The essence of a conventional nuclear power reactor is the controlled fission chain reaction of uranium-235 (235U) and plutonium-239 (239Pu). This produces heat which is used to make steam to drive a turbine. The chain reaction depends on having a surplus of neutrons to keep it going (fission of 235U requires one neutron input and produces on average 2.43 neutrons).
For many years there has been interest in utilizing thorium (Th-232) as a nuclear fuel since it is three times as abundant in the Earth's crust as uranium. Also, all of the mined thorium is potentially useable in a reactor, compared with the 0.7% of natural uranium, so some 40 times the amount of energy per unit mass might be available. A thorium reactor would work by having Th-232 capture a neutron to become Th-233 which decays to uranium-233, which then fissions. The problem is that insufficient neutrons are generated to keep the reaction going.
More recently, there has been interest in transmuting the long-lived transuranic radionuclides (the 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, the main long-lived ones are actinides.
Accelerator-driven systems (ADS) address both of these issues. ADSs are seen as safer than normal fission reactors 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'.
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 (e.g., 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 (at 97% of criticality).
A number of research facilities exist that explore this phenomenon, and there are plans for much larger ones. However, in all of these facilities, the heating of the target is largely that of the incident proton beam, and none comes close to using the generated neutrons to sustain a chain reaction.
A matter of dispute in Australia has been whether such devices might be a sufficient source of neutrons so that they are able to produce commercial quantities of radioisotopes, thereby rendering a new research reactor at Lucas Heights redundant. At present and forseeably, the scale of the facility is too small.
If the spallation target is surrounded by a blanket assembly of nuclear fuel, such as fissile isotopes of uranium or plutonium (or thorium capable of breeding uranium-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, currently 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, utilizing "near breeder" reactors.
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 spent nuclear fuel.
One approach is to start with fresh spent fuel from conventional reactors in the outer blanket region and progressively move it inwards. The fuel 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 (MOX) fuel in conventional reactors.
Two alternative strategies are envisaged: (1) 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 (2) the plutonium and minor actinides being managed together, providing better proliferation resistance but posing some technical challenges. Both strategies are capable of achieving major reduction in waste radiotoxicity, and the first would add only 10-20% to electricity costs.
However, along with fission products, the process generates spallation products from the target material, in direct proportion to the energy of the proton beam. 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 is significantly reduced, by 1000 years, and is less than that of the equivalent uranium ore.
The French Atomic Energy Commission is funding research on the application of this process to nuclear wastes from conventional reactors, as is the U.S. Department of Energy since its 1999 budget term. 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), a process 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 seem to rule out early use.
A study conducted by the Nuclear Energy Agency (NEA) showed that multiple recyclings of the fuel would be necessary to achieve major (e.g., 100-fold) reductions in radiotoxicity, and also that the full potential of a transmutation system can be exploited only with a commitment of one hundred years or more.
- WNA paper on Accelerator-driven nuclear energy
- 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 39/2/97
- Treulle, H. 2002, The answer is No - Does transmutation of spent nuclear fuel produce more hazardous material then it destroys?, Radwaste Solutions July-August.
- Nucleonics 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.