Atsuhiko M. Sukegawa

Fast collimated neutron flux measurement using stilbene scintillator and flashy analog-to-digital converter in JT-60U

A line-integrated neutron emission profile is routinely measured using the radial neutron collimator system in JT-60U tokamak. Stilbene neuron detectors (SNDs), which combine a stilbene organic crystal scintillation detector (SD) with an analog neutron-gamma pulse shape discrimination (PSD) circuit, have been used to measure collimated neutron flux. Although the SND has many advantages as a neutron detector, the maximum count rate is limited up to ∼1×105counts∕s due to the analog PSD circuit. To overcome this issue, a digital signal processing system (DSPS) using a flash analog-to-digital converter (Acqiris DC252, 8GHz, 10bits) has been developed at Cyclotron and Radioisotope Center in Tohoku University. In this system anode signals from photomultiplier of the SD are directory stored and digitized. Then, the PSD between neutrons and gamma rays is performed using software. The DSPS has been installed in the vertical neutron collimator system in JT-60U and applied to deuterium experiments. It is confirmed t...

Development of Flexible Neutron-Shielding Resin as an Additional Shielding Material

A soft-type neutron-shielding resin has been developed by improving an existing hard-type neutronshielding material using the epoxy-based resin as an additional shielding material. A flexible heat-resistant neutron-shielding material has been developed, which consists of a new polymer-based resin with boron. The neutron shielding performance of the developed flexible heat-resistant resin with the 252Cf neutron source is almost the same as that of polyethylene. The outgases of H, H2, NH4, H2O, CO, O2, C4H10, and CO2 from the developed resin have been measured at high temperature (up to 250°C) by thermal desorption spectroscopy methods. The soft-type resin and the newly developed heat-resistant resin will be applied to prevent the effects of neutron streaming and to control the movement of a vibrated pipe as the seal material around the plumbing in the future fast reactor and innovative fission reactor.

High-Heat-Resistant Neutron Shielding Resin

Abstract Three kinds of unique high-heat-resistant neutron shielding resins have been developed. One is an excellent resin in terms of light weight. Another is a high-neutron-shielding-performance resin with a heat-proof temperature of 200°C. The third is a resin improved up to a heat-proof temperature of 300°C. The neutron shielding performance of the resins was carried out using a 252Cf neutron source. The results show that the shielding performance of the resin with a heat-proof temperature of 200°C is better than the neutron shielding resins with heat-proof temperatures up to 200°C. And, the performance of the resin with a heat-proof temperature up to 300°C is almost the same as that of polyethylene. It can be concluded that high-heat-resistant resin is attractive as the neutron shielding material in a high-temperature environment.

Conceptual Design of Magnet System for JT-60 Super Advanced (JT-60SA)

At Japan Atomic Energy Agency (JAEA), the JT-60 is planned to be modified to a full-superconducting tokamak referred as JT-60 Super Advanced (JT-60SA) as one of the JA-EU broader approach projects. In JT-60SA, the magnet system with 1400 ton consists of 18 toroidal field (TF) coils, 4 stacks of central solenoid (CS) and 7 plasma equilibrium field (EF) coils. The TF coil with using the NbTi cable-in-conduit (CIC) conductor has the maximum magnetic field of 6.4 T in winding, the magnetomotive force of 8.2 Tm and the magnetic stored energy of 1.5 GJ which is 1.5 times larger than present superconducting fusion devices. The conductor for CS which is the Nb3Sn CIC conductor with the maximum magnetic field of 10 T was newly designed to increase the flux swing capability for the ITER plasma simulation and to accommodate the diminution of available space because of the TF coil with NbTi conductor. For CS, the faster maximum varying field of 2.88 T/s is required because of fast plasma initiation scenario of JT-60SA.

Absolute calibration of microfission chamber in JT-60U

We have conducted calibrations of a microfission chamber, which was installed between the vacuum vessel and the toroidal field coils, by both a Cf-252 neutron source and real deuterium plasmas in JT-60U. The detector employs both pulse counting and Campbell (mean square voltage) modes in the electronics to cover a wide dynamic range of the neutron source strength. The pulse counting and Campbell modes were calibrated by Cf-252 and deuterium plasmas, respectively. Point efficiencies, counts per neutron from a point at a single angle, were measured for 27 locations of the neutron point source in toroidal scan. The efficiencies were influenced by the various components such as the vacuum vessel, port, and graphite tiles. The point efficiencies can be integrated and averaged with angle to provide toroidal line efficiencies. The line efficiencies of the microfission chamber and the nearest neutron monitor of the U-235 fission chamber were 5.38x10(-9) and 1.77x10(-8), respectively. Then the calibration for the Campbell mode was also performed by using a real deuterium plasma. The detection efficiency in the Campbell mode was about three-tenths of that of the nearest neutron monitor, which is consistent with the calibration result obtained by using a Cf-252 neutron source for pulse counting mode.

Thermal and Mechanical Design of Error Field Correction Coil for JT-60SA

The inhomogeneous poloidal magnetic field of tokamak device, which is called error field, has to be reduced because the error field degrades the plasma performance. There are 18 sets of EFC coils installed inside the vacuum vessel for JT-60SA to compensate the error field. The conceptual design of EFC coils has been completed. The water-cooled hollow copper conductor was selected to reduce the conductor size since the available space for EFC coils is small. The outer size of the conductor and the diameter of the cooling channel were optimized in considering hydraulic and thermal characteristics. The design of the conductor was validated by the testing of a mock-up coil. The bar springs are used for the structure of EFC coils. The structural analysis was performed to optimize the parameters of bar springs. The results of structural analysis suggest that the structure of EFC coils can be used for both the conditions of plasma operation and baking operation with the use of Inconel625 for bar springs. In this paper, the specification of the JT-60SA EFC coils, the test results of mock-up coils, and the structural analysis results of the EFC coil structure are described. The manufacture of EFC coils has started based on these designs.

Comparison Between Simulations Using the PHITS Code and Activated Material Analysis

In this study, a comparison between simulations using the Particle and Heavy Ion Transport code System (PHITS) code and activated material analysis of JT-60U have been performed. Neutron transport has been simulated using the Monte Carlo method PHITS to determine the neutron fluency at the irradiation position. The activated analysis has been complemented by the foil-activation technique. The preliminary accuracy assessment of these PHITS simulations has been confirmed by comparing the reaction rates of gold, cobalt, and nickel foil activation. The simulation results by PHITS consistent with the measured reaction rate of each foil near the device.

Evaluation of two-stage system for neutron measurement aiming at increase in count rate at Japan Atomic Energy Agency-Fusion Neutronics Sourcea)

In order to increase the count rate capability of a neutron detection system as a whole, we propose a multi-stage neutron detection system. Experiments to test the effectiveness of this concept were carried out on Fusion Neutronics Source. Comparing four configurations of alignment, it was found that the influence of an anterior stage on a posterior stage was negligible for the pulse height distribution. The two-stage system using 25 mm thickness scintillator was about 1.65 times the count rate capability of a single detector system for d-D neutrons and was about 1.8 times the count rate capability for d-T neutrons. The results suggested that the concept of a multi-stage detection system will work in practice.

Comparison between simulations using the PHITS code and activated materials analysis toward JT-60SA radiation safety assessment

In the present study, a comparison between simulations using the PHITS code and activated material analysis of JT-60U have been performed. Neutron transport has been simulated using the Monte Carlo methods PHITS to determine the neutron fluency at the irradiation position. The activated analysis has been complemented by the foil-activation technique. The preliminary accuracy assessment of these PHITS simulations has been confirmed by comparing the reaction rates of gold, cobalt, and nickel foil activation. The simulation results by PHITS consisted with the measured reaction rate of each foils near the device.