core neutron beam port to further investigate tritium transport mechanisms at prototypical reactor conditions. The experiment monitored the time-dependent release of tritium from the salt-free surface and the permeation rate of tritium through the stainless steel Flibe-containing test stand. Measured results were benchmarked with a multiphysics tritium transport simulation to resolve complex effects in the test. Trends in tritium release rates from the irradiation were in agreement with the multiphysics simulation of the test, which combined computational fluid dynamics, radiative heat transfer in participating media, and tritium transport in STAR-CCM+. [ABSTRACT FROM AUTHOR]

The LiF–BeF2 system is used as a heat transfer medium in molten salt reactors (MSRs). The thermal diffusivity of Li2BeF4 was studied using the laser flash analysis (LFA) method in solid and transition states. While the thermal diffusivity is shown to decrease slightly in solid-state Li2BeF4, it drops significantly at temperatures close to phase transition. The heat capacity of Li2BeF4 was measured by differential scanning calorimetry (DSC). Some differences were observed between the results obtained in cooling and heating modes. Thermal conductivity was calculated using thermal diffusivity-, density-, and heat-capacity data. The good thermal conductivity of the Li2BeF4 compound in solid and liquid states justifies its use as a heat transfer medium for molten salt reactors.

A thermodynamic analysis of the equilibrium in the LiF–BeF2–BeO–C system at temperatures below 3000°C is carried out. The conditions and parameters of the carbothermic reduction of beryllium oxide dissolved in FLiBe are chosen on the basis of the analysis results. The occurrence of the reaction BeO(s) + C(s) = Be(s/l) + CO(g) is thermodynamically possible at temperatures above 2500°C. Such a high temperature undesirable for an analysis of a molten salt with a significantly lower liquidus temperature can be decreased by adding a metal that forms alloys with beryllium with a low activity coefficient via the reaction BeO(s) + C(s) = M(Be)(l) + CO(g). A thermodynamic analysis of the system 66 kmol % LiF + 34 kmol % BeF2 + 1 kmol % BeO + 10 kmol % C with the specified activity coefficient of beryllium (10–2 → 10–4 → 10–6 → 10–8) shows that the reduction of BeO occurs at 1640°C by 99.2% at the activity coefficient f(Be) = 10–4 in the alloy and by 99.98% at f(Be) = 10–6. The Cu–Be alloy with the activity coefficient of Be of 10–6 to 10–4 in the temperature range from 1000 to 1700°C can be used to decrease the reduction temperature. The determined reduction conditions (copper bath) and parameters (1640°C) are successfully tested on FLiBe–BeO samples with a known oxygen concentration on a Metavak-K gas analyzer. [ABSTRACT FROM AUTHOR]

The LiF–BeF2 system is used as a heat transfer medium in molten salt reactors (MSRs). The thermal diffusivity of Li2BeF4 was studied using the laser flash analysis (LFA) method in solid and transition states. While the thermal diffusivity is shown to decrease slightly in solid-state Li2BeF4, it drops significantly at temperatures close to phase transition. The heat capacity of Li2BeF4 was measured by differential scanning calorimetry (DSC). Some differences were observed between the results obtained in cooling and heating modes. Thermal conductivity was calculated using thermal diffusivity-, density-, and heat-capacity data. The good thermal conductivity of the Li2BeF4 compound in solid and liquid states justifies its use as a heat transfer medium for molten salt reactors. [ABSTRACT FROM AUTHOR]

Considerable work is focused on the development of molten salt fueled nuclear reactors. The treatment of the spent fuel in these systems has unique waste management issues. These typically halogen-based salts are known to be water soluble to various degrees. In addition to leachability issues, radiolysis may destabilize these salts when they are in solid form. FLiBe, a favored candidate for fluoride-salt-fueled reactor applications, presents significant waste management challenges. Conversion of FLiBe to fluorapatite may be an effective means to stabilize the salt for improved long-term stability and storage.

BackgroundFLiBe is commonly used as the coolant and carrier salt in liquid molten salt reactors (MSRs). Its certain moderating properties and thermal neutron scattering attributes affect the neutronic performance of the MSR, and this in turn influences the physical design and safe operation of the reactor. Consequently, studying FLiBe's thermal neutron scattering data is essential for MSRs.PurposeThis study aims to analyze the influence of of FLiBe thermal neutron scattering on neutronic performances of a 65-MW MSR.MethodsFirst, according to the requirements, a core model of a 65-MW MSR was established by using the general Monte Carlo procedure. Then, the neutronics performance of the MSR was calculated by considering the scattering cross-section of the free gas model and FLiBe thermal neutron scattering data (e.g., the neutron spectrum, effective multiplication factor, and nuclide reactivity rate). Finally, the changes in the influence of FLiBe thermal neutron scattering effect on neutronic properties under different energy spectra were compared.ResultsThe computation results show that, by considering the thermal scattering effect of FLiBe molten salt, the neutron energy spectrum in the core of the MSR becomes harder, 235U fission rate decreases, the keff value of the reactor decreases, but the density coefficient in the temperature reaction coefficient of the fuel keeps almost unchanged, and the Doppler coefficient decreases by 0.28×10-5 K-1. With the hardening of the energy spectrum, the variation in the 235U fission rate reduction decreases, and the decrease in keff caused by thermal neutron scattering changs from 9.2×10-4 to 2×10-4.ConclusionsTherefore, it is necessary to incorporate FliBe's thermal neutron scattering data into the physical calculations for the MSR core.

• New molten salts proposed as carriers for long-lived fission products transmutation. • Viscosity mechanism of ZrF 4 –BeF 2 is different from LiF–BeF 2. • Alkali metals are important in the material design of salts containing BeF 2. This study analyzed the influence of ZrF 4 on the viscosity of a ZrF 4 –BeF 2 binary salt towards developing BeF 2 -based molten salts for nuclear transmutation targets. Specifically, we performed molecular dynamics simulation using a polarization ion model and evaluated the viscosity, static structure, and applicability of the Stokes–Einstein equation for ZrF 4 –BeF 2 compared with LiF–BeF 2. Our study revealed that ZrF 4 –BeF 2 exhibits higher viscosity than LiF–BeF 2 because each ionic bond's longer lifetime extends the structural relaxation time. Furthermore, it is suggested that the Zr F and Be F relaxations contribute to the viscosity mechanism of ZrF 4 –BeF 2. Most of the elements in long-lived fission products (LLFPs) are multivalent ions such as Zr, Pd, and Sn, which bind strongly to anions and are not expected to contribute much to reducing the viscosity of BeF 2 mixed salts. In contrast, Cs, also an LLFP, is a monovalent ion, and CsF can reduce the viscosity of these salts. Mixing particular LLFPs with CsF will be one of the principles in designing BeF 2 -based salts for transmutation. [ABSTRACT FROM AUTHOR]

In this study, lattice physics calculations were carried out to evaluate the reactor physics characteristics of different advanced fuel lattices cooled with 7LiOH/NaOH or FLiBe and moderated externally by graphite and various types of metal hydroxides, such as 7LiOH, 7LiOD, Mg(OD)2, and ZrE(OD)4. The lithium in these compounds is enriched to 99.995 at. % 7Li/Li. Such lattice fuel concepts could be used in compact, thermal-spectrum, high-temperature (700°C) small modular reactors (SMRs). For an SMR with a bare core size of diameter = height = 163.3 cm, there are several lattice design concepts identified that could achieve modest power densities (up to 18 MW/m³) that are higher than those found typically in high-temperature gas cooled reactors (~ 2 to 10 MW/m³) [IAEA Technical Document 1382 (2019); Report PNR-131-20110914, Delft University, Netherlands (2011)], although lower than those found typically in SMRs based on light water reactor technology (for example, the NuScale SMR has a volumetric power density of ~47 MW/m³) [Proc. PBNC 2018, p. 270 (2018)]. In addition, there are lattice designs identified for the fixed core size that could achieve high fuel burnup (up to 126 MWd/kg), long core lifetimes (up to 24 years before refueling), very good fissile utilization (up to 640 MWd/kg-fissile), and very good relative uranium utilization (up to 44% of that achieved with a conventional pressure-tube heavy water reactor using natural uranium fuel). The best lattice concept found to maximize fuel burnup with 7LiOH/NaOH coolant was an 18-cm-pitch lattice with ZrE(OD)4 external moderator (126.5 MWd/kg). The best lattice concept for FLiBe coolant was a 16-cm-pitch lattice with 7LiOH external moderator (125.99 MWd/kg). Although it is recognized that there are numerous and challenging technical issues to be resolved, particularly with corrosion and materials science, the potential use of hydroxides as coolants and/or external moderators could lead to very important performance improvements for very small and compact SMRs. [ABSTRACT FROM AUTHOR]

• Molecular dynamics with a machine learning potential is used to show modest binding interaction of Cr and Ni in FLiBe. • Dissolved Cr and Ni tend to reside in each other's second nearest neighbor. • The existence of Cr in FLiBe increases the solubility of Ni at high salt potentials. In nuclear and/or solar applications that involve molten salts, impurities frequently enter the salt as either fission products or via corrosion. Impurities can interact and make complexes, but the impact of such complexation on the properties of the salts and corrosion rates has not been understood. Common impurities in molten salts, such as FLiBe, include Cr, Ni, and Fe. Here, we investigate the complexation of Cr and Ni in FLiBe using molecular dynamics based on a machine learning interatomic potential (MLIP) fitted using the atomic cluster expansion (ACE) method. The MLIP allows us to overcome the challenges of simultaneously needing accurate energetics and long time scale to study complexation. We demonstrate that impurity behavior is more difficult to capture than that of concentrated elements with MLIPs due to less sampling in training data, but that this can be overcome by using active learning strategies to obtain a robust fit. Our findings suggest that there is a weak but potentially significant binding free energy between CrF 2 and NiF 2 in eutectic FLiBe of - 0.112 e V. Under certain conditions this binding creates a significant short-range order between the species and lowers the redox potential of NiF 2 in the presence of CrF 2 in FLiBe, making Ni dissolution more favorable in the presence of Cr as compared to its dissolution in pure FLiBe. However, we find little impact of this complexation on the diffusivity of Ni and Cr. Overall, the methodology presented here suggests an approach to modeling complexation with MLIPs and suggests that interactions between dissolved cations could be playing a significant role in some salt thermophysical properties. [ABSTRACT FROM AUTHOR]

In the fight against climate change through the progress of fusion technology, the Affordable, Robust and Compact (ARC) fusion reactor design is under development at Commonwealth Fusion Systems (CFS) and Massachusetts Institute of Technology (MIT)- Plasma Science and Fusion Center, in collaboration with the Ente Nazionale Idrocarburi (ENI) S.p.A. The reactor features demountable superconducting toroidal field coils and a replaceable vacuum vessel immersed in a Fluorine Lithium Beryllium (FLiBe) molten salt blanket. The low-pressure molten salt cools the divertors and the blanket, shields the magnets and acts as tritium breeder and tritium carrier. Tritium must be recovered efficiently from the salt to fuel the reactor, minimizing its inventory and its migration outside the FLiBe loop. The current work is aimed at presenting a preliminary design of a Double-Wall Heat eXchanger (DWHX) to be installed in an ARC-class reactor primary cooling system. The main feature is the presence of a flowing sweep gas gap which captures tritium and prevents its diffusion toward the secondary system. For the DWHX, thermal-hydraulic and tritium transport analyses were performed, considering a supercritical water Rankine power conversion system on the secondary side to carry out the calculations. To assess tritium extraction capabilities of the DWHX, the tritium extraction efficiency has been evaluated. Calculation results highlighted how the current configuration is not adequate to remove tritium, in fact the tritium extraction efficiency is very low. On the other side, it seems to act adequately as tritium permeation barrier. [ABSTRACT FROM AUTHOR]

To develop the application of 95Nb as an indicator of redox potential for fuel salt in molten salt reactor (MSR), the specific activity of 95Nb in FLiBe salt and its deposition of 95Nb on Hastelloy C276 have been studied. Experimental results indicated that the amount of 95Nb deposited on Hastelloy C276 resulted from its chemical reduction exhibited a positive correlation with the decrease of 95Nb activity in FLiBe salt and the relative deposition coefficient of 95Nb to 103Ru appeared a well correlation with 95Nb activity in FLiBe salt. Both correlations implied that the measurement of 95Nb activity deposited on Hastelloy C276 specimen might provide a quantitative approach for monitoring the redox potential of fuel salt in MSR. [ABSTRACT FROM AUTHOR]

The 235,238UF4 was irradiated by photo-neutrons, distribution and behavior of the fission product 95Nb from irradiated 235,238UF4 in FLiBe salt were investigated by the measurement of its activity in the salt with the γ-ray spectroscopy. The experiments indicated that a part of 95Nb deposited on the surfaces of graphite and Hastelloy, as the moderator and the structural materials of molten salt reactor (MSR), respectively, and the majority of 95Nb maintained in molten salt. Addition of lithium metal made 95Nb in salt to be reduced and settled, leading to the decrease in its activity. Degree of the decrease was found to be correlated with niobium concentration. The experimental results supported the statement proposed early by ORNL, that 95Nb might be used as a redox indicator for MSR. Finally, the problem met with on-site monitoring for redox potential in MSR was pointed, and a possible protocol to resolve the problem was proposed. [ABSTRACT FROM AUTHOR]

In 1974, reference correlations for the viscosity of molten LiF-NaF-KF, LiF-BeF2, and Li2CO3-Na2CO3-K2CO3 were proposed by Janz and have been extensively used since then. However, in the last 45 years, many additional measurements have been published. This is why in this paper, new reference correlations for the viscosity of these salts are proposed. All available experimental data for the viscosity of these three molten salts have been critically examined with the intention of establishing improved or new reference viscosity correlations. All experimental data have been categorized into primary and secondary data according to the quality of measurement specified by a series of criteria. The reference correlation proposed for LiF-NaF-KF, with an uncertainty of 2.9% at the 95% confidence level, expands the temperature range of the previous correlation from (770–970) K to (732–1163) K and retains its uncertainty. The correlation proposed for LiF-BeF2, with an uncertainty of 4.9% at the 95% confidence level, expands the high temperature range of the previous correlation from (740–870) K to (793–1573) K, with a slight loss in its uncertainty. It is, however, a much better correlation as it is based upon measurements not available at the time of the previous one. Finally, the reference correlation for Li2CO3-Na2CO3-K2CO3, with an uncertainty of 3%, also expands the temperature range of the previous correlation from (920–1170) K to (738–1170) K and retains its uncertainty. [ABSTRACT FROM AUTHOR]