J. Adachi

A fusion power reactor concept using SiC/SiC composites

JAERI studied a concept of a commercial fusion power reactor (5.5 GW, electric output: 2.7 GW) having high environmental safety, high thermal efficiency and high availability. The reactor configuration was designed to achieve good maintainability, high performance breeding blanket, high efficiency power generation system and less radwastes. The design was based on the use of low activation structural material (SiC/SiC Composites) and helium as a coolant. (1) Easy maintenance is attained by sector replacement with the radiation environment less than 103 R/h in a reactor chamber. (2) The net thermal efficiency over 45% is attained by high temperature helium gas Brayton cycle. (3) Most of radwastes of DREAM reactor can be disposed in shallow land burial as a low level radwaste after cooling of several tens of years.

Blanket and divertor design for the Steady State Tokamak Reactor (SSTR)

Abstract The tritium breeding blanket and plasma facing components (first wall and divertor) have been designed for the Steady State Tokamak Reactor (SSTR). Low activation ferritic steel (F82H) was chosen for the structural material of the first wall and the blanket. The ceramic breeder pebble bed concept with beryllium neutron multiplier was adopted. The blanket is divided into two parts; a replaceable blanket and a permanent blanket. Electrical insulation made of functionally gradient material (FGM) was introduced in the blanket box structure to drastically reduce electromagnetic loads during a plasma disruption and to simplify the attaching mechanism of the blanket. Low-temperature and high-density divertor plasma conditions and a radiative cooling concept lower the heat flux to the divertor plate to below 3 MW/m2 and enable us to adopt the same cooling conditions as the blanket coolant. Puffing mechanisms of gas (deuterium) and iron are installed for the radiative cooling of the divertor plasma.

Prototype tokamak fusion reactor based on SiC/SiC composite material focusing on easy maintenance

If the major part of the electric power demand is to be supplied by tokamak fusion power plants, the tokamak reactor must have an ultimate goal, i.e. must be excellent in construction cost, safety aspect and operational availability (maintainability and reliability), simultaneously. On way to the ultimate goal, the approach focusing on the safety and the availability (including reliability and maintainability) issues must be the more promising strategy. The tokamak reactor concept with the very high aspect ratio configuration and the structural material of SiC/SiC composite is compatible with this approach, which is called the DRastically Easy Maintenance (DREAM) approach. This is because SiC/SiC composite is a low activation material and an insulation material, and the high aspect ratio configuration leads to a good accessibility for the maintenance machines. As the intermediate steps along this strategy between the experimental reactor such as international thermonuclear experimental reactor (ITER) and the ultimate goal, a prototype reactor and an initial phase commercial reactor have been investigated. Especially for the prototype reactor, the material and technological immaturities are considered. The major features ofthe prototype and commercial type reactors are as follows. The fusion powers of the prototype and the commercial type are 1.5 and 5.5 GW, respectively. The major/minor radii for the prototype and the commercial type are of 12/1.5 m and 16/2 m, respectively. The plasma currents for the prototype and the commercial type are 6 and 9.2 MA, respectively. The coolant is helium gas, and the inlet/outlet temperatures of 500/800 and 600/900°C for the prototype and the commercial type, respectively. The thermal efficiencies of 42 and 50% are obtainable in the prototype and the commercial type, respectively. The maximum toroidal field strengths of 18 and 20 tesla are assumed in the prototype and the commercial type, respectively. The thermal conductivities of 15 and 60 W/m per K are assumed in the prototype and the commercial type, respectively.

Maintenance and material aspects of DREAM reactor

A concept of a commercial fusion power reactors (Fusion Power: 5.5 GW, electric output: 2.7 GW) having high environmental safety, high thermal efficiency and high availability has been studied in JAERI. The gross reactor configuration was designed to achieve good maintainability, high performance breeding blanket, high efficient power generation system and little radwastes. Design was based on the use of low activation structural material (SiC/SiC composites) and helium as a coolant. In this paper, maintenance and material aspects of DREAM reactor design is discussed. The concluding remarks are as follows. (1) The difficulty of development of maintenance tool is alleviated by sector replacement and the radiation dose environment less than 10 Gy/h in a reactor chamber. (2) Design requirement and present status of SiC/SiC composites was investigated. (3) The SiC/SiC composite development program is planned to satisfy the requirements of DREAM reactor.

The concept of drastically easy maintenance (DREAM) tokamak reactor

Abstract One of the most important features of a power plant is high availability. An easy maintenance scheme is indispensable for high availability operation. For the easy maintenance scheme, a plasma configuration with high aspect ratio (with slender doughnut shape) of about 6 and a small number of 12 torus sectors is introduced. The adjacent torus sectors are jointed mechanically by a bolt-flange structure. The plasma vacuum region comprises a cryostat system and 12 horizontal access doors. A whole section of each torus sector is pulled out horizontally in a single radial straight motion between the adjacent toroidal field (TF) coils. The TF coil system is imbedded in the reactor building and can be repaired independent of the torus structure. The torus structure consists of a shield integrated vacuum vessel, a divertor structure and a tritium fuel breeding blanket structure. The divertor or blanket system is replaced from the transverse (toroidal) direction after the torus structure is pulled out. Even in the torus inboard region, there is a gap of 1 m between adjacent TF coils because of the high aspect ratio reactor configuration. Therefore, all the piping and feeder systems are gathered at the torus inboard region. The overall machine size is determined by a fusion power of 3 GW and almost 100% bootstrap current with a moderate Troyon coefficient of 3.5, magnetohydrodynamic safety factor of 5 and plasma elongation of 1.6.

Underwater cutting of stainless steel with the laser transmitted through optical fiber

By using a fiber-transmitted laser beam and high pressure oxygen gas, underwater laser cutting of thick stainless steel was demonstrated. In the field of decommissioning, underwater cutting of nuclear facilities is desirable. For this operation, it is very useful to apply the laser beam transmitted through optical fiber because of flexibility. We used chemical oxygen-iodine laser for fundamental experiments with laser beam power of 1-7kW. And for this experiments, we designed cutting heads with some ideas for nozzles and optics. The nozzles have various nozzle diameters and two kinds of shapes; one is conical, and the other is divergent. The latter was designed to make a supersonic gas flow by using high pressure oxygen gas. And we prepared several focusing optics with different focal lengths, which have influence on both the focal spot diameter and the depth of focus of the laser beam that incidents upon a workpiece. Cutting ability was measured by cutting a tapered workpiece. From this investigation it was clear that the nozzle-to-workpiece distance that was about 10mm had the large tolerance of deviation, and there was a suitable optics according to laser power.

Development of a remote handling system for replacement of armor tiles in the Fusion Experimental Reactor

Abstract The armor tiles of the Fusion Experimental Reactor (FER) planned by JAERI are categorized as scheduled maintenance components, since they are damaged by severe heat and particle loads from the plasma during operation. A remote handling system is thus required to replace a large number of tiles rapidly in the highly activated reactor. However, the simple teaching—playback method cannot be adapted to this system because of deflection of the tiles caused by thermal deformation and so on. We have developed a control system using visual feedback control to adapt to this deflection and an end-effector for a single arm. We confirm their performance in tests.

Feasibility study of remote erosion measurement of first wall for Fusion Experimental Reactor (FER)

Abstract A remote measuring system has been developed for application to detection aand dimensional measurement of eroded defects of protection tiles on the first wall in the Fusion Experimental Reactor. The present paper describes a remote defect measuring system, and results of the experimental study for the feasibility. The measuring system consists of a three-dimensional position sensor system and a manipulator to move it. The sensor system adopts a light sectional method using a laser slit light and an industrial TV camera, and an image fiber and a light guide are respectively used for transmission of an image and the light to protect the camera and the laser source against radiation. The system is able to detect defects of tiles in a large region and also to measure the form of only the defect part precisely. As a result of the experiment, it has been proved that the measuring system is practicable and applicable to the remote measurement of the first wall for FER.