How exactly do control rods in a nuclear reactor capture neutrons and slow down the fission occurring?


A nuclear reactor is essentially just a metal barrel with a bunch of neutrons whizzing about and causing some metal to fission and release even more neutrons, right? So how does the addition of a non-metal such as boron or a metal like cadmium into steel prevent the atoms within the control rods from destabilized as well? Is it something to do with the molecular structures of the resulting alloys?

In: Physics

The reactive materials are in thin rods, so a lot of the neutrons leave one rod and enter the next one.

The control rods sit in between them.

So control rods can’t do anything about the neutrons inside the solid rods of fuel, but they’re kept below critical mass on purpose. (Smaller than critical).

So the rods control how much reactivity you allow to travel in between the solid rods.

The maths and the how is a bit complex, but the mechanism is simple in theory.

Simple answer really, the maths is hard but the

It doesn’t really have much to do with Boron or cadmium being non-metal, they work to slow the reaction because they are good neutron absorbers. They happen to be isotopes that will readily grab a free neutron into their nucleus without much complaint. The amount of neutrons you have flying around is directly related to the reaction rate, so absorbing more of them is a good way to slow things down.

It’s like shooting cannonballs at a target in an empty field vs in a forest. The control rods go between the fuel rods. Fission still is undergoing in each fuel rod, but there aren’t as many neutrons flying around between rods.

Yes at an atomic level, those alloys can absorb extra neutrons.

Slightly less ELI5 than others: In a reactor you deal with 4 types of materials: neutron source (the fuel rod), reflector (neutrons bounce off it), moderator (slows neutrons down), and absorber (neutrons sink in it, heating it but never leaving).

The fuel rod will heat up quite a bit all on its own, but it will not reach any dangerous temperatures; most of the neutrons from natural decay it emits are too fast, too high energy, that they pass without interacting with nuclei of other atoms. Matters of quantum physics, the energy must be just right, not too low, not too high.

Add moderator – a material that slows neutrons down – these upon impact into nucleus of the radioactive material in a fuel rod will break it up, releasing more (fast) neutrons. So adding moderator between the rods will make the reaction rate climb rapidly as the number of slow neutrons rises, neutrons from one rod slowed down by passing through moderator hit another rod and cause it to release more neutrons – that won’t do much until they pass through moderator.

A lot of neutrons will just fly off the sides of the reactor. Add reflector, to redirect them back, to sustain and increase the reaction speed. Remove the reflector, the reaction will slow down.

And then, if you want to slow down the reaction, you have the absorber. It will swallow the slow neutrons without producing more in the process.

Different reactors use these four differently. Water, for example, is an excellent coolant but simultaneously a strong moderator. You can’t just remove coolant (everything is so hot it would melt very fast) so you have to get rid of all the slow neutrons flying between fuel rods – by inserting control rods made of absorber. That was old RBMK like Chernobyl; especially with control rods tipped with graphite (“slippery”, good for sliding through holes so the rods won’t jam, and very resistant thermally – but accidentally a very good moderator.)

Or alternatively, have a central rod surrounded with moderator/coolant, and a mirror of reflector all around. It will increase own rate of reaction by bouncing the neutrons off the mirror; if the mirror is lifted the rate will drop. If mechanism of lifting the mirror fails, the mirror will melt (made of material of lower melting point than the rod) and again, the reaction will stop.

Or have a mix of “pebbles” of fuel, moderator, and a special material that is pretty much transparent to neutrons at “operational” temperatures, but when overheated, reversibly transforms into absorber substance. This type of reactor is self-regulating; as it gets hotter the absorber reduces its rate of reaction.

so the way a reactor works at a gcse level is that a neutron is fired into a uranium 235 atom, which then becomes unstable , which then splits, which then releases more neutrons – the new atoms are the things that form nuclear waste. i imagine that the atoms can absorb many neutrons before becoming unstable – they only last for a certain amount of time ( 6 – 8 years) – my gcse explanation