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During a severe accident in a Light Water Reactor, large amounts of hydrogen gas are likely to be generated and released into the containment during core degradation, potentially raising a combustion hazard. Additional burnable gases (H2 and CO) may be released into the containment volume in case of Molten Corium/Concrete Interaction leading, in case of ignition, to pressure and temperature loads that may challenge the containment and safety components integrity. Mitigation measures are often employed to avoid severe damage when the atmosphere of NPP containment is combustible during an accident. NPP personnel as well as safety authorities need information on the status of the gas mixture inside the containment to select suitable and timely measures. Therefore, gas concentration monitoring systems are important in supporting personnel to make appropriate decisions. Usually, H2 monitoring systems are based on few (less than 10) sensors located in different area of the containment. Based on these gas concentration measurements, operators may take decisions as postponing the spray activation, venting line opening … Existing measurement techniques have limitations and drawbacks: they do not work under the conditions of the late phases of a severe accident, when oxygen is lacking and carbon monoxide is present in the atmosphere of the containment, or because they increase the risk of leakage from the containment. To overcome these limitations and as part of the French MITHYGENE project, the COMOS prototype was developed, based on Raman optical technology, and qualified under severe accident conditions. This monitoring technique offers several decisive advantages, such as local, distributed and chemically selective gas measurement. It aims to provide in situ accurate information on the gas mixture (H2, O2, N2, H2O, CO, CO2) in near-real time at several locations inside the containment. The prototype has a TRL of 5 to 6 and has given rise to three patent applications.
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