| Case Name |
Fire caused due to leakage of molten sodium (a cooling agent) from piping for a cooling system at a fast breeder reactor |
| Pictograph |
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| Date |
December 8, 1995 |
| Place |
Tsuruga, Fukui, Japan |
| Location |
Atomic power plant |
| Overview |
On December 8th, 1995. Molten sodium leaked from the "Monju" prototype fast breeder reactor, and a fire occurred. In circulation piping of molten sodium, an inserted thermo-well was vibrated by a symmetrical vortex, and it was broken by high-cycle fatigue. Molten sodium leaked outside through the broken thermo-well. Leaked sodium generated white smoke in contact with air. The cause was a design error. That is, the newest standard was not reflected in the design strength of the thermo-well inserted at a right angle to the flow. |
| Incident |
Molten sodium, which is a cooling agent, leaked from secondary cooling system piping of the prototype fast breeder reactor "Monju". The leaked molten sodium reacted with air and fumed. Radioactivity did not leak. |
| Processing |
Consumption and usage |
| Process Flow |
Fig2.Unit process flow
|
| Substance |
Sodium, Fig3 |
| Type of Accident |
Leakage, fire |
| Sequence |
19:47 on December 8th, 1995. A fire detector operated during a trial run at 40% output at the "Monju" fast breeder reactor.
19:46. Smoke was found at the site.
20:00. It was judged that a small-scale leakage had occurred and normal shutdown operation was started.
20:50. Alarms of fire detectors were activated. Increase of white smoke was confirmed.
21:50. An emergency shutdown (a manual trip) was performed and the nuclear reactor was stopped immediately.
22:55. Sodium drawing off started.
00:15 on December 9th. Sodium drawing off was finished. |
| Cause |
Downstream of a thermo-well, a symmetrical vortex occurred in the molten sodium that flowed in piping. The thermo-well was vibrated by the power of this fluid, and it was broken by high-cycle fatigue. As the thermocouple inserted into the thermo-well had bent, difference from other thermo-well was generated, vibration occurred notably. This resulted in high-cycle fatigue. |
| Response |
A normal shutdown. An emergency shutdown. Sodium drawing off. |
| Countermeasures |
Safety was checked regarding facilities, operation manuals, and education and training.
Advice from specialists in many fields, such as sodium technology, power generation plant technology, and quality assurance, should be received. |
| Knowledge Comment |
A nuclear reactor is highly dangerous, and requires greater safety measures than usual. It is always necessary to be proactive for safety in the nuclear energy field.
Evaluations of the potential hazards of special substances are insufficient. |
| Background |
Recognition of an apparatus that uses molten sodium was insufficient, and the problem lay in management at the design/manufacture stage. When a thermo-well was designed, vibration caused by a Karman vortex was taken into consideration. However, vibration caused by a symmetrical vortex was not taken into consideration. At the ASME, new knowledge about the vibration was reflected, and it had already been added to the design criteria. However, that knowledge was not reflected in the design of this nuclear reactor. Moreover, examination and evaluation of the influence of a minor or mid-scale leak and combustion of sodium on the safety of a structure were inadequate. |
| Incidental Discussion |
If a special material is used, sufficient prior examination is required not only about its advantages but also disadvantages. Moreover, after it is adopted, whenever abnormality appears or technology improves, it is necessary to update countermeasures. |
| Reason for Adding to DB |
Example of accident caused due to insufficient hazard evaluation of special substances at a nuclear power plant |
| Scenario |
| Primary Scenario
|
Poor Value Perception, Poor Safety Awareness, Inadequate Risk Recognition, Organizational Problems, Inflexible Management Structure, Insufficient Information Transfer, Insufficient Analysis or Research, Insufficient Practice, Forestudy of Influence at Accident, Planning and Design, Poor Planning, Poor Design, Bad Event, Mechanical Event, Vibration, Secondary Damage, External Damage, Leakage/Fire, Loss to Organization, Social Loss, Resesrch Interruption of Fast Breeder Reactor
|
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| Sources |
Fire and Disaster Management Agency, Cases of major accidents "Fire caused by leakage of sodium from piping. Accident cases of dangerous materials. pp.29, 310-312.
Power Reactor and Nuclear Fuel Development Corp. Monju fast breeder reactor secondary main cooling system sodium leak accident. Summary of investigation on the cause. (1997).
|
| Physical Damage |
A thermocouple and a heater were damaged by fire. Parts of a piping cover, ventilation duct, floor liner, grating and cable, etc. were damaged by fire. About 700 kg of metallic sodium leaked. |
| Financial Cost |
¥ 10 million. (Accident cases of dangerous materials) |
| Multimedia Files |
Fig3.Chemical formula
|
| Field |
Chemicals and Plants
|
| Author |
ITAGAKI, Haruhiko (Japan National Institute of Occupational Safety and Health)
TAMURA, Masamitsu (Center for Risk Management and Safety Sciences, Yokohama National University)
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