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Report No.
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New small sodium leak detection system, 2; An Experiment of moving fluid ionization detector and laser leak detector in fast breeder reactor Monju

Okazaki, Koki*; Naito, Susumu*; Nakayama, Kunihiko*; Makino, Shunichiro*; Shigeyama, Musashi*; Kuwako, Akira*; Izumi, Mikio*; Ito, Jun*; Okada, Hisashi*; Muto, Keitaro 

In fast breeder reactor (FBR), it is important to detect a small sodium leak surely for safety. As the coolant sodium used in FBR is low pressure, a small leak occurs from a crack before a pipe breaks, therefore it is possible to detect a sodium leak so as to shutdown a reactor before a pipe completely breaks. The sodium leaking from piping of cooling system reacts with oxygen etc. including with the ambient gas to form aerosols. In the second coolant system of the prototype liquid metal FBR Monju, the ambient air around piping is drawn into radiative ionization detector (RID). When sodium aerosols are detected by RID, chemical analysis of aerosols and the viewing of the leak point are carried out, then sodium leak is confirmed. As the signal of RID depends on temperature strongly and RID detects the aerosols which do not include sodium, the improvements of RID are demanded for reliability. Moving fluid ionization detector (MID) and laser leak detector (LLD) have been developed to improve these points. MID can reduce the dependence for temperature of the signal strength, and LLD can detect sodium atoms which are included in aerosols by means of laser-induced breakdown spectroscopy. An experiment was carried out for validating sodium leak detection system with MID and LLD under real environment installed in the second coolant system of Monju. In the experiments, sodium was not leaked, and ambient air around piping of the secondary coolant system was introduced through actual sampling line and measured by MID and LLD. In addition a sodium level in the ambient air was measured by means of ion chromatography. The experiment showed that the background signals of MID and LLD were stable and low and that the sodium level due to the background of salinity (NaCl) contained in the ambient air was lower than three orders of magnitude than the sodium level for small sodium leak. The plant applicability of MID and RID was validated by this experiment.

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