Improved nuclear fuel-rod cladding might prevent future Fukushimas. A substitute for traditional zircaloy could greatly reduce the danger of hydrogen explosions. David L. Publication Date :. Caption :. Credits :. But the greatest damage to the complex, and the greatest release of radiation, may have been caused by explosions of hydrogen gas that built up inside some of the 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 [ 1 2 ] appearing in the journal Nuclear Technology.
Because of the harsh environment fuel rods are exposed to — heat, steam, and neutrons that emanate from nuclear reactions — extensive further testing will be needed on any new cladding for use in commercial reactors, Kazimi says.
Other groups have investigated the use of SiC for cladding, but the material had never been subjected to the detailed tests and simulations that the MIT team carried out. Nuclear fuel rods are made of hundreds of small pellets of enriched uranium placed end-to-end inside hollow tubes of zircaloy that are about a half-inch across.
Zirconium absorbs very few of the neutrons that drive a nuclear reactor, so zirconium alloys made sense as a fuel cladding — as long as the reactor operated as planned. If a reactor loses its cooling water, however, the zirconium can make a bad problem worse. The job for Terrani's team, as the innovation engine of a consortium led by General Electric, was to create a zirconium-free alloy that would generate as little hydrogen as possible during incidents.
At the same time, however, it needed to match the performance of the nuclear fuel rod cladding in use today. The project was out of the ordinary for at least three reasons, Terrani explained. In the first place, the team was not interested in testing existing alloys to see if one might be appropriate. Instead, it designed the new alloy from scratch by putting together a diverse team that included experts in nuclear engineering, materials science, radiation effects, corrosion, thermomechanics and alloy fabrication.
We designed an alloy that we knew was going to work. I'm not surprised that this alloy behaves so well under different conditions; we designed it to do so.
Secondly, the team was able to identify and produce the alloy in six years, which is lightning fast in the nuclear industry. In the paper and packaging industries, zirconium compounds also make good surface coatings as they have excellent water resistance and strength. Most of the zirconium chemicals and zirconium metal produced worldwide is obtained from zirconium silicate ZrSiO 4 , or zircon , a natural mineral that is recovered from ancient mineral sands deposits.
After mining and production of a heavy mineral concentrate, zircon is separated, beneficiated and commercialised in the form of zircon sand and is either used directly in certain applications foundry sands including investment casting or processed for use in refractories , ceramic opacification , or to numerous zirconium compounds from which the metal can be produced.
The early development of zirconium metallurgy was essentially due to the nuclear power generating industry, and where zirconium alloys are now regarded as the proven structural material for nuclear fuel cladding in light water reactors. Charged with developing nuclear-propelled ships and submarines, Rickover realised that the reactor for use in vessels had to be compact and be able to operate when the ship was rolling or pitching, or at an angle when the submarine was diving or surfacing.
A pressurised water reactor PWR was envisaged, with the need for a nuclear fuel cladding material that would withstand corrosion at high temperatures and over long periods of time, maintain its integrity in an environment of intense radiation and would not absorb neutrons required for the nuclear reaction. Low neutron absorption is vital to any structural material used in a nuclear reactor because large numbers of neutrons produced by the reaction must be free to interact simultaneously with all the nuclear fuel confined inside hundreds of fuel rods.
This interaction sustains the necessary chain reaction throughout the reactor's core. Zirconium was the metal of choice for this application as it absorbs relatively few of the neutrons produced in a fission reaction and because the metal is highly resistant to both heat and chemical corrosion.
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