An investigation of thermal stratification phenomena in fast reactors. Sodium experiments and analyses

In fast reactors, the thermal stratification phenomena occur in the reactor vessel when a scram shutdown is imposed to the reactors. The thermal stratification phenomena can induce thermal fatigue in structural components in the reactor vessel. Sodium experiments were performed to study the thermal stratification phenomena. The experimental parameters were Richardson (Ri) and Reynolds (Re) numbers. And two dimensional analyses were carried out under variation of Ri and Re conditions using a multi-dimensional thermal-hydraulic code AQUA with an algebraic stress turbulence model and with QUICK and QUICK-FRAM for convection terms of the momentum and the energy equations respectively. The experimental results showed that the rising speed of the stratification interface was correlated to the ratio of Grashof number to Re rather than Ri. The two dimensional analyses could simulate measured vertical temperature distributions well under the higher Ri conditions. As for the height of the stratification interface, the analyses simulated the experimental trends; the height of the interface became higher as Ri decreased. (author)