Last update on February 2020

Water-corium interaction and steam explosion: the ICE project’s aims

One of the greatest potential risks during a nuclear reactor core meltdown accident is the explosive interaction between the corium (a mixture of fuel and melting structural material) and the water present. This type of interaction, known as a steam explosion, is similar to a detonation and could release enough energy to damage the internal structures of the vessel in the area of the reactor and, consequently, the containment. This would be more likely to occur during the phase when the corium is transferred from the vessel to the reactor pit, if it contains water.

The development of the physical phenomena leading to a steam explosion is well understood. Two phases normally come into play. The first is the "pre-mixture" phase, in which the corium jet escaping from the reactor vessel fragments into drops on contact with the water, which then vaporizes to a greater or lesser degree. This usually results in the creation of a "bed of debris", due to the rapid solidification of the corium in the water. Under certain conditions, which have not yet been sufficiently characterized (and are, therefore, difficult to predict), a second "explosion" phase may occur, in which various complex local phenomena (fragmentation, water encapsulation, etc.) may cause excessive pressure, breaking up the corium into finer fragments and increasing its contact surface with the water. This results in faster boiling and a renewed increase in pressure locally, leading to the same sequence: the phenomenon spreads gradually and escalates into an explosive process similar to a detonation. The triggering of the explosion is largely dependent on the contact conditions of the fluids and their physical properties.

The main objectives of the ICE project were:

• to better understand and model the fragmentation and dispersion of fuel jets in water during the so-called "pre-mixture" phase;
• to characterize and model the oxidation effect of the fuel jets;
• to better understand and model the heat transfer and boiling processes during the explosion phenomenon.

Initial findings

The initial phase of the project saw significant progress made, particularly in terms of the knowledge acquired on corium fragmentation. Accurate digital simulations (Figure 1) using Gerris code provided a better understanding of the fragmentation processes when a drop of liquid is added to another liquid. They also revealed fundamental differences in the (better documented) scenario of drops present in gas. These findings were confirmed by a series of experiments involving the fragmentation of drops of a liquid metal with a low melting point in water (carried out by the LEMTA laboratory in Nancy using GALAD, the on-demand drop generator), as well as a detailed analysis of a new corium jet fragmentation test in the KROTOS installation (performed at the CEA).

This particular test in the KROTOS installation also yielded data on the size distribution of the corium drops and enabled researchers, for the very first time, to characterize the conditions under which a bed of debris forms and evolves.

Figure 1: Example of the simulation of a drop in a liquid medium using Gerris code: demonstration of the interaction between the drop (gray) and the vortexes (blue) created in the ambient fluid.

The project team gained a far greater understanding of the oxidation of the fuel jet melted by the steam, by performing a direct digital analysis with a specific application called MESO (see Figure 2), a component of the MC3D calculation software, and by carrying out a new, dedicated test in the KROTOS installation. Although the results are yet to be consolidated, they tend to indicate very fast oxidation processes, which are not restricted by the distribution processes in the corium. This specific phenomenon is due to the very high temperatures at play.

Figure 2: Example of the simulation of pseudo-boiling processes in super-critical conditions (P = 240 bars) around a fixed cylinder, using the MC3D-MESO model. Colored background: fluid density, with low densities corresponding to pseudo-steam.

Finally, a detailed digital simulation of the boiling around fine fragments of corium under super-critical pressure (above 220 bars and characteristic of an explosion), again using the MESO model in the MC3D software, enabled researchers to develop a more thorough description of the processes at work and the amount of steam generated.

This work, together with the fragmentation experiments, led to the conclusion that the MC3D software’s explosion model could be improved and the principles for developing a new model were proposed (see below).

In order to overcome the shortcomings of the MC3D software, identified in light of this new knowledge, an extension to the ICE program was agreed to allow the application to undergo major structural revision. This new version (V4) will need to be based on a new and more flexible application (model) construction architecture. It will also need to improve its 3D reactor simulation capabilities and significantly reduce its computation time, through better meshing and by performing simultaneous calculations.

The extension to the project has also paved the way for a new thesis at the LEMTA laboratory, involving the use of the new JEDI (two-phase jet) installation, which should enable the observation and analysis of the fragmentation and dispersion of a liquid metal in water, specifically using advanced optical techniques including PDA (Phase Doppler Anemometry) and DUAL PIV (Particle Image Velocimetry). A second thesis, funded by the LEMTA laboratory itself, will add to this work by focusing on the modeling of the physical processes observed during the JEDI experiment.

Finally, the IRSN also began funding a thesis in 2019, the aim of this research being to propose an improved model of the explosion phase, in line with the conclusion drawn from the ICE project's previous work.