Study on the Jet Breakup and Fragmentation during Fuel-Coolant Interactions


Student thesis: Doctoral Thesis

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Award date14 Oct 2019


During a hypothetical core disruptive accident (CDA), the molten corium may contact with the coolant pool and creates violent fuel-coolant interactions (FCIs). The injection mode can be classified into coolant injection (CI) and melt injection (MI) modes. If certain conditions are realized, steam explosions (SEs) could occur as a consequence of the FCIs. The thermal energy stored in the corium melt may transfer to the water, and explosive vaporization occurs. For the MI mode FCIs, the melt may be injected in the form of a melt jet and threaten the integrity of Nuclear Power Plants (NPPs), such as Light Water Reactors (LWRs) and Sodium-cooled Fast Reactors (SFRs). 

Steam explosion is a violent multiphase and multiphysics phenomenon, which involves four stages: premixing, triggering, propagation, and explosion. The pre-fragmentation occurs in the premixing stage. Centimeter-sized melt droplets are stripped from the melt jet. In the large FCI experiments conducted in the KROTOS and TROI facilities, a pressure pulse is used to produce an external trigger, which induces vapor film collapse locally, and melt-coolant contact occurs, giving rise to fine fragmentation of the melt (typically fragments <100 μm) allowing rapid heat transfer to the water and subsequent high pressurization. While expanding, this zone produces further fragmentation around it, and this process escalates and propagates to all the premixture. Propagation is a self-sustained process that can reach supersonic velocities depending on the premixture characteristics. In this case, the shock may reach the water surface and generate a hundred MPa scale impulse loading to the surrounding structure. The fragmentation is crucial to this thermal detonation, while the chemical reaction is crucial to the chemical detonation. Indeed, the fragmentation of the melt jet and droplets is the key phenomenon leading to the steam explosion. The melt particle size distribution also has a strong effect on the coolability of the debris bed. However, the mechanism of the fragmentation is complicated and has not been sufficiently clarified, which prevents predicting the consequence of the steam explosion precisely.

Fuel Coolant Interaction experiment has been conducted at the CEA Cadarache KROTOS facility for our research group under the ALISA project. This test aims to study the effect of a reduced water pool depth on the interaction. A benefit of this geometry is that most of the water height can be observed by the new large X-ray visualization system at KROTOS. After triggering, a steam explosion occurred that have ejected a large part of the water and the corium debris outside of the test section. The pressurization induced by the explosion has blocked further jet pour for some time. Subsequently, a second Fuel Coolant Interaction occurred in a shallower pool without any spontaneous explosion. Various types of instability are assessed based on the X-ray image data. 

In order to further investigate the mechanism of the fragmentation phenomenon, the present study employs a commercial CFD code COMSOL for the 2D numerical analysis based on the phase field method. Firstly, the coolant jet injection is studied based on Park’s experiment. The penetration velocity is simulated as a benchmark case. The simulation result of the penetration velocity overestimates the experimental data slightly. Subsequently simulations under different conditions are carried out. At the top of the cavity, the Rayleigh-Taylor (R-T) instability is observed at the interface between coolant and melt. At the bottom of the cavity, The Kelvin-Helmholtz (K-H) instability is observed at the interface between the melt and the entrained air. The fingering instability follows due to the less viscous of the entrained air compared with the melt. As the density ratio increases, cavity penetration velocity decreases. Compared with Ikeda’s correlation and Sibamoto’s correlation, the penetration velocity of the simulation result is smaller. The Froude number effect on the penetration velocity is found to be insignificant.

Furthermore, the melt jet injection and breakup are simulated based on the test performed at the KTH facility. In order to study the hydrodynamic fragmentation of the melt jet, our numerical results are compared with the FCI experiment using Wood’s metal. The simulation result of the dynamic process of the melt jet ejection is in relatively good agreement with the experimental data. The jet breakup length is an important parameter, which is defined by the length of the coherent jet penetrating the water pool. Then the effect of the initial jet velocity, the jet diameter, the density ratio, the Froude number, and the instability theory on jet breakup length are presented. Our simulation result demonstrates the dynamic jet thinning phenomenon due to the K-H instability. Saito’s model is also validated through our simulation results. The numerical and analytical study of the melt jet pre-fragmentation is helpful for the understanding of the premixing stage of the fuel-coolant interaction.

In summary, the hydrodynamic behavior of the melt jet during FCIs is modeled using the phase field method based on the COMSOL software. The simulations under different situations provide the qualitative and quantitative illustrations of the jet pre-fragmentation and coarse breakup. All in all, this work is novel and useful for model development and reducing uncertainties in understanding the FCIs.

    Research areas

  • Nuclear severe accident, fuel-coolant interactions, steam explosion, numerical simulation, phase field method, corium behavior, breakup, fragmentation