Verification Of Thermal Stratification In The Reactor Pool Of SMR

Thermal stratification,due to the temperature gradient and buoyancy induced flow,is a common scenario in the passive heat removal systems(PHRS)of nuclear power plants where the fluid initially is stationary and the heat is to be removed by natural convection.This may occur in the reactor pool,which is a PHRS,of Small Modular Reactors(SMR)during main steam line break(MSLB)or loss of coolant accidents(LOCA).Since the containment will be releasing heat to the reactor pool in case of an accident,therefore it is important to study this natural flow field for any potential thermal stratification.NuScale is one of such kind of small modular reactor which has a broad development prospect as a well-designed light water reactor.The containment vessel of this reactor rests inside a pool of water,to cool down the steel containment naturally in case of any accident.Both,the experiments and simulations were performed in order to carry out the studies on the natural circulation inside the reactor pool.A scaled facility of approximately 1/21th of the size of an actual single module of the NuScale was used for experimentation.The decay heat to the reactor pool was provided by installing heating rods inside the containment.Different layers of thermocouples were employed in various locations inside the reactor pool and along the height of the small containment to measure the temperature in the key regions.To measure the velocity field inside the pool,Particle Image Velocimetry(PIV)technique was utilized.Same parameters were simulated in CFD.Simulations were accomplished using the commercially available CFD software ANSYS Fluent by employing various turbulence models.The experimental results from thermocouples and PIV showed that there exists a temperature gradient in the pool water that hinders the movement of the fluid naturally.The regions of high velocity were above the containment doom and along the heating wall.Numerical analysis showed that the k-s turbulence model gave quite close results with the experimental values.The temperature and velocity distributions validated the presence of thermal stratification in the cooling pool.