Thorium

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Thorium is a possible nuclear fuel. It is "fertile": it can be converted to fissile Uranium-233 by being bombarded with neutrons. It has various advantages, and some disadvantages, compared to Uranium as a fuel. Its use has been experimentally demonstrated in real world reactors. But it has also acquired an almost cult status among some advocates.

IAEA

Thorium fuel cycle — Potential benefits and challenges International Atomic Energy Agency; May 2005

Thorium is three times more abundant in nature compared to uranium and occurs mainly as ‘fertile’ 232Th isotope. From the inception of nuclear power programme, the immense potential of 232Th for breeding human-made ‘fissile’ isotope 233U efficiently in a thermal neutron reactor has been recognized. Several experimental and prototype power reactors were successfully operated during the mid 1950s to the mid 1970s using (Th, U)O2 and (Th, U)C2 fuels in high temperature gas cooled reactors (HTGR), (Th, U)O2 fuel in light water reactors (LWR) and Li7 F/BeF2/ThF4/UF4 fuel in molten salt breeder reactor (MSBR). 232Th and 233U are the best ‘fertile’ and ‘fissile’ materials respectively for thermal neutron reactors and ‘thermal breeding’ has been demonstrated for (Th, U)O2 fuel in the Shippingport light water breeder reactor (LWBR). ThO2 has also been successfully used as blanket material in liquid metal cooled fast breeder reactor (LMFBR) and for neutron flux flattening of the initial core of pressurized heavy water reactor (PHWR) during startup. So far, thorium fuels have not been introduced commercially because the estimated uranium resources turned out to be sufficient. In recent years, there has been renewed and additional interest in thorium because of: (i) the intrinsic proliferation resistance of thorium fuel cycle due to the presence of 232U and its strong gamma emitting daughter products, (ii) better thermo-physical properties and chemical stability of ThO2, as compared to UO2, which ensures better in-pile performance and a more stable waste form, (iii) lesser long lived minor actinides than the traditional uranium fuel cycle, (iv) superior plutonium incineration in (Th, Pu)O2 fuel as compared to (U, Pu)O2 and (v) attractive features of thorium related to accelerated driven system (ADS) and energy amplifier (EA). However, there are several challenges in the front and back end of the thorium fuel cycles. Irradiated ThO2 and spent ThO2-based fuels are difficult to dissolve in HNO3 because of the inertness of ThO2. The high gamma radiation associated with the short lived daughter products of 232U, which is always associated with 233U, necessitates remote reprocessing and refabrication of fuel. The protactinium formed in thorium fuel cycle also cause some problems, which need to be suitably resolved. The information on thorium and thorium fuel cycles has been well covered in the IAEATECDOC-1155 (May 2000) and IAEA-TECDOC-1319 (November 2002). The objective of the present TECDOC is to make a critical review of recent knowledge on thorium fuel cycle and its potential benefits and challenges, in particular, front end, applying thorium fuel cycle options and back end of thorium fuel cycles.

Protactinium

Dutch satirical news show on why we need to break the taboo around nuclear energy (comment) Reddit;

warning, nuclear physicist talking.
Anything you watch or read when they talk about Thorium, do the Protactinium test: Ctrl+F "Protactinium".
If you've heard about Thorium, you might remember that 232Th is not a nuclear fuel per se, it must be turn into the good stuff 233U; thats the one that will fission and give you your energy from fission, to turn into heat, steam, etc. Think of it like a recipe, you have butter and flower, you mix them to get the shortbread that you want. See how easy it is for everybody to get some shortbread?
Except everybody also like to gloss over that between the "butter/flower" step and the "shortbread" step, there's a "white phosphorous neurotoxic napalm" step that might make things a bit more complicated the kitchen. That's your 233Pa.
So it goes 232Th+n -> 233Pa -> 233U.
This is when you say: "but wait 233c, this is just like 239Pu is produced from 238U: 238U+n -> 239Np -> 239Pu, this is happening all the time in normal nuclear power plants. What's the difference?". The difference is the same as between 2 and 27.
239Np (the step between Uranium 238 and Plutonium 239) has a half life of 2 days, while 233Pa (the thing between Thorium and Uranium 233) has one of 27 days. If you leave 239Np in the core it will quickly turn into 239Pu, but you can't leave 233Pa in the core for a month or it will capture more neutrons and turn into something else than 233U. (there's also a matter of cross section: 233Pa has a much higher probability of capturing neutrons than 239Np). If you leave your butter and flower too long in the over you'll get a brick rather than a shortbread.
If you want to use Thorium, you must: expose your Th; extract your 233Pu; let it decay into 233U; feed the 233U back to your reactor.
By now you should understand why liquifying the fuel make so much more sense for Th than for U. It's not "MSR work so well with Thorium", it "if you want to continuously extract your 233Pa, you'd better do it with a liquid fuel".
this is where you say "Ok, but still don't see the issue, you just pump and filter your fuel to recover the 233Pa, and let it decay in a tank, and pump/filter the 233U back in for it to fission".
I'm going to assume that you know what a Becquerel and a Sievert are.
Remember the 27 days? with the density of 233Pa, that translates into 769TBq/g (Tera is for 1012 , that's a lot), and because of the high energy gamma from our friend 233Pa, that also means a dose rate at 1m from a 1g teardrop of 233Pa of 20,800mSv/h. Starting to get a picture?
Notice how all the numbers I've use are not "engineering limits" that few millions in R&D can bend, those are hardwired physical constants of Nature: half life, density, neutron capture cross section, gamma energy. Good luck changing those by throwing $ at them.
Now try to imagine technicians working in those plants, like doing some maintenance, replacing a pump (I haven't even touched the complex chemical separation system you need to extract your 233Pa from your fuel or 233U from your 233Pa, which will definitely need maintenance). Let's put it this way: if there is 1mg of 233Pa left in the component they are working on, they'll reach their annual dose limit in 1h.

Th fuel cycle

A safer route to a nuclear future? phys.org; 13 Jun 2012

discussion of recycling actinides as fuel, safety problems with multiple cycles with U fuel that Th fuel could solve, Ben Lindley, Cambridge Enterprises

Weinberg foundation

Thorium-Fuelled Molten Salt Reactors Report for the All Party Parliamentary Group on Thorium Energy; The Weinberg Foundation; Jun 2013

Thorium-fuelled Molten Salt Reactors (MSRs) offer a potentially safer, more efficient and sustainable form of nuclear power. Pioneered in the US at Oak Ridge National Laboratory (ORNL) in the 1960s and 1970s, MSRs benefit from novel safety and operational features such as passive temperature regulation, low operating pressure and high thermal to electrical conversion efficiency. Some MSR designs, such as the Liquid Fluoride Thorium Reactor (LFTR), provide continuous online fuel reprocessing, enabling very high levels of fuel burn-up. Although MSRs can be fuelled by any fissile material, the use of abundant thorium as fuel enables breeding in the thermal spectrum, and produces only tiny quantities of plutonium and other long-lived actinides.
Current international research and development efforts are led by China, where a $350 million MSR programme has recently been launched, with a 2MW test MSR scheduled for completion by around 2020. Smaller MSR research programmes are ongoing in France, Russia and the Czech Republic. The MSR programme at ORNL concluded that there were no insurmountable technical barriers to the development of MSRs. Current research and development priorities include integrated demonstration of online fuel reprocessing, verification of structural materials and development of closed cycle gas turbines for power conversion.

Thor-bores

Thor-bores and uro-sceptics: thorium's friendly fire Jim Green − Nuclear Monitor editor; Nuclear Monitor Issue: #8014458; 9 Apr 2015

Many Nuclear Monitor readers will be familiar with the tiresome rhetoric of thorium enthusiasts − let's call them thor-bores. Their arguments have little merit but they refuse to go away.

China

The U.S. government lab behind China's nuclear power push Reuters; 20 Dec 2013

Netherlands

Thorium molten salt reactors in EenVandaag - Dutch Public Television 20151105 Gijs Zwartsenberg

Thorium Energy Report

Thorium Energy Report website with pages of reports on work on Thorium reactors from:

Kirk Sorensen

Thorium can give humanity clean, pollution free energy Kirk Sorensen; TEDxColoradoSprings

Thorium Energy World conference

Thorium Energy World - papers

127 mostly quite technical papers presented during the Thorium Energy World conference
Grouped by subject area. Links to full papers where author(s) permit.