Holtec’s NRC-certified transport casks carry the used fuel inside an all-welded stainless-steel vessel known as multi-purpose canisters (MPCs). The MPC is, however, optional for transporting used fuel in a transport cask. The transport cask is designed to carry “bare” fuel without any MPC inside it. The transport cask, certified by the USNRC, must be demonstrated to maintain the radiological matter contained in the used fuel’s rods inside the cask’s internal space under a set of punishing accidents such as free fall simulating a hard collision, fire, and deep submersion. In other words, the transport casks are licensed by the NRC to serve as an autonomously leak-tight fortress capable of preventing leakage of its internal gaseous matter to the environment under a series of bounding accidents without any reliance on a leak-tight MPC which may be inside it. For the HI-STORE CIS facility, adopting an even more stringent posture of safety, Holtec has mandated that a shipment of used fuel can occur only after the fuel has been packaged in an MPC. This voluntary measure amounts to sequestering the used fuel inside the welded Canister which is itself sequestered from the environment by the corpus of the transport cask. Together, the MPC and the transport cask reduce the probability of a radiological release to the environment to several orders of magnitude below what is considered “non-credible” under USNRC’s definition.
In fact, the MPC has been recognized as an exceedingly stout barrier against leakage able to withstand deceleration loads in the wake of a postulated accident so severe that it would totally demolish the transport car and its associated structures! The railroad cars are restricted from traveling at a speed limit that is substantially below the level for which the cask is qualified. A Holtec paper, “MPC: A bulwark of safety in the post-9/11 age,” provides additional technical information on the invulnerability of the MPC to severe accidents here.
To summarize, the maximum impact load that can conceivably develop from a transport accident is well below the structural capacity of the cask to maintain leak-tightness and an order of magnitude below the capacity of its enclosed MPC to maintain radiological confinement. Therefore, the risk of radiological release from an MPC bearing the HI-STAR model cask subject to the most severe transport accident contemplated in the NRC regulations (which bound those possible in practice) is essentially zero.
Furthermore, the assessment of a cask encountering a severe transport accident has been estimated by the authorities to be “non-credible.” Industry experience with millions of miles of cask transports over the past four decades bear out the above estimate.
Thus, the occurrence of a radiological release from a Holtec cask must overcome three non-credible independent probabilities that are stacked against it. First, the occurrence of a transport accident would be a complete departure from the industry experience, and then if the transport accident is (counter-factually) assumed, two non-credible occurrences of failure of the transport cask’s containment boundary, and failure of the MPC confinement structure both must occur as a result of the accident to produce a release. Because of the above-described cascade of non-credible possibilities, the scenario of a radiological release from a Holtec transport cask is best termed as “impossible.”