During the course of a severe accident in a light water nuclear reactor,
large amounts of hydrogen can be generated and released into the
containment during reactor core degradation. Additional burnable gases
[hydrogen (H2) and carbon monoxide (CO)] may be released into
the containment in the corium/concrete interaction. This could
subsequently raise a combustion hazard. As the Fukushima accidents
revealed, hydrogen combustion can cause high pressure spikes that could
challenge the reactor buildings and lead to failure of the surrounding
buildings. To prevent the gas explosion hazard, most mitigation
strategies adopted by European countries are based on the implementation
of passive autocatalytic recombiners (PARs). Studies of representative
accident sequences indicate that, despite the installation of PARs, it
is difficult to prevent at all times and locations, the formation of a
combustible mixture that potentially leads to local flame acceleration.
Complementary research and development (R&D) projects were recently
launched to understand better the phenomena associated with the
combustion hazard and to address the issues highlighted after the
Fukushima Daiichi events such as explosion hazard in the venting system
and the potential flammable mixture migration into spaces beyond the
primary containment. The expected results will be used to improve the
modeling tools and methodology for hydrogen risk assessment and severe
accident management guidelines. The present paper aims to present the
methodology adopted by Institut de Radioprotection et de Sûreté
Nucléaire to assess hydrogen risk in nuclear power plants, in particular
French nuclear power plants, the open issues, and the ongoing R&D
programs related to hydrogen distribution, mitigation, and combustion.