[RadioAstronomer]

Given that we now are reasonably certain that the center of the earth is a fast breeder reactor with periodic buildups of Plutonium causing increased reactivity,

Got a link to a paper on this?

[SubMareener]

I don't have a link because I saw it printed on paper! It was either in "New Scientist" or "Science Weekly" or both. I think the bigger article was in "New Scientist". It really makes sense. How else would the earth stay so warm after 3 Billion years or so?

The article didn't go into the Uranium/Plutonium reactivity fluctuation. I added that from my experience in Naval Nuclear Power. The thing that really limited the life of the long-lifed cores was the build up of Plutonium 239 and the consequential reduction in Beta-bar. This made the plant operate too close to prompt critical, with possibility of large power fluctuations.