Benefits of natural convection in solid breeder blankets with poloidal coolant channels under LOFA conditions

This paper analyzes natural circulation flow in solid breeder blanket designs with poloidal coolant channels under a LOFA condition. Analyses couple the flow transient behaviors with transient heat transport models. While two-phase natural circulation exists in the system, a homogenous mixture model is used. The results of example calculations performed for an ITER solid breeder design concept indicate that in a 3-loop system with an elevation head (AZ) of 20 m, the removal of the afterheat (energy stored in the blanket elements and decay heat) depends on two-phase natural circulation flow with a quasi-equilibrium flow quality of 26% at the outlet of the blanket segment. However, if the available hydrostatic head is about 15 m or less, the amount of natural circulation flow is reduced due to significant increases of friction and acceleration losses in a two-phase system. To rely on natural circulation flow as an afterheat removal mechanism, the design of coolant system with its flow channels should be addressed analytically and experimentally to permit stable steady-state operation under conditions of the presence of vapor in the coolant channels.