Since 1978, the United States and other nations have been pushing to eliminate the use of highly enriched uranium (HEU)the kind of stuff that terrorists or a rogue nation might use to make an atomic bombfrom dozens of civilian research reactors around the world. However, achieving that goal will take far longer than officials had previously hoped, according to a new study by the National Academies of Sciences, Engineering, and Medicine. Only a few years ago experts had hoped to eliminate the use of HEU in civilian research reactors by 2018. But that objective cannot be reached until 2035 at the earliest, the report concludes. “Clearly there have been unexpected challenges, both technical and nontechnical, that have led to the significant extension of the timeframe,” said Julia Phillips, a former vice president of Sandia National Laboratories in Albuquerque, New Mexico, who chaired the report committee at a webcast press briefing today. A follow-up to a 2009 report, the new study was requested by Congress in 2012. Currently, global stock piles of HEU for civilian research total 60 tonnes, enough to make roughly 1000 bombs, and nonproliferation experts worry that civilian supplies may be less secure than far bigger military supplies. But nations have made progress over the past decade in reducing civilian use of HEU, notes Phillips, an applied physicist. Since 2009, 28 civilian research reactors have either converted to safer low enrich uranium (LEU) fuel or closed. However, 74 such reactors are either still using or planning to use HEU fuels, the report says. And progress is sure to slow, Phillips says. “The long and short of it is that the easy conversions have been made and the more difficult ones remain,” she says. On the technical side, nuclear scientists and engineers have struggled to develop replacement fuels for research reactors that are based on safer LEU. Research reactors are generally relatively small facilities that focus on materials science, reactor design, and the production of radioactive isotopes for medical purposes. Such research generally involves exposing materials to fluxes of neutrons from the reactors’ cores that are higher than those in larger commercial power reactors. To generate those high neutron fluxes, research reactors have relied on HEU. Uranium atoms come in different types, or isotopes, depending on the number of neutrons in their nuclei. The isotope uranium-235 has 92 protons and 143 neutrons in its nucleus, and it can undergo a nuclear fission chain reaction. In contrast, the isotope uranium-238 has three more neutrons in its nucleus and cannot undergo nuclear fission. Any mixture of uranium with more than 20% uranium-235 is considered highly enriched. Research reactors typically use uranium enriched closer to or above 90%a level known as weapons grade uranium. In comparison, power reactors use uranium enriched to about 4% uranium-235. Researchers would like to switch the research reactors to LEU fuel, but to maintain the necessary neutron flux would require ensuring a very high density of uranium in the fuel. Researchers in Europe and Asia are developing a fuel in which granules of a uranium molybdenum alloy are dispersed within an aluminium matrix and then surrounded by an aluminium cladding. In contrast, the United States is developing a “monolithic” fuel that consists of a single ribbon of uranium molybdenum alloy clad in aluminium. In principle, the denser monolithic fuel could be used in all current research reactors, whereas the dispersed fuel could not. However, progress with both fuels has been slower than expected. For example, early versions of the dispersed fuel exhibited a tendency to swell as the fuel was consumed. And the monolithic fuel presents significant manufacturing challenges. Development of both fuels will likely take another 10 to 15 years, the report estimates. On the nontechnical side, Russia, which has 32 of the 74 research reactors still using or planning to use HEU, has expressed little interest in converting them to LEU. In fact, Russia has converted only one civilian research reactor to LEU, and that one with assistance from the United States. “This is not a priority with the Russian government,” said William Tobey, an expert on nuclear nonproliferation at Harvard University and a member of the report committee at the press conference. Given the prospects, the new report makes seven recommendations, including trying to engage in further collaboration with Russia, developing more detailed reporting on the conversion and closings of civilian research reactors, and planning for new facilities to replace the eight current U.S. civilian research reactors, the youngest of which is 45 years old. However, one recommendation is sure to draw the most attention and controversy. The United States has designated 20 tonnes of 93% enriched weapons grade uranium for civilian research reactors. Given the prospect of waiting another 15 years for a LEU fuel, the committee recommends immediately diluting that entire supply to 45% enrichment, a level that still leaves it highly enriched, and using it to fuel the research reactors in coming decades. That step would make the fuel less of a proliferation threat, but would surely raise eyebrows, as the U.S. has previously rejected the idea of using such fuel. That interim approach would also require two conversions of each reactor instead of one. A 45% fuel design has already been “validated” by bodies such as the U.S. Nuclear Regulatory Commission, the panel notes, so it could be used in relatively short order. And committee members stress that the step would be only a stop-gap measure to improve nuclear security in the short run. “This is not instead of, but complementary to the ultimate goal of using low enrichment uranium in all reactors,” said Paul Wilson, a committee member and nuclear engineer at the University of Wisconsin, Madison, at the press briefing. Hanging over all of this is the continuing prospect that eliminating HEU from civilian research reactors may take even longer than the report suggestsespecially if Russia decides not to collaborate.
Science, 28 January 2016 ;http://www.sciencemag.org/ ;