Fast Reactors and Thorium

Special topic 1: Fast reactors

Nearly all of the reactors for the generation of electricity in operation are ‘thermal’ reactors, such as the PWR type described earlier. The term thermal refers to the slowing down of the fast neutrons emitted in nuclear fission (energies measured in MeV) to energies characteristic of the temperature of the reactor (measured in tens of meV). This slowing down is carried out by a material called the moderator and it happens by a simple transfer of momentum largely due to elastic collisions of neutrons with the nucleus of the moderator atoms. You will learn more about moderators in Week 2, where the physics of reactors is covered in more detail. Thermal reactors benefit from the enhanced chance of nuclear fission when thermal neutrons come close to and interact with a U-235 or Pu-239 nucleus.

The slowing of the fast neutrons is not essential, however, for a controllable nuclear reactor, and such thinking lies at the heart of a different class of reactors: the ‘fast reactors’. A fast reactor has no moderator, merely a coolant with which useful energy can be extracted (e.g. for electricity generation). Typical coolants used in experimental, prototype, and demonstration fast reactors have included liquid sodium, inert gases and lead (or lead alloys). Fast reactors have been investigated throughout the whole history of commercial nuclear electricity generation. Indeed the world’s first reactor was a small 25 kW (thermal power) research reactor, which was built in 1946 at Los Alamos in New Mexico USA and named ‘Clementine’. Despite their long history, fast reactors have so far failed to break through to commercial success.

There are some important technical differences and issues with fast reactor operation:

·         the neutron flux (neutrons per second per unit area) is typically much higher than in a thermal reactor

·         the reactor core can be very compact

·         loss of coolant accident (LOCA) safety relies on different principles for reactivity than in, for example, a PWR. Namely a decrease in reactivity (negative thermal coefficient of reactivity) rather than from a loss of moderator (these terms will be defined in Week 2)

·         fast reactors have the potential for much more efficient fuel utilisation especially when associated with closed nuclear fuel cycles (see Week 3).

The failure of fast reactors to break through relates to significant engineering challenges. Metal-cooled fast reactors have optically opaque coolants rendering fuel inspection difficult and they face further challenges specific to various types of fast reactor: e.g. the need to keep hot sodium coolant away from water; and the build-up of highly radioactive isotopes (i.e. polonium-210 in lead cooled systems) and gas-cooled fast reactors face a heat removal challenge, especially in accident scenarios.

The UK abandoned its substantial (sodium cooled) fast reactor programme in 1994. Arguably Russia has made the greatest progress with the technology (including the BN-600 and BN-800 power plants). Despite the challenges fast reactors continue to attract much technical interest in the UK and around the world given their potential fuel cycle efficiencies. India, for example, has a major fast reactor programme based at the Indira Ghandi Centre for Atomic Research near Chennai in the south-east of the country.

Special topic 2: Thorium

The element thorium is a slightly radioactive, naturally occurring heavy metal with very few industrial uses. Thorium-rich material is accumulating in places where valuable rare earth metals are being extracted and processed. Rare earth elements are widely used in magnets making possible, for example, lightweight battery-powered power tools.

The mineral from which rare earth elements are often obtained is monazite, which can be found as beach sands. Monazite is a phosphate mineral rich in both rare earths and thorium although the thorium frequently is of such low commercial value that it is not processed to metal but rather left in the form of a thorium rich residue of the rare earth processes. Nuclear physics considerations, including decay half-life, suggest that the world should see thorium in quantities roughly three times as abundant as uranium. Thus far, however, exploration and extraction is falling short of theoretical expectations with only 6.5–7.4MT of thorium reported compared with 5.4 MT of reasonably assured uranium reserves (Barthel and Tulsidas, 2011; OECD, 2010). One must acknowledge however that much rare-earth extraction activity occurs in China, but very little information emerges from China about thorium resources. This information gap may have a role in explaining the apparent shortfall in reported thorium reserves.

Thorium is termed a ‘fertile’ nuclear fuel; that is, fissile material must be created in order for fission to occur. Furthermore, unlike uranium, thorium essentially occurs naturally in only one isotopic form (thorium-232). More than 99.99% of naturally occurring thorium is Th-232; in contrast 99.3% of uranium is the fertile isotope uranium-238, the remainder being almost entirely fissile uranium-235. For those that are curious, there is a second thorium isotope (Th-230, sometimes called ’ionium’). Th-230 however is not like uranium -235, rather it is like radium. Thorium-230 is a daughter product of the radioactive decay of a heavier element. As such the geological presence of Th-230 is quite different from that of U-235. Th-230 is not necessarily geologically co-located with Th-232. The important point is that in nature one can find places with extremely isotopically pure geological deposits of Th-232. This is different from the story with uranium where there is always a U-235 component of around 0.7%.

As a fertile nuclear fuel, thorium (Th-232) is reminiscent of fertile uranium-238. It is noteworthy that in an entirely uranium-fuelled nuclear power station a significant part of the energy produced (>25%) actually arises from the fission of Pu-239, rather than U-235. This is because of the in-reactor conversion of U-238 to Pu-239.

The U-238 to Pu-239 process is:

                                             Beta decay                                                       Beta decay

U-238 + n →  U-239         →                                       Np-239                 →                                         Pu-239

                                             Half-life 23.5 min                                            half-life 2.36 days

Equivalent to the role of Pu-239 in a conventional uranium fuelled reactor, in a thorium-fuelled reactor the energy comes from the fission of U-233 produced in the reactor itself.

The Th-232 to U-233 process is:

                                             Beta decay                                                       Beta decay

Th-232 + n →  Th-233      →                                        Pa-233               .→                                       U-233

                                             Half-life 21.8 min                                            half-life 27.0 days

There are three viable candidate fissile materials for nuclear energy: U-235, Pu-239 and U-233. Of these, only U-235 is naturally occurring, and only U-235 and Pu-239 have so far played a role in commercial power generation. Thorium for power generation is currently a topic for research, development and demonstration.

Proponents of thorium point to various possible benefits, including its expected greater natural abundance than uranium, and claims are sometimes made concerning high levels of proliferation resistance. The latter point can be somewhat complex. While it is true that thorium fuel cycles involve almost no production of either Pu-239 or U-235, the essential role of fissile U-233 in thorium-based nuclear energy means there is inevitably a proliferation risk at some level. No fuel cycle should ever be considered proliferation proof. The most one can hope for is merely higher levels of resistance.

It is sometimes asserted that thorium was overlooked in the development of nuclear energy because of the existence of a military industrial complex pushing for U–Pu based nuclear energy because of the synergies with nuclear weapons programmes. However, such arguments are not entirely convincing. Th-232 is a fertile isotope with no naturally occurring fissile partner (there is no naturally occurring equivalent of U-235 for thorium). As such, for thorium fuels to get started one needs to source significant quantities of neutrons. In the twenty-first century, it is possible to get such neutrons from proton accelerators via neutron spallation; in the mid-twentieth century, however, such options were generally not available and you would need a nuclear reactor, as suitable proton accelerators had simply not been developed back then. Any twentieth century reactor would have been fuelled with uranium using the only fissile isotope found in nature, U-235. Twentieth century nuclear power simply had to start from U-235; there was nothing else available. 

While the resource and security benefits of thorium can sometimes be overplayed, it does represent an important option going forward. As such it is just one way in which to stretch the future of nuclear power beyond the era of open-cycle mined uranium (Grimes and Nuttall, 2010). Indeed research is underway around the world to explore the benefits of thorium fuelled nuclear power.

Bill Nuttall, with Stephen Ashley and colleagues from Cambridge University, have researched thorium in collaboration with Bhabha Atomic Research Centre in India. That work has confirmed the importance of closing the nuclear fuel cycle in order for substantial benefits to be seen from a move to thorium.

Nuttall et al. (2015) have concluded: ‘that thorium fuel offers little benefit over conventional uranium-fuelled approaches for open-cycle nuclear energy production limited to the Low-Enriched Uranium standard of 20% 235U. We suggest that short- to medium-term interest in thorium should be restricted to those countries with an interest in spent nuclear fuel reprocessing or with a need to reduce inventories of fissile material (such as separated plutonium).’

Further reading

India has long planned to implement a three-stage nuclear programme with thorium utilisation at its heart. You can download a PDF of the article from the MRS Bulletin website at https://www.cambridge.org/core/journals/mrs-bulletin/article/indias-thorium-based-nuclear-vision/DD73D2B3C1266299AA88624CE9A35ABE.

References

Barthel, F. and Tulsidas, H. (2011) World Thorium Resources and Deposits, presentation from the IAEA Technical Meeting on World Thorium Resources.

The OECD Nuclear Energy Agency and the International Atomic Energy Agency (2010) Uranium 2009: Resources, Demand and Production, ISBN: 978-92-640-47907.

Grimes, R.W. and Nuttall, W.J. (2010) ‘Generating the Option of a Two-Stage Nuclear Renaissance’, Science, Vol. 329, pp. 799–803, ISSN: 0036-8075

Nuttall, W.J., Ashley, S.F., Fenner, R.A., Krishnani, P.D. and Parks, G.T., ‘Technology Assessment of Near-Term Open-Cycle Thorium-Fuelled Nuclear Energy Systems, presentation at Thorium Energy Conference (ThEC15), October 12-15, 2015, Mumbai, India)

Acknowledgements

The authors are most grateful to Stephen F. Ashley for his assistance on matters relating to thorium.