DREAMS of a table-top nuclear fusion reactor just won鈥檛 go away. The latest glimmer of hope comes from an experiment in sonofusion 鈥� the supposed phenomenon in which imploding bubbles in specially prepared liquids are said to create temperatures high enough to trigger nuclear fusion.
The debate started in earnest in 2002 when nuclear engineer Rusi Taleyarkhan, who was then at the Oak Ridge National Laboratory in Tennessee, and his colleagues reported seeing sonofusion in a beaker of acetone in which all the hydrogen atoms had been replaced with the heavier isotope, deuterium. They bombarded the deuterated liquid with neutrons to seed it with bubbles a few nanometres across and then set up powerful standing waves within the liquid by blasting it with sound. The standing waves caused the bubbles to grow and then collapse in a flash of light.
The researchers claimed that the energy of the flash was fusing deuterium nuclei to create tritium and release neutrons (Science, vol 295, p 1868). But sceptics countered that the neutrons claimed to be products of fusion were the same neutrons that were beamed into the vessel in the first place.
Advertisement
Now, Taleyarkhan and his colleagues at Purdue University in Lafayette, Indiana, have addressed this criticism by eliminating the external neutron beam. Instead, they dissolved uranyl nitrate, a salt of natural uranium, in the deuterated liquid. The alpha particles from the radioactive decay of uranium created the bubbles, and the collapse of these bubbles causes nuclear fusion, the researchers say.
The team got the same result with four different liquids, including deuterated water, acetone and benzene. Significantly, the experiment failed with liquids that had no deuterium. To further answer critics, the researchers used four different types of particle detectors, and reported seeing 5000 to 7000 neutrons per second streaming out of the liquid at the energy levels expected for deuterium-deuterium fusion. The work will be published in the journal Physical Review Letters.
Even if the effect is real, don鈥檛 expect table-top reactors any time soon. The energy that powers the fusion is still orders of magnitude greater than the energy coming out. 鈥淚 cannot say when we will reach the break-even condition,鈥� Taleyarkhan says, 鈥渂ut I鈥檝e never seen a reason why it cannot be scaled up either.鈥�
鈥淓ven if the effect is real, don鈥檛 expect table-top reactors any time soon. The energy fed in is far greater than that produced鈥�
Larry Crum of the University of Washington in Seattle, who was once a critic of Taleyarkhan鈥檚 earlier work, is impressed by the latest results. However, he points out that the gold standard of proof is for other labs to replicate the results. 鈥淚t is worrisome that nobody else has duplicated this outside of Rusi鈥檚 lab,鈥� he says.
