Bypass flow computations using a one-twelfth symmetric sector for normal operation in a 350 MW prismatic VHTR

Johnson, R.*; Sato, Hiroyuki  

Calculations are performed using CFD for flow of the helium coolant in the gap and coolant channels along with conjugate heat generation and heat transfer in the fuel compacts and graphite of a 350 MW prismatic reactor. Various scenarios are computed by varying the gap width from zero to 5 mm, varying the total heat generation rate to examine average and peak radial generation rates and variation of the graphite block geometry to account for the effects of shrinkage caused by irradiation. It is shown that the effect of increasing gap width causes increased maximum fuel temperature while providing significant cooling to the near-gap region. The maximum outlet coolant temperature variation is increased by the presence of gap flow and also by an increase in total heat generation with a gap present. The effect of block shrinkage is actually to decrease maximum fuel temperature compared to a similar reference case.