Moving rods of spent nuclear fuel (SNF) to interim storage or a geologic repository requires road or rail travel. Although a heavy shielding cask protects the rods, long distance transportation subjects SNF to vibrations, sudden movements and other potentially agitating forces. To understand the effect, both the nuclear industry and its regulator need a reliable method of characterizing how well SNF will withstand the rigors of transportation.
With support from the U.S. Nuclear Regulatory Commission, researchers at the U.S. Dept. of Energy (DOE)’s Oak Ridge National Laboratory have devised the Cyclic Integrated Reversible-bending Fatigue Tester, or CIRFT. Combined with ongoing fuel transportation research, data from the CIRFT system will help facilitate cask designs and transportation protocols that ensure safe fuel transportation. DOE has designated this recently patented technology as one of the 2015 Used Fuel Disposition Campaign’s key elements for implementing the nation’s nuclear waste management strategy.
“The CIRFT system is the first tool developed to use bending and dynamic stimulation for fatigue testing of spent nuclear fuel,” said ORNL lead researcher Jy-An Wang. “If the fuel rod exhibits any anomalous behavior during normal conditions of transport, you can tell right away with this device.”
A nuclear fuel rod consists of half-inch cylindrical pellets stacked in a strong metal tube known as cladding. As the heart of a fission reactor, these fuel rods can face corrosion, embrittlement and other radiation-induced damage. Even after fuel rods leave a reactor, high amounts of radioactivity remain within the nuclear pellets.
Jy-An Wang, with fellow ORNL scientists Hong Wang, Hao Jiang, Bruce Bevard and Robert Howard, created two CIRFT apparatuses at Oak Ridge National Laboratory. DOE lauded the project as an Outstanding Performance for FY14, as the devices help characterize a mechanical property of high–burn-up spent fuel during transport.
Historically, cladding has been tested for mechanical properties with the fuel removed. The CIRFT system is the first test system that provides key mechanical properties for the fuel/clad system. “We found a way to test spent nuclear fuel under conditions of dynamic vibration without removing fuel pellets from the spent fuel,” said Jy-An Wang, who in April presented CIRFT at the American Nuclear Society’s International High-Level Radioactive Waste Management Conference and described its potential role to support global activities for nuclear fuel disposal.
CIRFT’s creators focused on devising a standardized method of assessing spent fuel rod durability during transport. Undetected flaws in spent nuclear fuel can be exacerbated in transit; if the cladding has degraded enough, or if the vibration load is large enough, the vibrations could trigger a break in a fuel rod. That could present challenges if the SNF rods ever have to be removed from their canisters. These tests, or a “prediction protocol” using CIRFT technology, could help anticipate any cladding breaches as well as support new, improved transportation cask designs.
“Before this advance, nobody really knew how high–burn-up spent fuel would perform during extended transport under extensive jostling conditions,” said Wang.
CIRFT’s simple, U-shaped design is crucial for reliable testing of hazardous materials. The two long arms of the device connect to electromagnetic motors that bend the rod sample in the targeted frequency cycles. Researchers measure the forces that represent the normal vibrations during transport with a combination of advanced displacement sensors and finite element analysis of the sample. The insight gained from these modeling measurements help ORNL’s scientists investigate potential points of deformation and stress in the cladding.
ORNL scientists discovered that SNF samples tend to degrade at the interfaces between adjacent pellets. This understanding of how vibrations affect the structure and behavior of used high–burn-up fuel rods is the first step in predicting fuel performance during transport.
“The data we generate with CIRFT can help the nuclear industry, including the regulatory bodies, to better understand how spent fuel responds under transport or to design transport casks that can minimize vibration effects on the fuel,” Wang said.
Source: Oak Ridge National Laboratory
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