Fig. 2-19 Schematic of the remote, multilocation analysis system for fuel process gases of a fusion reactor The laser light is branched by a laser distributor and transmitted to optical fibers. The laser light irradiates the gas at the measuring optical system installed in the equipment and the piping of gas processes in the glove box. Gas processes consist of purification, recovery, and separation of deuterium and tritium in the exhaust gas from the vacuum vessel. The characteristic scattered light signal coherent to hydrogen isotopes and impurities is transmitted to multichannel spectrometers and CCD detectors through the optical fibers. The spectrum of the scattered light is detected, and measured. This measurement can be made in several seconds.

Fig. 2-20 The time behavior of the ingredients of D2 (deuterium molecule), HD (hydrogen deuterium molecule) and H2 (hydrogen molecule) are known from the peak values

At the moment it is foreseen that fusion reactors will use a mixed gas of deuterium and tritium as fuel. Tritium is a slightly radioactive material and has a half-life of 12 years. Therefore, it is necessary to measure and manage the tritium so that it does not leak out. In the case of the ITER design, some kg of tritium in total are stored separately in individual components, and are circulating in the system. Self-managed safety standards assume a leak of less than some hundreds-g of tritiated water (HTO) at ground level for the design base accident. The ITER design has introduced a threefold confinement barrier for fuels to satisfy the safety standards. Furthermore, it is important to detect unusual phenomena as soon as possible and to take appropriate measures so as to reduce the amount of leaked tritium. For this, the most effective way is to know exactly the total fuel amount, the ingredients and its temporal change in the reactor system. The fuel gas is flowing on the rate of about 0.3 g/s in the ITER fuel circulation system. We have developed a system, where a laser light irradiates the fuel, and the scattered light after interaction with the gas molecules is measured to know quickly the ingredients of the fuel and its chemical changes. The features of this method are the measurement of the gas directly and continuously with high accuracy. The schematic of this system and an example of a measurement are shown in Fig. 2-19 and Fig. 2-20, respectively. We have measured the isotopic composition of hydrogen with an accuracy greater than 99% in about 1 minute at some points. This technology has the capability of wide range applications to the microanalysis of ingredients and their temporal changes in chemical processing.

Reference S. O'hira et al., Development of Real-Time and Remote Fuel Process Gas Analysis System Using Laser Raman Spectroscopy, Fusion Technol. 30, 869 (1996).

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