Nuclear Fuel Made Safer

Wednesday, October 30, 2013 @ 07:10 PM gHale

There could soon be new types of nuclear fuel pellets that would be safer in the event of a nuclear disaster.

New materials could end up encasing uranium-bearing fuel as an alternative to zirconium alloys, which have seen use as the outer layer of nuclear fuel pellets for the last 50 years, said a team of scientists from the University of Tennessee (UT) and Oak Ridge National Laboratory (ORNL), who will present their work at the AVS 60th International Symposium and Exhibition in Long Beach, CA.

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Using sophisticated computer analyses, the UT and ORNL team identified the positive impact of several possible materials that exhibit resistance to high-temperature oxidation and failure, on reactor core evolution, thereby buying more time to cope in the event of a nuclear accident.

“At this stage there are several very intriguing options that are being explored,” said Steven J. Zinkle, the Governor’s Chair in the Department of Nuclear Engineering at the University of Tennessee and Oak Ridge National Laboratory. There is evidence that some of the new materials would reduce the oxidation by at least two orders of magnitude.

“That would be a game-changer,” he said. The materials examined include advanced steels, coated molybdenum and nuclear-grade silicon carbide composites (SiC fibers embedded in a SiC matrix).

The next step, he added, involves building actual fuel pins from these laboratory-tested materials and exposing them to irradiation inside a fission reactor. Once they perform as desired, the new fuel concepts would likely end up tested in a limited capacity in commercial reactors to then enable larger deployment possibilities.

Though it would take years before any new fuel concepts end up used commercially, given the rigorous and conservative qualification steps required, Zinkle said, these new materials may eventually replace the existing zirconium alloy cladding if they prove to be safer.

The typical core of a nuclear power plant uses the heat generated by fission of uranium and plutonium in fuel rods to heat and pressurize water. Steam then generates to drive steam turbines for electricity production. Water continuously circulates as a coolant to harness the thermal energy from the fuel and to keep the core from overheating.

The cooling pumps are a critical part of the reactor design because even when a nuclear reactor shuts down, the power it generates from radioactive decay of fission products remains at 1 percent of its peak for hours after shutdown. Given that nuclear power plants generate a staggering sum of energy under their nominal operating conditions (~4 GW of thermal energy), even one percent power levels after shutdown prove substantial. That’s why it’s essential to have cool water circulating continuously even after the shutdown occurs. Otherwise you risk overheating and ultimately melting the core – like leaving a pot boiling on the burner.

That’s basically what happened at Fukushima. On March 11, 2011, engineers at the plant managed to initially safely shut down the plant following a massive earthquake, but then a large tsunami knocked out the backup generators running the water pumps an hour later. What followed were explosions associated with hydrogen generated from the reduction of steam during high temperature oxidation of core materials, and releases of radioactive fission products. The accident displaced the local population and will take years and require a significant cost to clean-up.

Fukushima has had a profound impact on the safety culture of the industry, said Zinkle. Despite the fact that not a single U.S. nuclear power plant was unsafe or shut down in the wake of the accident, the Nuclear Regulatory Commission has issued new requirements to enhance their safety, including increasing the requirements for backup power generation on site.

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