Since last one decade production of ‘green’ and ‘clean’ energy is the main focus of research among the researchers and scientists. In this regard several scientists have worked on alternative energy resources like:  solar energy, wind energy, nuclear energy etc. However, in present scenario, these sources only contribute near about 15-20% of the total energy demand and there is an urgent need to improve the contribution of these sources for power generation in order to reduce the emission of greenhouse gases. However, economics is main hurdle to make this dream sustainable.  Among the available alternative sources, nuclear energy is the most viable one, which can overawe the ghost of greenhouse gas emission and have potential to provide economical and sustainable power. In addition, it requires small amount of fuel and generate small quantity of waste for the disposal. However, the capital costs of nuclear plants are quite high compared to the fossil fuel based power plant. The other major concern related to nuclear based power is safety of the plant.  To overcome these issues Generation IV nuclear reactors have been introduced with safe, reliable and economical nuclear technology. Liquid cooled pebble bed reactor (PBR) is the one, being preferred among the six other Generation IV technologies due to its high thermal efficiency. Graphite moderated liquid cooled PBR is a combination of two older technologies, which uses 6 cm diameter TRISO particle as fuel and a molten-salt (generally a combination of different fluoride salts) as a coolant. Molten salts like lithium-beryllium fluoride salts, sodium-zirconium fluoride salts etc. are considered to be the primary coolant. In liquid cooled PBR, both pebble and liquid flows upward in co-current fashion against gravity. Further, density of molten salt (~1900 kg/m³) is slightly higher than the density of fuel pebbles (~1800 kg/m³), therefore, pebbles naturally float on the liquid against the gravity under moderate buoyancy. During its path pebbles transfer their heat generated through the nuclear fission to the coolant. Pebbles are recirculated to the reactor in a controlled manner for maximum utilization of fuel pebbles. The heat transfer between pebbles and coolant strongly depends on the relative distribution of pebbles and their velocity profile in the selected coolant. Therefore, detail understanding of pebbles flow and their interaction with the coolant inside the reactor is required for the optimal design of liquid cooled PBR. However, most of the studies conducted on liquid cooled PBR are mainly focused on radiation physics and pebbles preparation and basic hydrodynamic studies of these reactors are still missing.

In current work, a hydrodynamically scaled down cold flow model of liquid cooled pebble bed reactor is developed. In cold flow model water is used as a coolant and 1 inch polypropylene ball of density 839 kg/m³ is used as pebbles. Both liquid and solids flow co-currently upward against the density. Liquid is separated from the top of the column and pebbles are recirculated back to the column at a controlled rate (1 to 10 pebbles per minute). To understand the effect of liquid inlet velocity and pebble circulation rate on pebble distribution inside the reactor, experiments are performed for different liquid inlet velocities and circulation rate. Gamma-ray densitometry experiments are performed to find the distribution of pebble in the bed across the radial and axial positions. Further, dense discrete phase model (DDPM) of ANSYS 14 is used to model the flow phenomena in liquid cooled PBR. Particle-particle interaction in DDPM is model by spring-dash pot model. Simulations are conducted for different liquid velocities. Finally simulation results are validated against the experimental data.