Akio Sagara

Design and development of the Flibe blanket for helical-type fusion reactor FFHR

Blanket design is in progress in helical-type compact reactor FFHR-2. A localized blanket concept is proposed by selecting molten-salt Flibe as a self-cooling tritium breeder from the main reason of safety: low tritium solubility, low reactivity with air and water, low pressure operation, and low MHD resistance which is compatible with the high magnetic field design in force-free helical reactor (FFHR). Numerical results are presented on nuclear analyses using the MCNP-4B code, on thermal and stress analyses using the ABAQUS code, and heat exchange efficiency from Flibe to He. R&D programs on Flibe engineering are also in progress in material dipping-tests and in construction of molten salt loop. Preliminary results in these experiments are also presented.

Design Progress on the High-Temperature Superconducting Coil Option for the Heliotron-Type Fusion Energy Reactor FFHR

Abstract Feasibility studies on applying high-temperature superconductors (HTS) to the heliotron-type fusion energy reactor FFHR are being carried out. Using HTS, we consider that the three-dimensional helical coils with a ~40 m diameter can be constructed without preparing a huge winding machine. A practical method for realizing this concept is proposed. The electromagnetic stress inside the helical coil packs is examined using an FEM analysis for double-pancake windings. The effect of error magnetic field generated by the shielding currents in HTS tapes is also examined.

Innovative liquid breeder blanket design activities in Japan

Abstract In order to clarify key engineering issues and to enhance key R&D activities for D-T fusion blankets, many design activities on innovative liquid blanket systems are on going as collaboration studies in Japan. Recently an improved long-life Flibe blanket has been proposed, and the self-cooled Li/V blanket design has started. For Flibe systems, much progress has been made on tritium permeation barrier, energy conversion system, free surface designs, and thermofluid loop experiments. For Li/V systems, evaluation studies have proceeded on Be-free nuclear properties and allowable crack fraction on multilayered MHD insulation coatings.

Optimization of a Mechanical Bridge Joint Structure in a Stacked HTS Conductor

The mechanical bridge joint (bridge-type lap joint) of a stacked high-temperature superconducting conductor has been investigated for a “remountable” or a segment-fabricated high-temperature superconducting magnet. In a previous study, joint resistivities were evaluated experimentally for the bridge joints of single-layer and double-layer stacked GdBCO coated conductors. However, the joint resistivity increased with an increase in the number of layers due to nonuniform contact pressure distribution caused by a gap or misalignment in the joint region. In this study, therefore, we were aiming at the reduction of joint resistance by achieving more uniform contact pressure distribution. In addition, we investigated the effects of temperature while applying pressure to the joint and positioning the joint structure to investigate its application in an actual large-sized magnet. First, we inserted an indium film between the joint surfaces to make contact pressure uniform. Experimental results showed that joint resistivity with the indium film did not depend on the number of layers. In addition, applying force at room temperature was more effective in decreasing joint resistivity than that at 77 K. Finally, we examined the effect of the joint structure with screw bolt tightening. The result showed that the structure of the convex plate has better joint performance than others.

High Performance Corrosion Resistance of Nickel-Based Alloys in Molten Salt Flibe

Molten salt LiF-BeF2 (Flibe) is one of candidates for self-cooled tritium breeder in fusion blanket system. The Ni based alloys of Hastelloy C-276 (6.28Fe, 15.67Cr, 0.42Mn, 15.83Mo, 3.34W, Ni as balance), Inconel 600(7.02Fe, 15.75Cr, Ni as balance) and Inconel 625 (4.12Fe, 21.94Cr, 9.10Mo, Ni as balance) are candidates of structural material of blanket loop components at down stream. Corrosion characteristics of these alloys were investigated by corrosion test in static Flibe at 500°C and 600°C for 1000 hours. The corrosion rates were estimated from the weight losses of specimens, and those of Hastelloy C-276, Inconel 600 and Inconel 625 in Flibe at 600°C were 3.4μm/year, 2.8μm/year and 1.1μm/year, respectively. The mass balance between the weight losses of specimens and the increase of impurity in Flibe by the exposure was investigated, and it was found that the corrosion was mainly caused by the depletion of Cr from the alloys. The corroded surface had high Ni concentration after the Cr depletion by corrosion, and this is expected to be corrosion resistant in Flibe.

Flow Visualization and Heat Transfer Characteristics for Sphere-Packed Pipes

Particle image velocimetry visualization to identify the complex flow structures in a sphere-packed pipe is carried out by using a matched refractive-index method with a sodium iodide solution as the working fluid. The following three flows were confirmed as representative flow structures in the pipe: a meandrous bypass flow with a high-flow velocity due to the wall effect, two pairs of unstable twin vortices accompanied by a strong impinging flow to the pipe wall, and a spouting flow from the central area of the pipe. In an experiment on heat transfer using water as the working fluid, the wall-temperature distribution is measured with thermocouples and infrared thermography, which makes clear a relation between the flow structures and the local heat transfer performance. Though an area with a high wall temperature is formed by the flow stagnation located at a contact point between the sphere and the heating wall, the colliding effect of the high velocity and of the meandrous bypass flow with the spheres significantly affects the heat transport from the stagnation areas. On the other hand, the heat transfer performance is quite high in a large gap area between the upstream and downstream spheres because of the influence of the strong impinging flow and the vortices that are both induced by the meandrous bypass flow.

Experimental research on heat transfer enhancement for high Prandtl-number fluid

Abstract The experimental research on heat-transfer enhancement for such high Prandtl-number fluid as Flibe has been performed with a large molten salt circulating experimental loop named as “TNT loop” (Tohoku-NIFS Thermofluid loop). Through the experiments, a packed-bed tube is employed as the enhancer for molten salt. It is clarified that the enhancement of packed-bed tube is superior to that of turbulent heat transfer from the viewpoint of the same flow rate. Also, the 1/4-diameter bed is superior to the 1/2-diameter one at the same flow rate. Furthermore, at low flow rate, a little differences of heat transfer performance can be seen between the stainless-steel bed and copper bed. At high flow rate, however, the heat-transfer coefficient ratio strongly depends on the flow rate in the case of the 1/4-diameter copper bed only. As a result, it is considered that the thermal energy is expanded from a heated wall deeply and fast through packed bed at low flow rate. On the contrary, it is also considered that the convective heat transfer in the vicinity of a heated wall is strong at high flow rate. The evaluation from the viewpoint of the pressure drop shows that the turbulent heat transfer is superior to that with packed bed. However, the ratio of heat transfer with bed to turbulent one is steeply improved at low flow rate. Taking account of MHD effect, avoidance of erosion and electrolysis of Flibe, the enhancement under low flow-rate condition can be suitable in a fusion reactor.

First Operation of the Flinak/LiPb Twin Loop Orosh2i-2 with a 3T SC Magnet for R&D of Liquid Blanket for Fusion Reactor

A Flinak/LiPb twin-loop Orosh2i-2 was constructed in the Fusion Engineering Research Project of NIFS to establish an engineering basis for the “real function” of a liquid blanket for fusion energy reactors. It was constructed with a 3 T superconducting magnet, which is the strongest in the world for these purposes. In this study, 120 litter Flinak and LiPb were successfully prepared in-situ in a purified glove-box. Designed flow velocity up to 1.5 m/sec was achieved for both. The magnetic field and velocity dependence on pressure drop agreed well with theoretical predictions. Tentative plans for phased experiments within the next few years have been presented, including operation of Supercritical-CO2 Turbine (SCOT) power generation system in the Flinak loop.

Performance of a Mechanical Bridge Joint for 30-kA-Class High-Temperature Superconducting Conductors

In this report, we propose segment-fabricated high-temperature superconducting (HTS) magnets as candidates for the FFHR-d1 heliotron-type fusion reactor. The FFHR-d1 requires 100-kA-class superconducting conductors used at 12 T for a pair of helical coils. We fabricated and tested two 30-kA-class GdBCO conductors with bridge-type mechanical lap joints (mechanical bridge joints). This report details the design of the joint section and the experimental results of those samples, especially, those of their joints. We improved the geometry of the joint region in a second sample, based on our results from the first. The second sample has sufficiently low joint resistance (less than 5 nΩ), and we could apply 70 kA to it without causing quenching at the joint. Its joint resistance was also acceptable for providing the electric power required to run the cryoplant for the segmented HTS helical coils.

Experimental research on molten salt thermofluid technology using a high-temperature molten salt loop applied for a fusion reactor Flibe blanket

Experimental research on molten salt thermofluid technology using a high-temperature molten salt loop (MSL) is described in this paper. The MSL was designed to be able to use Flibe as a coolant, however, a simulant, heat transfer salt (HTS) has to be used alternatively since Flibe is difficult to operate under avoiding a biohazard of Be. Experiment on heat-transfer enhancement, that is required for applying to cool the high heat flux components of fusion reactors, is ongoing. Preliminary experimental results showed that an internal structure of a mixing chamber in the MSL was important to obtain accurate bulk temperatures under severe thermal conditions. For operating the loop, careful handling are needed to proceed how to melt the salt and to circulate it in starting the operation of the MSL. It is concluded that several improvements proposed from the present experiences should be applied for the future Flibe operation.