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PARIS (AFP)- New high-efficiency nuclear fuel meant to burn longer and stronger may prove unstable in an emergency and hard to dispose of, according experts cited in a report published Wednesday.

By further enriching the uranium used to power nuclear reactors, operators have been able to extract more electricity from a given amount of fuel, a measure expressed in gigawatt-days per ton of uranium, or GWd/tU.

Ramping up fuel efficiency has worked especially well in the pressurized water and boiling water reactors used in the U.S. and elsewhere.

The objective has been to extract more power from fuel and produce less radioactive waste, one of the most vexing problems associated with nuclear energy.

A new generation of nuclear plants in the U.S. and U.K. is poised to use reactors designed for "burn-up rates" of 60 GWd/tU, according to the U.K. weekly New Scientist, which canvassed experts.

"At these rates, uranium fuel rods should burn for around a year longer than today's best burn-up fuel," the magazine said.

But tests conducted by Michael Billone at Argonne National Laboratory in Illinois, presented last month at a conference in Washington, showed that burn- up rates above 45 GWd/tU would violate the U.S. Nuclear Regulatory Commission's safety standards unless new methods were devised for packaging the fuel, the magazine reported.

A sudden loss of cooling water - as happened during the partial meltdown of a reactor core in 1979 at Three Mile Island in Pennsylvania - would pose such a danger, according to the simulations.

The U.S.'s Electric Power Research Institute said such a loss of coolant is not possible in modern reactors, but the NRC has still launched a three-year review of its safety standards.

"We are actively preparing to revise NRC's safety criteria to account for the burn-up effect," a commission spokesman told New Scientist.

Disposal is also a potential problem because the new high-efficiency fuel is up to 50% more radioactive than fuel currently in use, thus generating far more heat during storage.