 INES 1 |
Secondary sodium leakage in the prototype fast breeder reactor Monju |
The prototype fast breeder reactor Monju was started up on December 6, 1995. When power output was being increased for plant trip tests, an alarm sounded, indicating high sodium temperature at the outlet of the intermediate heat exchanger (IHX) on the Secondary Heat Transport System (SHTS). A fire alarm sounded at the same time. Further, an alarm sounded indicating that there was a sodium leak in the SHTS. Then the reactor power output was decreased and the reactor was shutdown manually.
In the accident, aproximately 0.7 tons of secondary system sodium leaked inside the reactor auxiliary building. As the secondary system sodium does not pass through the reactor core, it is not radioactive. A seondary system sodium leakage of this extent does not exert any notable direct influence on the reactor cooling, so the reactor safety was maintained.
The sodium leakage occurred at the spot where the thermometer was fitted to the SHTS. The cause of sodium leakage was a break of the thermometer well tube protecting the termocouple of the termometer fitted horizontally inside the pipe from the lateral side of the main pipe near the exit of the IHX of the C-loop of SHTS.
Justification of the rating:
This incident was rated level 1 based on the results of the assessment. Both "off-site" and "on-site" critera were not applicable. The incident was rated level 0 based on the defence-in-depth criterion. However, allowing for additional factors, the level was upgraded by one level because there was the possibility of common cause failures, procedural inadequacies and safety culture deficiencies.
Event date: Fri, 08-12-1995
