Self-cooled Liquid Metal Blanket Research Articles

Pre-conceptual design and proof of principle assessments of self-cooled Toroidally symmetric lead-lithium (TSLL) blanket

A new self-cooled liquid metal blanket concept called TSLL (Toroidally Symmetric Lead-Lithium) blanket is proposed and assessed, including analysis for magnetohydrodynamic (MHD) flows, structural analysis, and heat transfer and neutronics assessments using the ARC reactor with demountable magnets designed by the Commonwealth Fusion Systems (CFS) as a testbed. The proposed blanket utilizes lead-lithium (PbLi) alloy as breeder/coolant and reduced activation ferritic/martensitic (RAFM) steel as structural material. A special feature of the new concept is the toroidally symmetric flow in the blanket integrated first wall and the breeding zone to reduce the MHD pressure drop, while using anchor links to strengthen the first wall construction. Provided analysis suggests acceptable MHD pressure drop, required mechanical integrity and high tritium breeding ratio of ∼ 1.64. As a result of these assessments, the new blanket concept can be recommended for more detailed studies as a promising blanket candidate for implementation in future fusion devices.

Numerical simulation of MHD flows in a coupled U-turn rectangular duct with different wall conductance ratios

Liquid metal flow in a conducting coupled U-turn rectangular duct subjected to an external uniform magnetic field is numerically analyzed using our in-house MHD solver developed in OpenFOAM software. The coupled U-turn duct is the structure used in self-cooled liquid-metal blankets, which includes the duct wall, partition wall, inflow channel, outflow channel, and connection channel. The characteristics of the fluid velocity, induced electric current, electric potential, and pressure distribution depending on the wall conductance ratio of the duct wall and the partition wall are investigated. As the partition wall divides the inflow and outflow, the three-dimensional induced electric currents close loops at the end of the partition wall in the connection channel. When the duct wall is insulating and the partition wall is weak conducting, the three-dimensional electric current is significant in the connection channel and the start of the outflow channel. The three-dimensional electric current results in the pressure fluctuation. The flow forms a visible primary vortex at the end of the partition wall because of a strong adverse pressure gradient. The pressure along the flow direction has a significant fluctuation near the connection channel. When the duct wall is weak conducting, high-velocity jets form in the side layers near the partition wall and the duct wall. The velocity distribution is a strong asymmetrical M-type profile in the U-turn rectangular duct with conducting duct walls. The normalized electric potential distribution in the different walls along the magnetic field direction is independent of the Reynolds number when the Reynolds number and the Hartmann number is moderate. When the Hartmann number and Reynolds number is high, the normalized electric potential varies with the Reynolds number. The conductance ratio of the duct wall has a decisive effect on the pressure gradient. The pressure drop with conducting duct wall is two orders of magnitude higher than that of cases with an insulating duct wall. As the duct wall is insulating, the wall conductance ratio of the partition wall does not affect the pressure gradient remarkably. While the duct wall is conducting, the increase of the wall conductance ratio of the partition wall results in an increase of the pressure drop significantly.

Friction Factors in Fully Developed MHD Laminar Flows for Oblique Magnetic Fields and High Hartmann Numbers in Rectangular Channels

Self-cooled liquid metal blanket of a thermonuclear fusion reactor has the most critical issue of strong MHD effects which influence the thermal efficiency of the reactor. Special attention needs to be paid to the MHD friction factors for flow in rectangular channels at low as well as at high Hartmann numbers. The effect on MHD friction factors of a magnetic field oblique to the channel walls for various channel wall conductivities and aspect ratios at different Hartmann numbers is investigated numerically. Even for relatively small channel wall conductivity at high Hartmann numbers, velocity jets are developed near the channel walls. Due to these jets, MHD friction factors increase by the orders of magnitude which can be observed in the numerical results. The finite-difference method has been used to discretize the coupled second-order linear partial differential equations of MHD by using both uniform and nonuniform grids. Nonuniform discretization of the mesh is more efficient than the uniform mesh due to steep velocity gradients near the walls. For parallel plates, an analytical solution for MHD friction factor has been developed, and then compared with the numerical solutions. It is found that an oblique magnetic field has a significant effect on MHD friction factors for different channel wall conductivities and aspect ratios at various Hartmann numbers.

Electrical insulation test of alumina coating fabricated by sol–gel method in molten PbLi pool

Development of electrical insulation coatings, which insulate an induced electric current from electrical conducting walls, is a key technology for the research and development of self-cooled liquid metal blanket including lead–lithium (PbLi) fusion blankets. As for magnetohydrodynamic (MHD) thermofluid study, an electrical insulation coating is extremely important from the viewpoint of the secure electrical insulating wall condition. The present study employs a sol–gel (SG) method to fabricate an Al 2O 3 coating, and discusses the feasibility of the SG coating as an electrical insulation coating for the PbLi through the electrical insulation test in the molten PbLi pool and the SEM with EDS analysis on the SG coating structure. The present study shows that the SG coating will be a potential electrical insulation coating for PbLi with both the operation time and the temperature limitation.

Review of blanket designs for advanced fusion reactors

The dominating fraction of the power generated by fusion in the reactor is captured by neutron moderation in the blanket surrounding the plasma. From this, the efficiency of the fusion plant is predominated by the technologies applied to make electricity or hydrogen from the neutrons. The main blanket concepts addressed in this paper are advanced ceramic breeder concepts, dual coolant blankets as well as self-cooled liquid metal and Flibe blankets. Two important questions that are addressed are: (i) Can we draw a bottom line conclusion on the most promising concept(s)? (ii) What are the common issues to be resolved independently from individual design and layout proposals to define a feasible route towards advanced fusion reactors? For ceramic breeder concepts, a key issue in the long term could be the limitation of beryllium as the considered multiplier in terms of world sources and achievable temperature levels. For liquid metal blankets, attractive long-term visions have been developed but major technological challenges also exist for the in-vessel blanket technology and the corresponding sub-systems. The paper proposes a strategic conclusion derived from the review of blanket designs for advanced fusion reactors.

Joule heating in magnetohydrodynamic flows in channels with thin conducting walls

Joule heating in liquid metal magnetohydrodynamic flows is investigated with reference to self-cooled liquid metal blankets for tokamaks. Pressure-driven flow of an electrically conducting fluid confined between two parallel, infinite walls with a transverse magnetic field is studied. The walls are electrically conducting, which implies strong currents flowing within the thin conducting walls. The problem is solved both analytically and numerically. It is shown that the Joule heat cannot be neglected in certain range of parameters relevant to fusion blanket applications. The magnitude of the Joule heat released inside the channel and the walls depends on the thermal conductivity of the outside surface of the channel walls. For thermally conducting outside surface of the walls the Joule heat can become significant for high values of the Hartmann number and moderate average velocity. The effect is even more pronounced for thermally insulating outside surface of the walls. For example, for lead–lithium flow with stainless steel walls the temperature increase along the flow exceeds 200 °C over the length of the blanket, which is almost three times higher than that for thermally conducting outside surface of the walls. The main reason for such a strong rise in temperature is the heat released inside the walls. The heat produced in the fluid region is quickly convected towards the exit from the channel. The heat released inside the walls can only leave the domain by diffusion into the fluid region and thus is accumulated along the channel length.

Fabrication of CaO insulator coatings by MOCVD for application in vanadium/lithium blankets

The blanket system is one of the most important components in a fusion reactor because it has a major impact on both the economics and safety of fusion energy. The primary functions of the blanket in a deuterium/tritium-fueled fusion reactor are to convert the fusion energy into sensible heat and to breed tritium for the fuel cycle. The self-cooled liquid metal blanket concept requires an electrically insulating coating on the first-wall structural material to minimize the magnetohydrodynamic (MHD) pressure drop that occurs during the flow of liquid metal in a magnetic field. Calcium oxide is a possible coating material because it is an excellent electrical insulator and it has large negative free energy to prevent the attack from liquid lithium. In this paper, details are presented on the metallorganic chemical vapor deposition (MOCVD) method that was used to fabricate the CaO coating. The as-deposited CaO coating was annealed in calcium vapor to prevent coating spallation. Composition and phase analyses of the coating were performed by energy dispersive X-ray (EDX) analysis and X-ray diffraction (XRD). Scanning electron microscopy (SEM) showed that the coating did not crack after several thermal cycles from room temperature to 715 °C. The resistance of the coating was high enough for application as an insulating coating for the self-cooled lithium blanket in fusion systems. The information on Li compatibility is only preliminary and additional effort is needed to evaluate their long-term performance in fusion-relevant conditions.

The EU advanced dual coolant blanket concept : Norajitra, P. et al. Fusion Engineering and Design, 2002, 61–62, 449–453

Electrical insulation of the structural material against the liquid metal (Pb-17Li) is necessary to reduce the MHD pressure drop in a self-cooled liquid metal blanket. The direct insulation by means of an adherent insulating layer seems to be a good solution of the problem. Al2O3 was shown to be sufficiently compatible with the liquid alloy. Al2O3 layers can be produced on MANET steel with an Al layer which is finally converted into a FexAly layer.MANET steel is dipped under inert atmosphere into molten Al. The samples are then heat-treated at different temperatures in order to develop the intermetallic layer by interdiffusion. The high-temperature oxidation of the FexAly layer is performed in air.The properties of the insulating layer and the intermediate are determined and will be discussed in this paper.

Magnetohydrodynamic Heat Transfer Research Related to the Design of Fusion Blankets

Lithium or any lithium alloy like the lithium lead alloy Pb-17Li is an attractive breeder material used in blankets of fusion power reactors because it allows the breeding of tritium and, in the case of self-cooled blankets, the transfer of the heat generated within the liquid metal and the walls of the cooling ducts to an external heat exchanger. Nevertheless, this type of liquid-metal-cooled blanket, called a self-cooled blanket, requires specific design of the coolant ducts, because the interaction of the circulating fluid and the plasma-confining magnetic fields causes magnetohydrodynamic (MHD) effects, yielding completely different flow patterns compared to ordinary hydrodynamics (OHD) and pressure drops significantly higher than there. In contrast to OHD, MHD flows depend strongly on the electrical properties of the wall. Also, MHD flows reveal anisotropic turbulence behavior and are quite sensitive to obstacles exposed to the fluid flow.A comprehensive study of the heat transfer characteristics of free and forced convective MHD flows at fusion-relevant conditions is conducted. The general ideas of the analytical and numerical models to describe MHD heat transfer phenomena in this parameter regime are discussed. The MHD laboratory being installed, the experimental program established, and the experiments on heat transfer of free and forced convective flow being conducted are described. The theoretical results are compared to the results of a series of experiments in forced and free convective MHD flows with different wall properties, such as electrically insulating as well as electric conducting ducts. Based on this knowledge, methods to improve the heat transfer by means of electromagnetic/mechanic turbulence promoters (TPs) or sophisticated, arranged electrically conducting walls are discussed, experimental results are shown, and a cost-benefit analysis related to these methods is performed. Nevertheless, a few experimental results obtained should be highlighted:1. The heat flux removable in rectangular electrically conducting ducts at walls parallel to the magnetic field is by a factor of 2 higher than in the slug flow model previously used in design calculations. Conditions for which this heat transfer enhancement is attainable are presented. The measured dimensionless pressure gradient coincides with the theoretical one and is constant throughout the whole Reynolds number regime investigated (Re = 103 → 105), although the flow turns from laminar to turbulent. The use of electromagnetic TPs close to the heated wall leads to nonmeasurable increase of the heat transfer in the same Re regime as long as they do not lead to an interaction with the wall adjacent boundary layers.2. Mechanical TPs used in an electrically insulated rectangular duct improved the heat transfer up to seven times compared to slug flow, but the pressure drop can increase also up to 300%. In a cost-benefit analysis, the advantageous parameter regime for applying this method is determined.3. Experiments performed in a flat box both in a vertical and a horizontal arrangement within a horizontal magnetic field show the expected increase of damping of the fluid motion with increasing Hartmann number M. At high M, buoyant convection will be completely suppressed in the horizontal case. In the vertical setup, the fluid motion is reduced to one large vortex leading to a decreasing heat transfer between heated and cooled plate to pure heat conduction.From an analysis of the experimental and theoretical results, general design criteria are derived for the orientation and shape of the first wall coolant ducts of self-cooled liquid metal blankets. Methods to generate additional turbulence within the flow, which can improve the heat transfer further are elaborated.

Heat transfer in liquid metal cooled fusion blankets

Experiments on the magnetohydrodynamic (MHD) heat transfer under fusion relevant conditions have been performed in order to quantify the heat transfer capability of such flows especially of the highly heat loaded first wall of a self-cooled liquid metal blanket. Results on heat transfer without and with additional turbulence promotion by means of mechanical and electromagnetic turbulence promoters (TP's) are presented for both electrically insulated ducts and thin walled, electrically conducting ducts of rectangular shape. The experimental results are compared with data from theoretical models. A cost-benefit analysis for the use of turbulence promoters is given and engineering correlations are derived which give guidelines for a MHD adapted blanket design. The heat transfer measured in a thin walled conducting rectangular duct is improved by turbulent transport at fusion conditions by a factor of about two compared to slug flow. The corresponding dimensionless pressure gradient keeps constant although the flow changes its state from laminar to turbulent. Using electromagnetic TP's installed close to the heated wall no measurable enhancement of the heat transfer has been observed within the range of Re=10 3–10 5. Mechanical TP's used in the test section with insulated walls improve the heat transfer up to 7 times compared to slug flow, but the corresponding pressure drop increases within the measured range of Reynolds numbers by up to 300%. Based on the analysis of the results design criteria are derived for the orientation and shape of the first wall coolant ducts of self-cooled liquid metal blankets.

Effects of Imperfect Insulating Coatings on the Flow Partitioning between Parallel Channels in Self-Cooled Liquid Metal Blankets

Fully developed liquid-metal flow in a system of three straight rectangular ducts is investigated. The ducts are electrically coupled by common conducting walls covered with an imperfect insulating layer. A numerical model of magnetohydrodynamic (MHD) flow in the system is described. Since no additional assumptions, such as in the core-flow solution, have been made, this model can be used for the analysis of MHD flow in parallel ducts with nearly perfect insulating coating. Any orientation of the applied uniform magnetic field is possible. Electrical conductivities of the dividing and exterior walls, and of the insulating layers in individual channels can be varied independently, as well as characteristics of insulation imperfections in each channel. A restriction of equal pressure gradients in all ducts is imposed, and the flow partitioning between parallel channels is examined. Results of the numerical simulation of the influence of insulation imperfections on flow distribution and velocity profiles are...

Alloying of aluminum and its influence on the properties of aluminide coatings: oxidation behavior and the chemical stability in Pb [sbnd]17Li

Electrical insulation of the structural material is necessary to reduce the MHD pressure drop in a self-cooled liquid metal blanket. This coating has to be compatible with liquid Pb 17Li up to 450°C. Specimens with different types of coatings were exposed to static Pb 17Li for 1200 h at 450°C in order to study their compatibility. Iron and a ferritic steel were coated with an aluminide layer by means of an aluminizing process. Iron metal plate was hot dip aluminized at Thyssen, Germany. The preheated sheet was coated for this purpose by exposing for a few seconds to a melt of Al with 10 wt% Si. The ferritic steel, MANET, was immersed into a melt of the same composition. In this case, cold specimens were dipped into the melt at 700°C for up to 10 min. The formation of the required oxide scale on top of the aluminide layer was performed by using two different methods: high temperature oxidation in air and anodic oxidation at room temperature. All the exposed specimens were examined before and after the corrosion experiments. The analytical method used is EDX measurements on the cut of the specimens and metallographical examinations.

Development of insulating coatings for liquid metal blankets

It is shown that self-cooled liquid metal blankets in tokamaks with high magnetic fields are feasible only with electrically insulating coatings at the duct walls. The requirements for the insulation properties are estimated by simple analytical models. Candidate insulator materials are selected, based on their insulating properties and thermodynamic considerations. Different fabrication technologies for insulating coatings are described. The status of the knowledge on the most crucial feasibility issue, i.e. the degradation of the resistivity under irradiation, is reviewed.

Magnetohydrodynamic investigations of a self-cooled Pb-17Li blanket with poloidal-radial-toroidal ducts

Abstract For self-cooled liquid metal blankets, the magnetohydrodynamic (MHD) pressure drop and velocity distributions are considered as critical issues. This paper summarizes MHD work performed for a DEMO-related Pb-17Li blanket, where the coolant flows downwards in rear poloidal ducts; turns around by 180° at the blanket bottom; is diverted from poloidal ducts into short radial channels which feed to toroidal First wall coolant ducts; flows through the subsequent radial channels; is collected again in poloidal channels and leaves the blanket segment at the blanket top. To reduce the pressure drop and to decouple electrically parallel channels, flow channel inserts are used for all the ducts except the first wall ducts. A previous pressure drop assessment resulted in significant values for duct geometries with flow distribution or collection, and multichannel effects for the system of U-bends. As a result of the uncertainty of these assessments, corresponding investigations were carried out recently. Characteristic results are presented in this paper. It is shown that, for both geometries, the pressure drops are considerably lower than those previously assessed. First results from experiments on the velocity distribution in a radial-toroidal-radial U-bend are also presented. Here, it is shown that, with an increasing interaction parameter, the liquid preferentially flows close to the First wall. Additionally, a pair of strong vortices was observed in a toroidal duct. Both effects are supposed very favourable for heat transfer.

The effects of imperfect insulator coatings on MHD and heat transfer in rectangular ducts

Abstract The integrity of the electrically insulating coating at the coolant channel walls represents a feasibility issue for a self-cooled liquid metal blanket. The effects of cracks in the insulating layer on MHD pressure drop are investigated, for various crack locations, sizes and resistivities. It is shown that, once the crack resistance exceeds the Hartmann layer resistance, the MHD pressure drop increases significantly. Using the 2-D fully-developed MHD flow code, it has been found that the crack location has a large impact on the MHD pressure drop: for the same crack sizes and resistivities, the pressure gradient increases as the cracks move towards the central plane. The effect of the insulation on heat transfer is discussed. The heat transfer capability is reduced in the presence of the insulator coating.

Flow balancing in liquid metal blankets

Abstract Non-uniform flow distribution between parallel channels is one of the most serious concerns for self-cooled liquid metal blankets with electrically insulated walls. We show that uncertainties in flow distribution can be dramatically reduced by relatively simple design modifications. Several design features which impose flow uniformity by electrically coupling parallel channels are surveyed. Basic mechanisms for “flow balancing” are described, and a particular self-regulating concept using discrete passive electrodes is proposed for the US ITER advanced blanket concept. Scoping calculations suggest that this simple technique can be very powerful in equalizing the flow, even with massive insulator failures in individual channels. More detailed analyses and experimental verification will be required to demonstrate this concept for ITER.

Recent Designs for Advanced Fusion Reactor Blankets

A series of reactor design studies based on the Tokamak configuration have been carried out under the direction of Professor Robert Conn of UCLA. They are called ARIES-I through IV. The key mission of these studies is to evaluate the attractiveness of fusion assuming different degrees of advancement in either physics or engineering development. This paper discusses the directions and conclusions of the blanket and related engineering systems for those design studies.ARIES-I investigated the use of SiC composite as the structural material to increase the blanket temperature and reduce the blanket activation. Li2ZrO3 was used as the breeding material due to its high temperature stability and good tritium recovery characteristics. To reduce the activation caused by the using of Zr, isotopic tailoring is required. Also, W was selected as the divertor target. The activation caused by the Zr and W, even with isotopic tailoring, reduced the safety advantage for the SiC blanket.The ARIES-IV is a modification of ARIES-I. The plasma was in the second stability regime. Li2O was used as the breeding material to remove Zr. A gaseous divertor was used to replace the conventional divertor so that high Z divertor target is not required. We investigated the possibility of breeding without the use of Be. However, tritium self sufficiency could not be assured with the uncertainties in the neutronic data. The safety advantage of ARIES-IV was enhanced by the removal of the high activation materials.The physics of ARIES-II was the same as ARIES-IV. The engineering design of the ARIES-II was based on a self-cooled lithium blanket with a V-alloy as the structural material. Even though it was assumed that the plasma was in the second stability regime, the plasma beta was still rather low (3.4%). To achieve an acceptable neutron wall loading, the magnetic field is rather high. This put an extra burden on a self-cooled liquid metal blanket. It was determined that a self-cooled lithium blanket with bare walls was not acceptable for a reactor with ARIES-II type parameters. Therefore, an insulating coating is required to assure an acceptable design window to reduce the MHD pressure drop.The ARIES-III is an advanced fuel (D-3He) tokamak reactor. The reactor design assumed major advancement on the physics, with a plasma beta of 23.9%. A conventional structural material is acceptable due to the low neutron wall loading. From the radiation damage point of view, the first wall can last the life of the reactor, which is expected to be a major advantage from the engineering design and waste disposal point of view. Organic coolant was selected as the reactor coolant to reduce the operating temperature compared to He, and to reduce the coolant pressure and improve thermal efficiency compared to water. However, the use of organic coolant raised safety and decomposition concerns.

Preparation and characterization of [formula omitted] layers on MANET steel

Electrical insulation of the structural material against the liquid metal (Pb-17Li) is necessary to reduce the MHD pressure drop in a self-cooled liquid metal blanket. The direct insulation by means of an adherent insulating layer seems to be a good solution of the problem. Al 2O 3 was shown to be sufficiently compatible with the liquid alloy. Al 2O 3 layers can be produced on MANET steel with an Al layer which is finally converted into a Fe x Al y layer. MANET steel is dipped under inert atmosphere into molten Al. The samples are then heat-treated at different temperatures in order to develop the intermetallic layer by interdiffusion. The high-temperature oxidation of the Fe x Al y layer is performed in air. The properties of the insulating layer and the intermediate are determined and will be discussed in this paper.

Corrosion of insulating layers on MANET steel in flowing Pb17Li

The suppression of MHD forces in the self-cooled liquid metal blanket can be performed by direct insulating layers of alumina. Such layers, which were produced on an aluminide intermediate layer on MANET steel, should be compatible with liquid Pb17Li at temperatures around 450°C. The corrosion behaviour of such specimens of MANET covered with an aluminide layer and an insulating alumina surface layer was studied in the PICOLO loop in flowing Pb17Li at 450°C. Samples were exposed up to 1000 h in the molten alloy. The corroded specimens were inspected for corrosion effects. Up to the longest time of exposure, corrosion effects could not be detected by several examination methods. The resistivity of the layers and their adherence are not degraded by the liquid alloy.

Liquid metal flow in a U-bend in a strong uniform magnetic field

Magnetohydrodynamic flows in a U-bend and in a right-angle bend are considered with reference to the toroidal concepts of self-cooled liquid-metal blankets. The ducts composing the bends have rectangular cross-sections. The applied magnetic field is aligned with the toroidal duct and perpendicular to ducts supplying liquid metal. For high Hartmann numbers the flow region is divided into cores and boundary layers of different types. The magnetohydrodynamic equations are reduced to a system of partial differential equations governing wall electric potentials and the core pressure. The system is solved numerically. The results show that the flow is very sensitive to variations of certain parameters, such as the wall conductance ratio and the aspect ratio of the toroidal duct cross-section. Depending on these parameters, the flow exhibits a variety of qualitatively different flow patterns. In particular, structures of helical and vortex type are obtained. A high-velocity jet occurs at the plasma-facing first wall and there is mixing of the fluid in the toroidal duct. These factors lead to desirable heat-transfer conditions.