Precalculations of the onset phase of natural circulation in a sodium loop

One-dimensional numerical predictions of the onset phase of natural circulation in a sodium loop are compared to actual measurements in the JANUS test section. Two tests are considered: one with favourable initial conditions - i.e. positive buoyancy head - the other one with adverse initial conditions - i.e. negative buoyancy head. Both give rise to thermal stratification in the pipe system after a thermal transient in the air cooler. The discrepancies between the 1-D model and the experiment are discussed and an attempt to improve the quality of the prediction with multi-D models is proposed