Last update on October 2013
The corium-water interaction (ICE) project, coordinated by IRSN, was selected by the French National Research Agency (ANR) during a call for projects by the “Tomorrow’s Nuclear Energy” investment in the future program. Starting in late 2013, it should last five years and end in 2018. Its goal is to improve knowledge of phenomena that may occur during a corium flow in water during a core melt accident in a nuclear reactor. It will improve knowledge of the phenomena involved and the predictive capacity of the MC3D software, a numeric simulation tool developed by IRSN. Project partners include:
IRSN: responsible for modelling and development of the MC3D software;
CEA: responsible for experiments in the KROTOS
LEMTA (University of Lorraine): responsible for analytical experiments and modelling work;
EDF and AREVA: along with ANR, are co-funding the project.
One of the significant risks during a core melt accident is the explosive interaction between the corium (mixture of molten fuel and structural materials) and water such as occurred in three reactors at the Fukushima nuclear power plant in 2011. Such a “steam explosion” interaction resembles a detonation and the energy released may be sufficient to degrade the reactor containment.
This type of interaction is difficult to control, but understanding and characterising the underlying phenomena are essential for proper management of the later phases of an accident. The objective of the ICE research project is thus to better understand all of these phenomena in order to identify methods for limiting the consequences.
For more than 20 years, IRSN has committed significant research efforts to this issue. It has supervised numerous experimental programs at the CEA and continues to play a significant leadership role in advancing international scientific research on subject. In particular, as part of a joint effort with the CEA and EDF, IRSN develops and improves fuel-coolant interaction models used in the MC3D Computational Multiphase Fluid Dynamic (CMFD) software. These models better integrate the results of previous experimental programs, particularly results from the OECD’s SERENA program. Nevertheless, certain aspects of the problem remain poorly understood or modeled.
Project objectives and procedure
The series of physical phenomena leading to a steam explosion is known: as soon as it enters the water, the corium jet is fragmented into droplets and causes intense vaporization of the water, which in turn causes a shock wave which causes the corium to become even more fragmented, further increasing the contact surface with the water. The phenomenon propagates with a cascading effect, developing an explosive process and producing a large quantity of steam. Initiation of an explosion is not assured, but depends largely on the fluid contact conditions and their physical properties: in an initial mixture (or pre-mixture), the corium must remain “liquid” in order to be able to be fragmented. It must also besufficiently dispersed without provoking vaporization that is too intense so that water remains close to the corium droplets.
The ICE project has four main objectives:
improve modelling of the fragmentation and dispersion of corium jets in the water during “pre-mixing”;
characterise and model oxidation effect of corium during the premixing and during the explosion : no computer tool can currently simulate this phenomenon in anticipated conditions;
improve the characterisation of the corium solidification, phenomenon that limits fragmentation and thus explosion;
validate or improve modelling of the explosion phenomenon: there is still uncertainty about the process controlling formation of the shock wave.
More generally, the goal of the project is to improve the capacity of the MC3D software so that it can realistically predict, on the scale of the reactor, all significant phenomena involved. The research includes theoretical and experimental stages. With the experimental facilities to be used, both “integral” and “analytic” experiments can be performed. Since the integral experiments are performed with corium in the CEA’s KROTOS facility, it is possible to study all phenomena involved in the most realistic conditions possible. The analytical experiments provide the opportunity for more in depth analysis of each of the phenomena concerned. The IRSN coordinates the entire project, develops models and integrates the models developed in the MC3D software. The CEA will supervise experimental programs using corium and associated analyses. Its main mission is to expand the experimental database of the properties (thermodynamic, physical and chemical) of corium resulting relating to the impact on the physical phenomena involved. The University of Lorraine’s Laboratory of Energetics and Theoretical and Applied Mechanics (LEMTA) will oversee experimental study and improving the modelling of the corium jet fragmentation.