Erm....you know how a fission reactor works don't you?
Just checking.

Worth reading up on how Chernobyl happened, the control rods became jammed in the atomic pile and thus couldn't moderate the reaction.
(Bicycle Repairman "you're also a bit of a wanker", Thu 18 Oct 2012, 13:53, 1 reply, 13 years ago)

I'm not saying in reality it's physically possibly to stop a fission reaction.
just that theoretically if you remove fuel it'll stop.
( the mighty badger Aphrodite, on a bar stool, by your side, Thu 18 Oct 2012, 13:55, Reply)

Practically speaking, if you stop putting fuel pellets into a fusion reactor
It'll stop right away. A fission reactor remains hot for ages after, not to mention that the fuel itself can become flammable and thus cause further contamination - see Calder Hall for more details.
(Bicycle Repairman "you're also a bit of a wanker", Thu 18 Oct 2012, 13:58, Reply)