The way it was explained to me, is that the graphite would typically slow the reactivity, but because the tips were in the reactor they were themselves irradiated. When they're inserted they bring it down, but there would be an initial surge from the tips going in. This could be fixed by not having the ends of the rods in, but would be more mechanically complicated.
forbiddenthought t1_jc3vuo7 wrote
Reply to Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite? by Figorama
The way it was explained to me, is that the graphite would typically slow the reactivity, but because the tips were in the reactor they were themselves irradiated. When they're inserted they bring it down, but there would be an initial surge from the tips going in. This could be fixed by not having the ends of the rods in, but would be more mechanically complicated.