Resuming the operation for the radio frequency plasma heating device for ion cyclotron heating

March, 2004
National Institute for Fusion Science
Since the fire accident on January 9th, 2003, NIFS tried to find out what caused the fire and developed measures to prevent recurrence. We now completed safety measures for the radio frequency plasma heating device in question. We are planning to resume operation in the middle of this month. The following is the summary of safety measures taken at this time.
<Safety Measures>
1) In order to prevent the main cause -- operative malfunctions on the vacuum circuit breakers for high-voltage systems -- or to prevent accidents if the malfunction occurred, we took all possible safety measures.
- Execution of reinforced maintenance and inspection
- Additional installation of new serial vacuum circuit breakers
- Temperature control to prevent operative malfunctions due to low temperature
2) We repaired the control system and fully installed the interlock.
3) We revised the operation manual and carried out an education program for the safe operation.
  By implementing the above measures, we reassured the reliable cut-off operation with a vacuum circuit breaker for high voltage systems. Also, the repair for control systems enabled more effective safety operations for the entire radio frequency plasma heating device.
  From now on, we will be highly conscious of the safe operation. Then, we will do our best to gain understanding and trust to NIFS from local people.
<Additional Information>
1. Process report

After the completion of an experiment on January 9, 2003, fire broke out from a clover board on the radio frequency heating device DC power supply for the ion cyclotron heating in the NIFS heating device room. Fire damaged the board. Please find its causes and prevention measures in the report dated January 22, 2003, "Causes of some fire from radio frequency heating device DC power supply and their prevention measures". The major causes of fire are the following.

  1. A vacuum circuit breaker for high-voltage systems operated improperly (did not operate).
  2. A remote control operation panel did not reflect the status of a vacuum circuit breaker properly.
  3. An interlock system, that protects machines from going on the next operation at the time of a vacuum circuit breaker malfunction, was not built in the control system.

Fire appears to have broken out due to the causes above. When more than one of those causes occurred together, a grounded circuit became connected with a vacuum circuit breaker turned on. Then, the ground resistance was heated because of a DC high-voltage condition, resulting in the fire breakout.

     
2. Summary of the prevention measures
We fully performed the safety measures described above and decided to build a new system that has a thoroughgoing safety. Items of the prevention measures are listed below.
1) Prevention of the vacuum circuit breaker malfunction
  • Extra inspection was performed to all the vacuum circuit breakers.
  • As for vacuum circuit breakers in the radio frequency heating device, Additional serial vacuum circuit breakers were newly installed.
  • From now on, reinforced regular checkups will be performed for the high-voltage devices including vacuum circuit breakers.
  • Temperature control to prevent the vacuum circuit breaker malfunction due to condensation and low temperature will be performed.
2) Modification of the control operation and the operation indicator regarding the shut-down operation, addition of protective functions, and the improvement of control systems are performed as below.
  • In order to proceed the shut-down operation at high-voltage properly, we improved the interlocks for the display and sequence.
  • In the case that downstream voltage still remains even though a high-voltage shut-down was operated or that there are no open signals from a circuit breaker, an additional operation to completely cut off the voltage was built in the system.
  3) Revision of the operation manual and education for the safety operation
Reviewing the operation procedure at the time of fire, we reviewed the whole process. Then, we carried out an education program to fully familiarize with the safety operation.
By taking the above measures, complete cut-off operation with a vacuum circuit breaker for high-voltage systems and safety operations by the control system should be fulfilled.

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