A European research team took an important step this week toward proving that an unusual combination of a nuclear reactor and a particle accelerator could be used to eliminate highly radioactive and long-lived nuclear waste produced in conventional nuclear reactors. In Paris yesterday, researchers from the Belgian Nuclear Research Centre (SCK-CEN) in Mol, the French National Research Council (CNRS), and France's Atomic Energy Commission (CEA) reported that they had successfully operated a research reactor called Guinevere by boosting its nuclear reactions with an externally produced beam of neutrons.
Such a set-up is known as an accelerator-driven system (ADS). The reactor in an ADS contains too little fissile material in its core to sustain a nuclear chain reaction. The shortage of material means there isn't enough neutrons. To make the reactor run, extra neutrons must be supplied from another source, such as a proton beam hitting a heavy metal target. That knocks out neutrons in a process called spallation.
The concept of an ADS is almost as old as nuclear power itself: American physicist Ernest O. Lawrence, the inventor of the cyclotron accelerator, suggested the use of particle beams in conjunction with a reactor during the 1950s. The idea gained more ground in the 1980s when Nobel-winning particle physicist Carlo Rubbia proposed building an ADS to generate electricity, which he dubbed the energy amplifier. Since then, most research has focused on using an ADS to transmute high-level nuclear waste into less harmful material. The first working ADS was demonstrated by researchers at Kyoto University in Japan in 2009.
Hamid Aït Abderrahim, deputy director of SCK-CEN, says that Guinevere, which is based around a 1-kilowatt lead-cooled reactor, is a "reduced" model of a much larger ADS, called Myrrha, whose construction will start in 2015. "What is unique in this experiment is that we are reproducing exactly the characteristics of an ADS on an industrial scale," says Abderrahim.
Although future ADS systems will use protons beams and a heavy metal target to produce neutrons, Guinevere's neutron source—deuterium ions hitting a tritium target—is powerful enough to simulate how a bigger machine like Myrrha will behave, says Abderrahim. Guinevere will also aim to demonstrate the safety of an ADS. Belgian authorities will require proof that a much larger accelerator-controlled reactor cooled with liquid lead is safe. "Guinevere will allow us to experimentally validate all the control techniques and exploitation procedures of an ADS system," says Abderrahim.
Hopes are high that such systems will be used for the transmutation of isotopes known as lower actinides that are part of high-level nuclear waste. Some of the fission products that cannot be destroyed in an ADS will still need to be stored in an underground depository because of the remaining radiotoxicity, explains Joachim Knebel of the Karlsruhe Institute of Technology in Germany. "Of course, the heat load and the volume and amount of radiotoxicity are then much less" following ADS treatment, says Knebel.
Europe and Japan are setting the pace in ADS research, which has been slower to catch on in the United States. "There is a lot of informal, self-organizing activity regarding ADS," says Stuart Henderson, associate director for accelerators at the Fermi National Accelerator Laboratory in Batavia, Illinois. "We are trying to engage the nuclear energy community as we design a high-power proton accelerator, called Project-X." But convincing funding agencies is still an uphill task, he adds. "It's good science and worth pursuing, but we are not quite there yet."