A large size bubble introduced into the region of the core that has a positive void reactivity worth, could give rise to more than one dollar’s worth of reactivity, and such a bubble could enter at a rate that might give rise to a very rapid reactivity rate of addition.

Section 4.2.1 discussed the sources of bubbles in some detail and showed that the likelihood of a large coherent bubble’s arriving at the core inlet plenum was very remote. A bubble of tens of liters in size would be required before large reactivity ramps would result, and good design of the primary system and its cover gas surfaces prohibit the appearance of such a bubble.

Thus, although it is indeed necessary for the plant design to consider gas entrainment and accumulation very seriously, a reactivity addition from the introduction of external voids into the core is not a credible CDA initiator.