Meliloo-stand: investigating freezing in a lead pool

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SESAME Newsletter #2 (February 2017) 

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Meliloo-stand: investigating freezing in a lead pool

Figure: Experimental vessel (right) of the Meliloo – Stand facility with the different obstacles to be inserted during the experimental campaigns (left), located at the CV REZ Research Centre in Rez (Czech Republic)
Figure: Experimental vessel (right) of the Meliloo – Stand facility with the different obstacles to be inserted during the experimental campaigns (left), located at the CV REZ Research Centre in Rez (Czech Republic)

Freezing of the coolant in liquid metal cooled reactors is identified as a potential safety issue. In liquid metal reactors, the highest risk of coolant freezing is thought to occur during shut-down, maintenance, refuelling, and whenever an overcooling from the secondary system or the (passive) emergency cooling system is established. However, solidification might be expected even in normal operation in case there will be any flow stagnation zones in the vicinity of cooled surfaces. The local solidification of the liquid metal will affect the natural convection pattern in the pool with possible impairment of the core cooling.

The Meliloo – Stand facility is being designed to investigate the solidification process at a reduced scale but with a geometry relevant to the pool HLM reactors.

The test facility Meliloo was originally designed for testing the thermal-hydraulics and corrosion processes of the Pb-Li eutectics. The loop was modified for SESAME needs to the Meliloo-stand, which works with pure lead. The main functions of the facility are testing, controlling and checking of the solidification process under different working regimes and collecting of the experimental data. The operating regimes of the facility are driven by controlled heating power and air cooling.

The main experimental vessel is a cylindrical pool of internal diameter and height about 30 cm. Air cooling is provided to the external side while heating is provided by heating rods at its centre. This configuration allows to generated a natural convection pattern, and by adjusting the balance between air cooling and rod-heating, to develop a freezing front from the lateral border. Different internal obstacles will be inserted in turn to study their effect on the evolution of the solidification pattern. The geometrical pattern is essentially axial-symmetrical, however by selectively blocking the cooling on a specific sector, the induced natural convection pattern will result un-symmetrised.

The vessel is heavily instrumented with thermo-couples to monitor the evolution of the freezing front. The free-surface level is monitored. Remarkably, by fast draining, opening of the cover and withdrawing of the obstacle, the solidification front will be scanned. The analysis of the measurement and their processing will be performed in such a way to give a quite complete set of reference data. These data will be used for comparison, improvement and validation of the numerical modelling being performed  in parallel.

The design facility has been finalised during 2016 and its construction concluded early 2017. The experimental campaign will proceed during this year.