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Content archived on 2024-04-19

Molten fuel/Coolant interaction


The consequences of releases of large amounts of corium into the lower plenum of the reactor pressure vessel (e.g. steam explosions) have been investigated in many severe accident studies. First estimations from separate effects studies, like fluidization, concluded that some hundreds of kgs could trigger a severe molten fuel coolant interaction (MFCI), while more recent results from multi-D codes indicated quantities in the order of tens of tons. The amount of melt involved in a MFCI is mainly determined by the premixing phase for which the necessary constitutive laws (fragmentation of corium jets and drag laws) are still poorly defined for three phase mixtures (corium, liquid water and vapour). After the initial premixing phase the MFCI can develop in the propagation phase due to some triggers, including fine fragmentation and subsequent steam explosions with detonation waves.

The project includes several analytical and experimental studies, going from simple to more complex configurations, to be used for premixing and propagation code validation with a series of newly developed multidimensional/multicomponent codes. The study of the fuel fragmentation is undertaken with droplet experiments where the transition is analyzed from the thermal fragmentation, observed at the onset of the MFCI, to the hydrodynamic one, expected when a pressure wave travels through the mixture. Results of this and other specific experiments are being used to improve the initial conditions for detonation codes in the propagation phase modelling.
In order to validate multidimensional Premixing Codes, experiments using solid spheres were decided in order to get rid of fragmentation problems and so concentrate on drag and heat transfer.
During this year, many experiments using isothermal spheres of steel or aluminium were performed and analysed in the BILLEAU cold facility (CEA). A report giving the conditions of the spheres (velocity, volumetric concentration) just before entering the water and the evolution of the ball jet into water was sent to all the partners.

Development of the BILLEAU 1000 and BILLEAU 2200 was pursued and some preliminary tests performed in the BILLEAU 1000 facility.

In KfK, the construction of the QUEOS facility where 7 ltrs of hot (approximately 2600 K) spheres will be dropped into saturated water is finished and preliminary tests performed in order to check the measurement devices (temperature of spheres, vapour production)
The spheres will be made of Molybdenum coated with Rhenium in order to avoid oxidation. Their manufacturing has been tested.

From AEA, results of two experiments MIXA 0.1 and 0.6 were distributed to the partners in order to be recalculated with Premixing Codes. These experiments are similar to the previous ones, except for the fact that the arriving droplets are in a molten state: they are made of UO2/MO at about 3500 K and water is saturated.

In Oxford, design and construction of the equipment for the measurement of drag and heat transfer in gas-liquid flow through arrays of spheres is under progress.

In KfK, a test section named PREMIX is under construction and will use about 10 kg of thermite generated molten alumina (from 2313 K to 2900 K) which will be dropped into saturated water. A new melt generator has been designed allowing for a better separation of these thermite materials. Qualification of measuring devices (steam flow, pool level swell) has also been performed.
Exploring tests were also performed without the observation of explosions. These experiments are of the FARO type but are expected to give finer measurements useful for Code Validation.

Droplet fragmentation under strong water flow

In IKE, an apparatus in order to study the transition between thermal and hydrodynamic fragmentation of melt drops during the escalation of the propagation phase has to be rebuilt in order to be used with higher temperature melt (> 1270 K). A new triggering device necessary to produce the strong water stream has been constructed.

Validation of 2D premixing codes using corium droplets

This task was performed:

- in AEA with CHYMES
-in KfK with IVA3
- in CEA with MC3D (ex TRIO-MC)
- in ENEL with IFCI

During this year, these codes have been used to precalculate and/or to recalculate the two tests already performed in the FARO facility i.e.:

- FARO Scoping Test 1: 18 kg of melt
- FARO Quenching Test 2: 44 kg of melt.

Jet fragmentation modelling

This work consists in the improvement of the already existing model IKEJET (IKE) and the development of a CEA model.

In IKE, extended calculations of experimental cases with and without film boiling have been performed.

A theoretically based correlation for the jet breakup length has been developed justifying the use of the Froude number (first used by Saito). No global agreements can be found for the jet breakup length and the fragment size. At the same time, improvements in the numerics of the calculation of the perturbation complex wave velocity has been performed taking into account the turbulent velocity profile of the surrounding flow.
Work programme

A state-of-the-art report is being produced to cover some key areas, such as the premixing, the triggering and the fragmentation mechanisms as well as the fuel-coolant heat transfer and the achievable energy conversion rates obtained from the analysis of experiments. The results coming from the premixing experimental programme are used to validate in three phases the constitutive laws needed to describe the premixing of corium and water. CEA-DRN/Grenoble is validating the MC-3D code against specific premixing experiments in their BILLEAU-2200 facility (5 litres of solid oxide spheres at temperatures up to 2500 K) and against global experiments of the FARO facility of JRC Ispra. A special model is also developed for the thermally induced fragmentation resulting from vapour film destabilization. KfK is validating their IVA3 code against premixing experiments at their QUEOS facility (10 kg of Mo spheres at temperatures up to 2600 K), against jet experiments at their PREMIX facility and against propagation experiments performed with simulant materials at low and high temperatures, like those of the KROTOS facility of JRC Ispra. IKE/Stuttgart is improving their IKEJET code (to be implemented into the COMETA code of JRC Ispra) by developing a jet break-up model under film boiling conditions using a 3-layer approach (melt, water and vapour film). They are also improving the hydrodynamic fragmentation module of their 1D-IDEMO detonation code. Detonation calculations are also performed by ENEL-VDN/Milan in the IFCI code (from SANDIA Nat. Lab.), using a new adaptative meshing technique, in connection with some KROTOS experiments. AEA/Winfrith is further developing their codes for premixing (CHYMES) and for propagation (CULDESAC) on the basis of specific FARO and KROTOS tests at JRC Ispra. The University of Oxford finally is performing very fundamental experiments for heat transfer and drag laws in a 2-phase flow through a rigid array of hot spheres at 800 C.

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Commissariat à l'Energie Atomique (CEA)
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Centre d'Études de Grenoble Avenue des Martyrs
38041 Grenoble

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