In the nuclear power plant, the off gas linear monitor had been installed for the purpose of a detection of fuel leakage. However, its sensitivity to detect the early stage of fuel leakage was not enough, because it measured only the gross gamma-ray. So a high sensitive off gas monitor has been desired. HITACHI, Ltd., is commercialized the Off-Gas Nuclide Analysis System which can be accurately detected radioactive noble gases as a index to fuel leakage, and delivered it in 19 plants since 2002. It has been continuously working well and able to detect the early stage of fuel leakage 7 times. It can be detected the minute fuel leakage, so it is possible to operate the plant till next periodic check of the plant by executing PST*. And even if it is the case in which the plant must be stopped, the extent of pollution by fuel failure can be minimized.

Main features of the Off-Gas Nuclide Analysis System are followings: (1) Maintenance-free continuous measurement is enabled by adopting electricity cooling system for a Ge detector. By attempting the miniaturization of the system, a compact field implementation plan is realized and also a periodic check can be simplified. (2) Thickness of a Ge detector is optimized to detect Xe-133 (80keV) as a main index nuclide to fuel leakage, so detection sensitivity is improved. (3) By adopting the mass flow control system wcich can control the flow stably in low, it is possible to reduce a volume and length of a decay tube for 13N which is the main interference nuclide, then a installation of the system is able to be simplified. (4) Moving average method which is effective to reduce noise is applied to a trend display of nuclides measured value, and it can be set at optimum time for the PST execution by flexibly setting a moving average time.

*PST: Power Supression Test

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