How is a Nuclear Reaction controlled?

[MolotovCocktail]

One thing that I have been wondering about was, how are nuclear reactions in a nuclear power plant controlled? It seems to me that one really cannot control the neutrons that cause the chain reaction, and I know that the reaction cannot be stopped once it starts. I'm guessing that it is probably the speed of the reaction that is controlled, rather than the reaction itself. What is it that keeps a nuclear power plant from becoming a nuclear bomb?

[insane_alien]

well, it is actually the neutrons that are controlled. the reactor exists in a slightly super critical state normally. so if there were no controls it would heat up and eventually melt down. so it needs to be controlled.

 

the neutrons emitted from a fission are fast neutrons which are crap for starting more fission reactions. so first they need to be slowed down by a moderator like graphite. then, you need to limit the number of them so you can control the rate , or if need be, shut it down completely.

 

this is done using boron and (cadmium?) rods. these absorb neutrons without releasing more. since you take neutrons out, the reaction slows down. and this is all balanced nicely to keep a constant output.

[Klaynos]

Just too add to IA the 'control rods' can be pushed in and out to slow or speed up the reaction.

[swansont]

well, it is actually the neutrons that are controlled. the reactor exists in a slightly super critical state normally. so if there were no controls it would heat up and eventually melt down. so it needs to be controlled.

 

By definition the reactor is critical; one fission results in, on average, one neutron that induces another fission. The effective neutron multiplication factor is 1.

 

The control rods are one aspect of control, but many plants are also affected by the water temperature, as this affects the efficiency of the moderation (thermalization) process.

 

What really allows control of the reactor, though, is delayed neutrons. A fast neutron from fission doesn't take very long to thermalize and induce another fission — a few tens of microseconds. With that kind of feedback time, there is no way one could control the reactions quickly enough to prevent a runaway reaction. But there are two classes of neutrons produced: prompt neutrons, from the fission itself, and delayed neutrons, which come from beta decays of the fission products. (Some daughters are so neutron-rich that after beta-minus decaying they eject a neutron). The delayed neutron fraction is smaller than 1%. As long as the fractional change in the fission rate is smaller than the delayed neutron fraction, the average cycle time is a reasonable fraction of a second. But if the change in fission rate is greater, one can become (super)critical from the prompt neutrons alone, and the cycle time shortens to that tens of microseconds, and you have problems. (Chernobyl, or SL-1)

 

http://en.wikipedia.org/wiki/Prompt_critical

[insane_alien]

sorry, i should have said slightly super critical when the control rods are removed. sorry bout that. swansont is right in that under normal operating conditions the reactor is only critical.

[MolotovCocktail]

ahh... so it is the flow of neutrons that are controlled. My bad, I was only thinking about the way neutrons flow in an uncontrolled reaction, and not thinking about the control rods used in the reaction process. Thank you very much!

[YT2095]

one thing I have been curious about is the use of Cadmium as a control, it has a very Low melting point 594k (321c) you can melt it with a cig lighter.

how does it survive in a reactor?

[insane_alien]

its in an alloy along with the boron. can't remember what the other stuff in the control rods are made of though.

[MolotovCocktail]

Actually, I do have another question. how do you get rid of the waste? And can spent nuclear fuel be reused?

[insane_alien]

you shut the reactor down and pull out the used rods then send them off for processing. while its empty maintenance is done on the reactor before new fuel is put in.

 

the waste can be processed to get unused fuel back and make it into new fuel rods.

[mclrn227]

i have always been facinated by nuclear power and reactions. from civil to military uses, it is a topic that can be endlessly explored.