5.52. In a severe accident, a large amount of hydrogen might be released to the atmosphere of the containment, possibly exceeding the ignition limit and jeopardizing the integrity of the containment. In the event of interactions between molten core material and concrete, carbon monoxide might also be released, contributing to the hazard. To assess the need to install special features to control combustible gases, an assessment of the threats to the containment posed by such gases should be made for selected severe accident sequences. The assessment should cover the generation, transport and mixing of combustible gases in the containment, combustion phenomena (diffusion flames, deflagrations and detonations) and the consequent thermal and mechanical loads, and the efficiency of systems for the prevention of accidents and the mitigation of their consequences.

5.53. Uncertainties remain concerning the production of hydrogen during severe accident sequences; these uncertainties are essentially linked to such phenomena as flooding of a partially damaged core at high temperatures, the late phase of core degradation, the slumping of molten core material into residual water in the lower head of the reactor pressure vessel, and the long term interactions between molten core material and concrete. For new plants, these uncertainties should be taken into account in the design and layout of the means of mitigation of the consequences of the combustion or deflagration of hydrogen, and in the design of the containment.

5.54. The efficiency of the means of mitigation of the consequences of combustion or deflagration should be such that the concentrations of hydrogen in the compartments of the containment would at all times be low enough to preclude fast global deflagration or detonation. Possible provisions in the design for achieving this goal are, for example, an enhanced natural mixing capability of the containment atmosphere coupled with a sufficiently large free volume, passive autocatalytic recombiners and/or igniters suitably distributed in the containment, and inerting. For new plants the amount of hydrogen expected to be generated should be estimated on the basis of the assumption of total oxidation of the fuel cladding.

5.55. The leaktightness of the containment for the most representative accident sequences should be ensured with sufficient margins to accommodate severe dynamic phenomena such as a fast local deflagration, if these phenomena cannot be excluded.

5.56. Even in an inerted containment, the concentrations of hydrogen and oxygen generated over a long period of time by water radiolysis may eventually exceed the ignition limit. If this is a possible threat, a hydrogen control system, passive hydrogen recombiners or other appropriate systems for mitigation and monitoring (e. g. systems for oxygen control and measurement) should be installed.

5.57. Provision should be made for hydrogen monitoring or sampling. The concentrations of other combustible gases and oxygen should also be monitored.