Thorium

Thorium (Th)
The element thorium (Th) has 90 protons in its nucleus. Naturally occurring thorium is 100% 232Th. Its melting point is 2023 K and its boiling point is 5081 K. Thorium has been used to fuel over a dozen reactors in the past, significant among these were two light water breeder reactors at Shippingport. A 100 MWe (1977-82) and a 285 MWe reactor (1962-80) [WNA 2010, Olson et al. 2002].

World Nuclear Association, Updated October 2009, “Thorium”. http://www.world-nuclear.org/info/inf62.html [Accessed 9th Mar 2010]

Olson, G.L., McCardell, R.K. and Illum, D.B., 2002, “Fuel Summary Report: Shippingport Light Water Breeder Reactor” Idaho National Engineering and Environmental Laboratory, LLC, USA. INEEL/EXT-98-00799, Revision 2. http://www.inl.gov/technicalpublications/Documents/2664750.pdf [Accessed 5th Feb 2010]

Research and Development (R&D)
The uranium fuel cycle has been the subject of significantly more R&D than the thorium fuel cycle. It will require investment in order to develop an industrial scale thorium processing industry. It is the opinion of ThorEA that the reason thorium has been the subject of less R&D than uranium is because uranium has better properties for generating plutonium during its fuel cycle. During the development of Generation I nuclear power stations governments desired the production of plutonium for their weapons programs. Following this, the existing uranium fuel processing industry made it the fuel of choice for the commercial nuclear power industry for economic reasons. It is anticipated that a developed thorium processing industry will be economically competitive with the uranium industry.

Resources
The NEA and IAEA have predicted that at 2006 consumption rates there is 100 years worth of uranium left in the Earth that can be mined for a price of less than $130 US dollars /kg [NEA/IAEA 2008]. Thorium is 3-4 times more abundant than uranium in the Earth. In the thermal fuel cycle, thorium produces approximately 40 times more energy per kilogram mined than uranium [WNA 2010].

It has been considered that terming an energy source renewable implies that its supply will last for as long as the sun continues to burn in its steady state, 5 billion years [Cohen 1983]. The consumption efficiency of uranium fuel is increased significantly by using fast breeder reactors. Uranium is present in ocean water (3.3 parts per billion supplied by 3.2 $$\times$$ 1013 tonnes per year from rivers). It has been identified that it is possible to consume 6500 tonnes of uranium from sea water for hundreds of millions of years. This would reduce the uranium content of the water by only 25% and is enough fuel for 6500 GWe of electricity from fast breeder reactors. The world currently generates approximately 750 GWe of electricity. The expense of extracting the uranium fuel from sea water is expected to be balanced by the reduced demand for fuel due to its being burned in a fast fission cycle.

NEA/IAEA, 2008, “Uranium 2007: Resources, Production and Demand” Nuclear Energy Agency and the International Atomic Energy Agency, ISBN 978-92-64-04766-2, Paris, France: OECD.

World Nuclear Association, Updated October 2009, “Thorium”. http://www.world-nuclear.org/info/inf62.html [Accessed 9th Mar 2010]

Cohen, B. L., 1983. “Breeder Reactors: A Renewable Energy Source”, American Journal of Physics, 51 (1), pp. 78

Gamma Rays Aassociated with the Thorium Fuel Cycle


The natural decay chain chart shows the nuclei involved with the natural radioactive decay chains, overlaid on the chart are black arrows, these indicate the key nuclear reactions in the thorium cycle that can lead to high-energy &gamma;‑ray emission, as described by the IAEA report “Thorium fuel cycle — Potential benefits and challenges”[IAEA 2005].

Natural thorium (232Th) has a 14 billion year half-life. Following the decay of 232Th, the remaining nuclei in this natural decay chain have short half-lives, $$T_{1/2}(total) < 7$$ years. Near the end of the natural chain, the daughter nuclei that follow the &beta;‑‑decay of 212Pb and 208Tl characteristically emit a 0.78 and 2.6 MeV &gamma; ray, respectively. By one of a few multi-step reaction channels the thorium fuel cycle can transmute 232Th into 232U. 232U &alpha;-decays into the daughter nucleus 228Th, one of the 232Th natural decay chain nuclei. This therefore leads to the emission of the 0.78 MeV (66% chance) and the 2.6 MeV &gamma; rays within a half-life of $$T_{1/2} = 5$$ years. In effect, the thorium reactor catalyses the natural thorium decay chain, leading to intense high-energy &gamma;‑ray activity. It is interesting to note that one of the key 232U populating mechanisms, 233U(n,2n)232U, has a minimum threshold centre of mass kinetic energy of 6.37 MeV below which the reaction does not take place.

IAEA, 2005, “Thorium fuel cycle — Potential benefits and challenges”, International Atomic Energy Agency, IAEA-TECDOC-1450

High-Energy &gamma; Rays
Increasing &gamma;‑ray energy increases the depth of material a &gamma; ray will penetrate through. The depth of penetration is described by the equation, $$ I_d = I_0 \times exp^{–(m \times d)}$$, where $$I_d$$ is the &gamma;‑ray intensity after traversing a medium of depth, $$d$$; $$I_0$$ is the initial &gamma;‑ray intensity; and $$m$$ is the absorption coefficient. For the relationship between $$m$$ and &gamma;‑ray energy, see the schematic example in Figure on the right.

Increasingly high energy &gamma;‑rays are increasingly difficult to shield. Inadequate shielding results in human exposure to radiation doses during handling the material. For fuel reprocessing, this incurs economic expense due to the large degree of shielding required to protect workers. Proliferation of spent fuel is difficult due to the radiation protection that is required by people who are in close proximity to the material.

Gamma‑Ray Activity in Spent Thorium Fuel
[[File:Seperated_uranium_activity.jpg|thumb|250px|Beta-activity and &gamma;-ray dose due to 232U in separated uranium from spent fuel in a thermal critical reactor. The spectrum in this figure is taken from [Pigford 1999].]Chemical reprocessing techniques for spent thorium fuel are capable of extracting nearly pure uranium from the nuclide mix. Broadly speaking separated uranium can be expected to contain 100’s –1000’s of particles per million (ppm) of 232U. The precise abundance is subject to the reactor design and fuel mix specifications. 232U has a half-life for &alpha;-decay of 72 years. Following separation, decay of 232U increases the 228Th content in the separated reprocessed fuel and therefore its high-energy &gamma;‑ray activity. The &gamma;‑ray activity continually increases until it peaks 10 years following separation of the uranium, see the Figure to the right [Pigford 1999].

Similarly to the case for recovering uranium from spent thorium fuel, the separation of thorium (to recycle 232Th) includes 228Th and therefore the associated high-energy &gamma;‑rays are present once again. The timescale over which the &gamma;‑rays are emitted is significantly briefer for separated thorium compared to uranium, as there is no 72 year waiting point half-life for the &gamma;‑decay of 232U. For separated thorium, the activity reaches nominal levels over a time scale of 3 –16 years.

[Pigford 1999] Pigford, T.H., 1999, “Thorium fuel cycles compared to uranium fuel cycles” J. Phys. IVFrance 9 pp. 7‑73.

Fuel Storage
Spent thorium fuel contains significant quantities of 233Pr, ($$T_{1⁄2} = 27$$ days), this nuclide is of limited significance to the uranium fuel cycle. Protactinium is very challenging to separate from uranium [Pigford 1999]. It is important to recover protactinium as it is a breeder of fissile 233U. The decay of 233Pr into 233U is also important due to its continually creating fissile material following the shutdown of a reactor core. This requires that there is close monitoring of the core reactivity.

[Pigford 1999] Pigford, T.H., 1999, “Thorium fuel cycles compared to uranium fuel cycles” J. Phys. IV France 9 pp. 7-73.