The Energy Amplifier
Imagine a nuclear reactor producing no long-lived nuclear waste with a greatly reduced risk of meltdown. Add that the same reactor can use abundantly available thorium as fuel, and you have Carlo Rubbia's proposed “Energy Amplifier.” In 1993, the Nobel Laureate outlined how such a reactor could be built and has since lobbied for resources to construct a test reactor. The proposed design entails significant technical and economic challenges, but the potential benefit of such a reactor could be vast, providing enough energy for mankind for up to 200,000 years.
Nuclear fission is a process in which a heavy nucleus splits into two smaller nuclei, releasing energy and free neutrons. While fissile nuclei can be split directly by bombarding them with neutrons, fertile nuclei need to absorb a neutron first, and only then can they decay into fissile nuclei. In a conventional nuclear power plant, the reactor is driven at criticality. This means there are enough fissile nuclei in the reactor to achieve a chain reaction where an equal number of neutrons are generated and absorbed. Rubbia suggested producing the neutrons indirectly through the use of a proton accelerator, enabling the reactor to run subcritically. The required neutrons are knocked out from lead atoms by bombarding them with high-energy protons. Herein lies the main technological and economic challenge of the design: building a high-energy and high-power proton source.Rubbia suggested using a three-stage cyclotron design seen in particle accelerators. No cyclotron with a sufficiently high power and energy output has ever been built. Recent developments at the Fermi National Accelerator Laboratory near Chicago may make such a cyclotron more feasible. The Energy Amplifier was also tested on a small scale in CERN, Geneva in 1995. The free neutrons were successfully generated by a proton beam and caused fission in a uranium target.The Energy Amplifier can run on fuel using both fissile and fertile elements. It can also potentially be used to reduce plutonium stockpiles. Moreover, it can tap into the vast reserves of thorium found on Earth. Thorium is a fertile material that can be converted into fissile uranium. The Energy Amplifier can also destroy actinides, the main components of long-lived nuclear waste. Elimination of such nuclear wastes would make long-term storage solutions unnecessary. The attractive waste characteristics of the Energy Amplifier can only be realized by reprocessing. Reprocessing of nuclear fuel separates the fission products from the rest of the fuel. The remains are then reformed with some extra thorium to create new fuel rods. The small amount of actinides created would therefore be recycled and always kept in the reactor. Unfortunately, the chemical processes to reprocess fuel from the Energy Amplifier have not yet been fully developed.A conventional nuclear reactor has a risk of going supercritical, meaning that more neutrons are generated from the fission reactions than absorbed. This leads to an uncontrolled chain reaction which can cause disasters like Chernobyl. Since the Energy Amplifier runs subcritically, this risk is greatly reduced. The Energy Amplifier is also designed to use a closed lead convection cooling system, which would enable cooling of the reactor without supply of power. This would eliminate the risk of accidents that create a loss of cooling, such as the accident at the Fukushima Dai-ichi nuclear power plant in Japan. The Energy Amplifier will shut itself down if it overheats, even without human intervention.
Of major concern is the proliferation of nuclear weapons through spent fuel reprocessing. The most commonly used reactors allow for breeding of plutonium, which can easily be separated from the spent fuel to create nuclear weapons. In the Energy Amplifier, only small amounts of plutonium would be generated. However, the greatest proliferation danger of the Energy Amplifier seems to be from breeding fissile fuel using the proton beam that drives the reactor. It would be possible to direct the beam into a lead target to generate neutrons for breeding plutonium from fertile uranium which could then be used to make nuclear weapons. Therefore, the Energy Amplifier does not solve the proliferation issue.
While the technology is very promising, the main obstacle is economic—the great risk of investing a very large sum of money in an unproven technology when significant problems may occur during development. The advanced particle accelerator would need the biggest investment. The process for reprocessing spent nuclear fuel necessary to get the full benefits of the Energy Amplifier is also not well-explored since the thorium nuclear fuel cycle has not yet been fully implemented. Alternative nuclear reactor technologies may also prevent the Energy Amplifier from entering the market.
The Norwegian company Aker Solutions recently bought Rubbia's patent and is raising money to make a prototype reactor. Some parameters have been adjusted from Rubbia's original design to enable the use of a smaller, less costly particle accelerator. The new design seeks to run the reactor closer to criticality, thus requiring fewer neutrons from the accelerator to produce power. Aker Solutions has not yet commented on whether running nearer criticality could have a negative impact on the safety of the reactor.
The Energy Amplifier is a promising prospect for the nuclear power generation industry as it potentially solves the major problems of both long-term fuel supply and long-lived radioactive waste. The safety features of the Energy Amplifier also compare very favorably to other nuclear reactors. However, the proliferation concern does not appear to be fully addressed. There are also significant technical problems remaining. Fifteen years after the original paper by Rubbia, the Energy Amplifier has caught the eye of Aker Solutions. Perhaps this firm will eventually be able to fully develop and commercialize this novel reactor technology.
Cover image by Fermilab, source: "Considering an Alternative Fuel for Nuclear Energy," The New York Times.