Safer nuclear fuels: New uranium pellets with degradation-resistant cladding for enhanced safety

The scientists, who will present their work at the AVS 60th International Symposium and Exhibition in Long Beach, Calif. next week, investigated new materials that could be used to encase uranium-bearing fuel as an alternative to zirconium alloys, which have been used as the outer layer of nuclear fuel pellets for the last 50 years.

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 that have been 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 it is established that they perform as desired, the new fuel concepts would likely be tested in a limited capacity in commercial reactors to then enable larger deployment possibilities.

Though it would take years before any new fuel concepts are widely 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.

Nuclear Safety, Post-Fukushima

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 is then generated that is utilized to drive steam turbines for electricity production. Water is continuously circulated 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 is shut 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 1 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 is expected to 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 found to be unsafe or shut down in the wake of the accident, the Nuclear Regulatory Commission has issued a number of new requirements to enhance their safety, including increasing the requirements for backup power generation on site (above and beyond the previously elevated requirements after 9/11/2001).

For instance, U.S. nuclear plants are now required to have enough onsite electrical battery capacity to keep their pumps going full tilt for 24 hours in the event of a total power failure. Prior to Fukushima, plants were only required to have 8 hours of battery power with additional portable power sources.

The events in Japan have also spurred new research into other ways of increasing safety, including the search for new ways to construct fuel pellets that could replace existing ones and survive longer in the case of a severe accident.

What Nuclear Fuel Looks Like

Nuclear fuel pellets are something like jawbreakers with a core made out of one kind of candy surrounded by layers of tougher candy. In most of the world's nuclear reactors, the center contains a brittle uranium blend, and the solid outside shell ("fuel cladding") is made of a metallic alloy containing the element zirconium.

Zirconium alloy was selected as the material of choice more than 50 years ago by the U.S. Navy because it has many desirable properties, such as low neutron absorption, which means that it does not interfere with the nuclear fission. It has become so reliable over the past decade that U.S. reactors typically run almost all the time and only shut down for a couple weeks once every two years or so to reshuffle and replace old fuel and for routine maintenance.

"It's a great technology and works well under normal operating conditions," Zinkle said.

But in the event the water pumps fail and the core overheats, this type of cladding is susceptible to oxidation and rupture, which can increase the burden on the safety systems for the nuclear reactor and push it over the cliff earlier. Changing the cladding may help make the fuel pins survive longer -- a relatively straightforward fix, like retrofitting a classic car with modern airbags.

The major advantage of this approach is that it would allow nuclear plants to achieve greater safety without making major capital investments to modify the mechanical operations of the plant. It would only require operators to swap out the old fuel pellets for new ones when they shut down the plant for routine refueling, which they do every two years or so.