Last update on April 2019
MITHYGENE project (to improve our understanding of the hydrogen risk and how to
manage it under severe accident conditions), launched in early October 2013, is
one of 23 projects awarded funding as a result of call for projects on nuclear
safety and radiological research under France's ’Investment
in the Future’ program. It aims to improve our understanding of the hydrogen risk - i.e. the risk that the hydrogen
produced during a PWR core meltdown explosion may explode - and how to manage
that risk under severe accident conditions.
is focused on better understanding the phenomena governing this risk, and on
developing prototype instrumentation for measuring gas concentrations suitable
for use under the conditions produced in the reactor containment during a
severe accident. Advances made on the project are being used to improve severe
accident management procedures and will also be used in explaining the events
that occurred during the accident at Fukushima-Daiichi. The findings of the
project should also help improve the practices adopted by industry operators to
eliminate the risk of hydrogen explosion at their own facilities, be they
nuclear or non-nuclear.
Background and objectives
a core melt accident, hydrogen may be produced due to oxidation of metals as
the accident progresses. This mainly occurs when the reactor core becomes
degraded and the metal cladding around the fuel rods oxidizes under the effects
of steam. It also occurs if the corium (magma resulting from core meltdown)
pierces through the reactor vessel; metals in the corium are oxidized by the
gases released during decomposition of the concrete of the foundation raft,
located beneath the reactor vessel.
released inside the containment can build up in certain areas and lead to the
formation of clouds of inflammable gas. If these clouds then combust, they
generate pressure and temperature loads liable to damage the reactor
containment structure and the equipment contained within it, including
types of hydrogen explosion occurred during the accident at the
Fukushima-Daiichi Nuclear Power Plant in 2011, raising the issue of assessing
this risk for the French nuclear power plant fleet and of whether the available
measures implemented to protect against explosions and mitigate their
consequences are adequate. In pressurized water reactors (PWRs) in France, the
design combines large-volume containments with the installation of passive
autocatalytic recombiners, which have been installed in all units in the French
nuclear power plant fleet since 2007. In spite of the performances indicated
for these recombiners, the studies carried out by IRSN highlight the difficulty
of demonstrating that the formation of a hydrogen-oxygen mixture liable to
result in local flame acceleration phenomena can be excluded at any time and at
any point within the reactor containment.
hydrogen explosion risk therefore needs to be integrated into severe accident
management guidelines drawn up by nuclear power plant operators setting out
procedures for implementing safeguard systems. For example, the use of the
Containment Spray System, designed to lower pressure and reduce fission
products in the reactor containment in the event of an accident, may
"de-inertize" the containment due to steam condensing on water
droplets; this could lead to the formation of an inflammable cloud. In
addition, turbulence caused by the spray of water droplets may promote flame acceleration
in the event of combustion. Trying to find a balance between the negative and
positive effects on safety has resulted in modifying management of the
containment spray system under severe accident conditions, delaying its use to
allow the recombiners a chance to significantly reduce concentrations of
in-depth understanding of the phenomena involved is therefore a key factor in
optimizing the measures taken to manage an accident situation. This observation
was highlighted in the ASN's report on the stress tests performed following the
Fukushima-Daiichi accident. Apart from the two issues mentioned above, and
which are acknowledged by the community of experts, the report demonstrates the
need to study the risk of hydrogen explosion in the annulus space between the
two walls of the reactor containment in the 1300 MWe series and in the
containment venting/filtering systems (U5 filter).
Program overview and areas of research
The MITHYGENE project, which is coordinated and led by IRSN, is divided into three phases, each focused on a number of research topics. The partners involved in the project are a university partner - the Institut Icare - institutional partners - IRSN, the CEA and the Forschungszentrum Jülich in Germany - and an industrial partner, Arcys. Two industrial players sponsor the project: EDF and Air-Liquide.
Phase 1 (2013-2016)
The objective was to improve the predictability of computer codes used to assess the risk of hydrogen explosion and develop real-time in situ instrumentation for measuring gases, qualified for severe accident conditions.
Four research areas were identified:
Phase 2 (2016-2018)
More practical in nature, Phase 2 aimed to apply the knowledge gained from Phase I.
Three areas of application were identified:
- WP5: laid the groundwork for industrializing the prototype Raman spectrometer for measuring hydrogen concentration developed within the framework of WP4;
- WP6 focused on summarizing the results obtained and improving industry practice relative to managing the risk of hydrogen explosion at industrial facilities;
- WP7 aimed to give a full explanation of the accidents that occurred at the Fukushima-Daiichi NPP.
The project was initially intended to run for five years (2013-2018) but has been extended for a further three years with a view to industrial roll-out.
The MITHYGENE project has been extended in order to move on from the lab prototype developed during the second phase (WP4 and WP5) to producing the first industrial measurement prototype. This will provide real-time information on the composition of the atmosphere inside the containment and in the annulus space between the two containment walls under severe accident conditions. These studies will be performed by a consortium made up of IRSN, CEA-LIST and Arcys.
Four new areas for research and development have been identified:
- WP1 will focus on drawing up detailed technical specifications and the qualification program for the industrial measurement instrumentation, based on the research carried out in the two previous phases of the project;
- WP2 aims to develop the Raman instrumentation (a probe and its related optoelectronics system) around which the equipment for measuring hydrogen concentrations in the containment atmosphere will be based.
- WP3 will concentrate on validating and qualifying the complete industrial measurement system;
- WP4 will then transfer the system to industry as a risk management tool. This will entail proposing, based on studies of different scenarios, improvements to severe accident management procedures by including information on the composition of the containment atmosphere.