Monday, 2 May 2016

Don’t criticize what you can’t understand

Your old road is rapidly agin’
Please get out of the new one if you can’t lend your hand
For the times they are a-changin’
If only Jim Green (national anti-nuclear campaigner for Friends of the Earth, Australia), had listened to Bob Dylan. Poor Jim hasn't much of a clue about science so he does not know what he's talking about. He attempts to criticise thorium powered nuclear reactors here but does a bad job because he does not know much about the technology pathways being promoted. For instance, let's just look at his WNA quotation. There's so much wrong with it:
"A great deal of testing, analysis and licensing and qualification work is required before any thorium fuel can enter into service. This is expensive"
-- The anti-nuclear movement are the reason for this. We could have zero-carbon, cheaper energy, thorium reactors working in a few years were it not for the catalogue of regulation and agencies (official and unofficial) blocking the development of better nuclear power.
"Other impediments to the development of thorium fuel cycle are the higher cost of fuel fabrication"
-- Liquid fuels like molten salts could be far cheaper. Such fuels require no fabrication. Liquid molten salt fuels are proposed for nearly every thorium reactor
"the cost of reprocessing to provide the fissile plutonium driver material"
-- Thorium reactors do not need plutonium to start. They could start with uranium-235. That will only require enrichment; which is what currently happens.
"the high cost of fuel fabrication (for solid fuel)"
-- Nearly all the thorium reactor plans are for molten salt reactors - not solid fuel reactors. Such liquid fuels have no 'fabrication'.
"Separated U-233 is always contaminated with traces of U-232"
-- were it to be true, that would be a good thing. It would make the U-233 made in thorium reactors proof against weapons proliferation. According to weapons experts as only 50 ppm U-232 will make uranium-233 unsuitable for weapons. Reprocessing can be run either entirely robotically, or manually behind safe screens. As was demonstrated for the IFR over 2 decades ago, and as done at some reprocessing plants today. Contaminating U-232 will pose no problem. Au contraire, several reactor designers want to make sure their fuel will contain such U-232!

Note: Thorium reactors work by breeding thorium-232 to uranium-233.

   Th-232 + n -> Th-233; 
   Th-233 + e- -> Pa-233; Th-233 ½-life: 22 min
   Pa-233 + e- -> U-233;  Pa-233 ½-life: 27 days

Uranium-233 is the fissionable material. The product 'bred' from thorium (Th-232). The other materials: Th-233, and Pa-233 are just steps along the way. U-233 behaves differently to other materials. When its nucleus is hit by a neutron, it is generally not captured to increase the atomic weight, causing transmutation [i.e. U-233 + n -> U-234 ]. Instead, U-233 is fissionable. It splits in two when hit by a neutron. One atom becomes two smaller, energetic, atoms, plus 2 or 3 neutrons, plus electromagnetic rays. This is where the energy in nuclear fission is made: from the release of nuclear binding energy. This fission is the reaction we want. We do not want fissionable materials to be wasted by capturing a neutron to increase in atomic mass. With such neutron capture both the potentially fissionable atom (e.g. U-233) and a neutron are wasted, because U-234 is not fissionable. There are only 3 isotopes available to us which are fissionable by 'thermal' (moderated) neutrons: U-233, U-235, and Pu-239. U-233 is the best of them. In the thermal neutron spectrum U-233 has the best neutronics all all fissionable materials:

Notice the final two columns in this table. With U-233 only 7.7% of neutrons are wasted by capture ( U-233 + n -> U-234 ). The U-234 made does not fission. Proportionally U-235 wastes almost twice as many neutrons and plutonium wastes more than 3 times as many by capture. Uranium-233 also shows a much better neutron economy over a wider neutron energy range than either U-235, or Pu-239 (see chart below). The average eta value (number of neutrons produced for every neutron absorbed) for U-233 = 2.27 in a standard PWR compared to 2.06 for U-235 and 1.84 for Pu-239. Eta must be at least 2 for breeding (to sustain the reaction) because 1 neutron is needed to cause another fission and 1 to breed another U-233 (from Th-232). In practice a breeder reactor needs eta much larger than 2 because many neutrons are lost (absorbed by the reactor, the moderator, or captured by U-233, or even by the intermediate Pa-233.

Thorium has far less toxic waste

There is another thorium advantage: any U-234 made can eventually absorb another neutron to make U-235, so it gets a 2nd chance to fission.

The consequence of this is that the thorium/uranium-233 fuel cycle has a waste stream which is far less radioactive after the fission products have decayed. It contains very little long-lasting radioactive transuranics. The chart below has logarithmic scales. Thorium transuranics show the green curve. All kinds of fission include the blue curve (fission products). The point where the blue curve crosses the dotted orange line happens after 300 years. After this point, thorium waste will be as safe as natural uranium ore, which is generally considered safe. In contrast radioactivity of uranium/plutonium waste from a conventional reactor such as a PWR, or BWR will not fall below uranium ore radioactivity levels for hundreds of thousands of years at best (red).

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