Ridley_Himself t1_jce1e0z wrote
Reply to comment by Hiddencamper in Why were the control rods in the reactor featured in the HBO series 'Chernobyl' (2019) tipped with graphite? by Figorama
So, if I'm reading this correctly, the design flaw is that, if you need to insert the control rods to kill a reaction, the graphite tips have to move past the portion of the fuel with the highest rate of reaction?
Hiddencamper t1_jcewc2g wrote
The “design flaws” are:
Reactor design with active boiling and positive void coefficient, such that your power profile is essentially inverse to your normal control rod position. Additionally you are severely impacted by things like trips of a reactor coolant pump.
No mechanical limits on location of the graphite followers (not just control system limits, but a physical hard stop)
The graphite followers having to move past the fuel is the result of the two above plus the operators making some very dumb decisions.
If those graphite followers were never removed as much as they were, if they essentially stayed in the lower portion of the core, they would have been fine. But when you pull them out far enough, then during the reactor trip they will initially add reactivity.
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