• Blog Stats

    • 71,499 hits
  • Archives

  • Enter your email address to subscribe to this blog and receive notifications of new posts by email.

    Join 146 other followers

  • Copyright notice

    This blog entry and all other text on this blog is copyrighted, you are free to read it, discuss it with friends, co-workers and anyone else who will pay attention.

    If you want to cite this blog article or quote from it in a not for profit website or blog then please feel free to do so as long as you provide a link back to this blog article.

    If as a school teacher or university teacher you wish to use content from my blog for the education of students then you may do so as long as the teaching materials produced from my blogged writings are not distributed for profit to others. Also at University level I ask that you provide a link to my blog to the students.

    If you want to quote from this blog in an academic paper published in an academic journal then please contact me before you submit your paper to enable us to discuss the matter.

    If you wish to reuse my text in a way where you will be making a profit (however small) please contact me before you do so, and we can discuss the licensing of the content.

    If you want to contact me then please do so by e-mailing me at Chalmers University of Technology, I am quite easy to find there as I am the only person with the surname “foreman” working at Chalmers. An alternative method of contacting me is to leave a comment on a blog article. If you do not know which one to comment on then just pick one at random, please include your email in the comment so I can contact you.

Thorium fueled reactors

Dear Reader,

It has come to my attention that the thorium based fuel cycle is being discussed in magazines such as “Chemistry World” which is the magazine of the Royal Society of Chemistry. As with all technology it is important that we have a honest and reasonable debate about it.

One attractive thing about the thorium fuel cycle is that it tends to form less of the transuranium elements such as plutonium, one idea for a nuclear fuel would be to make a mixture of thorium and plutonium dioxides. The idea is that the plutonium will provide the seed fuel while new fuel can be made from the thorium. Natural thorium  (232Th) can be converted by thermal neutrons into  233Th which will decay via  233Pa into  233U.

In many ways a thermal reactor is better than a fast one, I assume that many of my readers have heard of the term “fast breeder”, the idea of a fast breeder reactor is that it uses fast neutrons to make more fuel than it consumes. Commonly a fast breeder is fueled with a mixture of  238U and something fissile such as  235U or  239Pu. The reason why a fast neutron spectrum is better is that thermal neutrons can cause fission of 239Pu but the fission to capture ratio for fast neutrons is more favoring fission than capture. The capture (nγ reaction) of neutrons with 239Pu tends to form a neutron poison (240Pu) which is activated further to form 241Pu which undergoes beta decay to form minor actinides such as 241Am and even curium. These minor actinides can be a right royal pain. Another problem is that in a thermal reactor the formation of 236U by the nγ reaction of 235U can occur, the 236U is long lived and can be activated further to make short-lived 237U which can decay into 237Np. The 237Np can then form by another capture reaction 238Np which does a beta decay into the house of horror bugbear isotope of plutonium (238Pu). It is interesting that while the greens complain about the “evils of plutonium” they never seem to mention the fact that a lot of plutonium formed in power reactors is more alpha active than pure 239Pu. They seem to be trapped in their thinking by the long half life of the nicest plutonium isotope, 239Pu is not very radioactive gram for gram when compared with many other things such as radium.

As the 239Pu undergoes less activation and more fission in a fast reactor it is a logical choice for making and using plutonium, but on the other hand a fast reactor is bad for the thorium based fuel cycle. Here the desired outcome is neutron capture by natural thorium. The intended reactions are the neutron activation of 232Th to form 233Th (t½ 22 min) which decays by beta decay to 233Pa (t½ 27 days) which in turn undergoes a beta decay to 233U (t½ 159200 years). While 233U can be used for both reactor fuel and bombs, it is interesting to note that it is normally contaminated with some 232U. The decay of 232U forms high energy gamma emitters which will increase the dose rate near the 233U, this could make bomb and fuel fabrication more difficult.

The unwanted reaction in the thorium containing reactor is the n,2n reaction on 232Th to form 231Th, the 231Th then does a beta decay to get to long lived 231Pa. The next neutron capture then forms 232Pa which decays into 232U.

Some of the daughters of 232U (208Tl and 212Bi) emit very high energy gamma rays (up to 2.6 MeV) which will be much more troublesome than the gamma rays from 241Am which is commonly found in plutonium which has been allowed to age for some years. The majority of the gamma rays from 241Am are much lower in energy (60 and 33 keV) are much lower in energy and thus can be shielded against with a lead apron (circa 1 mm Pb) or a sheet of glass attached to a glove box. To attenuate the 208Tl gamma rays a very thick layer of shielding would be required making glovebox work impossible unless the glove box worker is willing to incur a large hand dose and happens to look rather like Mr Tickle of the Mr Men.

The key thing to understand is that a slow or thermal neutron has too little energy to do the n,2n reaction on the natural thorium. While a thermal neutron is able to do the neutron capture which we want. With some luck we can consider some reactor designs which reduce the formation of 232U.

Advertisements

One Response

  1. Very interesting…..more so I am wondering if you are the mark i know??? 🙂

Go on, Have your say !

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: