Jul
30
A Path to Safer Longer Running Nuclear Power
July 30, 2013 | 1 Comment
Silicon Carbide Nuclear Fuel Cladding Cross Section. Click image for more info.
For many people the Fukushima Daiichi nuclear plant disaster marked another reason nuclear fuel should be discarded as a power source. The plant initially went into shutdown by the magnitude 9.0 quake. Then its emergency generators then failed because the water of the tsunami wave inundated them. 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.
The 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.
Nuclear fuel rods are made of hundreds of small enriched uranium pellets placed end-to-end inside hollow tubes of zircaloy that are about a half-inch across. The tubes are filled with inert helium gas to improve the heat conduction from the pellets to cladding that is cooled by the water that circulates outside the tubes. These tubes are then packed together in bundles that are inserted into the reactor core, where they heat water to produce steam to drive a turbine generator to produce electricity.
The MIT research team 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 appearing in the journal Nuclear Technology.
Mujid Kazimi, the TEPCO Professor of Nuclear Engineering at MIT, who is senior author of the papers said, “We are looking at all sides of the issue, regarding replacing the metallic cladding with ceramic.” 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.
So far the MIT team has taken the SiC testing much further than has been done in the past. Kazimi adds SiC is, “very promising, but not at the moment ready for adoption” by the nuclear industry.
Materials science has come a long way since the design for the zircaloy material was adopted. Other groups have suggested 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. Kazimi and his colleagues not only tested the material’s response under normal operating conditions, with temperatures of 300º C (572º F), but also under the more extreme conditions of an accident, with temperatures up to 1500º C (2732º F).
Silicon Carbide Nuclear Fuel Capsule Sample in 1500º Furnace. Click image for more info.
To test SiC cladding under normal operating conditions, the MIT team used a three-layer cladding design that features a middle layer made of a composite of SiC fibers reinforced with more SiC. The tubes were tested inside MIT’s research reactor in special loops that replicate the coolant temperature and chemistry conditions in large power reactors.
The irradiation apparatus was designed by MIT research scientist David Carpenter and research engineer Gordon Kohse. The effects of irradiation were studied by graduate student John Stempien and others, working with Kazimi. The results showed good strength retention during mechanical testing, Stempien says.
Graduate student Youho Lee and research scientist Tom McKrell conducted high-temperature oxidation studies on SiC. Under the extreme conditions of an accident, the corrosion rate was 100 to 1,000 times less than that of zircaloy. Zircaloy loses strength as temperature increases, becoming 2% weaker for every 10º C increase in temperature and losing all strength at about 1300º C. Stempien says the strength of the SiC ceramic remains essentially constant to temperatures well above 1500º C.
Kazimi explains the potential advantages of SiC cladding extend beyond reducing the risks in an accident. 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.
Kazimi also notes the ability to leave fuel rods in place longer would reduce the spent fuel produced by each reactor, resulting in less volume for disposal.
The idea is not fully worked out. There are still further tests to be done. In particular, while zircaloy tubes can have their ends capped by welding a metal disk onto each end, ceramic can’t be welded, so a suitable bonding agent will need to be found. “We need to join the ceramic to ceramic in a way that can withstand the conditions in the nuclear core,” Kazimi says. “That’s not as perfected a science as it is for metals.” Other details, such as the optimal thickness of the tubes for durability and for heat transfer, also need to be determined.
Additionally, the SiC material needs to be tested further to determine its response to various stresses. “The fracture behavior is different,” co-author Lee says. In particular, while metal deforms predictably under pressure, a ceramic tends to fracture in a way that is “more statistical,” he says: It can only be predicted as a statistical likelihood of certain failure modes.
All that is quite encouraging. The problems remaining may need quite a lot of thinking and further development. But the solutions that come from decades of experience where the unforeseen, rare and surprise events show the weaknesses are not cause for stopping an important power supply and energy resource rather they show how to build in much more safety and improve the energy extraction.
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A fascinating subject, thanks.
Materials science is destined to facilitate advanced nuclear power, along with advanced production of synthetic fuels using surplus nuclear heat.