Effect of void propagation to sodium void reactivity in transient analyses of fast reactors with sodium-plenum

The effect of void propagation to sodium void reactivity in fast reactors with sodium-plenum has been investigated. The sodium boiling occurs around the top of reactor core, and propagates in the sodium plenum. In the transient analysis of severe accidents the sodium void reactivity in sub-regions of the reactor is calculated considering the void propagation and it is seen the reactivity is different from the void reactivity postulated that only sodium in a sub-region is boiling. We found that the two results of void reactivity are different by up to twice depending on the location of sub-region in the sodium plenum.