|Fig. 1: ADSR schematic (Source: Wikimedia Commons)|
The concept of an accelerator-driven subcritical reactor (ADSR) is based on the idea of coupling a fission reactor to a particle accelerator. Currently, the production of energy from nuclear power plants requires the use of critical reactors, in which the number of neutrons released by fission is just enough to sustain a controlled chain reaction. An ADSR relies instead on a subcritical reactor, in which more neutrons are absorbed than generated and a self-sustained chain reaction is not possible. To run such a device at a constant fission (and power) level, additional neutrons are created by bombarding a heavy metal target inside the reactor with a high-energy proton beam supplied by an accelerator (a process known as spallation). A fraction of the reactor's energy output is in turn used to power the accelerator. The main advantage of an ADSR is increased safety, by theoretically eliminating the risk of criticality accidents.  However, several technical challenges need to be overcome before a fully operational system could be built. Apart from the production of energy, ADSRs have also been proposed as an effective way to transmute radioactive isotopes contained in the spent fuel of existing reactors, thus contributing to the solution of the problem of nuclear waste disposal.
A potential substantial increase in the demand for nuclear energy would necessitate the adoption of a more efficient use of the world reserves of nuclear fuel. This, in turn, would require the use of breeder reactors, which produce the fissile fuel they consume by irradiating suitable fertile nuclei with neutrons. The choice of thorium (Th-232) is particularly attractive, partly because it is more abundant than the uranium alternative (U-238). Upon capturing a neutron, a Th-232 nucleus becomes Th-233. The decay chain of the latter produces the fissile uranium isotope U-233, which has fission characteristics similar to those of U-235. Another advantage of thorium is that its irradiation produces minimal amounts of plutonium and other transuranic elements such as neptunium, americium and curium. This substantially reduces proliferation risks (due to Pu-239 and its use in nuclear weapons) as well as the radiological hazards posed by heavy minor actinides in the spent reactor fuel.
The most prominent proposal for an ADSR using thorium fuel and optimized for energy production is that of Carlo Rubbia, which referred to this concept as the Energy Amplifier.  This is a reactor with a fast neutron spectrum and a subcritical core which contains thorium and an initial quantity of fissile material, such as plutonium from a conventional light water reactor. Molten lead is chosen as coolant, both for its thermodynamic properties and because it does not slow down the neutrons. Apart from extracting heat from the core, the coolant also acts as a spallation target for neutron production. To drive the system, a proton accelerator with an energy of 1 GeV and a beam current of about 10 mA is required. To achieve such specifications, one can use accelerators with circular geometry (such as cyclotrons) or linear accelerators (linacs).  Since the power output (with a nominal thermal power of about 1500 MW) is to be regulated by variations of the proton beam intensity, no control rods are necessary. Reprocessing of the spent fuel consists of thorium replenishment and fission product removal. The minor actinides are reloaded in the reactor as their high fission probability with fast neutrons allows for their more efficient incineration (i.e. destruction by fission).
Apart from safety, another notable advantage of a subcritical system is that it can utilize fuel with less than optimal neutronic properties. In particular, a conventional critical reactor largely relies on delayed neutrons (i.e. neutrons emitted after a fission event by one of its fragments) for keeping the chain reaction under control. Compared to uranium, minor actinides emit a smaller number of delayed neutrons and would therefore be considered unsafe for use in the existing systems. On the other hand, an ADSR has a higher tolerance level with respect to the fuel's delayed neutron fraction and could be used for large-scale minor actinide incineration. 
Relevant ADSR proposals, focusing more on waste transmutation and incineration, have been put forth by Bowman.  They generally refer to a thermal subcritical reactor in which the fuel (in molten salt form) consists of a quantity of actinides to be incinerated (plutonium or heavy MAs such as americium or curium). An additional amount of solid thorium may also be used for extra neutron production. The liquid fuel circulates continuously through an extraction facility which removes the stable and short-lived fission products. The spallation neutrons are generated by directing a 1 GeV proton beam against a lead target inside the reactor and are then thermalized using heavy water as moderator. Such a thermal system requires less fissile material to run but is more complex to build than a fast reactor.
The main technical challenge for ADSRs has to do with developing a reliable accelerator with sufficient power to drive the reactor (an average proton beam power of 10 MW is required for the Energy Amplifier). At present, proton accelerators with energy in the 1 GeV range are limited to about 1.5 MW in terms of beam power. For instance, the synchrotron for the Spallation Neutron Source at Oak Ridge National Laboratory has a design power of 1.44 MW (at 1 GeV), though upgrades to 3 MW are envisioned.  Another important figure of merit is the beam availability, which should be improved from the current 85-90% level to at least 95% if commercial ADSR operation is to be achieved.  Recently, the MYRRHA project has sparked renewed interest in pursuing an R&D program for ADSRs.  MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) is a fast, subcritical system under development by the Belgian Nuclear Research Centre (SCK-CEN) with EU (and possible international) funding. The main objective of the project is to demonstrate the ADSR concept at a power level (100 MW) which would allow a future extrapolation to an industrial-scale reactor and also to study minor actinide transmutation techniques. The driving accelerator is a 600 MeV proton linac with an average current of 4 mA. A Lead-Bismuth eutectic is used as coolant while mixed oxide (MOX) fuel forms the core of the reactor. A critical mode is also available for reactor research and radioisotope production. Total cost is estimated at $1.2 billion, with the facility projected to be fully operational in 2023.
Accelerator-driven subcritical reactors are hybrid systems that promise a range of important advantages over conventional reactors. These include increased safety (and therefore better public acceptance of nuclear power), less proliferation issues, greatly reduced radioactive waste production and the potential to use them for transmuting existing nuclear waste. Though a lot of additional research and development (mainly in developing the required accelerator technologies) is necessary before this concept can be realized, a large-scale project such as MYRRHA shows that there is active interest in pursuing the idea of an ADSR.
© Panos Baxevanis. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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