clarification (Score 2, Informative)

I should note that the ability to overcome the xenon transient is dependent on the geometry and neutron moderator configuration of the reactor. There probably are some reactors out there that would not be able to start up immediately. A link further down the page indicates that this is the case for heavy water-moderated CANDU reactors, which use natural unenriched uranium. The 20-hour startup I mentioned above occurred at a boiling water reactor.