Thorium

Unfortunately, only a small part of the nuclear industry selected it to develop civil power plants. Instead of that, the great majority decided to invest in the Uranium-based technology, originally developed for the submarines. The history showed us this was maybe not the best choice, as the fission of Uranium generates very unstable products, in the family of Minor Actinides.

With a proper use of Thorium, none of the actual problems faced by the nuclear industry would have been encountered.

Thorium history

As stated by the IAEA(*), “during the pioneering years of nuclear energy, from the mid-1950s to mid-1970s, there was considerable interest worldwide to develop thorium fuels and fuel cycles in order to supplement uranium reserves. The feasibility of thorium utilization has been demonstrated in high temperature gas cooled reactors (HTGR), light water reactors (LWR), pressurized heavy water reactors (PHWRs), liquid metal cooled fast breeder reactors (LMFBR) and molten salt breeder reactors (MSBR).

The initial enthusiasm on thorium fuels and fuel cycles was not sustained among the developing countries later, due to new discovery of uranium deposits and their improved availability”.

A second reason for the disinterest in Thorium-based reactors, lies in the fact that the Uranium-based industry received an enormous support from the military lobby, because their civil reactors could produce a by-product they dramatically needed… called Plutonium.

After the Three Miles Island accident and the Chernobyl catastrophe in 1986, the growth of nuclear power dramatically slowed down, particularly in the USA and Europe.

Even the safe Thorium power plants operating in Germany (THTR300) has been stopped !

Thorium today

Recent concerns about global warming, and the Kyoto accords limiting CO₂ suggest that future energy demands cannot be met solely through the burning of fossil fuels. And on the other end, the sustainable energy sources will never be able to provide 100% of the energy needed by our modern civilization. Taking those constraints into account, a return to some reliance on the nuclear option is required.

In this context, Thorium-based cycles that offer proliferation-resistance, that have improved waste form characteristics, and that permit reduction of plutonium inventories receive renewed interest in in several developed countries.

IAEA argues also that in the mid-term, Thorium will be mandatory to replace Uranium in the actual nuclear industry. “the annual world requirements of uranium is expected to grow from the present level of some 66.000 tonnes ‘U’ to nearly 82.000 tonnes ‘U’ by the year 2025. Nowadays, the world uranium production (36.042 tonnes) provides about 54% of world reactor requirements (66 815 tonnes), with the remainder being met by secondary sources, including civilian and military stockpiles, uranium reprocessing and re-enrichment of depleted uranium. However, by 2025, secondary sources will decline in importance and provide only about 4–6% of requirements, depending upon the demand projections used. At that juncture, introduction of thorium fuel cycle will play a complementary role” (IAEA, 2005)

Thorium benefits and challenges

According to IAEA (IAEA, 2005), Thorium fuels and fuel cycles have the following benefits and challenges :

Benefits

•        Thorium is 3 to 4 times more abundant than uranium, widely distributed in nature as an easily exploitable resource in many countries and has not been exploited commercially so far.

•        Thorium fuel cycle is an attractive way to produce long term nuclear energy with low radiotoxicity waste.

•        Unlike natural uranium, which contains ~0.7% ‘fissile’ ²³⁵U isotope, natural thorium does not contain any ‘fissile’ material and is made up of the ‘fertile’ ²³²Th isotope only”. To be able to fission, this fertile element has to be transformed into fissile ²³³U by the capture of one neutron. The fission further happens by absorption of a second neutron.

•        For the ‘fissile’ ²³³U nuclei, the number of neutrons liberated per neutron absorbed is greater than 2.0 over a wide range of thermal neutron spectrum, unlike ²³⁵U and ²³⁹Pu. Thus, contrary to ²³⁸U–²³⁹Pu cycle in which breeding can be obtained only with fast neutron spectra, the ²³²Th–²³³U fuel cycle can operate with fast, epithermal or thermal spectra.

•        Thorium dioxide is chemically more stable and has higher radiation resistance than uranium dioxide. The fission product release rate for ThO₂–based fuels are one order of magnitude lower than that of UO₂. ThO₂ has favorable thermo-physical properties because of the higher thermal conductivity and lower co-efficient of thermal expansion compared to UO₂.

•        ThO₂ is relatively inert and does not oxidize unlike UO₂, which oxidizes easily to U₃O₈ and UO₃. Hence, long term interim storage and permanent disposal in repository of spent ThO₂–based fuel are simpler without the problem of oxidation.

•        Thorium–based fuels and fuel cycles have intrinsic proliferation-resistance due to the formation of ²³²U during the production of ²³³U. And some of the daughter products of ²³²U, like ²¹²Bi and ²⁰⁸Tl, emit strong gamma radiations.

•        In ²³²Th–²³³U fuel cycle, much lesser quantity of plutonium and long-lived Minor Actinides (MA: Np, Am and Cm) are formed as compared to the ²³⁸U–²³⁹Pu fuel cycle, thereby minimizing the radiotoxicity associated in spent fuel.

Challenges

•        The melting point of ThO₂ (3350°C) is much higher compared to that of UO₂ (2800°C). Hence, a much higher sintering temperature (>2000°C) is required to produce high density ThO₂ and ThO₂–based mixed oxide fuels.

•        As ThO₂ and ThO₂–based mixed oxide fuels are relatively inert, they do not dissolve easily.

•        The significant amount of gamma emitting products necessitates remote and automated reprocessing and re-fabrication in heavily shielded hot cells, which increases in the cost of fuel cycle activities.

•        In the conversion chain of ²³²Th to ²³³U, ²³³Pa is formed as an intermediate, which has a relatively longer half-life (~27 days) as compared to ²³⁹Np (2.35 days) in the uranium fuel cycle. Thereby this requires a longer cooling time for completing the decay of ²³³Pa to ²³³U.

•        The three stream process of separation of uranium, plutonium and thorium from spent (Th, Pu)O₂ fuel, though viable, is yet to be developed.

•        The database and experience of thorium fuels and thorium fuel cycles are very limited, as compared to UO₂ and (U, Pu)O₂ fuels, and need to be augmented before large investments are made for commercial utilization of thorium fuels and fuel cycles. For example, some radionuclides produced during the reaction such as ²³¹Pa, ²²⁹Th and ²³⁰U have radiological impact and have to be managed in the fuel cycle.

(*) IAEA-TECDOC-1450 “Thorium fuel cycle — Potential benefits and challenges” May 2005

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