Failure of the nuclear reactors thermocouples.

The leak cannot be stopped and may generate a major accident.

CONTENT

1)Purpose of thermocouples in power nuclear reactors.

2)Self damaging effects of leaks.

3)A potentially disastrous leak.

4)Permanent loss of coolant.

5)Core melting down.

6)Risk of radiation release.

7) Conclusion

APPENDIX: Emergency cooling of reactor core

1)Purpose of thermocouples in power nuclear reactors.


The in-core temperature of the nuclear reactors is monitored by thermocouples; these wire-shaped sensors are assembled together inside columns which penetrate the head of the reactor vessel. The sealing system of the thermocouple columns is rated at the highest level of the safety requirements, because its failure would be very worring:
- leak of the nuclear eactor head;
- perturbation of the temperature mesurements;

2) Self damaging effetcs.

If the design of parts is poor, a small leak is likely to deteriorate very abruptly due to its damaging side effects: 1)The flow of flashing water or the mixture of water and steam will induce shocks and vibrations; 2)the sharp temperature transient induces stress which may cause the sudden blast of the sealing parts .

3) A potentially disastrous leak

The failure of the thermocouple sealing system is potentially disastrous because the leak cannot be shut down; no closing device is fitted between the reactor vessel and the thermocouple sealing system.

4) Permanent loss of coolant

The leak of the thermocouple sealing system would be sustained by the coolant injected into the reactor vessel ; the emergency injection is required as an attempt to remove the heat from the core and slow down the nuclear reaction. The leak can last weeks...

5) Core melting down.

Disrupting the emergency injection would stop the leak, but the core would dry up ; the nuclear reaction and the temperature would rise and cause the melting down of the core. In that case highly explosive hydrogen could be generated and the reactor building could blow up.

6) Risk of radiation release .

During emergency core cooling, large amounts of contaminated coolant are processed outside the containment (pumping, heat removal...See the appendix). A high the risk of radiation release to the environment has to be taken into account.

Damages to property and to individuals could be so huge that they can hardly be evaluated , especially in very populated countries like Belgium.

7) Conclusion

The thermocouple columns are rated at the highest safety level.

A leak of thermocouple column occurring in normal operating conditions may take weeks with no guaranty of keeping under control the situation.

The accident of Three Mile Iland is an example of what can happen in case of persistent loss of coolant .

Unfortunately , the Administration and the Safety authorities have neglected nuclear rules and regulations. In repeated circumstances they have behaved with complacency , keeping silent about obvious breaches; consequently, safety has been impaired worlwide and more specifically at Tihange(Belgium) and Koeberg (South Africa) nuclear power plants .

APPENDIX: emergency core cooling