Patent Number: 054105749
Section: summary

FIELD OF THE INVENTION 1. Background of the Invention This invention relates to an internal component or structure of a fusion reactor and, more particularly, to an internal component or structure of a fusion reactor having a cooling structure for removing heat caused by a thermal load from a plasma or nuclear reaction. 2. Discussion of the Background These days, many countries including Japan are energetic in making research and development of nuclear fusion reactors which are expected to one of a number of promising future energy sources. In fact, large-scale plasma testing apparatus have been built to enable the establishment of prospects of various physical properties, such as plasma physics, impurity control, plasma heating, fuel injection and tritium breeding, in nuclear fusion reactors including tokamak reactor which is considered as being the representative of nuclear fusion reactors. Current designs of nuclear fusion reactors have encountered difficulties in protecting from heat the internal reactor components for confining plasma within the reactor. More specifically, internal components of a nuclear fusion reactor includes tritium non-breeding inboard blankets and tritium breeding outboard blankets which are densely arranged in the circumferential direction within a torus-type vacuum vessel, as well as divertors which are arranged densely in upper and/or lower spaces inside the vacuum vessel for the purpose of discharging He which is a plasma reaction product, thus forming a reactor internal structure. The arrangement is such that the plasma generated in the torus internal space defined by the above-mentioned reactor internal structure is confined by a toroidal field coil or a poloidal field coil. The reactor internal components of a nuclear fusion reactor has a cooling structure for removing heat which has been transferred to the reactor internal structure from the plasma, by means of a coolant such as water introduced into the cooling structure. The cooling structure, which is disposed comparatively close to the plasma, tends to receive thermal and particle loads from the plasma. In addition, the cooling structure is also placed under action of many tritium rays radiated from the plasma, because the current design of nuclear fusion reactor relies upon deutrium-tritium reaction known as D-T reaction [D+T.fwdarw..sup.4 He(3.5 MeV)+n(14.1 MeV)]. Tritium, which is radioactive, requires greatest care in handling. In the reactor inner structure of a nuclear fusion reactor, tritium which does not substantially exist naturally is generated by the action of neutrons derived from the plasma, while nuclear reaction heat generated during generation of tritium is carried away through the cooling structure in the blankets. Therefore, the cooling structure in the reactor inner structure is subjected to nuclear reaction heat and tritium. Thus, the cooling structure must be operated in such an environment which supplies much heat, particles and tritium to the cooling structure. The cooling structure therefore is constructed typically as illustrated in FIG. 16. More specifically, the cooling structure 1 has an internal cooling water passage 2 for allowing cooling water as the coolant to circulate therethrough. Such cooling structure is adapted for the inboard and outboard blankets and divertors disposed in the reactor internal component or structure. The cooling structure for cooling the reactor internal component tends to exhibit minute cracking and damage due to the aforementioned thermal and particle loads and due to other causes such as application of electromagnetic forces produced during operation of the nuclear fusion reactor. Such minute cracks or damage of the cooling structure tends to allow leakage of the coolant. The leakage of the coolant, when occurred inside the vacuum vessel, undesirably lowers the level of the vacuum inside the vessel, seriously affecting the operation of the nuclear fusion reactor. The outboard blankets as a part of the reactor internal structure contains lithium oxide for the purpose of breeding tritium. In addition, a high temperature, e.g., several hundreds of degrees centigrade is established inside the blanket during operation of the nuclear fusion reactor. A risk therefore exists that the coolant leaking from the cooling structure contacts the lithium oxide so as to cause a chemical reaction, as well as rapid evaporation of the coolant, thus impeding the functions of the components inside the reactor. Furthermore, the cracks and damage, even though they may be small at the beginning, can grow so as to lead to a critical accident or problems such as rupture of the cooling structure and loss of the coolant, if a suitable measure such as repair is not taken. The tritium radiated from the plasma permeates through the material of the cooling structure so as to be diffused to reach the coolant, thus contaminating the coolant. Furthermore, operation of a nuclear fusion reactor often experiences an abnormal state such as disruption in which plasma is suddenly extinguished. In the event of such an abnormality, a huge electromagnetic force is generated, tending to destroy or damage the cooling system. SUMMARY OF THE INVENTION In view of the above-described circumstances, an object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art and to provide an internal component or structure of a fusion reactor which is improved in terms of safety by multiple protection against thermal/particle loads received from a plasma and other places by a cooling structure. Another object of the present invention is to provide an internal component or structure of a fusion reactor in which a leakage of a coolant due to small cracking and damage in a cooling structure can be immediately detected and tritium contamination of the coolant can be limited. These and other objects can be achieved according to the present invention by providing an internal component of a fusion reactor in which an internal structure assembly is housed in a toric vacuum vessel in an arrangement along a circumferential direction thereof and in which a high-temperature plasma in which hydrogen and hydrogen isotopes are maintained in a plasma state is confined in a toric internal space defined in the internal structure assembly, the internal component comprising: a cooling structure of a multi-wall structure having walls formed in the internal structure assembly; and a flow channel formed in said cooling structure for a cooling fluid for extracting heat caused by plasma and a nuclear reaction. In a preferred embodiment, the internal structure assembly comprises a plurality of outboard blanket assemblies each having a surface facing the plasma, a plurality of inboard blanket assemblies each having a surface facing the plasma and a plurality of divertor assemblies each having a surface facing the plasma, the outboard blanket assemblies and the inboard blanket assemblies and the divertor assemblies being arranged along the circumferential direction of the toric vacuum vessel and each of the outboard blanket assemblies and the inboard blanket assemblies and the divertor assemblies is provided with the cooling structure formed on the surface thereof facing the plasma. In detailed embodiments, the cooling structure is formed of an inner wall member in which a flow channel for the cooling fluid is formed and an outer wall member surrounding the inner wall member with a gap formed therebetween. A leak detection mechanism is provided so as to communicate with the gap for detecting a leak of the cooling fluid into the gap between the inner and outer wall members. A gap is formed between the walls and a hydrogen processor capable of communicating with the gap is provided to separate and store hydrogen and hydrogen isotopes entering the cooling structure. The hydrogen processor is provided for a gas circulation system in which a gas, i.e. inert gas such as helium, circulates, an internal space of the hydrogen processor is partitioned into a processed gas chamber forming a part of the gas circulation system and a processing chamber for storing hydrogen and hydrogen isotopes by a hydrogen permeable membrane permeable to hydrogen and hydrogen isotopes. The hydrogen processor is arranged to oxidize at least one of hydrogen and hydrogen isotopes separated by the hydrogen permeable membrane. A hydrogen getter is accommodated in the processing chamber of the hydrogen processor and separated hydrogen and hydrogen isotopes are absorbed and stored by the hydrogen getter. A gap is formed between the multiple walls and gas and liquid circulation systems are communicated with the gap, the circulation systems including means for measuring a change of a state of a pressure, a water content and a temperature in a gas and a liquid existing in the gap to detect a leak of the gas and a cooling liquid flowing through the cooling structure. A gap is formed between the multiple walls of the cooling structure and a pressure detection mechanism capable of communicating with the gap is provided to detect a leak out of the cooling structure of a gas existing in the gap. A detector for detecting a gas is provided at an exhaust port communicating with the internal space of the toric vacuum vessel to detect a leak of the gas out of the cooling structure through an internal space of the vacuum vessel. A plurality of exhaust ports communicating with the internal space of the toric vacuum vessel are arranged in a circumferential direction of the vacuum vessel and detectors for detecting a gas are respectively provided at the exhaust ports to detect a place through which the gas leaks out of the cooling structure. A gap is formed between the walls of the cooling structure and metallic wires having a high heat conductivity are provided in the gap. The metallic wires are formed of a same material as that of the cooling structure or formed of a material having a heat conductivity higher than that of a material forming the cooling structure. The walls of the cooling structure are closely fitted to each other with partial gaps formed between the walls as grooves through which a fluid is caused to flow. The cooling structure has a thickness which is reduced at a side facing the high-temperature plasma. The cooling structure has a rectangular shape in cross section or may have a circular shape in cross section composed of inner and outer pipe members. The cooling structure is integrally formed with the surface, facing the plasma, of each of the outboard blanket assemblies, inboard blanket assemblies and divertor assemblies. The cooling structure may be separately formed from the surface, facing the plasma, of each of the outboard blanket assemblies, inboard blanket assemblies and divertor assemblies and the cooling structure is then secured to the surface thereof. According to the characters and structures of the present invention described above, in the internal components or structures of the fusion reactor in accordance with the present invention, the cooling fluid channels are formed in the cooling structures, thereby achieving multiple protection against thermal and particle loads received from the plasma by the cooling structures. To improve heat conduction between the walls in the cooling structures, the walls are closely fitted to each other or metallic wires are provided in the gaps formed between the walls, and the thickness of the walls is changed for further improvement in heat conduction, thereby achieving multiple protection without impairing the internal component cooling function. In the cooling structures, flow grooves through which a fluid flows are formed as partial gaps between closely-fitted surfaces of the walls, and a function of detecting a leak of the coolant through the flow grooves or gaps between the walls is provided, thereby making it possible to detect even a small coolant leak due to cracking or damage in the cooling structures. It is therefore possible to prevent rupture or damage in the cooling structures with the advancement of damage or cracking in the internal structure of the fusion reactor or a loss-of-coolant accident due to rupture or damage in the cooling structures. If the kind and the mixture ratio of fluids flowing through the gaps formed between the multiple walls of the cooling structures or through flow grooves formed as partial gaps are suitably selected, a place in the cooling structures where cracking or damage occurs can be ascertained. It is thereby possible to rapidly take measures for repair and maintenance of the internal components or structures of the fusion reactor. The reliability of the fusion reactor is thereby improved. Further, a function of disposing of tritium passing through the cooling structures through the gaps formed between the walls of the cooling structures or through flow grooves forming partial gaps is provided, and a hydrogen storage alloy is used to form a part or the entire of the material of the multiple walls. The amount of the tritium passing and diffusing through the cooling structures and entering the coolant can thereby be reduced effectively, thus limiting tritium contamination of the coolant.