Interesting ? Thorium instead of uranium in nuclear reactors

[eppur_se_muova]

India, with the world's largest supply of thorium, as well as political incentives related to non-proliferation strictures, is looking into this much more intensively than anyone else. Thorium reactors have been operated in several countries in the past, but new approaches are reviving interest.

Most of the usual problems with nukes apply, but many are significantly diminished. Several design tweaks reduce the amount of plutonium produced, easing proliferation worries. The plutonium is also less amenable to extraction and weapons manufacture, because of its different isotopic composition. Thorium dioxide is safer to handle (less corrosive, higher mp) than uranium oxide. And it appears (though need more details here) the amount of waste produced is less. If so, that would be good news, as it opens the possibility of modifying current reactors to burn an alternative fuel with greater efficiency and less waste disposal.
http://www.americanscientist.org/template/AssetDetail/assetid/25710/page/1
http://www.world-nuclear.org/info/inf62.htm

One interesting variation is the "energy amplifier", invented by Carlo Rubia, a name familiar to most physicists as the former chief of CERN. This would result in a nuclear reaction in a subcritical mass of thorium, which would thus be incapable of runaway.
http://en.wikipedia.org/wiki/Energy_amplifier
http://www.world-nuclear.org/info/inf35.htm

Several regular posters in this group know more about nukes (pro or con) than I. I'm guessing this is not the first post on this particular topic, but would like to hear if there's any more recent news on this front. Also, would the recent US-India deal on nuclear technology derail the effort to switch to thorium? I have a sneaking suspicioun that it might.

A Googling of india+thorium produces a

[Warpy]

1. Anything that reduces the amount of radiocative garbage

that existing plants produce is a good thing.

I won't be pro nuclear power until they figure out what to do with all the garbage, period.

Most of it is toxic for far longer than humanity has had recorded history.

[Massacure]

3. Recorded history is far longer than 400 years.

And spent fuel is far from "garbage".

[NNadir]

4. And I won't be pro fossil fuels until they figure out what to do with CO2.

Of course, the toxic waste of coal, oil and gas, isn't solid, doesn't have a half-life and will exist for as long as the earth does.

And there's the small matter of the fact that fossil fuel waste is generated on a scale of billions of metric tons per year, while the entire history of nuclear power has yet to approach 100 thousand metric tons of spent fuel.

Then there is the inconsiderable fact that fossil fuel waste kills millions of people every year, and no one has ever died from the storage of spent nuclear fuel.

Other than that the two situations are slightly comparable, except that so called nuclear waste has maxima determined by their equilibria, and mercury, coal ash, oil slicks, carbon dioxide, heavy metals and sulfur oxides are only limited by the supply of coal. On the brighter side, NOx from fossil fuels is subject to an equilibrium condition.

[NNadir]

5. Self delete, response to wrong post.

[MADem]

2. I'm giving this a kick because I would like to see it discussed

I know nothing of this subject, but I find your questions interesting.

If no one bites, bring it up again--given the energy issues we are facing, it does look like, absent a major push/breakthrough with "natural" energy alternatives, nuke is the path of least resistance.

We surely will have to face these issues sooner than we think.

[NNadir]

6. Here's a quick overview.

1) The very first commercial reactor in the United States, at Shippenport, operated on thorium fuel as a test. The test was very successful, but the process was not carried out because uranium fuel proved to be so cheap.

2) Thorium is not fissionable, it is fissile. This means one must as a practical matter have either enriched uranium or plutonium to use thorium. Thorium is converted to uranium-233, and it is the uranium-233 that is fissioned.

3) The United States evaluated two kinds of breeder reactors in the 1960's. One was a plutonium fast liquid breeder that had some potential for dual use, weapons and the other was a thorium thermal breeder invented by the great nuclear engineer Alvin Weinberg, the molten salt reactor. The molten salt breeder reactor experiment was a spectacular success, and included the decontamination of radioactive fuel and isolation of the uranium-233 in a few hours, using a fluorine sparge and separation of the U233F6 as a gas. Weinberg pushed the development of the reactor at Oak Ridge and was fired for his efforts. The molten salt reactor (MSR) was suitable for preventing nuclear war, whereas the liquid metal fast reactor as conceived then was more suited for making weapons grade plutonium. (This was not true of the IFR, a later liquid metal reactor design.)

4) CANDU reactors, invented in Canada, but copied now widely in India, are perfectly suited for using thorium as a breeder fuel with high neutron economy. The doubling time for fuel is about a decade longer for a Thorium/U-233 fueled CANDU than for a liquid metal fast reactor, but it is possible to achieve very high burn-ups under these conditions.

5) Existing Pressurized Water Reactors can be net plutonium burners and near breeders, essentially tripling fuel burn up (in the thorium rods) and life time (going from 2 years to about 9 years between refueling) if a fuel rod mixing scheme known as the Radkowsky configuration is used. It must be said that the Radkowsky configuration is just one of many possible plutonium burning fuel cycles and faces competion from other inovative fuel schemes like CORAIL and APA. From what I understand, the Radkowsky cycle has more advantages, but the latter two are better funded and will probably be in commercial operation sooner.

6) The difficulty with all thorium based cycles involves the necessity for remote handling of the fuel using robotic systems. This is because of a nuclear reaction involving thorium in which an incident fast neutron results in the emission of two neutrons, leading to the formation of Pa-231. The Pa-231 in turn captures another neutron to yield U-232. U-232 is analogous to Pu-238 in having a half-life of less than 100 years. It rather quickly establishes equilibrium with thallium-208 which is a powerful short lived gamma emitter. The minus side is the need for expensive remote handling. The plus side is that it makes U-233 almost impossible (not completely impossible though) to divert for use in nuclear weapons. U-233 based nuclear weapons have been tested, but they are very difficult to build, maintain and conceal..

7) Because of factor number 6, it is generally cheaper to use virgin uranium, at least while uranium prices remain relatively low.

8) Thorium is about 3 times more abundant than uranium in ores. Currently thorium is discarded in dumps (which are radioactive) from the tailings of the processing of monazite for their lanthanide content. The lanthanides are valuable because of their use in color television sets. Formerly thorium was used in gas lantern mantles, except people went nuts when they realized that 100% of the thorium on earth is radioactive. Thorium resources have only incompletely been explored, since currently the world has far more thorium than it can use. The stuff is being dumped. For the long term, however, uranium may be more available, since uranium forms a slightly soluble carbonate complex that can be recovered from seawater. Thorium concentrations in seawater are far lower and it does not seem likely that thorium can be recovered from this source.

9) Thorium oxide is one of the most refractory substances known, and it is still used to make high temperature ceramic crucibles. This resistance to melting obviously has implications in the use of nuclear fuel, since it is practically impossible to melt thorium oxide. Thorium oxide also has excellent heat transfer properties that are very desirable. The space of ternary thorium uranium plutonium mixtures and alloys and all of their properties has been the subject of some research - it is certainly an area worth of more attention.

10) The conversion of thorium to uranium-233 involves the formation of the intermediate nucleus Pa-233 with a half-life of about 27 days, and which has a moderately high capture cross section for thermal neutrons. The presence of this isotope has certain implications for breeding ratios in nuclear reactors, making molten salt reactors significantly better (if the goal is thermal breeding) than the more common commercially available CANDU type reactor. The Gen IV nuclear reactor program is specifically calling for the use of MSR's as a) thorium breeders featuring in line processing and isolation of protactinium-233 for controlled decay out of the neutron flux, b) minor actinide burning and c) consumption of plutonium. Many types of molten salt reactors are possible, including some with fast neutron spectra.

11) The other type of reactor that has operated on a thorium fuel cycle is the HTGCR, high temperature gas cooled reactor. These reactors, which use helium as a coolant are considered as important cogs in the use of nuclear power for chemical processing, including the production of hydrogen and liquid carbon based fuels. They can be breeders or near breeders achieving very high burn ups. It is worth noting that the first commercial reactor of this type, the Ft. St. Vrain reactor in Colorado was a horrible commercial and technical failure. The performance was so poor that the reactor was converted to a natural gas plant. However it is believed that these operational difficulties can be surmounted in newer versions. My personal opinion is that these reactors aren't all bad, and I prefer them to the competing pebble bed type, since the pebble bed is, in my view, wasteful of perfectly good nuclear resources. The main reason for the preference for pebble-beds is the irrational fear of all things nuclear by the public, the public containing a large proportion of complete idiots and fools.

12) The main attraction of the thorium fuel cycle is that it makes the manufacture of nuclear weapons extremely problematic. Effectively it makes huge amounts of rather uncommon uranium isotopes available that easily denature uranium and prevent the manufacture of highly enriched uranium for bomb use. These are U-232, U-234, and U-236. (The latter is available from the use of enriched uranium in nuclear reactors.) The world resources for virgin uranium are so huge however, that these isotopes can be at best, of extremely limited utility for these purposes. However the specifics of using fuels with these isotopes, especially U-236, have not, to my knowledge, been worked out in great detail. On some level, U-236 is thought of as a neutron poison, and its presence has implications on neutron economy and reactor physics, leading, therefore, to certain potential economic penalties. My personal opinion is that the weapons minimization potential is too great to be overlooked. I really, really, really like the thorium fuel cycle for this reason.

13) The cycle further allows for huge reduction in plutonium inventories and the contamination of almost all newly formed plutonium with large amounts of Pu-238, a powerful heat and alpha source that makes plutonium use in weapons very difficult. In addition, the cycle allows for the consumption of plutonium without creating very much new plutonium, and does so while not losing the important resource of breeding capable fissionable material.

India plans the CANDU route, since it has significant commercial experience with CANDU operations.

Those are some points about the thorium fuel cycle. The cycle is not new, and has been considered almost since the dawn of the nuclear age. Like the plutonium (MOX) cycle however its commercial application has been complicated by slow growth of nuclear power because of public misapprehension of its vast environmental advantages, the unexpected discovery that uranium was far more common than appreciated, the existence of a well established uranium enrichment and MOX recovery cycle, and the need for remote processing.