Control of Hydrogen

The control of hydrogen and the possible back-fitting of safety systems are live issues in the operation of present day reactors. The sources include the Zircaloy/steam (and some contribution of steel/steam) reactions and also contributions from metal reactions in MCCIs. There may also be a contribution to the hydrogen source from long-term radiolysis of water.

The design measures that can be taken include the presence of a large containment volume. This helps to keep hydrogen concentration levels to below detonation limits (10-13%). There also needs to be good mixing preferably by natural circulation. There should be a minimal number of compartments to prevent the build-up of local hydrogen concentration.

In some designs, the containment atmosphere can be pre-inerted. This may be a useful approach for smaller containments but a disadvantage is that the containment is inaccessible during operation. For such containments, special attention will need to be paid to gas build-up by radiolysis during the periods when the containment is not de-inerted, e. g. during refuelling.

Various hydrogen reduction devices are available. Igniters, catalytic or battery-driven recombiners are useful for dissipating the hydrogen at lower concentrations but are less suitable for large concentrations. Recombiners may also be appropriate for longer term hydrogen management.

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