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Experimental studies have been developed on a new freeze plug concept for safety valves in facilities using molten salt. They are designed to allow the closure of an upstream circuit by solidifying the molten salt in a section of the device and to passively melt in case of a loss of electric power, thus releasing the upper fluid. The working princi...
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... A sufficiently short opening time of the valve is required to drain the reactor quick enough to prevent the temperature from exceeding dangerously high values, as stated by Tiberga et al. (2020a). Apart from experimental investigations such as those performed by Giraud et al. (2019), the design of the freeze valve relies heavily on numerical simulations to obtain predictions of its melting time. Experimental studies suitable for numerical validation have mainly focused on the melting of pure metals or paraffin wax in rectangular or cylindrical enclosures, such as those performed by Gau and Viskanta (1986), Jones et al. (2006), and Faden et al. (2019a) and do not include the full complexity of the MSFR freeze valve design. ...
... 1016/j.anucene.2023.110093 Received 13 April 2023; Received in revised form 7 July 2023; Accepted 15 August 2023 freeze valve designs (Giraud et al., 2019), to compare the solid-liquid phase change modelling capabilities and their coupling with fluid flow and conjugate heat transfer models of different numerical codes. The benchmark consists of five different stages and with each stage, complexity is added as the model evolves towards a more realistic representation of the MSFR freeze valve. ...
... A numerical benchmark is proposed based on the MSFR freeze valve design described by Giraud et al. (2019). The freeze valve is a key passive safety feature unique to the MSFR, and is designed to melt in the case of a reactor anomaly, draining the fuel salt into an emergency drainage system. ...
... Since no operation is needed for reactor shutdown, the freeze plug is regarded as a passive safety system. The feasibility of the freeze plug has already been proven in the MSRE that is the earliest MSR developed in Oak Ridge National Laboratory (Richardson, 1962;Chisholm et al., 2020), and several advanced designs have been proposed (Rubiolo et al., 2017;Tiberga et al., 2019;Giraud et al., 2019;Jiang et al., 2020). ...
... The SAMOFAR project, which performs experimental and analytical work supporting the development of a 300 MW th thorium-fueled MSR, has worked on the development of a freeze plug design for use in the European Molten Salt Fast Reactor (MSFR). (Giraud et al., 2019;Makkinje, 2017;Shafer, 2018;Swaroop, 2016;Tiberga et al., 2019;Van Tuyll, 2016) Chisholm et al. (Chisolm et al., 2020) also summarize the SAMOFAR contributions to freeze-plug development to date. The ongoing research by SAMOFAR and Chisholm et al. (Chisolm et al., 2020) suggests that the freeze-valve/freeze-plug technology for MSRs is within a TRL range of 4-5. ...
The primary uses of molten salt in energy technologies are in power production and energy storage. Salts remain a single-phase liquid even at very high temperatures and atmospheric pressure, which makes molten salt well-suited to advanced energy technologies, such as molten salt reactors, or hybrid energy systems. The molten salt cooled reactor is an advanced nuclear reactor concept that utilizes molten salt as either a coolant for solid fuel or as a fuel salt. The liquid phase provides orders of magnitude higher heat capacity per cubic meter than the gas phase. This, coupled with the low-pressure environment required to maintain the liquid phase, provides significant advantages in terms of compact-sized systems constructed with relatively thin walls.
The heat from a heat-generating process is transferred to a heat transfer media and can be extracted later using a secondary power cycle. There are several types of facilities that use thermal energy storage with molten salts, such as concentrated solar power plants (CSP plants) or nuclear hybrid energy systems (NHES). A CSP plant is a power production facility that uses a broad array of reflectors or lenses to concentrate solar energy onto a small receiver. Since molten salt remains in the liquid phase, it has excellent heat retention properties, meaning heat from a solar-generation process can be stored for an extended period for later use. A Nuclear Hybrid Energy System (NHES) refers to several energy systems combined to generate energy more efficiently, such as nuclear reactors, renewable energy sources, process heat applications, and energy storage.
The selection of a salt type for a nuclear reactor or a thermal storage system requires careful consideration of the chemical and thermodynamic properties of the candidate salts. Different energy technologies will require different salt types, based on temperature and fluid property requirements. Fluoride salts are often the primary candidate salts for nuclear reactor systems. Chloride salts are another category of candidate salt that have been considered for power production because chloride salts often exhibit similar behavior to fluoride salts. Nitrate-nitrite salts contain NO3 and NO2 and are used in solar applications.
As with other nuclear reactors, molten salt systems involve radiological and chemistry challenges, including tritium production and corrosion. Tritium production can be problematic in a reactor system because it can be a hazard to human operators. Tritium is difficult to contain; therefore, the production of tritium must be minimized. Corrosion of structural materials is also an area requiring further study in molten salt systems. Corrosion in a molten salt system differs from standard nuclear reactor systems due to the lack of a passive oxide film on the surface of structural materials, making it necessary to mitigate corrosion by either purifying the salt, controlling its redox potential in a reducing state, or using redox buffers. Additionally, since molten salt reactors are constructed with much thinner structural members due to low-pressure loads, reducing reactor operating lifetime and/or marginally increasing the structural thickness to provide additional corrosion allowance may be acceptable design approaches.
The behavior of volatile fission products in Fluoride and Chloride salts is also a consideration due to the volatility of insoluble fission products that precipitate and plate out on surfaces affecting thermal hydraulic parameters. Fission products such as cesium (Cs), iodine (I), strontium (Sr) and other salt seekers, have a complexing nature with fluoride (F) and iodine (I) and their behavior in molten salt reactors will have to be addressed. Molten salt reactors present a particular challenge for recycling fission products including solubility, volatility, and precipitation behavior, and how the fission products change the corrosivity of the salt melt. Current nuclear fuel recycling technology will not accommodate molten salt streams and will need to be redesigned. New molten salt recycling designs and chemistry will ultimately need to be applied to a variety of fluoride and chloride salts mixtures.
All thermal energy facilities either utilize the heat energy produced directly, such as space or process heating, or convert a portion of the heat energy to some other energy form such as electricity. The energy conversion cycles utilized in most (if not all) other types of base-load power plants (e.g., coal-fired power plants, gas-fired power plants, and nuclear power plants) are the Rankine and Brayton power conversion cycles. MSRs are also coupled with both Rankine and Brayton power-conversion cycles to transform heat into other energy forms. The feasibility for cycle coupling with MSRs depends on a variety of factors; two of the more important factors are technology readiness level (TRL) and efficiency. The application of existing steam cycle designs will likely require modifications to equipment. Steam generators and reheaters will present a particular problem in accommodating the molten salt. The adaptation of the plant to the molten salt reactor will require trade studies to obtain information necessary for further design.
... No specific reliability estimates have been identified in the open literature for freeze valves. Recent work has investigated the associated thermodynamics of salt plug freezing and melting (Giraud et al., 2019;Makkinje, 2017;Shafer, 2018;Swaroop, 2016; van Tuyll, 2016); however, there has not been analysis conducted to evaluate freeze valve safety and/or reliability. Therefore, a thorough review of freeze valve operating experience has been recently conducted, and an original quantitative reliability assessment has been developed. ...
... Experimental data collected from the Forced Fluoride Flow for Experimental Research (FFFER) and Salt at Wall: Thermal excHanges (SWATH) facilities to improve the accuracy of the SAMOFAR analysis detailed by Giraud et al. (2019) have corroborated Tiberga et al.'s conclusion that the MSFR freeze valve should be thermally isolated from the heat in the core to maximize stability of the heat balance in the plug and the system control setpoints. The FFFER operates using a FLiNaK eutectic salt, and the inner diameter of the piping is 57 mm (2.2 in.). ...
... The heat balance of the freeze valve system in the FFFER loop was significantly altered by heat transferred from nearby lines containing flowing salt. Thus, small alterations in the physical configuration of the FFFER loop resulted in significant changes in the operational setpoints and performance of the freeze valve (Giraud et al., 2019). ...
Reliable mechanical valves that can withstand the corrosive and high-temperature conditions in Molten Salt Reactors (MSRs) have not yet been demonstrated. In their place, freeze valves (sometimes called freeze plugs) represent a unique nuclear design solution for isolating salt flow during operations. Additionally, in some cases, they are intended to perform safety-related functions. As implied by the name, operation of a freeze valve depends upon the controlled phase change of salt, and the ability of increased reactor heat from transients to induce modulation of the “valve” has frequently been equated to safety function passivity. In this article, a review of literature pertaining to freeze valve design, performance, and thermodynamic analyses is presented first. The Molten Salt Reactor Experiment (MSRE) was the first MSR to implement the use of freeze valves and is the only liquid-fueled MSR with significant operating experience; yet, the success of the MSRE freeze valve to thaw on demand or remain frozen during power operation depended on two subsystems comprising power-operated sensors, mechanical valves, and other active components. Accordingly, as executed at the MSRE, the freeze valve system was not a fully passive safety feature. Performance of original Process Hazards Analysis (PHA) studies on an MSRE safety-related freeze valve design indicates failure of multiple individual active components could result in the impairment of the freeze valve function, such that the freeze valve itself is a system of components – rather than a single component. Fault Tree Analysis (FTA) based on PHA results provides estimated failure rates for two important failure modes of freeze valves. The results suggest that designers of freeze valve systems may face a trade-off between the reliability of the freeze valve to thaw on demand and the reliability of the freeze valve to remain frozen during normal reactor operations; however, a sensitivity study showed that the likelihood of a spurious thawing of the MSRE freeze valve system could have been reduced without significantly affecting the likelihood of a failure to thaw.
... It utilized an external high-frequency induction coil to melt the salt plug and freeze the salt using the cooling gas supplied by an external high-pressure fan. Giraud et al. [20] constructed two experimental facilities to improve the freeze valve design for molten salt application. Aji et al. [21] conducted a fundamental experiment to determine the thawing time of a typical salt. ...
To improve the reliability and reduce energy consumption, a conceptual design of a freeze valve is proposed for the thorium-based molten salt reactor (TMSR) concept. Fins were utilized in this new design to enhance heat transfer and realize passive shut-off function, which could not be realized by the previous design. An experimental apparatus using the fluoride salt FLiNaK was constructed to conduct a series of preliminary solidification and melting experiments. In addition, the enthalpy-porosity method of ANSYS® Fluent solver was applied to simulate the solidification process of the salt at a specified operating temperature. Temperature distributions of the fluoride salt, solidification/melting time, and frozen plug effect were analyzed under natural convection heat transfer in an open space. The calculated salt temperatures exhibited good agreement with the experimental values. The results indicated that the range of effective operating temperature is 530–600 °C for the finned freeze valve. In this study, the ideal set operating temperature of the finned freeze valve was chosen as 560 °C to achieve competent performance. Moreover, 560 °C is additionally the highest set operating temperature for maintaining excellent cooling performance and sustaining deep-frozen condition of the salt plug. At this set operating temperature, the simulation data indicated that the molten salt in the flat part of the finned freeze valve will completely solidify at 10.5 min. The percentage of solid salt in the flat and lower transitional parts of the valve reaches 29.60% in 30.0 min. Furthermore, the surface temperature of the proposed freeze valve is 11.10% lower compared with that of the TMSR freeze valve at a cooling gas supply of 173 m³/h. Therefore, the new freeze valve was proven to be capable of reducing the energy consumption and realizing the passive shut-off function.
Freeze plug is an important passive safety system used in the molten salt reactors (MSRs). It enables automatic drainage of the liquid fuel from the core to the storage tanks in an emergency to stop nuclear fission chain reaction without any operator's action and electric power supply. The opening time, that is the time taken for the freeze plug to open, is therefore of considerable importance to ensure passive safety of the MSRs. In our previous studies, systematic numerical simulations were carried out to understand how the fundamental design parameters such as the tube diameter and wall thickness of the freeze plug affected the opening time. In this work, a simple analytical model was developed for rough estimation of the opening time. It was shown that the opening time calculated by the present simple model was in fairly good agreement with that by the full simulation using the mass, momentum and energy conservation equations for the salt and the heat conduction equation within the wall material. The present simple model was hence shown to be useful particularly for the schematic design of the improved MSR freeze plugs.
The shallow nitrogen-vacancy center of diamond exhibits excellent sensitivity and resolution in the magnetic detection and quantum sensing areas. Compared with other methods, low-energy carbon ion implantation doesn't need high-purity diamond and introduce new impurity atoms, but the formation mechanism of nitrogen-vacancy center is not clear. In this paper, shallow nitrogen-vacancy centers were created in the diamond by low energy carbon ion implantation and vacuum annealing, and the transformation mechanism of nitrogen-vacancy centers in diamond was studied by Raman spectroscopy, X-ray photoelectron spectroscopy and positron annihilation analysis. The results showed that shallow nitrogen-vacancy can be obtained by carbon ion implantation combined with vacuum annealing. After implantation, superficial layer of diamond showed the damage zone including lattice distortion and amorphous carbon, and carbon-vacancy cluster defects (carbon atoms were surrounded by vacancy clusters) were generated. During the vacuum annealing process, the damaged area gradually transformed to the diamond structure through the recovery of the distortion area and the solid-phase epitaxy of the amorphous carbon area, accompanied by the continuous dissociation of carbon-vacancy cluster defects. When samples were annealed at 850 and 900, the℃℃ structure of the damaged area was partially repaired. While annealing at 950, not only the damaged laye℃ r was basically recovered, but nitrogen atoms captured the single vacancy obtained by the dissociation of carbon vacancy clusters, forming the nitrogen-vacancy centers.
