NUCLEAR WASTE Particle accelerators have the potential to address critical issues for the future of nuclear energy By Elizabeth Clements icture a football field covered seven yards Pdeep in metal containers full of used nuclear fuel. That’s the amount of radioactive waste— roughly 65,200 metric tons—that nuclear power plants in the United States produced in the last four decades. According to the Nuclear Energy Institute, a policy organization for the nuclear technologies industry, the US nuclear industry generates 2000- plus metric tons of nuclear waste every year. Given the nation’s energy appetite, this rate of accumulation will not decrease any time soon. Yet today the only potential long-term storage option for nuclear waste in the US would be an underground geological repository, which so far doesn’t exist. A particle accelerator may contribute to an alternative solution. 22 NUCLEAR WASTE A powerful enough accelerator could gen- “A lot of technologies, including ADS, have been erate a beam of particles to help transform proposed to try to resolve the [nuclear] waste spent nuclear fuel into a re-useable form. It issue,” says Albert Machiels, a technical executive could reduce the time required for long-term at the Electric Power Research Institute, a non- geological storage from 300,000 years to 500 profit company that conducts research and devel- years. And it could use an abundant natural opment for all things related to electricity. resource, thorium, as a safer, cleaner, more “To demonstrate its viability at a scale necessary proliferation-resistant fuel for energy production to make a significant impact will require sustained in nuclear reactors. R&D for long periods of time,” he says. Recent advances in accelerator technology could make this concept, called Accelerator Protons for safer fission Driven Systems or ADS, a reality in the relatively At the heart of most nuclear reactors is a process symmetry | volume 9 issue 1 february 2012 near future. While countries in Asia and Europe called fission in which heavy atoms split into are actively pursuing its applications and building lighter ones, releasing tremendous amounts demonstration facilities, however, the United of energy. States does not have an active ADS program. Uranium, the fuel source in the majority of Accelerator and nuclear physicists and engi- nuclear power plants, naturally undergoes fission neers are pushing for this to change. all the time, emitting neutrons as it splits. Those 23 neutrons hit other uranium atoms, causing them For an accelerator to treat nuclear waste to split and release more neutrons, and so on in effectively, it must work continuously. In acceler- an escalating chain reaction. If this continues, the ator-speak, that means it must generate particles reactor is said to be “critical,” and the reaction in a continuous wave, rather than in discrete continues until the uranium runs out. Most nuclear bunches. At the time the studies took place in the reactors use moderators to slow down emitted 1990s, linear accelerators had demonstrated neutrons and control the fission process. The that they could produce continuous-wave beams, energy released by all this atom-splitting gener- but the technology wasn’t yet reliable enough for ates steam, which spins a turbine to drive a gener- use in ADS. In the last decade, however, major ator and produce electricity. advances in accelerator science and technology In ADS, however, the road to fission starts with have allowed scientists to demonstrate that a lin- a linear accelerator. ear accelerator could reliably produce the contin- Although the details of proposed designs differ, uous-wave beam that ADS requires. the basic concept accelerates a beam of protons “In the last 10 years, scientists around the to 1-2 billion electronvolts of energy and slams world changed the picture by developing super- them into a heavy-metal target in the core of conducting radiofrequency and other tech- a reactor. Each proton that hits the target gen- nologies that make it practical to create a con- erates about 30 neutrons, which drive the fission tinuous-wave linear accelerator,” says Yousry process. Turn off the proton beam, and the process Gohar, a senior nuclear engineer at Argonne stops. Such a reactor, one that does not sustain a National Laboratory. chain reaction on its own, is said to be “subcritical.” Scientists have selected superconducting radio- An ADS system could use this subcritical frequency, or SRF, as the technology of choice process to transmute the most troublesome long- for next-generation particle accelerators due to lived components in radioactive waste into safer its efficiency and ability to sustain high-power forms with shorter lifetimes. beams of a few megawatts or more—close to the level of power needed for ADS. Besides Breakthrough in accelerator supporting fundamental physics research, an technology accelerator using SRF technology could also In the 1990s and early 2000s, several studies in open doorways to applications related to nuclear the US and abroad evaluated the potential for energy. using ADS to treat highly radioactive components But why use an accelerator as a dedicated of nuclear waste by transmuting long-lived neutron source to transmute nuclear waste in radioactive isotopes to safer ones with much the first place? shorter half-lives. However, at the time ADS “The safest and most efficient reactor process was expensive and at an early stage of develop- for transmutation would use ADS,” says Eric ment. The studies found the technology had Pitcher, a nuclear engineer at Los Alamos National not been adequately demonstrated, and as Laboratory. “Because the reactor operates in a a result the small existing R&D program was subcritical mode, it is useful for specific applica- eventually terminated. tions like burning problematic constituents in used nuclear fuel.” The fuel process Nuclear reactor Spent fuel Uranium, plu- is separated tonium and chemically. neptunium MAJOR ACTINIDES Americium are sent back Spent Fuel Uranium (94.5%) and curium to the reactor Plutonium (1%) are removed. as fuel. MINOR ACTINIDES Neptunium Curium Americium Americium ADS Americium and and curium transmutes curium become are sent americium safer isotopes, to an ADS and curium. with storage Graphic: Sandbox Studio reactor. time reduced from 10,000 to 500 years. 24 Transmutation of protons with a subcritical ADS reactor Accelerator An accelerator provides an intense, continuous beam of protons. Spallation target The protons hit heavy nuclei and “shake loose” neutrons, Graphic: Sandbox Studio which enter the reactor vessel. Transmutation The neutrons hit and split long-lived nuclei, such as americium and curium, creating energy and short-lived nucleus nuclei that are easier to process and store. Subcritical operation When the accelerator is switched off or loses power, the reactor no longer has enough neutrons to sustain nuclei the transmutation process and the nuclear reactions automatically slow down. Fuel reprocessing electricity, and the last thing they want to do is On average, the uranium fuel that drives a com- to take problematic fuel.” mercial nuclear power plant lasts three years Roughly 10,000 years must pass before the before it stops producing energy efficiently. The americium decays to the same level of radiotoxic- plant operator then removes the spent fuel from ity as the uranium ore used to produce the nuclear the reactor and stores it on site. Power plants in fuel in the first place. ADS would transmute ameri- some countries reprocess some fuel for reuse cium into short-lived fission products, reducing this in their reactors, but in the United States no time to less than 500 years. nuclear waste is reprocessed. “ADS is well suited to treat the most problematic In 1982, Congress passed legislation creating issue in nuclear waste—americium and curium,” the Nuclear Waste Fund to establish a waste Pitcher says. “ADS will be expensive because you disposal program. For every kilowatt-hour pro- have to build an accelerator. So you want to duced, nuclear power plants contribute one- focus on those constituents that are more difficult tenth of a cent to the fund. Since 1983 the fund to burn in a classical reactor.” has collected $35.8 billion and spent $10.8 billion, One proposed process would chemically extract according to the Nuclear Energy Institute. Yet the plutonium and uranium from nuclear waste 30 years later, a disposal program is yet to and send them back into a nuclear reactor as fuel. be established. Pitcher explains that neptunium can go along Uranium makes up 94.5 percent and plutonium with the plutonium and uranium, because those makes up 1 percent of used nuclear fuel. The three elements together do not present repro- remaining constituents are the minor actinides, cessing risks. Carrying the neptunium along with a group of elements comprising neptunium, the plutonium and uranium, something that no curium and americium, and other fission products. country that reprocesses spent fuel currently does, “While curium is the most radioactive and would leave manageable amounts of americium hence the most problematic to process,” Pitcher and curium for ADS transmutation. symmetry | volume 9 issue 1 february 2012 says, “americium dominates the radioactivity “If you isolate the uranium, plutonium and nep- level in a repository in the 1,000-year time frame, tunium from the fission products and other which can limit the amount of high-level waste actinides in the used fuel,” Pitcher says, “you can placed in a repository. The utilities operating make new fuel from them and deliver it to a today don’t want to take recycled fuel that has reactor site in the classical way that happens today. americium in it. They just want to produce While those components are radioactive, you can 25 still manufacture, inspect, and ship the new fuel Turning off the accelerator immediately stops without exposing workers to significant radiation.” the fission reactions.