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Old 06-24-2019, 02:35 PM
jjakucyk is offline
Join Date: May 2016
Location: Cincinnati
Posts: 437
Yes I understand the positive void coefficient. It's not about temperature so much as about steam vs water. Water is a weak neutron absorber, and when it's used only as a coolant but not as a moderator (I assume you need a lot more of it to act as a moderator than you find in an RBMK reactor), if it boils the density goes down and thus the neutron absorption goes down, leading to an increase in reactivity and power. That's why it's called positive *void* where void means essentially a steam bubble, or air.

What I'm talking about however is the negative temperature coefficient, which is different. That's simply the process where as the fuel gets hotter, reactivity goes down due to some sort of doppler effect. This is how you can, apparently, increase the output of a reactor simply by increasing the water flow. More flow means cooler water means more reactivity. So in the case of a nuclear submarine, for instance, going from half throttle to full throttle may not require any change in control rod position since the cooler water inflow will by itself increase the reactivity and power.

At Chernobyl, I'm assuming the excursion happened either too quickly for the negative temperature coefficient to matter, or its overall effect was too weak in the face of the runaway reaction.