Sunday, April 29, 2012

Uranium or Thorium?

My last post focused on the uranium as a renewable resource. This seemingly radical view, is supported by well known facts. The question then is, with the renewable nature of uranium, why should we need thorium?

Although uranium is virtually infantly renewable in sea water, the amount of uranium that can be recovered from sea water may not be large enough to supply all of the human energy needs, required from muclear power. In addition to sea water, a very large amount of uranium can potentially be recovered from shale gas fracking wells. This is because shale contains more energy in the form of uranium (and thorium), than it contains in the form of natural gas. It may be possible to recover uranium and thorium from natural gas fracking wells at a very low cost, since the wells have already been drilled. Dr Tracy Banks, A geologist from the University of Buffalo, has studied the presence uranium ajacent to natural gas deposits. Dr Banks dtated.
'Marcellus shale naturally traps metals such as uranium and at levels higher than usually found naturally, but lower than manmade contamination levels, . . . 'My question was, if they start drilling and pumping millions of gallons of water into these underground rocks, will that force the uranium into the soluble phase and mobilise it? Will uranium then show up in groundwater? . . . 'We found that the uranium and the hydrocarbons are in the same physical space. 'We found that they are not just physically - but also chemically - bound. . . . 'That led me to believe that uranium in solution could be more of an issue because the process of drilling to extract the hydrocarbons could start mobilising the metals as well, forcing them into the soluble phase and causing them to move around.
Thus it would appear that uranium is recoverable from natural gas wells. We still need to know if uranium can be recovered from gas wells at a reasonable cost. and how much uranium might ber recovered from gas wells..

All things considered we will never completely run out of uranium. and we probably have enough recoverable uranium resources to power the world for a long time to come. We will also lmow long before we start running out of low cost uranium when we will run out. Thus a we are running out of uranium sort of argument as a justification for thorium cycle reactors, simply will not fly. The global thorium supply is severak times larger than the supply of renewability and unconventional recovery uranium. Thus unlike uranium, thorium is not s renewable energy source, but it is a sustaunable energy source, and infact even more sustainable than uranium.

If uranium cycle reactors the only option, we would have to get by with all uranium fuel cycle reactors. However, before we dismiss thorium, we ought to consider if thorium cycle or thorium cycle - uranium cycle reactors might offer advantages.

I recently offered a post on David LeBlanc's research on the Denatured Molten Salt Reactor (DMSR). This reactor offers the advantage of being safer, and very likely significantly less expensive than Light Water Reactors. In addition it offers a low cost solution to the transe uranium waste problem, that will only leve fission products, to deal with. Many fission products are useful, or produce stable, useful and in some cases very valuable elements, and will be no more radioactive, than a uranium mine within 300 years. Finally, the DMSR is the most nuclear proliferation resistent reactor ever designed. Only some one who is utterly insane would use a DMSR as a nuclear proliferation tool, in preference to other, less expensive, and technologically simpler routes to the development of nuclear weapons. Only a fool would believe that the DMSR would be preferable to a centrifuge as a proliferation, because the core uranium of the DMSR would still have to be seperated by a centrifuge, or other uranium enrichment technology before it would be serviceable as erapons grade uranium. Even if a would be proliferator were to chose to seperate the U-233 ands U-235, he or she would face a further complication in the form U-232. It would be simply crazy to use a DMSR as a nuclear proliferation tool.

Dr. LeBlanc's ressearch has primarrily focused on research carried out at ORNL during the late 1970's. The DMSR was designed to include a start charge of 20% U-235 and 80% U-238. A 1000 MWe DMSR start charge was composed of 3450 kg of U-238 and 4 tines as much U-238. In addition to the uranium 110 tons of thorium was to be included in the core salts. Dr. LeBlanc notes that
thorium improves neutron production
at least in a thermal reactor. To understand why this is a concern, i consulted WASH 1097, (1969) The Use of Thorium in Nuclear Powered Reactors. In a thermal reactor operating at 600 C, U-233 was estimated to produce 2.29 neutrons prt mrutron capture. In contrast. U-235 produced 2.06 neutrons per neutron capture. and Pu-239 produced 1.79 neutrons per neutron capture.

Conventional reactors create some fissionable fuel, usually Pu-239. But Pu-239 is not a particularly good nuclear fuel in thermal and epithermal LWRs. This is because the nutron capture to fission rate of Pu-239 is only only about 65%. In contrast the Neutron Capture to fission rate of U-233 is relatively constant regardless of neutron speed. In thermal reactors thorium fissions after neutron capture 92% of the time. WASH-1097 states,
The thermal-spectrum-averaged value of eta for U-233 is relatively insensitive to changes in the moderator temperature. On the other hand, an increase in the moderator temperature will result in a slight decrease of the eta of U-235 and a greater decrease of the eta of Pu-239.
Thus in a thermal range, U-233 is the supereior nuclear fuel. Conversion Ratio refers to the relationship between nuclear fission events, and the production of new nuclear fuel. In conventional Light Water Reactors, the conversion ratios rarely run above 0.6 to 1. Naval reactors are fueled by pure U-235, with no U-238 or Th-232 in their cores. Thus naval reactors have a conversion ratio of 0 to 1. There is no question that in thermal and epithermal reactors, thorium produces a superior conversion ratio, and in fact it has been demonstrated that a light water breeder reactor is possible using the Thorium-U-233 conversion cycle. Thus thorium breeding possible in thermal reactors and thorium cycle reactors with less than a 1 to 1 breeding ratio can still offer far better conversion ratios than uranium cycle light water reactors can.

We have seen that the total amount of uranium in the DMSR is 17,500 kgs, compaired to around 100,000 kgs of thorium. Thus we have a little more than a 5 to 1 thorium to uranium to thorium ratio, and a .13 to one U-238 to Th-232 ratio. Thus it is far more likely that a fission produced neutron will hit a thorium than it will hit a U-238 atom. ThIjod gar more U-233 than Pu-239 will ne produced by the D

The use of thorium in a thermal converter reactor will vonsiderably decrease the demand for uranium. Dr LeBlanc notes that a conventional LWR operating on a once through fuel cycle will trquire 6400 thousand tonnes of utanium ore. during a 30 year operating period.while a once through DMSR will use 1810 tonnes of uranium ore and a DMSR with batch reprossing of fuel salts, and return if all utanium and trans-uranium elements to a DMSR core, will lead to only 1000 tones of uranium ore used. It is no small advantage of the DMSR that it will produce very little nuclear waste. Since return of actinide nuclear waste to a DMSR core after batch reprocessing, the remaining fission daughyrt products will become safe after 300 years. However, long before the 300 year period is up, non-radioactive daughter products, mamy of which can be used by industry, can be extracted from the waste repository. Thus the use of large ammounts of thorium in the DMSR will help to dplve the problem of uranium fuel cycle waste.

The use of the DMSR will provide major benefits. First the technology has latge been tested already. The DMSR draws heavily on the technology already used in the ORNL Molten Salt Reactor Experiment. Secondly the DMSR will ease public worries about nuclear power as well as lowering nyclear captiol costs because:
A. It offer deterministic safety. That is, it will be safe becaused all of its safety features are based on the laws of nature.
B. The DMSR either significantly decreases the long term nuclear waste problem or completely solves it.
C. The DMST is not a useful tool for nuclear proliferation.
D. The DMSR is likely to cost significantly less than LWRs of comperable power output.

4 comments:

Ivan Raszl said...

Reblogged here: http://thoriumforum.com/uranium-or-thorium

Thanks for the great post.

jimwg said...

Good issue!

I'd really like to see a (however fanciful or whimsical, whatever) feature examining how a current nuclear plant might be retro-fitted to use thorium as opposed the costs and siteing issues of a whole new plant and decommissioning. I'm sure there are sincere and over-the-top schemes that might fly!

James Greenidge
Queens NY

NNadir said...

I've never actually been an either/or kind of guy on the uranium/plutonium v. thorium/U-233 cycle.

The real reason for using thorium in fuels has nothing to do with proliferation issues, which are in fact, essentially a tempest in a teapot.

The real reason involves faster fissile growth with the existing infrastructure, as well as an impetus to phase out mining.

I have a rather long commentary on this topic that I partially finished a long time ago, but I abandoned it for time reasons.

I may publish it yet.

Nathan2go said...

Dr. LeBlanc has an interesting fact buried near the end of his 2010 Nuclear Engineering and Design paper: The DMSR works almost as well with no Thorium! See sect. 7.2 & 7.6: http://www.energyfromthorium.com/forum/download/file.php?id=727

It turns out that most of the DMSR benefits arise because of the liquid fuel. The thorium does improve fuel economy, pushing down the 30 year U ore requirement from 1500 to 1000 tons (still a 4x improvement of LWRs), by LeBlanc's calculation assuming recycling.

The no-thorium DMSR also makes an interesting first generation MSR, which can use regular 5% LEU (unlike the standard DMSR with 20% LEU, which might require specialized enrichment facilities). Also, it could make fuel recycling simpler, as the spent U will have low fissile content, and could be simply discarded, rather than requiring a dedicated re-enrichment facility (the spent fuel from the standard DMSR has about 10% enrichment: too valuable to discard, but needing re-enrichment for DMSR).

Dr. LeBlanc also cites the possibility of higher power density (leading to lower startup fissile requirement) without thorium, because with thorium protactinium losses increase with power density.

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