Fire safety, sodium leaks

The risk of fire in the power unit is associated with the presence of electrical cables and equipment, and the oil and hydrogen necessary for operation of the pumps and tertiary circuit equipment. Provisions in the power unit for fire safety of these items are traditional for conventional power stations and have been proved by long-term operation. 

Particular attention was paid to fire safety for the primary and secondary sodium equipment and pipelines. Technical measures are provided to limit the scope of sodium leaks and fires. Thus all reactor-related auxiliary sodium system pipes leaving the reactor vessel are jacketed up to a second isolation valve (including the valve casings) and are located in partially leak-tight rooms. 

In partially leak-tight rooms sodium fires are extinguished by actuation of a filtered firefighting rarefaction vent system which reduces the oxygen concentration. To promote extinguishing nitrogen can be supplied to the room from special gas receivers. 

During thirteen and a half years of operation of the BN-600 reactor there have been 28 events with sodium leakage out of primary and secondary equipment. The main causes of sodium leakages were  

There have been four emergencies with primary sodium leaks, three of which occurred on the pipeline for removal of sodium from the cold-trap. Recently (Oct.7, 1993) the largest happened in the same place. The total amount of sodium escaped during the event was assessed to be approximately 1000 kg. 

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Prevention, detection and mitigation of sodium leaks, improved resistance of nuclear systems to fires and choice of concrete for minimisation of interactions remain important directions for safety research. In France a new aluminous concrete which does not interact with sodium has been proposed. The EFR (European fast reactor) anchored safety vessel option was tested with this concrete. There is a need to continue the R D on sodium-resistant concrete to minimise damage to structures in the event of sodium leaks. 

As regards aspect of fire danger related to sodium coolant, it should be noted that about one third of all 61 sodium leaks occurred in all reactors (including BN-350) were caused by the errors of operators or repair personnel. About half of the leaks occurred during repair work, in the start-up and adjustment stage or in the system for sodium coolant preparation (which could not impact reactor safety). No one of the leaks has imposed threat to reactor safety. 

In fact, the sodium leak that occurred at Monju involved the secondary heat transport system, which means that the leaked sodium was not radioactive. Therefore, no radioactive material was released to the environment by the sodium leak and the safety facilities were not affected. Knowledge about sodium fire has been accumulated through operation experiences. So far, there have been about 150 sodium leak events in the UK, France, Germany, and the former Soviet Union. However, no reports have been released of safety facility damage due to the sodium leak or combustion, and the reactors that experienced the sodium leakage were repaired and successfully resumed their operations. 

In addition to the safety evaluation analyses of the IHTS for large leaks, R D work on sodium water reaction carried out to date includes small water leak experiments for the determination of a design base leak rate, development of reliable and real time detection system of water leaks using the acoustic signal as well as hydrogen detection, among others. Sodium fire characteristics and... 

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