In situ gas monitoring by fiber-coupled Raman spectrometry for H2-risk management in nuclear containment during a severe nuclear accident

During a severe nuclear accident leading up to core melting, molten corium–concrete interaction (MCCI) and core-coolant water reaction both release large amounts of hydrogen (H2) gas in the containment atmosphere. According to the Shapiro–Moffette ternary diagram and depending on local partial pressures of H2, air, and water vapor, deflagration/detonation may occur with potential deleterious impact over equipment and structures. CO and CO2 are also of interest, as revealing gases for MCCI. Pressurized Water Reactors (PWRs) of French Nuclear Power Plants (NPPs) are equipped with passive autocatalytic recombiners (PARs), partly mitigating the H2 risk. However, the H2-risk management strategy may be significantly improved by performing in situ monitoring of H2, O2, N2, H2O, CO, and CO2 partial pressures at several locations inside the containment, to account for potential local combustion risk. Raman spectrometry (RS) involves only one laser and spectrometer equipped with a 2-D charge-coupled device (CDD). Raman probes are chemically selective and may be radiation-hardened. Custom-made fiber-coupled Raman probes, linked with a readout unit, were qualified in a climatic chamber, a flame-propagation tube, a 60Co irradiation cell, a 3-D shaking table, a steam jet and the MISTRA facility (1/10 reduced-scale containment mock-up dedicated to thermo-hydraulic tests).