Holtec International said Friday that it had achieved an industry first in creation of a large rectangular cask for activated waste that passed a succession of 30-foot “free drop” tests without any breach of its containment boundary.
To earn the U.S. Nuclear Regulatory Commission (or International Atomic Energy Agency's) certification, a transport cask must pass a series of “free drop” tests in which a scaled replica of the loaded cask is shown to maintain its radiation blockage capability substantially unimpaired when dropped from a height of 30 feet onto an “essentially unyielding surface,” the company noted.
Under the U.S. NRC and international regulations, the cask's orientation at impact with the surface involved is required to be selected to induce what would potentially be maximum damage to the cask. In other words, it must be deliberately dropped to exhibit reliability through what would be the cask's most destructive type of 30-feet fall -- falls that deliberately test the cask's strength in the most grueling type of fall.
For the HI-STAR ATB-1T cask (or HI-STAR 330), the “free drop” tests were a particularly daunting challenge because, in contrast to a fuel-bearing transport cask, which is cylindrical, HI-STAR ATB-1T has a rectangular footprint. The cask tested is 12-feet by 5.9 feet storage cask, complete with corners, facets and edges – vulnerable points that are not associated with cylindrical casks.
Further, the company said, where cylindrical fuel casks are always outfitted with impact limiters, the 120-ton (loaded weight) HI-STAR ATB-1T cask has no impact limiter to cushion the impact upon collision with the target or the unyielding surface.
To minimize crew dose during loading and unloading, the Holtec rectangular cask is equipped with a quick connect/disconnect controlled cask locking system in lieu of a conventional lid that would be locked in place with bolts.
A quarter scale model of the cask, fabricated by the Holtec Manufacturing Division and instrumented by the Sandia National Laboratory in Albuquerque, New Mexico, was subjected to three successive “punishing drops” in three discrete orientations in accordance with a test plan approved by the U.S. NRC and a client, Holtec said.
"To the delight of the assembled engineers," Holtec said, the cask passed the tests, even when challenged by three direct collisions that included a top-down oblique drop, a center of gravity over corner drop and a puncture drop test. The cask "sustained no damage to its containment boundary or dislodging of its closure lid," said Holtec.
Further, the damage the tested prototype did sustain was predicted. “According to Hotec's design team, the cask exhibited the physical deformation contours in line with those predicted by the computer simulations for these tests,” the company said. Holtec predicted the rectangular cask would become “a reliable workhorse for nuclear plants, especially those undergoing decommissioning, to haul away their highly activated metal waste efficiently and with minimum crew dose.”
Holtec posted a video of the tests, which are re-posted here.
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