In article ,
(Anders Eklöf) wrote:

snip

No - it's not fission. Uranium - as well as thorium and potassium
generate heat through radioactive decay, which is not the same as
fission. Fission is one kind of radioactive decay, but it's very rare in
nature, including Earth's core - unless you count alpha decay as
fission. Also, U, Th and K are not only present in the core, but
throughout the planet, though the heavier elements may be more
concentrated in the core.

_Quirks & Quarks_ had an item this week about the possibility that
bacteria-concentrated U-235 formed small nuclear piles during the early
stages of Earth's transition to an aerobic environment. "Hot" isotopes
would have been much more abundant back then.

See http://www.cbc.ca/quirks/archives/09-10/qq-2009-11-14.html, near
the bottom of the page; includes a link to the paper.

--
Odysseus