[logfromblammo]

And if this sort of thing interests you, also read up on "neutron poisons" and the "iodine pit".

Basically, it takes about 500 hours (3 weeks) before a chain-reacting fission core reaches equilibrium with neutron-absorbing reaction products. A core that is late in its fuel cycle might not be able to achieve a self-sustaining chain reaction from a shut down state until after the radioactive neutron absorbers have been left a while to decay.

The longer the fuel rod has been in the reactor, the less you are able to modulate its output.

You really do have to look at Europe (especially France) for operating nuclear plant best practices, because most plants in the US are not built to a common design, and they may not be able to adopt improvements pioneered at another site. The US solution is usually to put the upgrades into the next-generation reactor designs, which never quite get to replace the old existing designs because of radfear and NIMBYism.