System analyses for lead-bismuth-cooled natural circulation reactors

In a natural circulation reactor, many secondary flow vortexes are expected to appear in a coolant system at nominal power conditions. The secondary flow vortex restricts the main flow area and causes the thermal stratification during a transient. Thermal stratification by the secondary flow vortex has an effect on thermal stress in a reactor vessel. Such two-dimensional effects in the natural circulation reactor should be considered in a system analysis. The combined multi-dimensional plant dynamics code (MSG-COPD) was used to conduct the two-dimensional system analysis for the natural circulation reactors. Transient analysis was performed for the STAR-LM designed by Argonne National Laboratory (ANL). In case of a hypothetical loss of heat sink condition, thermal stratification was observed in the lower plenum. However, the stratification was disappeared by modifying the intermediate-shell configuration. The core flow distribution in the 400MWe LBE cooled natural circulation reactor designed by JNC was evaluated by using the code. Natural circulation reactors have an inherent flow distribution characteristic. It means that much flow rate is distributed to the higher power channel inherently by the effect of buoyancy force balance. A simplified design method to evaluate the core flow distribution in the natural circulation reactor is compared with the one-dimensional flow network model, and also with the two-dimensional (2D) MSG-COPD calculation. It was made clear that more large flow rate is distributed to the hot channel than the value evaluated by the simplified design method, because of the non-mixing flow effect at the core exit. (author)