Generating nuclear power requires moving spent radioactive fuel safely over thousands of miles from reactor sites for reprocessing or disposal. To ensure that this is being carried out with the minimum of risks, Sandia National Laboratories (SNL) recently completed a nuclear "triathlon" that involved moving a simulated cargo of spent fuel rods over 14,500 miles to record the stress and jolts that fuel undergoes in transit.
According to SNL, up to 2,600 tonnes of spent fuel is produced by the reactors that supply the United States with 20 percent of its electricity. Transporting this fuel is a serious and delicate operation because it not only involves containing the fuel rods in line with the strict regulations of the International Atomic Energy Agency (IAEA), but it must also be done in a way that reassures the public that every possible precaution is being taken.
For over 30 years, the consignments have been stored in 125-tonne Type B casks, which contain up to about 24 tonnes of used fuel in the form of fuel rod assemblies. These consist of rectangular bundles of zirconium rods stuffed with fuel pellets of enriched uranium 235 or plutonium. The casks are forged out of solid steel with heavily gasketed steel hatches held on by specially reinforced pins that can withstand 100 tonnes of force each. When loaded and sealed, these casks are so strong that they can take a direct hit at their weakest point by a 100 tonne diesel locomotive traveling at 100 mph (160 km/h) and still come away with little more than scratches and superficial dents.
The problem today is that the fuel rods themselves are very fragile. Sitting inside the intense radioactive environment of a critical nuclear reactor for several years, the metal cladding of the rods becomes very brittle and the assemblies require careful handling if they're not to shatter. A similar problem occurs in any routine transportation of fuel rods, where repeated strains place them in danger of unexpectedly snapping.
To understand more about the stresses that these rods undergo, SNL in collaboration with Spanish and South Korean partners as well as and Pacific Northwest and Argonne national laboratories, loaded a brand new cask with three dummy fuel rod assemblies. Instead of spent nuclear fuel, the rods were stuffed with lead rope and pellets made of lead or molybdenum. Normally the cask would hold 32 assemblies, but these were special assemblies kitted out with accelerometers and strain gauges to record every bump, rattle and jolt of the journey.
This isn't the first test that SNL has conducted, but it is the most ambitious to date. Three previous tests included reproducing transport strains on a laboratory shaker table, then on a similar table with 50,000 lb (23,000 kg) of concrete to simulate a transport of casks over city streets and rough roads, and a third to simulate a rail journey.
The fourth and most recent "triathlon" test used a Spanish nuclear flask, which was sent on a 14,500 mile land and sea voyage that began with 250 miles over main roads and highways using a heavy-haul truck from northern Spain to a seaport, where the cask was transferred to a barge and shipped 1,000 miles along the coast to Belgium. There, it was placed aboard a cargo ship headed for Baltimore, 4,000 miles and a fortnight away. Then the cask was set on a flatbed rail car and passed through 12 states as it covered another 2,000 miles to the Transportation Technology Center Inc facility near Pueblo, Colorado for controlled runs on a 50 mile test track. Finally, the whole thing went in reverse as the cask was sent back to Spain.
As the cask traveled, the sensors collected data on shock and vibrations, which was recorded by a bespoke system. The final result is 8 terabytes of data that will take a year to analyze. According to SNL, this treasure trove will allow engineers to evaluate computer models used to estimate fuel rod stresses.
"Preliminary results show very low shock and vibration levels, which we will compare to the mechanical properties of fuel that's come out of a nuclear power reactor," says Paul McConnell, project manager for the tests. "Ultimately, we want to understand if the fuel can withstand the accumulation of shocks and vibrations during the journey that could potentially cause a fuel rod to break."
Source: SNL