The greatest damage to the Fukushima complex, and the greatest release of radiation, may have been caused by explosions of hydrogen gas that built up inside some of the reactors.
That hydrogen buildup was the result of hot steam coming into contact with overheated nuclear fuel rods covered by a cladding of zirconium alloy, or “zircaloy” — the material used as fuel-rod cladding in all water-cooled nuclear reactors, which constitute more than 90 percent of the world’s power reactors. When it gets hot enough, zircaloy reacts with steam to produce hydrogen, a hazard in any loss-of-coolant nuclear accident.
A team of researchers at MIT is developing an alternative that could provide similar protection for nuclear fuel, while reducing the risk of hydrogen production by roughly a thousandfold. Tests of the new cladding material, a ceramic compound called silicon carbide (SiC), are described in a series of papers.
Cross-section view of the proposed silicon carbide cladding for nuclear fuel rods. The fuel pellets are in the center, shown as a gray crosshatch. Then, after a thin layer of inert helium gas, the three layers of cladding are shown in black (solid SiC), green (composite material made up of SiC fibers infused with SiC), and blue (another solid layer of SiC). IMAGE COURTESY OF MUJID KAZIMI AND YOUHO LEE
The potential advantages of SiC cladding extend beyond reducing the risks in an accident, Kazimi explains. Because SiC reacts slowly with water, even under normal conditions it degrades less and can remain in a reactor core longer. That could allow reactor operators to squeeze extra energy out of fuel rods before refueling: The rods are typically replaced after four or five years in a reactor, and degradation of the cladding is a major limitation on their longevity.