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 tonne of uranium (GWd/tU).

Ramping up fuel efficiency has worked especially well in the pressurised water and boiling water reactors used in the United States 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 United States and Britain is poised to use reactors designed for "burn-up rates" of 60 GWd/tU, according to the British 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," it 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 US Nuclear Regulatory Commission's (NRC) 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.