MIDI, a Facility Dedicated to Study the Thermal Hydraulic Characteristics of Spent Fuel Pools

Abstract

In the framework of the post-Fukushima actions, a research program called DENOPI has been conducted by the French Institute for Radiation Protection and Nuclear Safety (IRSN) to study the behavior of spent fuel pools during loss-of-cooling accidents. This project, supported by the French Government and carried out in collaboration with partners, is composed of experiments and modeling works. As part of this project, the test facility named MIDI was implemented in the IRSN laboratories at Cadarache (France). MIDI facility is an experimental tool dealing with the thermal hydraulic phenomena that may occur at an integral scale in a spent fuel pool undergoing a loss-of-cooling accident before the stored fuels get uncovered. This paper first describes MIDI, a small-scale mock-up of a spent fuel pool, focusing on the design strategy and a presentation of its instrumentation. Then, it presents an overview of the main results. Within the DENOPI project, observations and obtained data highlight the competition during the accidental transient of three vaporization modes: the evaporation at free surface, the nucleate boiling onto the fuel assemblies and the gravity-driven flashing of superheated water above the heating zone. The operationality of the MIDI facility has been demonstrated. As a first step, additional works are planned as part of the new OECD POLCA (Pool during Loss-of-Cooling Accident) project due to start in 2024. In a general way, MIDI experimental results constitute an useful database for computational validation and for supporting the safety demonstration of large pools in natural convection configurations.