Kei-ichiro Shibata

Benchmark experiment on silicon carbide with D–T neutrons and validation of nuclear data libraries

Abstract Silicon carbide (SiC) is one of the low activation structural materials for fusion reactors. A benchmark experiment on SiC was conducted at the deuterium–tritium (D–T) neutron source facility FNS in JAERI, and the validity of the cross section data for SiC was investigated. An experimental assembly made of sintered SiC (457×457 mm, and 711 mm in thickness) was bombarded by D–T neutrons, and neutron spectra, gamma-ray spectra, dosimetry reaction rates and gamma-ray heating rates were measured in the assembly. Computational analyses of the experiment were performed by Monte Carlo transport calculations with the MCNP-4B code and several recent evaluated nuclear data files. The following findings were pointed out. The calculations with JENDL Fusion File and JENDL-3.2 predicted adequately the neutron fluxes from 14 MeV to 0.3 eV as well as the gamma-ray fluxes at any positions up to ∼600 mm depth in the SiC assembly. Results for the calculation with FENDL/E-2.0 were similar to those with the two JENDL calculations, but the neutron fluxes in the energy range from 1 to 14 MeV were calculated slightly smaller. The cross section data for silicon in FENDL/E-1.0 were found inadequate for nuclear design calculations.

Consideration on Current and Coil Block Placements With Good Homogeneity for MRI Magnets Using Truncated SVD

This paper describes a new method to determine the current and coil block placements with good homogeneity for magnetic resonance imaging (MRI) magnets, and discusses the relationships between the placements and homogeneity, using regularization of the truncated singular value decomposition (SVD). In the first step, the main coil (MC) is modeled as filament loop currents (FLCs) on a solenoid and the shield coil (SC) is modeled by coil blocks. The FLCs are determined so as to get a homogeneous magnetic field using a superposition of eigenmodes obtained by the SVD. In the second step, the FLCs are replaced by MC blocks. The findings are as follows. 1) The obtained FLC distribution in the axial direction has several peaks, which can be suitably replaced by MC blocks and the number of the peaks is equal to the number of MC blocks. 2) There is an optimum length to minimize the absolutely summed current and the 1542-mm length is the optimum for a 525-mm radius with six MC blocks. 3) A one-eigenmode (one MC block) increase or decrease changes the length by

Coil Block Designs With Good Homogeneity for MRI Magnets Based on SVD Eigenmode Strengths

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Application of a Simple Evaluation Method for Gamma-Ray Streaming to Multi-Bent Ducts Taking Account of Only Components Passing through the Duct

87 or

Development of Simple Evaluation Method for Duct Streaming

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Design Analysis to Reduce the Equivalent Dose Rate of the Neutral Beam Injection System in a Fusion Experimental Reactor

105 mm, respectively. 4) Six MC blocks can provide 1-ppm peak-to-peak homogeneity in the 400-mm diameter spherical imaging volume and this is adequate for MRI magnets.

Improvement of a Simple Evaluation Method for Duct Streaming by Taking Account of the Deep Penetration Component

This paper describes an improved method to determine coil block (CB) placements with good homogeneities for magnetic resonance imaging magnets. Each magnet has a main coil (MC) for strong magnetic fields in a volume of interest and a shield coil (SC) to make the magnetic fields weak outside the magnet. The CB placements are done in three steps and at every step, the SC magnetic moments are tuned. In step 1, approximate placements are done with axially distributed circular filament loop currents. According to the distribution, rough placements of CBs are done for the MC. In step 2, the CB cross-sectional shapes and positions are optimized to get good homogeneity with real numbers of winding turns. In step 3, the CB shapes are approximated with integer numbers of winding turns and conductor sizes, and the CB positions are tuned to get as good homogeneities as the step 1 result. In all steps, optimizations are done through truncated singular value decompositions. In steps 2 and 3, the placements are done with respect to eigenmode strengths, which are obtained in step 1. For a magnet with six MC-CBs, seven symmetric eigenmode strengths are tuned. The CB placements for magnets can be obtained with good homogeneities and integerized CB winding turns using the method.

Dose Analysis on High Performance Vault Storage System of Spent Nuclear Fuel

A simplified formula for γ-ray streaming through ducts is devised to apply to multi-bent ducts by optics analogy with considering only components passing through the duct without enormous preparatory calculations. Dose rates estimated by the formula are compared with experimental values in two types of two-stage bent ducts for about lMeV energetic γ-ray source. Regarding the two-stage, 45-degree bent duct with a diameter of 15 cm, estimated dose rates at the positions of the bend points and the exit agree with the experimental values within 30% and a factor of 2, respectively. Regarding the two-stage, 90-degree bent square duct with a 1.8 m side, estimated dose rates at the positions of the bend points and the exit agree with the experimental values within 30%. From these results, it is concluded that the formula is applicable to the midway in shielding design.

Evaluation of Nuclear Heat and Dose Rate for JT-60SU

A simple formula for radiation streaming through bent ducts is derived by optics analogy with considering only components passing through the duct not required for enormous preparatory calculations. Dose rates estimated by the derived formula are compared with experimental values in two types of two-stage bent ducts for gamma-ray source and four types of one-stage bent ducts for 14MeV neutron source. Regarding the two-stage, 45-degree bent duct with a diameter of 15 cm, estimated dose rates at the positions of the bend points and the exit agree with the experimental values within 30% and a factor of 2, respectively. Regarding the two-stage, 90-degree bent square duct with a 1.8m side, estimated dose rates at the positions of the bend points and the exit agree with the experimental values within 30%. For 90-degree one-stage bent square ducts with 10cm, 20cm and 40cm sides in iron and polyethylene shields with 14MeV neutron source, dose rates by the formula agree with experimental values within factor 3 at the exit. From these results, it is concluded that the derived formula is applicable to the midway in shielding design.

An Experimental Study on the Superconducting Coil Behaviors Using Electromagnetic Pulse Signals During Ramping

As applied to the common design of the neutral beam injection (NBI) system in the International Thermonuclear Experimental Reactor (ITER) Conceptual Design Activity, a design is proposed and examined that reduces the equivalent dose rate of the NBI system in order to enable access to the outside of the iniector. Modifying the current system is necessary because the equivalent dose rate in the NBI room after reactor shutdown is higher than the design limit for radiation workers. The NBI maintenance concept is based on full-remote maintenance. There are, however, some problems that must be solved before full-remote maintenance could be realized-such as connection and disconnection of the electric power cables and complicated coolant pipes, and location of the maintenance equipment-this concept solves the aforementioned problem by enabling worker accessibility to the outside of the injector The following design points are suggested to reduce the equivalent dose rate. The vacuum vessel should be composed of aluminum to reduce the induced radioactivity. Polyethylene, which has high shielding ability for neutrons, should be installed between the vessel and magnetic shield located outside the vacuum vessel to reduce not only neutron flux coming to the magnetic shield but also gamma-ray flux, caused by in-vessel components, leaking to the NBI room. The equivalent dose rate in the NBI room 1 week after reactor shutdown can be reduced to 28 μSv/h by applying the foregoing measures. Thus, the prospect exists for realizing access to the outside of the injector.