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Hydrogen Combustion Mitigation

Two approaches have been adopted to mitigate threats to containment structural integrity posed by hydrogen combustion [25]. The Mark I and Mark II containments for boiling water reactors are purged with nitrogen to prevent ignition of hydrogen. As demonstrated by the accidents at the Fukushima Daiichi plant, inerting the reactor containment does not prevent hydrogen combustion events in the reactor building should hydrogen leak from either the reactor coolant system or from the containment.

The second approach to mitigation of the hydrogen combustion threats is to burn hydrogen as it is released to reactor containments and prevent accumulations of hydrogen to levels where combustion could threaten the structural integrity of the reactor containment. In the USA, Mark III boiling water reactor containments and ice condenser containments for pressurized water reactors are equipped with glow plug igniters. These glow plugs initiate chemical reaction of hydrogen with oxygen from the air so there is not an accumulation of hydrogen. The glow plugs do require electrical power and special measures have been necessary to assure the availability of electrical power for the igniters in the event of a loss of all power for the plant (Station Blackout).

Canadian CANDU reactors and many types of reactors in Europe are equipped with catalytic hydrogen recombiners. These catalytic systems consist of finely dispersed noble metals (commonly palladium but in some cases platinum or platinum and palladium alloys) on large surfaces within structures to facilitate natural convection over the surfaces [26]. Molecular hydrogen, H2, absorbs and probably dissociates on the surfaces to produce surface-bound atomic hydrogen. Atomic hydrogen will react readily with gaseous oxygen to form eventually water vapor. Heat evolved by the reaction raises the temperature of the surface to facilitate natural convection of gases over the surfaces.

The catalytic recombiners do not require external power sources. They will begin to catalyze the reaction of hydrogen with air at hydrogen concentrations of about 1%. The combiners are susceptible to “poisoning” [27]. That is, the reactive noble metal particles dispersed on the surfaces can react with other atmospheric gases to either prevent hydrogen absorption or to occlude the surface from interaction with hydrogen. Poisoning of the catalyst surfaces can occur during normal operations and especially during shutdown operations when maintenance activities can introduce vapors and fumes into the containment atmosphere. Sulfur containing vapor species are especially reactive toward catalysts. The catalysts need to be tested regularly for poisoning or degradation during plant shutdowns for maintenance. Some testing has been done to ascertain the susceptibility of catalysts to poisoning during the progression of reactor accidents [26].

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