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0 pointsonenukecourse4y ago0 comments

Disclaimer - I only took one reactor course 17 years ago.

The rods soak up neutrons. the reactor is designed so that its power would increase if the rods weren't there. The rods are what keeps the k == 1 (basically how the neutrons change in time. K == 1 is no change).

So don't think of the rods as a throttle valve in your car, i.e. this throttle angle corresponds to this much torque and therefore power. The rods are best thought of as controlling the rate of change of reactor power.

This is why the reactor is built so that the neutrons increase when fully withdrawn - because you have to start it! If the reactor is being started, the rods are withdrawn until the power wanted is achieved. Then the rods are lowered.

Note this is a far more complicated problem; this is a very serious operation because at the very start, except for your neutron starter, you only have "prompt" neutrons that can very quickly cause an increase of power. Once the reactor has stabilized the "nuclear waste" inside it releases non-prompt neutrons that are part of the neutron budget and are easier to control.

So, while waste in the reactor is being accumulated and while the fuel burns up, the position of the rods to achieve stable power changes! Furthermore these changes are different in different parts of the reactor so each rod is individually changed (actually I think there are two sets of rods, not all of them are individually actuated)

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retzkek4y ago

I have a degree in Nuclear engineering, although I haven’t been practicing in a while. Your explanation is accurate and well described.

> actually I think there are two sets of rods, not all of them are individually actuated

commercial reactors have 5-10 “banks” arranged symmetrically, although only a subset of those are typically used during power operations [1], so you weren’t far off. Rods are rarely controlled individually, the only case that comes to mind is for identifying and then limiting the reactivity in a “leaker” fuel bundle (one where the cladding has been breached by a foreign object, so it is leaching contamination into the coolant - this is undesirable but expected).

[1]: edit for more information: PWRs and BWRs differ substantially here:

PWRs primarily use boric acid in the coolant to limit reactivity at the beginning of a fuel cycle, which is diluted out over the course of the cycle. One bank of rods is used for fine reactivity control. This gives them a generally even power distribution across the reactor.

BWRs primarily use their control rods, so start the fuel cycle with substantially more inserted. A significant part of designing the fuel layout for a cycle is also designing the control rod patterns to be used over the cycle, to control excess reactivity while maximizing fuel use. Power distribution across the reactor can vary wildly, so it’s a much more challenging engineering problem.

I just remembered I actually have a detailed visualization illustrating (modeled) power distribution in a BWR. You can easily see where control rods are inserted and how that has shifted the power around. https://www.kmr.me/posts/coremap/#content

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