New Uranium fuel: fact or fantasy?
doi:10.1038/nindia.2008.204 Published online 20 May 2008
India's theoretical physicists have made a bold prediction that Uranium-249 when struck by a neutron could undergo an exotic mode of fission with the release of as many as six prompt neutrons. This is more than double of what is emitted during the fission of its lighter cousin U-235, the fuel of today's nuclear reactors and atomic bombs.
While the Indian group is excited that the unusual behavior of U-249, if substantiated, could make it an interesting nuclear fuel, experts elsewhere are skeptical or outright dismissive. They say that even if the prediction comes true, U-249 (which has 14 more neutrons than in U-235) may be difficult to produce and even if produced has uncertain practical utility.
For decades, reactor builders and weapons designers had focused on the fissile isotopes U-235, U-233 and Plutonium-239 in the so-called 'valley' of stability — a narrow strip in the vast nuclear landscape of stable and unstable nuclei.
For the last six years Suresh Kumar Patra and Lakshmidhar Satpathy, at the Institute of Physics in Bhubaneswar have been searching for possible fissile nuclei among heavier Uranium isotopes in the 'slope' of the valley. Their hope was that Uranium nuclei with neutron number close to the 'magic number' 162 should meet the conditions for fission.
Now they claim to have identified these fissile nuclei. Along with Rajni Kant Choudhury, head of nuclear physics division at the Bhabha Atomic Research Centre in Mumbai, they report that a chain of Uranium isotopes (U-246 to U-264) with neutron number N=154-172 will be 'highly vulnerable to fission by neutron capture.'
The researchers chose U-249 as a representative of this group for detailed exploration of the fission process of U-250 (compound nucleus formed by neutron capture of U-249) using the 'relativistic mean field' (RMF) formalism, a standard model employed by fission theorists. They show that the two evolving fragments in the final phase of its fission develop a highly neutron-rich neck with neutron-to-proton density of 3.54 in contrast to 2.73 for U-236 (formed by the neutron capture of U-235).
"The excess neutrons in the neck are bound to 'drip' out at the time of scission since the two heavy fragments cannot hold so many neutrons," Patra explains. Their calculation shows 5-6 neutrons will be emitted per fission of U-250 in contrast to 2-3 emitted in the fission of U-235. Patra is confident the prediction could be verified once U-249 is synthesized in one of the radioactive ion beam (RIB) facilities where high speed projectiles of heavy ions slam on targets to create new nuclei.
We do not see radically more neutrons emitted from U-250 than from U-240
But Peter Moller of the Los Alamos National Laboratory in the United States, who has just concluded a very large-scale calculation of fission barriers for over 5000 nuclei, is skeptical. "A hand-waving argument is we do not see radically more neutrons emitted from U-250 than from U-240," he told Nature India. He says the RMF model used by the Indian group has not been extensively benchmarked against known fission properties and does not have predictive capabilities.
In response to that criticism Patra says the RMF calculation only provided additional support to the prediction already drawn from various mass formulae including finite-range droplet model of Moller et al. "Ours is not a model dependent result," he says — an argument that some nuclear theorists are prepared to accept.
"This fission mechanism could indeed be possible for very neutron-rich Uranium isotopes," Jϋrgen Gerl, a nuclear physicist at Gesellschaft fϋr Schwerionenforschung (GSI), at Darmstadt, Germany, told Nature India. But he said that producing U-249 will be "rather challenging" and in any case "the production rates would not allow for any technical application like energy generation."
Adds Dave Morrissey, leading nuclear chemist at Michigan State University in East Lansing, "My impression is that they (Indian group) have performed some new and interesting calculations of the properties of very heavy uranium isotopes." But he says the predictions may never be tested as "there is no plausible mechanism available to produce such neutron-rich uranium nuclei at any RIB facility (running or planned)."
But Patra is optimistic. He says his group has information that GSI Darmstadt will be able to synthesize highly neutron-rich isotopes including U-249, in the international Anti-proton and Ion Research (FAIR) facility to be commissioned in 2011.
Patra admits that U-249, even if produced, is unlikely to be utilized for any practical purpose with present technology. The difficulty arises because U-249 which decays by emitting an electron has a half-life of about 12 seconds which means that if someone wants to exploit the unique property of U-249 for energy generation, it should be done within this time.
"This is doubtful to achieve today," says Lídia Ferreira, professor of nuclear physics at the Instituto Superior Técnico in Lisbon where Patra presented his work last November. "But it may not be impossible in future to let the U-249 isotope undergo fission in the apparatus where it was produced, solving the problem of its short half-life."
"Of course we cannot know what will be possible in the far future," Gerl admits. "But for now the interest in this subject is purely academic: the structure of U-250 and its decay properties will help to explain the synthesis of heavy nuclei in stellar environments."
Valangiman Ramamurthy, former director of the Bhubaneswar institute says even if U-249 synthesis and its utility in energy generation are speculative, Patra et.al. have broken new ground with the proposition of a new mode of fission with emission of multiple neutrons. "Neutron-rich Uranium nuclei hold many surprises for physicists in the future and this group has unmasked one of these."
- Satpathy, L. et al. Fission decay properties of ultra neutron-rich uranium isotopes. Pramana 70, 87-99 (2008)
- Moller, P. et al. Nuclear Ground-State Masses and Deformations. At. Data Nucl. Data Tables 59, 185-381 (1995)