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Enhancing Nuclear Safety


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Research programs

MITHYGENE project

Last update on April 2019


The 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.
 
MITHYGENE 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

During 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.
 
Hydrogen 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 safety-related equipment.
 
Various 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.
 
The 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 hydrogen.
 
An 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:
  • WP1: perform experimental and numerical studies on hydrogen distribution taking into account the effect of mitigation measures. The results of these studies have helped to improve the performance of computer models in predicting situations that are not addressed by national and international programs;
  • WP2: perform experimental and numerical studies of hydrogen flame propagation, taking into account the impact of water vapor, the presence of water droplets, and thermodynamic conditions. The experiments carried out have made it possible to finesse the available data using detailed instrumentation and to make up for the lack of data on hydrogen flame propagation in wet environments. The numerical research has improved the combustion models in the computer codes and extended the scope for validating them;  
  • WP3: focused on performing experimental and numerical studies on the resistance of structures following hydrogen combustion. Various modes of combustion have been studied with a view to improving our knowledge of the effects of combustion on structures;
  • WP4 4 focused on developing and qualifying prototype instrumentation for real-time in situ measurement of hydrogen concentrations in the reactor containment atmosphere under severe accident conditions.


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.
 

Extension (2018-2021)

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.


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Characteristics

Dates: 2013-2021

Partners: CEA-DEN, CEA-DRT, ICARE, Forschungszentrum Jülich, ARCYS

Sponsors: Air Liquide, EDF    

Involved IRSN laboratories

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