The prototype sodium-cooled FBR MONJU was shut down due to a sodium leak incident on Dec. 8, 1995, and has remained shut-down since. Currently, a safety assessment of the proposed new countermeasures against future leaks is being carried out. One of these countermeasures, a new sodium leak monitoring system to detect small scale leakage with high reliability, will be installed throughout the entire secondary cooling circuit area. An initial assessment suggested that smoke sensors would best meet the requirements for this monitoring system from the standpoints of sensitivity, reliability, and installation cost. To confirm the suitability of smoke sensors for sodium leak monitoring, their sensitivity to sodium aerosol has been measured experimentally, and their ability to detect small scale sodium leakage has been estimated by analytical methods.

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