[ExI] Silence in the sky-but why?

John Clark johnkclark at gmail.com
Thu Sep 26 20:07:29 UTC 2013

On Thu, Sep 26, 2013 at 3:16 PM, Tomasz Rola <rtomek at ceti.pl> wrote:

> Since U is only slightly (3-4 times) less omnipresent in the crust, you
> should say now "we will run out of U in 400 million years, so we don't need
> Th at all".

All of today's reactors use only the rare U235 isotope, and that is only
one part in140 of mined Uranium; Thorium has only one isotope and Thorium
reactors can use 100% of it. But even so we are nowhere near to running out
of Uranium or Thorium, but there are other advantages to Thorium.

To burn the remaining 99.3% of Uranium, the U238, you'd have to use a
exotic fast neutron breeder reactor. Thorium reactors use slow neutrons and
so are inherently more stable because you have much more time to react if
something goes wrong. Also breeders produce massive amounts of Plutonium
which is a bad thing if you're worried about people making bombs. Thorium
reactors produce an insignificant amount of Plutonium.

Thorium reactors do produce Uranium 233 and theoretically you could make a
bomb out of that, but it would be contaminated with Uranium 232 which is a
powerful gamma ray emitter which would make it suicidal to work with unless
extraordinary precautions were taken, and even then the unexploded bomb
would be so radioactive it would give away its location if you tried to
hide it, destroy its electronic firing circuits and degrade its chemical
explosives. For these reasons even after almost 70 years nobody has a
Uranium 233 bomb in its stockpile.

A Thorium reactor only produces about 1% as much waste as a conventional
reactor and the stuff it does make is not as nasty, after about 5 years 87%
of it would be safe and the remaining 13% in 300 years; a conventional
reactor would take 100,000 years.

A Thorium reactor has an inherent safety feature, the fuel is in liquid
form (Thorium dissolved in un-corrosive molten Fluoride salts) so if for
whatever reason things get too hot the liquid expands and so the fuel gets
less dense and the reaction slows down.

There is yet another fail safe device. At the bottom of the reactor is
something called a "freeze plug", fans blow on it to freeze it solid, if
things get too hot the plug melts and the liquid drains out into a holding
tank and the reaction stops; also if all electronic controls die due to a
loss of electrical power the fans will stop the plug will melt and the
reaction will stop.

Thorium reactors work at much higher temperatures than conventional
reactors so you have better energy efficiency; in fact they are so hot the
waste heat could be used to desalinate sea water or generate hydrogen fuel
from water.

Although the liquid Fluoride salt is very hot it is not under pressure so
that makes the plumbing of the thing much easier, and even if you did get a
leak it would not be the utter disaster it would be in a conventional
reactor; that is also why the containment building in common light water
reactors need to be so much larger than the reactor itself. With Thorium
nothing is under pressure and there is no danger of a disastrous phase
change so the expensive containment building can be made much more compact.

  John K Clark
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