Following the nuclear disaster at Fukushima in March of 2011, the U.S. government increased allocation of funding for research into the development of stronger protective coating for nuclear fuel rods. Prior to the disaster, about two million dollars were being spent per year on improving the construction of nuclear fuel rods. Following the disaster, the funding rose to over thirty million a year.
The goal of the new funding is to promote the creation of nuclear fuel rods that are more difficult to damage and melt in extreme conditions. In addition, the protective coating on the nuclear fuel rods must be reformulated to reduce the danger of chemical reactions that make current fuel rods brittle and generate explosive hydrogen gas. The hope is that new fuel rod designs could give plant operators more time to respond to an emergency before explosions or a catastrophic meltdown occur.
Nuclear fuel has not changed significantly for decades. Naturally occurring uranium is mostly U-238 with less than one percent of U-235. Low enriched uranium for current U.S. reactors has had the percent of U-235 boosted to five percent by isotopic separation. Uranium dioxide powder from the fuel fabrication process is compressed into thimble sized pellets. The pellets are inserted into metal tubes up to fifteen feet long. The tubes are composed of a zirconium alloy that resists corrosion in the reactor. They are able to withstand the heat being generated and they separate the fuel pellets from the circulating coolant in the reactor. However, if the cooling systems fail and water level drops to expose the fuel rods to air, the zirconium cladding reacts with the steam to produce hydrogen gas. The reaction that produces hydrogen also produces heat, increasing the loss of coolant.
The Electric Power Research Institute is working on a type of cladding made of molybdenum which can resist higher temperatures that zirconium. The University of Tennessee are coating zirconium cladding with ceramics to increase heat resistance. The Westinghouse Electric Company is working on creating cladding out of silicon carbide. At the University of Illinois, they are attempting to create a new coating that could be applied to standard zirconium fuel rods to prevent hydrogen generation and heat damage. They are also trying to create chemicals that they can add to fuel rods that would migrate out of the rod and form a protective coating if the temperature rises.
Of course, any new approach to fuel rod construction must be economically feasible. Some researchers are hoping to offset the increased cost of new fuel rod design by extending the lifespan of the new fuel rods. Any new fuel rod design will have to be thoroughly tested before implementation. Even with thorough testing, we won't really know if the new designs are better than current fuel until another nuclear disaster occurs and it might take years of operation before any design problems surface. In any case, it will probably require new regulatory action by the Nuclear Regulatory Commission to insure that all U.S. reactors adopt any new fuel rod design.
Nuclear fuel assembly:
