The COMOS system – An innovative monitoring system for containment atmosphere in severe accident

In the event of a severe accident in a light water nuclear reactor, substantial amounts of hydrogen may be produced and released into the containment due to reactor core degradation. Additionally, if molten corium interacts with concrete, further combustible gases, such as hydrogen and carbon monoxide, may be emitted. As demonstrated during the Fukushima Daichi accident, the combustion of H 2 and CO can generate high-pressure peaks, potentially compromising the integrity of the reactor containment. To mitigate the formation of combustible gas mixtures, many European countries have implemented safety measures, including the use of Passive Autocatalytic Recombiners (PARs) or igniters, which help consume H 2 and CO upon release. However, studies indicates that even with these systems in place, completely preventing the formation of combustible mixtures remains challenging. This could lead to localized combustion, flame acceleration, and ultimately poses a risk to the integrity of containment structures and safety components. To safeguard containment integrity, severe accident management guidelines (SAMG) recommend strategies such as activating spray systems or emergency core cooling, which may inadvertently increase the risk of gas explosions. Therefore, an effective gas monitoring system is of strong interest for supporting decision-making and implementing SAMG measures. In response to this need, the COMOS system – a fiber-coupled nuclearized gas probe prototype utilizing optical Raman technology – was developed under the French MITHYGENE Project. This system aims to enhance the management of combustible gases during severe accidents in water-cooled reactors, including Small Modular Reactors (SMRs), offering an innovative approach to improving nuclear safety.