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Old 06-19-2019, 08:26 AM
jjakucyk is offline
Join Date: May 2016
Location: Cincinnati
Posts: 437

Chernobyl Control Rod Graphite "Tips"

It seems the straw that broke the camel's back at Chernobyl was the "positive SCRAM effect" caused by the graphite "tips" on the control rods. Of course they're not really tips (we'll use the term displacers going forward), as they're nearly as tall as the control rods themselves and the reactor core as a whole, with a critical 1.25M column of water above and below. There's a good diagram here: even though the text is poorly translated, and the Vlogbrothers also put together a great video with a similar diagram here: that show the position of the control rods, the displacers, and the relative neutron flux/reactivity distributed over the height of the core.

What I can't understand, not being a nuclear physicist myself is why the neutron flux went so high at the bottom of the reactor when the rods were inserted. The graphs show a huge spike in reactivity when the bottom of the displacers are aligned with the bottom of the reactor core (position "c" in the diagram). I understand why the reactivity goes up there, because neutron-moderating graphite is replacing neutron-absorbing water, but I don't understand why the reactivity goes up apparently an order of magnitude or more compared to what, in my mind, is exactly the same condition as in the center of the reactor. I would think that it would only shift the graph shown in position "a" downward, removing the falloff at the end. Even the "bulge" at position "b" is a bit of a head-scratcher to me because for most of the height of the reactor the displacer is still in the same position it's always been.

Is this a temporal issue, where if the rod insertion was stopped at position "c" under normal circumstances, that flux spike would equalize after some period of time, perhaps even a very short time? Was steam being generated at the bottom of the reactor rather than at the top (how?), causing the positive void coefficient to become apparent here but not yet in the rest of the reactor? Is it because the xenon poisoning was concentrated in the center of the reactor (thus the slight decrease in flux shown in graph "a" at center as compared to 80% top or bottom), so the little bit of extra reactivity caused by the water displacement caused runaway reactivity due to the xenon being burned off? Help me out here. This is the one and only bit of the situation I haven't been able to wrap my head around.