Messing that up would be catastrophic. It is also the sort of dynamical system susceptible to bifurcation. The conveyor could potentially invert due to a threshold change in temperature or salinity. http://en.wikipedia.org/wiki/Bifurcation_theory
Not only is the concentration different, but also the composition. I haven't heard of plutonium occurring in nature.
Not entirely related, but an interesting read: http://en.wikipedia.org/wiki/Goi%C3%A2nia_accident
Not when you bring the timescales into account, recognize that you'd have to scale operations up massively from present levels (15,000 or so plants vs. the 436 presently operating in the world). This would mean commissioning and decommissioning about a plant per day.
Add to this questions over fuel availability (known reserves are good for about 80 years at present levels of use, 6 if used for all human energy needs), and the need to find some way for creating transport and storage fuels, and nuclear is at best a small part of a much larger energy solution.
My favorite kind of result. Much like how "tree huggers" celebrate blocking a nuclear reactor or bird-killing wind farm, and as a result the old sooty coal plant keeps running instead. Some victory. Some progress.
Sidenote, I'm sometimes a little befuddled that we would be afraid of putting radioactive material in the ground. The ground is already full of the stuff, and we don't spend much time worrying about an earthquake spitting up a boulder of yellow cake in our backyards. Ok, the concentration is different, but sometimes it seems like worrying about putting salt in the oceans.