Limestone-Siliceous and Siliceous concretes thermal damaging at high temperature

Pré-publication : Construction and Building Materials 228 (2019) 116671Limestone-siliceous and siliceous concretes are used in reactor pits of French nuclear power plants. In case of severe nuclear reactor accident, failure of the reactor vessel would lead to interaction between molten corium (hot melt of nuclear fuel) and concrete. This paper focuses on the thermal degradation of both limestone-siliceous and siliceous concretes till 1000°C. Thermo-Gravimetric Analysis (TGA) and Mercury Intrusion Porosimetry (MIP) are used to measure mass loss and porosity modification. As concretes are heterogeneous materials, sampling and representativeness have been addressed. TGA experiments show larger mass loss for limestone-siliceous concretes due to the decarbonation of calcium carbonate gravels when T>800°C. MIP experiments demonstrate a 100% increase of porosity for limestone-siliceous (resp. siliceous) concretes when T> 500°C (resp. T> 800°C). The consequences of these results are discussed in the frame of experimental tests on prototypical corium systems aimed at describing the key-phenomena involved in molten corium concrete interaction

Needs for large mass prototypic corium experiments the PLINIUS-2 platform

International audienceCorium is the molten material formed after meltdown of a nuclear reactor core during a severe accident. In order to improve the understanding and modelling of corium behavior, experiments are needed both for LWRs and GenIV fast reactors. Experiments using low temperature simulant materials, thanks to lower costs and constraints, allow the testing of a larger number of configurations and the determination of correlations. But some crucial corium phenomena cannot be reproduced at low temperatures such as the importance of radiation heat transfer or the presence of a large (up to 1000 K) liquidus-solidus interval. Consequently, some experiments are performed with high temperature simulant materials alumina thermite as well as refractory oxides. However, it is not feasible to simulate all the aspects of corium phenomenology, especially its high temperature physico-chemistry. Therefore, even though the use of depleted uranium implies a series of protective and regulatory measures, the need for prototypic corium experimented is supported through several examples Another important aspect of experiment design deals with scaling. Small or medium scale corium experiments are easier to operate and only a few large scale (>100 kg) facilities have been built. Several effects are only visible with significant masses, as for instance, the formation of a corium cake during FCI or all the phenomena controlled by crust strength, such as underwater spreading or corium jet ablation. CEA is currently designing a new large prototypic corium platform PLINIUS-2 for both LWR and SFR corium experimental research. Its main characteristics will be presented

Molten Core Concrete Interaction Test in VULCANO Facility Preventing Initial Interfacial Crusts

International audienceIn the frame of the Severe Accident Facilities for European Safety Targets (SAFEST) project,Karlsruhe Institute of Technology (KIT) and the Society for installation and Reactor Safety (GRS)have proposed to realize a Molten Corium Concrete Interaction (MCCI) test in the VULCANO facilitylocated at the PLINIUS experimental platform, CEA Cadarache. The MCCI test, named VBES-U5,was carried out on July 20th, 2017. 50kg of thermite has reacted to melt a prototypic corium in asiliceous concrete test section, which was then heated by induction. The test section was 2Dcylindrical with an inner diameter of 250 mm, an outer diameter of 500 mm, an inner height of 300mm and outer height of 475 mm. MCCI was carried out for 40 min which conducted to an axialablation of 10 mm and a radial ablation of 60 mm. Great care has been taken to prevent initial crustformation (corium composition such that initial contact temperature is above solidus, high powerduring the first minutes). Nevertheless, a pronounced radial ablation has been observed for thissiliceous concrete, similarly to previous VULCANO tests in which initial crust formation was likely tooccur

DISCOMS : Capteurs Répartis pour le surveillance du corium et la sûreté

International audienceThe Fukushima-Daiichi nuclear disaster showed that the need for safety must always prevail. This paper discusses the development of remote monitoring technologies to improve Nuclear Power Plants (NPPs) safety, in operation (Pressurized Water Reactors), under construction (the EPR reactors, i.e. the GEN 3 PWR), or for any other next generations of reactors. At Fukushima, the total loss of electrical power supplies has quickly led most of the instrumentation inoperative and the operator (TEPCO) with no way to monitor the status and the evolution of the accident. To overcome these important drawbacks, advantage can be taken from the considerable potential of distributed sensing technologies based on both "Optical Fiber Sensors" (Raman, Brillouin, and Rayleigh Reflectometries) and long-length "Self Powered Neutron Detectors" (SPNDs). The goal consists in inquiring about the status of the third barrier of confinement and to define possible mitigation strategies in case of severe accident, namely: i) reactor pressure vessel breakthrough and corium relocation outside the vessel, ii) concrete basemat erosion and iii) corium cooling. Such monitoring should consist in "sensing cables" embedded in concrete basemat below the reactor vessel and interrogated from a rear base where operators can work safely. In this context, DISCOMS, which stands for "DIstributed Sensing for COrium Monitoring and Safety", is a five-year project, managed by the French National Research Agency (ANR), dealing with the NPP safety improvement, from normal situation to severe accidents. Monitoring phases include reactor vessel breaching, corium flow, along with post-accidental period (corium cooling ex-vessel). Thus, optical fibers selected for their resistance to ionizing radiations and long length SPNDs, both judiciously deployed within the reactor concrete basemat, and the structures around it, will provide a useful real-time or on-demand monitoring, in normal operation, and more important in accidental and post-accidental situations