Last update on March 2015
The MITHYGENE project, launched in early October 2013, is one of 23 projects awarded funding after a call for projects on nuclear safety and radiation research for “Tomorrow's Nuclear Energy”. Its goal is to improve knowledge of hydrogen risk and how to manage it during a severe accident. It will improve hydrogen risk assessment tools and contribute to development of a prototype for real-time measurement of gas concentrations at various points in the reactor containment and qualify it for severe accident conditions.
Advances from this project will help interpret the events that took place at Fukushima and improve mitigation measures and severe accident management procedures. The project will also improve practices and policies adopted by industry for eliminating the risk of hydrogen explosions in their facilities, nuclear or otherwise.
Background and objectives
During a core melt accident, hydrogen may be produced by the oxidation of metals during two stages of the accident: when the reactor core is degraded by oxidation of the fuel rod cladding; and during the interaction between the corium and the concrete of the containment foundation raft if the corium pierces the reactor vessel.
If the hydrogen reaches the reactor containment, and in the event its distribution is highly heterogeneous, the flammability limit of the gaseous mix (air and hydrogen) may be exceeded: combustion may be initiated quickly if sources of inflammation are present (hot spots, electricity, etc.). During flame propagation, certain rapid combustion regimens may be attained (rapid deflagration, detonation, etc.) and may generate pressure loads likely to threaten the reactor containment structure and the resistance of the equipment that it contains, including that related to safety.
The occurrence of various types of hydrogen explosions during the accident at the Fukushima Nuclear Power Plant in 2011 raises the issue of assessment of this risk for the French nuclear power plant fleet and whether available measures for protection from explosions and limiting their consequences are adequate.
For pressurized water reactors (PWRs) in France, the chosen strategy combines containments with large volumes and the installation of passive autocatalytic recombiners, which have been installed in all units in the French nuclear power plant fleet since 2007. Despite the performances indicated for the recombiners, the studies performed highlight the difficulty of demonstrating that the formation of a hydrogen-oxygen mixture conducive to local flame acceleration phenomena can be excluded at any time and at any point in the reactor containment. The research has been carried out as part of level 2 probabilistic safety assessments on core melt accident scenarios.
The severe accident management guides limit associated risks with precautions for implementing safeguard system, but detailed understanding of the phenomena remains an important issue. This observation was highlighted in the ASN's report on the stress tests performed after the Fukushima accident. It shows the need to pursue research to assess the risk of a hydrogen explosion in the containment annulus of 1300 MW reactors and the venting-filtration systems (U5 filter) of reactor containments.
In addition, operation of the sprinkler system, intended to lower pressure and reduce fission products in the reactor containment in the event of an accident, may also influence distribution of hydrogen in the reactor containment. The water from this system contributes both to making the atmosphere in the reactor containment more uniform and “de-inertized” through steam condensation, which may result in the formation of a flammable cloud. In addition the turbulence caused by the spray of water droplets may promote acceleration of flames in the event of combustion. The tension between the negative and positive effects on safety thus raises the issue of spray system management in the event of a severe accident.
The goal of the MITHYGENE project is to provide information that will improve the effectiveness of measures implemented to mitigate hydrogen risk.
Program overview and research topics
The MITHYGENE project consist of two parts (fundamental and applied topics), each with its own set of research areas.
Using basic research, its objective is to improve models for hydrogen related phenomena and at developing an in-situ and severe accident compatible diagnostics for gas concentrations measurement.
Four research areas are then addressed:
- (WP1, led by CEA/DEN) perform experimental and numerical studies on hydrogen distribution taking into account the effect of mitigation means.The results of these studies will improve the capacity of the computer codes to predict accident situations not yet addressed by national and international programs;
- (WP2, led by ICARE and IRSN) perform experimental and numerical studies of hydrogen flame propagation into gaseous atmosphere containing water vapor and water droplets at different initial conditions. The experiments thus carried out will fill the lack of data on the hydrogen flame propagation in wet medium. The synthesis of the experimental data will be used to enhance the flame acceleration criteria, to improve the models of combustion implemented in the computer codes and to extend their validation domain;
- (WP3, led by CEA/DEN) perform experimental and numerical studies of the concrete and metallic structures response to hydrogen explosion loads. Various loads corresponding to various modes of combustion will be studied. The results will be used to improve the knowledge on dynamic effects generated by combustion on concrete and metallic structures;
- (WP4, led by CEA/DRT) develop a prototype device for in situ and real time measurement of gas composition inside the reactor containment in case of severe accidents.
The applied topics’ objective is the enhancement of safety measures related to hydrogen risk based on the knowledge gained from the fundamental topics WP1 to 4.
Three areas have been identified for applications:
- (WP5, led by ELTA) aims to industrialize the prototype developed in the framework of WP 4;
- (WP6, led by IRSN) aims to propose improvements of the practices and doctrines adopted by manufacturers to prevent hydrogen explosion risk in their own installations;
- (WP7, led by IRSN) aims to give a complete explanation of the hydrogen explosions having occurred in Fukushima Daiichi NPP.
IRSN is coordinating and leading the project, which includes a university partner, ICARE; institutional partners, IRSN, CEA and the Forschungszentrum Jülich in Germany; and a partner from industry, Areva-Elta. Three industrial companies, EDF, Areva and Air Liquide, are providing funding for the project.
The project is planned to last five years (2013-2018). Initial testing will begin in late 2014.