4.162. Passive means such as passive autocatalytic recombiners and/or active means such as igniters should be provided for removing hydrogen.

4.163. If it is determined by means of analysis that the hydrogen concen­tration would increase slowly over a long period of time, the actuation of active means of hydrogen removal may be by manual means. In this case, it may be assumed that off-site power is available for active means. If analysis shows with sufficient certainty that the accumulation of hydrogen under LOCA conditions is slow, a mobile control system for combustible gases (i. e. a mobile recombiner) may be used. In this case, appropriate provisions should be made in the design and in the procedures for the use of such a system. The provisions for shielding should be such as to permit connection of the mobile system without causing any undue exposure of operators to radiation.

4.164. A single failure during the use of active hydrogen control systems need not be postulated provided that:

— Repair or means of substitution can be shown to be practicable.

— The generation of hydrogen is slow enough that hydrogen concentration limits will not be exceeded, either during the predicted repair time or during the time necessary to introduce substitute means (such as by putting a mobile recombiner into operation).

Homogenization

4.165. The containment design either should incorporate active means (such as sprays and mixing fans qualified for operation in a combustible gas mixture) or should facilitate the action of mechanisms (such as large volume dispersion

or natural circulation) to enhance the uniform mixing of the containment atmosphere within and between compartments. This is to ensure that localized hydrogen concentrations do not reach combustion limits following an accident. Alternatively, it should be shown by analysis either that uncontrolled local ignition will not occur or else that safety systems and components can survive local ignition.

Inerting

4.166. One possible way to avoid hydrogen combustion is to inert the containment atmosphere during reactor operation (usually with nitrogen). This is mainly applicable to small containments such as those of boiling water reactors.