Numerical and experimental investigation of a new conceptual fluoride salt freeze valve for thorium-based molten salt reactor

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℃ for the finned freeze valve.In this study,the ideal set operating temperature of the finned freeze valve was chosen as 560℃ to achieve competent performance.Moreover,560℃ 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 m^3/h.Therefore,the new freeze valve was proven to be capable of reducing the energy consumption and realizing the passive shut-off function.

Study of heat transfer by using DEM–CFD method in a randomly packed pebble-bed reactor

The pebble-bed reactor is one of the most promising designs for the nuclear energy industry. In this paper,a discrete element method-computational fluid dynamics(DEM-CFD) approach that includes thermal conduction, radiation, and natural convection mechanisms was proposed to simulate the thermal-fluid phenomena after the failure of forced circulation cooling system in a pebble-bed core. The whole large-scale packed bed was created using the DEM technique, and the calculated radial porosity of the bed was validated with empirical correlations reported by researchers. To reduce computational costs, a segment of the bed was extracted, which served as a good representative of the large-scale packed bed for CFD calculation. The temperature distributions simulated with two different fluids in this DEM-CFD approach were in good agreement with SANA experimental data. The influence of the natural convection mechanism on heat transfer must be taken into account for coolants with strong convective capacity. The proposed DEM-CFD methodology offers a computationally efficient and widely applied method for understanding the heat transfer process in a pebble-bed core. The method can also be easily extended to assess the passive safety features of newly designed fluoride-salt-cooled pebble-bed reactors.

Avoiding sealing failure of flanged connection for tubes made of dissimilar materials subjected to elevated temperature

Carbonic composite materials and ceramics appear to be excellent structural materials for parts subjected to very high temperatures in molten salt reactors(MSRs), in which the reactor core outlet temperature is normally above 700℃. Because of the high temperature,there are major challenges in the sealing of flanged connections for tubes made of alloys and nonmetallic materials. In this study, an improved method for sealing bolted flange connections for tubes made of dissimilar materials at high temperature is analyzed. The study focuses on the compensation mechanism for the difference in thermal expansion between the bolts and the flanges. An angle is introduced for the sealing surface in the flanged connection to provide effective sealing. The arctangent of the angle is the ratio of the thickness between the theoretical core of the sealing surface and the outside end face of the flange to the distances between the axis of the flanged joint and the theoretical core of the sealing surface of the flange; the sealing surface of the flange, which is made of the same material as the fastening assemblies, faces the fastening assemblies. To ensure effective sealing, the frictional coefficient between the two sealing surfaces should not exceed the tangent of the angle. This result does not agree well with the solution given by previous researchers. Further, in the modified flanged connection, the compression of each bolt in the clamped condition is increased to maintain the compaction force unchanged without increasing the number of bolts on the flanged joint.

Experimental study on the penetration characteristics of leaking molten salt in the thermal insulation layer of aluminum silicate fiber

The molten salt leakage accident is an important issue in the nuclear safety analysis of molten salt reactors.While the molten salt leaks from the pipeline or storage tank,it will contact the insulation layer outside;hence,the processes of penetration and spreading play an important role in the development of leakage accidents.In this study,the penetration and diffusion of leaking molten salt(LMS)in an aluminum silicate fiber(ASF)thermal insulation layer were studied experimentally.A molten salt tank with an adjustable outlet was designed to simulate the leakage of molten salt,and the subsequent behavior in the thermal insulation layer was evaluated by measuring the penetra-tion time and penetration mass of the LMS.The results show that when the molten salt discharges from the outlet and reaches the thermal insulation layer,the LMS will penetrate and seep out from the ASF,and a higher flow rate of LMS requires less penetration time and leaked mass of LMS.As the temperature of the LMS and thickness of the ASF increased,the penetration time became longer and the leaked mass became greater at a lower LMS flow rate;when the LMS flow rate increased,the penetration time and leaked mass decreased rapidly and tended to flatten.

DEM-CFD simulation of modular PB-FHR core with two-grid method

For designing and optimizing the reactor core of modular pebble-bed fluoride salt-cooled high-temperature reactor(PB-FHR),it is of importance to simulate the coupled fluid and particle flow due to strong coolantpebble interactions.Computational fluid dynamics and discrete element method(DEM) coupling approach can be used to track particles individually while it requires a fluid cell being greater than the pebble diameter.However,the large size of pebbles makes the fluid grid too coarse to capture the complicated flow pattern.To solve this problem,a two-grid approach is proposed to calculate interphase momentum transfer between pebbles and coolant without the constraint on the shape and size of fluid meshes.The solid velocity,fluid velocity,fluid pressure and void fraction are mapped between hexahedral coarse particle grid and tine fluid grid.Then the total interphase force can be calculated independently to speed up computation.To evaluate suitability of this two-grid approach,the pressure drop and minimum fluidization velocity of a fluidized bed were predicted,and movements of the pebbles in complex flow field were studied experimentally and numerically.The spouting fluid through a central inlet pipe of a scaled visible PB-FHR core facility was set up to provide the complex flow field.Water was chosen as Liquid to simulate the molten salt coolant,and polypropylene balls were used to simulate the pebble fuels.Results show that the pebble flow pattern captured from experiment agrees well with the simulation from two-grid approach,hence the applicability of the two-grid approach for the later PB-FHR core design.

FEA-based structural optimization design of a side cooling collimating mirror at SSRF

Based on finite element analysis of thermal mechanical behavior, structural optimization design was proposed for a side cooling collimating mirror subjected to high heat load for a beamline at SSRF(Shanghai Synchrotron Radiation Facility). The temperature distribution,stress concentration effect, maximum equivalent(vonMises) stress, and slope error of the mirror were analyzed.In particular, the cooling water channels of the traditional structural design were optimized, and the modified designs were further optimized. Although the traditional structural and the improved designs could meet requirements for the temperature and thermal stress, the deformation gradients were relatively large for several structural designs, and this led to larger slope error. The further improved structural designs could be of better performance.

Fine structure characterization of an explosively-welded GH3535/316H bimetallic plate interface

An explosion-welded technology was induced to manufacture the GH3535/316H bimetallic plates to provide a more cost-effective structural material for ultrahigh temperature,molten salt thermal storage systems.The microstructure of the bonding interfaces were extensively investigated by scanning electron microscopy,energy dispersive spectrometry,and an electron probe microanalyzer.The bonding interface possessed a periodic,wavy morphology and was adorned by peninsula-or island-like transition zones.At higher magnification,a matrix recrystallization region,fine grain region,columnar grain region,equiaxed grain region,and shrinkage porosity were observed in the transition zones and surrounding area.Electron backscattered diffraction demonstrated that the strain in the recrystallization region of the GH3535 matrix and transition zone was less than the substrate.Strain concentration occurred at the interface and the solidification defects in the transition zone.The dislocation substructure in 316H near the interface was characterized by electron channeling contrast imaging.A dislocation network was formed in the grains of 316H.The microhardness decreased as the distance from the welding interface increased and the lowest hardness was inside the transition zone.