Temperature ramps for severe accident instrumentation in nuclear reactor cavity concrete

Christophe Journeau (1) , Viviane Bouyer (1) , Arthur Denoix (1) , Jean-François Haquet (2) , Anne Boulin (2) , Quentin Souquet (2) , Laurent Maurin (3) , Gwenael Jouvin (3) , Hamid Makil (4)

1 LEAG - Laboratoire d'Etudes et d'Expérimentation pour les Accidents Graves
2 SMTA - Service Mesures et modélisation des Transferts et des Accidents graves
3 LCFO - Laboratoire Capteurs Fibres Optiques
4 LCAE - Laboratoire Capteurs et Architectures Electroniques

Distributed Optical Fiber Sensing and Self-Powered Nuclear Detectors are potential innovative instrumentation for the management of severe accidents. The former can monitor temperature, strain, and/or presence of hot melt while the latter is more dedicated to gamma radiation measurements and can also be coupled to thermocouples for temperature measurement purpose. These sensors could be installed in the basement concrete to monitor corium progression (arriving in lower head, later on in the reactor pit, reaching a predefined level in concrete before melt through…). In order to assess the temperature evolution that these sensors may experience, thermal conduction in concrete structures was modelled coupled with TOLBIAC-ICB calculation results for several typical scenarios. This confirmed that, due to concrete low thermal conductivity, ablation progresses more rapidly than conduction, except in a small zone close to the ablation front. Therefore experimental temperature profiles from prototypic corium-concrete interaction experiments can be used to specify the temperature profiles that sensors inserted in the reactor cavity can withstand during such severe accident. A first experiment, VULCANO VB-U10, has been carried out to study the behavior of 5 distributed optical fiber sensors that were installed inside a concrete crucible experiencing ablation by prototypic corium. These instruments have been used in this test to monitor the concrete temperature and/or its ablation. During this test, 50 kg of prototypic uranium-containing corium have interacted for 98 minutes with a lime-siliceous concrete leading to an axial ablation of 25 mm and a radial ablation of 80 mm. Optical fiber length measurements with Rayleigh OFDR technique have been found to be coherent with data from thermocouples installed in the concrete as in previous VULCANO experiments. Raman DTS measurements provided satisfactory temperature evolution results. This good performance of the distributed temperature measurement has been validated in conditions representative of a severe accident.

Keywords: Severe Accident; Distributed Optical Fiber Sensor; Thermocouple; Corium; MCCI; High Temperature; instrumentation; nuclear instrumentation; Optical Fiber Sensor; Self-Powered Nuclear Detector; detector; modelling; corium-concrete interaction; Vulcano

Domains: Instrumentation and Detectors [physics.ins-det]; Nuclear Experiment [nucl-ex]

Christophe Journeau, Viviane Bouyer, Arthur Denoix, Jean-François Haquet, Anne Boulin, et al.. Temperature ramps for severe accident instrumentation in nuclear reactor cavity concrete. SAMMI 2020 - OECD/NEA Specialist Workshop on Advanced Measurement Method and Instrumentation for enhancing Severe Accident Management in an NPP addressing Emergency, Stabilization and Long-term Recovery Phase, OECD; NEA, Dec 2020, Fukushima, Japan. ⟨cea-03955109⟩