A subcritical reactor is a nuclear fission reactor that produces fission without achieving criticality. Instead of a sustaining chain reaction, a subcritical reactor uses additional neutrons from an outside source. The neutron source can be a nuclear fusion machine or a particle accelerator producing neutrons by spallation.
Such a device with a reactor coupled to an accelerator is called an Accelerator-Driven System (ADS).
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A subcritical reactor can be used to destroy heavy isotopes contained in the used fuel from a conventional nuclear reactor, while at the same time producing electricity. The long-lived transuranic elements in nuclear waste can in principle be fissioned, releasing energy in the process and leaving behind the fission products which are shorter-lived. This would shorten considerably the time for disposal of radioactive waste. However, some isotopes have threshold fission cross sections and have a small effective fraction of delayed neutrons and therefore require a fast reactor for being fissioned, and for safety reasons preferably a subcritical reactor if they constitute a significant fraction of the fuel. The three most important long-term radioactive isotopes that could advantageously be handled that way are neptunium-237, americium-241 and americium-243. The nuclear weapon material plutonium-239 is also suitable although it can be expended in a cheaper way as MOX fuel or inside existing fast reactors.
Besides nuclear waste incineration, there's interest in this type reactor because they are seen as safer than normal fission reactors.[1] In most critical reactors, the nuclear chain reaction can potentially increase exponentially until the heat destroys the reactor, causing an expensive and potentially dangerous accident (see the chernobyl disaster as an example). With a subcritical reactor, the reaction will stop automatically unless continually fed neutrons from an outside source.
Most current ADS designs propose a high-intensity proton accelerator with an energy of about 1 GeV, directed towards a spallation target made of thorium that is cooled by liquid lead-bismuth in the core of the reactor. In that way, for each proton interacting in the target, an average 20 neutrons are created to irradiate the surrounding fuel. Thus, the neutron balance can be regulated so that the reactor would be below criticality if the additional neutrons by the accelerator were not provided. The main advantage is inherent safety, even if the nuclear fuel under consideration lack Uranium's self-regulating properties—like delayed neutrons and Doppler coefficient—that make standard nuclear reactors safe. Whenever the neutron source is turned off, the reaction ceases.
There are technical difficulties to overcome before ADS can become economical and eventually be integrated into future nuclear waste management. The accelerator must provide a high intensity and be highly reliable. There are concerns about the window separating the protons from the spallation target, which is expected to be exposed to stress under extreme conditions. The chemical separation of the transuranic elements and the fuel manufacturing, as well as the structure materials, are important issues. Finally, the lack of nuclear data at high neutron energies limits the efficiency of the design.
Some laboratory experiments and many theoretical studies have demonstrated the theoretical possibility of such a plant. Carlo Rubbia, a nuclear physicist and former director of CERN, was one of the first to conceive a design of a subcritical reactor, the so-called "energy amplifier". In 2005, several large-scale projects are going on in Europe and Japan to further develop subcritical reactor technology.
Subcritical reactors have been proposed both as a means of generating electric power and as a means of transmutation of nuclear waste, so the gain is twofold. However, the costs for construction, safety and maintenance of such complex installations are expected to be very high, let alone the amount of research needed to develop a practical design (see above). There exist cheaper and reasonably safe waste management concepts, such as the long-term geological disposal or transmutation in fast-neutron reactors. However, the solution of a subcritical reactor might be favoured for a better public acceptance—it is considered more acceptable to burn the waste than to bury it for hundreds of thousands of years. For future waste management, a few transmutation devices could be integrated into a large-scale nuclear program, hopefully increasing only slightly the overall costs.
While originally thought that an ADS would be a part of a light water reactor design, other proposals have been made that incorporate an ADS into other generation IV reactor concepts.
One such proposal calls for a gas cooled fast reactor that is fueled primarily by Plutonium and Americium. The neutronic properties of Americium make it difficult to use in any critical reactor due to neutronic properties that tend to make the moderator temperature coefficient more positive, decreasing stability. The inherent safety of an ADS, however, would allow Americium to be safely burned. These materials also have good neutron economy, allowing the pitch-to-diameter ratio to be large, which allows for improved natural circulation and economics.