Friday, March 28, 2008

The Uranium Fuel Cycle

WASH-1097 remains an invaluable source of information on the thorium fuel cycle. It explains why the thorium fuel cycle creates such a small problem with transuranium isotope. First, however, it is important to understand why there is a problem in the uranium fuel cycle.

When U238 absorbs a neutron a transformation process is triggered. After a couple of sub-nuclear events (beta radiation), the two neutrons in the atom become protons. This process turns the uranium-239 atom into plutonium-239. Pu239 is fissile. But Pu239 has some characteristics that make it something less than a desirable fuel, in ordinary moderated thermal neutron reactors. Fission is most likely to occur with low energy neutrons. Yet, Pu239 has a healthy appetite for these low energy neutrons, while only fissions about 2 times out of 3 when it absorbs low energy neutrons. The net effect is that Pu239 doesn't "pull its weight" in the reactor when it is fissioned by low-energy neutrons. It doesn't produce enough neutrons per absorption to make up for the neutrons lost in absorption.

In the ideal uranium fuel cycle, a Pu239 nucleus absorbs a neutron, splits and emits three neutrons. One of them is absorbed by another fissile atom (U235 or Pu239) atom which splits. The other is absorbed by a U238 atom which is transformed into U239. As you get more and more of the absorption products of Pu239 building up in the nuclear fuel (Pu240, Pu241, etc), the neutronics become more and more unfavorable.

The heart of the problem is the fact that low energy neutrons split Pu239 atoms only about 2/3rds of the time. This is all laid out very nicely in WASH-1097. In the other 1/3rd of the time, Pu239 becomes Pu240. If Pu240 absorbs a neutron it becomes Pu241 and if Pu241 absorbs a neutron, 75% of the time it fissions. Thus by the WASH-1097 account, 25% of the time when Pu 241 absorbs a neutron it becomes Pu242. Thus after absorbing 4 neutrons, nearly 9% the atoms that started out as U238 are still plutonium. This is what is called a poor neutron economy. The neutron economy of fast breeders is better, because a neutron absorption in Pu239 is more likely to cause a fission with more energetic neutrons. Hence the desirability of fast breeder reactors in a transuranium reactor economy.

As we have seen conventional fast breeders use sodium as a coolant, and sodium is really nasty, dangerous stuff. In addition, as Kirk Sorensen points out, using liquid sodium as a coolant, limits the thermal efficiency of a reactor. Thus not only are LMFB reactors inherently dangerous, they
are also not as efficient as power producers as liquid fluoride reactors.

But here we must ask, why are we producing plutonium in breeder reactors? If we are producing it to go into conventional reactors, we are not producing very good nuclear fuel.

- Charles Barton


Joffan said...

Interesting summary of the neutron process for plutonium, thanks.

But I have to say that dismissing sodium as "really nasty, dangerous stuff" is more of an anti-nuclear tactic than I really like to see from you. It's an industrial-strength material that requires industrial-strength handling. Maybe the liquid fluoride process is easier and has more benefits - it looks that way to me - but the sodium cooled process doesn't look either infeasible or inevitably flawed.

Charles Barton said...

If you put something into a reactor, you have to ask, "what is the worse thing that can happen if it escapes?" That is why reactors are put in massive containment structures, to contain steam explosions and prevent the escapes of radio-isotopes. With sodium, the problem is compounded, because it is potentially far more explosive than hot water under pressure.

I am not the first to raise the question about putting sodium in a reactor. Eugene Wigner and Alvin Weinberg did that a long time ago. They may not have used the words "nasty, dangerous stuff," but they implied them. I am going to post a Weinberg statement on the LMFBR.

Anonymous said...

Charles, I did a fairly significant edit of this article on the version you posted on Energy From Thorium. You might want to take a look at that and compare and contrast to avoid getting comments here about some errors in the text.


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