|Fig. 1: A propelled rail car carrying a spent nuclear fuel flask (orange), immediately after impacting a concrete wall during a test designed to prove the flask's integrity in a crash scenario. (Source: Wikimedia Commons)|
The most dangerous result of nuclear energy production is the abundance of nuclear waste that must be disposed. In particular, high level waste (HLW) is predominantly comprised of depleted fuel rods from nuclear reactors, that are often stored temporarily on-site submerged in pools.  The potential for disaster in these spent fuel pools is high. For example, a buried HLW storage tank at the Kyshtym Complex, a Soviet plutonium production faciliy, exploded in 1957 after a critical cooling system malfunctioned. The resulting airborne radioactive emissions initially contaminated at least fifteen thousand square kilometers, and resulted in the evacuation of about ten thousand inhabitants from that area. 
To address the issue of safe long-term storage, additional disposal methods must be developed. The most likely of these is the use of deep geological storage sites. [3,4] However, to make use of such sites, spent fuel waste must be transported (by truck or by rail) to the storage site.  While there has never been an incident that has resulted in a dangerous release of radioactive waste, nuclear materials are much more exposed during transport, leading many to worry about the practice's safety and security. 
In part to ensure that nuclear transport flasks can double as safe long-term storage containers for high level waste, and in part to build public confidence in the safety of waste transportation, nuclear transport flasks are subjected to rigorous testing regimens. [7,8]
The NRC requires that a series of tests be run on candidate flasks to simulate accident conditions, among them a free fall of 30 feet onto a hard surface, a puncture drop test onto a 6 inch steel bar, exposure to an 800°C fire for thirty minutes, and total immersion under water.  In addition, several full-scale tests have been performed, an example of which can be seen in Fig. 1. In 1984, the United Kingdom's Central Electricity Generating Board (CEGB) staged "Operation Smash Hit," in which a freight locomotive impacted a stationary rail car carrying a transport flask at one hundred miles per hour. The test was important because it was broadcast publicly as a demonstration of the safety of spent fuel transport, and, along with other full-scale tests, showed that scale-model trials were effective predictors of actual performance. 
Safety research continues to this day. A 2014 study examined the potential dangers of bending and vibrational forces on spent fuel rods while in transit and the resulting deterioration of fuel rod cladding.  Finally, the RADTRAN code project, developed by Sandia National Laboratories, can be used to calculate risk of radiation exposure given a specified transport route and estimate casualties and contamination area if an incident were to occur. The model takes into account many factors, among them, the proposed route (over air, land, or sea), the specific type of waste being transported, the population density along the route, and even the amount of time required for emergency responses like evacuations. 
It is not enough, however, to protect nuclear waste from transportation accidents. Spent nuclear fuel provides an attractive target for would-be nuclear terrorists. Sandoval et al. developed an experimental basis in 1983 on which to predict the effects of an attack by high explosive device on a waste flask being transported in an urban area.  The line of investigation was updated by Luna et al. two decades later by examining a new explosive device and updated spent fuel casks.  While both reports concluded that potential casualties from the given scenario would only reach into the low hundreds, if that, the risk must still be considered. This is especially the case if attackers acquire full access to the internally-stored material, use additional dispersal devices, or target emergency response teams after the initial attack.  Advances in explosive penetration technology since Luna et al. could also make transport flasks more vulnerable. A 2006 study on spent fuel transportation weakness in Pakistan showed not only that approximately 160 people would succumb to severe cancer in the event of an urban "dirty bomb" scenario, but a radioactive plume would coat the surrounding land and require intensive cleaning efforts.  If a permanent repository for high level nuclear waste is ever created, close attention will have to be paid to the security procedures employed.
© Jean-Luc Watson. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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