Saburo Toda

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.

Numerical analysis of electromagnetic phenomena in superconductors

Governing equations for describing electromagnetic phenomena in superconductors were derived by using the Helmholtz theorem. The Meissner effect, which is common to type-I and type-II superconductors, is treated by introducing the magnetizing vector M. The current flow due to flux invasion into the superconductor, which is a particular phenomenon in the type-II superconductor, is evaluated using the current vector potential together with constitutive equations expressing critical state models. Numerical results based on a two-dimensional finite-element code showed excellent agreement with analytical solutions. >

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.

Numerical and experimental analysis of eddy current testing for a tube with cracks

A novel method for calculating the impedance signal in eddy current testing (ECT) for a tube with surface cracks is developed using the two-dimensional current vector potential method. Three-dimensional current flow around a surface crack is approximated by introducing quasi-conductivity in the crack region. A final matrix relating the current vector potentials is obtained by decreasing the width of the crack to zero, and then the elements in the crack region are not necessary any longer. Numerical results show good agreement with the experimental ones, which indicates the validity of the proposed method and the possibility of designing a more efficient probe of ECT using the code. >

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.

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.

Numerical analysis of fluid flow with free surface and phase change under electromagnetic force

In this study, fluid flow of liquid metal with free surface and melting of metal under electromagnetic force was investigated numerically. The computational code was developed by the authors based upon a finite difference method to simulate the free surface flow and the eddy current simultaneously. The VOF method was introduced to treat the free surface, and magnetic vector potential was used for eddy current analysis. The free surface shape predicted by the code agreed with that obtained experimentally. Furthermore, the code was improved to calculate the phase change (melting) of solid metal by Joule heating due to eddy current. Numerical results of melting of metal were also demonstrated.

Application of shape memory alloys to compacting and element-quickly replaceable design in high-power density fusion reactors

Abstract Quick replacement has been accomplished by using a shape memory alloy (SMA) coupling in the conceptual design of a cassette compact toroid reactor (CCTR). Further, by using an SMA driving element, a compact large gate valve can be newly devised. This gate valve will enable in-situ handling without breaking vacuum, so that the baking of the vacuum boundary for every replacement becomes unnecessary, except the initial baking. In these applications of SMA, the compacting and quickly replaceable technology that are needed for dealing with the problems associated with very high neutron loading in a compact reactor become available by using presently available or promised materials rather than an assumed material.

Coupled thermo-electromagnetic analysis of N type-II superconductors

Using a newly developed computational code, a coupled thermoelectromagnetic analysis of type-II superconductors is performed to evaluate distributions of transport current and temperature in the conductor. Spread of current quenching due to artificial thermal spike and recovery from it are simulated by the code. The numerical results show hysteresis in the current distribution in the superconductor as a consequence of the thermal disturbance. >

Quick replacement of the fusion core parts in a cassette compact toroid reactor

A study of a conceptual design for a “cassette” compact toroid reactor, which emphasizes quick replacement, has been carried out. The quick replacement, accomplished by using a functional material, the SMA joint, is necessary to release the first wall from high neutron loading. The SMA joint permits the connection or disconnection of the joint part by means of a simple operation involving the control of the SMA temperature, without the need for an extremely intelligent robot. Pheripheral structural mechanisms have also been devised for the blanket and the shielding construction.