Hydrogen is released into the nuclear plant containment area under LOCA conditions. In the longer term under normal operation, radiolysis and corrosion can also increase hydrogen levels to the extent that a potentially explosive mixture of hydrogen and air could be present. To mitigate hydrogen concentration, passive catalytic recombiners are installed in the containment. Recombiners use a catalyst made of porous material treated with metals such as platinum and palladium. The catalyst provides sites where hydrogen and oxygen atoms from air come into close vicinity and chemically react to form water, thus reducing hydrogen concentration.