Or, once a nuclear reactor is started, cannot it not be shut down?
http://latimesblogs.latimes.com/lanow/2013/03/san-onofre-mitsubishi.html
For what it's worth. I haven't done a great deal of research to find out if it's just partially decommissioned or what have you, but it's a place to start.
Fuel rods can be switched out, so they could just remove all of them and leave the core empty.
Current generation reactors actually require electrical input to operate.
Any fission reactor, as far as I know, has a moderating element (such as graphite) designed to moderate its operation (the speed of the reaction). If the fuel rods are completely immersed in such a moderating material, the reaction will completely stop. With old reactors though, once you got to the point of overheating, there was nothing else you could do (but one would have to be pretty incompetent to let things go so far). Newer generation reactors, as Caley says, are passively safe, which means that they need energy to work and in an emergency will automatically shut down.
"Newer generation reactors, as Caley says, are passively safe, which means that they need energy to work and in an emergency will automatically shut down. "
So, was Fukishima an older type?
Define stop? If you separate the fuel rods then the reaction stops. But radioactive material from the reaction and in the fuel rods themselves will continue generate heat from radioactive decay. This is what causes the melt in the meltdown. That is the radioactive stuff in the fuel rods becomes so hot that it melts the rod containers themselves and falls on the floor of the reactor. The radioactive debris then begins melting the bottom of the reactor floor. This was the extent of the now infamous Three Mile Island nuclear mess (Not really a disaster). Also note that not all of the radioactive material is solid and some will dissolve in water meaning a meltdown through the floor of the containment structure could release gas into the atomosphere or contaminate the water table with very hazardous radioactive materials and dangerous chemicals.
limitgov: So, was Fukishima an older type?
As far as I know there are no new (fourth generation) operational reactors anywhere, though a few are scheduled. Every reactor now in use is of the older kind which will melt in extreme circumstances.