Related Pages
(March 2009)
The essence of a conventional nuclear reactor is the controlled fission chain reaction of U-235 and Pu-239. This produces heat which is used to make steam which drives a turbine. The chain reaction depends on having a surplus of neutrons to keep it going (a U-235 fission requires one neutron input and produces on average 2.43 neutrons).
For many years there has been interest in utilising thorium (Th-232) as a nuclear fuel since it is three to five times as abundant in the Earth's crust as uranium. A thorium reactor would work by having Th-232 capture a neutron to become Th-233 which decays to uranium-233, which fissions. The problem is that insufficient neutrons are generated to keep the reaction going, and so driver fuel is needed – either plutonium or enriched uranium. Just as with uranium, if all of it and not a mere 0.7% of uranium is to be used as fuel, fast neutron reactors are required in the system.
More recently there has been interest in transmuting the long-lived transuranic radionuclides (actinides - neptunium, americium and curium particularly) formed by neutron capture in a conventional reactor and reporting with the high-level waste. If these could be made into shorter-lived radionuclides such as fission products, the management and eventual disposal of high-level radioactive waste would be easier and less expensive. As it is, most radionuclides (notably fission products) decay rapidly, so that their collective radioactivity is reduced to less than 0.1% of the original level 50 years after being removed from the reactor. However, a significant proportion of the separated high-level wastes is long-lived actinides.
Accelerator-driven systems (ADS) address both these issues. They are seen as safer that a normal fission reactor because they are subcritical and stop when the input current is switched off. This is because they burn material which does not have a high enough fission-to-capture ratio for neutrons to enable criticality and maintain a fission chain reaction. It may be thorium fuel, or actinides which need 'incineration'. An ADS can only run when neutrons are supplied to it.
Spallation
The capability of high-current, high-energy accelerators to produce neutrons by spallation from heavy elements has been used in the structural research of such materials. In this process a beam of high-energy protons (usually >500 MeV) is directed at a high-atomic number target (eg tungsten, tantalum, depleted uranium, thorium, zirconium, lead, lead-bismuth, mercury)and up to one neutron can be produced per 25 MeV of the incident proton beam. (These numbers compare with 200-210 MeV released by the fission of one uranium-235 or plutonium-239 atom.) A 1000 MeV beam will create 20-30 spallation neutrons per proton. Some are captured but the others go on to cause fissions at the rate of about 400 per source proton (@ 97% of criticality).
A number of research facilities exist which explore this phenomenon, and there are plans for much larger ones. The protons come from linear accelerators or cyclotrons. However, in all of them the heating of the target is largely that of the incident proton beam, none comes close to using the generated neutrons to sustain a chain reaction.
Energy Amplifier, ADS
If the spallation target is surrounded by a blanket assembly of nuclear fuel, such as fissile isotopes of uranium or plutonium (or thorium which can breed to U-233), there is a possibility of sustaining a fission reaction. This is described as an Accelerator-Driven System (ADS). In this, up to ten percent of the neutrons could come from the spallation, though it would normally be less, even where actinide incineration is the main objective.
In such a subcritical nuclear reactor the neutrons produced by spallation would be used to cause fission in the fuel, assisted by further neutrons arising from that fission. One then has a nuclear reactor which could be turned off simply by stopping the proton beam, rather than needing to insert control rods to absorb neutrons and make the fuel assembly subcritical. The fuel may be mixed with long-lived wastes from conventional reactors (see below). India is actively researching ADS as an alternative to its main fission program focused on thorium.
There have been proposals to develop a prototype reactor of this kind, based on the thorium-U-233 fuel cycle and using fast neutrons. Professor Carlo Rubbia is the main advocate, but at a national level, India is the country with most to gain, due to its very large thorium resources.
India is already running a very small research reactor on U-233 fuel extracted from thorium which has been irradiated and bred in another reactor. When this started in 1996 it was hailed as a first step towards the thorium cycle there, utilising "near breeder" reactors.
The core of an ADS is mainly thorium, located near the bottom of a 25 metre high tank. It is filled with some 8000 tonnes of molten lead or lead-bismuth at high temperature - the primary coolant, which circulates by convection around the core. Outside the main tank is an air gap to remove heat if needed. The accelerator supplies a beam of high-energy protons down a beam pipe to the spallation target - the lead or lead-bismuth - inside the core, and the neutrons produced enter the fuel and transmute the thorium into protactinium, which soon decays to U-233 which is fissile. The neutrons also cause fission in uranium, plutonium and possibly transuranics present, releasing energy. A 10 MW proton beam might thus produce 1500 MW of heat (and thus 600 MWe of electricity, some 30 MWe of which drives the accelerator). With a different, more subcritical, core a 25 MW proton beam would be required for the same result. Today's accelerators are capable of only 1 MW beams.
What was claimed to be the world’s first ADS experiment was begun in March 2009 at the Kyoto University Research Reactor Institute (KURRI), utilizing the Kyoto University Critical Assembly (KUCA). The research project was commissioned by Japan’s Ministry of Education, Culture, Sports, Science and Technology (MEXT) six years earlier. The experiment irradiates a high-energy proton beam (100 MeV) from the accelerator on to a heavy- metal target set within the critical assembly, after which the neutrons produced by spallation are bombarded into a subcritical fuel core.
"Incinerator"
The other role of a subcritical nuclear reactor or ADS is the destruction of heavy isotopes, particularly actinides but also longer-lived fission products such as Tc-99 and I-129. Here the blanket assembly is actinide fuel and/or used nuclear fuel.
One approach is to start with fresh used fuel from conventional reactors in the outer blanket region and progressively move it inwards. It is then removed and reprocessed, with the uranium recycled and most fission products separated as waste. The actinides are then placed back in the system for further transmutation by fission.
In the case of atoms of odd-numbered isotopes heavier than thorium-232, they have a high probability of absorbing a neutron and subsequently undergoing nuclear fission, thereby producing some energy and contributing to the multiplication process. Even-numbered isotopes can capture a neutron, perhaps undergo beta decay, and then fission. A fast neutron spectrum enables maximum fission with minimum build-up of new actinides due to neutron capture. This process of converting fertile isotopes to fissile ones is called breeding.
Therefore in principle, the subcritical nuclear reactor may be able to convert all transuranic elements into (generally) short-lived fission products and yield some energy in the process. Much of the current interest is in the potential of ADS to burn weapons-grade plutonium, as an alternative to using it as mixed oxide fuel in conventional reactors.
Two alternative strategies are envisaged: The plutonium and minor actinides being managed separately, with the latter confined to a small, dedicated part of the fuel cycle while plutonium is burned in fast reactors; and the plutonium and minor actinides being managed together, providing better proliferation resistance but posing some technical challenges. Both can achieve major reduction in waste radiotoxicity, and the first would add only 10-20% to electricity costs.
However, as well as fission products, the process generates spallation products from the target material, in direct proportion to the energy of the proton beam. Some of these are volatile and will find their way into the cover gas system above the coolant, posing a major maintenance challenge. Their radiotoxicity is likely to exceed that of the fission products in the short term, which is relevant to operation and storage rather than final disposal. Ultimately the burning of actinides means that overall radiotoxicity of them is reduced greatly by the time 1000 years has elapsed, and is then less than that of the equivalent uranium ore.
For longer-lived fission products such as Tc-99 and I-129 (213,000 and 16 million years respectively), these can acquire a neutron to become Tc-100 and I-130 respectively which are very short-lived and beta decay to Ru-100 and Xe-130 which are stable.
The French Atomic Energy Commission is funding research on the application of this process to nuclear wastes from conventional reactors, as is the US Department of Energy. The Japanese Omega project envisages an accelerator transmutation plant for nuclear wastes operated in conjunction with ten or so large conventional reactors. The French concept similarly links a transmutation-energy amplifying system with about eight large reactors.
Other research has been proceeding in USA, Russia and Europe.
Commercial application of partitioning and transmutation (P&T), which is attractive particularly for actinides, is still a long way off, since reliable separation is needed to ensure that stable isotopes are not transmuted into radioactive ones. New reprocessing methods would be required, including pyroprocessing. The cost and technology of the partitioning together with the need to develop the necessary high-intensity accelerators seems to rule out early use.
An NEA study showed that multiple recycling of the fuel would be necessary to achieve major (eg 100-fold) reductions in radiotoxicity, and also that the full potential of a transmutation system can be exploited only with commitment to it for one hundred years or more.
A 2008 Norwegian study summarised the advantages and disadvantages of an ADS fuelled by thorium, relative to a conventional nuclear power reactor, as follows, and said that such a system was not likely to operate in the next 30 years:
See also: Processing of Nuclear Wastes paper in this series.
Sources:Boldeman, J.W., 1997, Accelerator driven nuclear energy systems, AATSE symposium "Energy for Ever".Arkhipov, V., 1997, Future Nuclear Energy Systems: generating electricity, burning wastes, IAEA Bulletin 39/2/97Treulle, H. 2002, The answer is No - Does transmutation of spent nuclear fuel produce more hazardous material then it destroys?, Radwaste Solutions July-AugustNucleonics Week 7/11/96.Euradwaste summary 3/2/00.Bertel, E. et al 2003, P&T: A long-term option for radioactive waste disposal? NEA News 20.2.Thorium Report Committee 2008, Thorium as an Energy Source - opportunities for Norway. Norwegian Ministry of Petroleum and Energy.