|Fig. 1: Dry Cask Storage.  (Courtesy of the NRC)|
Spent fuel is the nuclear fuel that has been "burned" in a nuclear reactor. It is often highly radioactive and it generates huge amount of decay heat as a result of beta decay of fissile products, although the fissile chain reaction has ceased. Quantitatively, spent fuel, five minutes after reactor shutdown, still releases about 800 kilowatts of heat per metric ton of uranium.  Even though the production rate of decay heat will continue to slow down over time (for instance, decay heat will fall to 0.4% of the original core power level after a day), spent fuel has to cool down and store securely before being sent for reprocessing or long term disposal. 
Spent fuel pool is normally used as an interim storage method. Spent fuel from a nuclear reactor is carefully transferred to and sits deeply in a water-filled pool, where water actively cools down the spent fuel by circulation through heat exchangers and provides shielding from radiation. Generally, spent fuel pool locates on site nearby the nuclear reactors and many of them are designed to be natural-disaster-proof, earthquake-proof for example. This is a good way to store discharged nuclear waste temporarily, for at least several years and more.
Unfortunately, the current capacity of spent fuel pools is not enough for the increasing generation of spent fuels, especially after Yucca Mountain in Nevada, which should have been opened for more than 20 years ago as a permanent repository of nuclear waste, were shut down in 2009.  Consequently, the Nuclear Regulatory Commission (NRC) has loosened its policy in maximum inventory of spent fuels in a spent fuel pool that was originally designed to handle small inventories. This poses great danger to spent fuel pools, in spite of the new regulation of incorporation of neutron-absorbing materials, Boron-10 for instance, in the fuel-racks. During an emergency situation that leads to failure of cooling, water in the pool will boil off, exposing the radioactive spent fuel to atmosphere if one fails to refill the water promptly. More seriously, the uncovered spent fuels will get hotter and hotter due to the ineffective air-cooling to densely packed fuels, and will eventually catch fire, releasing tremendous amount of radioactive airborne substances in the area. For this reason, study has shown that terrorists could plausibly mount a successful attack on a spent fuel pool than a nuclear core reactor.  Apparently, there is a need for another temporary storage method to alleviate the problem of overfilled pools, in particular after the alarm raised by the accident of cooling malfunction of spent fuel pools in the Fukushima.
The idea of dry cask storage is simple. Spent fuel that has cooled in spent fuel pool for at least one year can be encapsulated in a steel dry cask, which is welded or bolted closed when it is moved out from water. The cask is pumped with inert gas inside, and then is contained into another cask made of steel, concrete, or other radiation shielding material. Subsequently, this leak-tight and radiation-shielded dry cask can be stored either horizontally in concrete over-pack or vertically on a concrete pad. One design for casks oriented vertically is called the thick-walled cask, whereas cask with over-pack is normally the design for horizontal storage. The former makes use of the very thick exterior wall as the protection to radiation for each cask, while the latter uses thin outer wall for each cask and relies on the concrete bunker to provide radiation protection. Thanks to its standalone protection, thick-walled cask erected vertically is more prevalent nowadays. A schematic structure of dry cask in both orientations is given in Figure 2.
Regardless of the cask type, the cooling mechanism of dry casks follows these heat-transfer events: 
Heat release in fuel matrix due to radioactive decay
Heat conduction in the fuel and through the cladding
Convection heat transfer from fuel rods due to natural convection of gaseous coolant inside vertically or horizontally oriented casks
Thermal radiation inside casks, radiation heat transfer between the rows of fuel rods and between the fuel and basket-surrounding elements
Heat conduction through internal elements of the cask and through its thick body wall
Natural convection and thermal radiation from the cask's outer surface to the environment
|Fig. 2: A schematic structure of dry cask in both vertical (left) and horizontal (right) orientations.  (Courtesy of the NRC)|
The dry cask storage is less prone to catastrophes. Different from spent fuel pool, dry casks exploits passive cooling by natural convection that is driven by the decay heat of the spent fuel itself. In other words, dry cask is not vulnerable to loss of coolant, which, in comparison, will result in cascade of accidents in spent fuel pool. Moreover, given the fact that nuclear power plants are usually surrounded by ample exclusion area, one can spread out the casks when each of them contains only small amount of radioactive substances. That means, to cause a huge amount of airborne release or wide spread fire, a big number of casks must fail or be attacked simultaneously, not to mention that each cask has its strong protection wall. Other advantages of dry casks include no moving parts, no electricity, relatively simple maintenance (check of vent blockage), and dual-purposes of storage and transportation vehicle.
Two main reasons hindering in moving older spent fuel from pools to dry casks are the high cost and the low availability of casks. It costs about 1 million USD for each cask and another half million USD to load each one with fuel.  The concrete pad for casks to sit on costs another 1 million USD. A rough estimated cost to move all of the fuel in the United States that has cooled in pools for at least five years could cost 7 billion USD. In addition to high cost, the low production rate of the cask is another limiting factor. It has to improve in order to catch up the increasing need for temporary spent fuel storage. There are other issues of dry casks such as additional chance of human errors and radiation risks. The extra step of moving spent fuels from pools to casks, compared to sitting in the pools until long term disposal, poses higher odds to accidents caused by human mishandling; furthermore, it imposes additional radiation doses to workers who transfer the spent fuels from the water.
Further developments on the cost, security, and stability of dry casks are necessary. Research on finding cheaper material and manufacturing process for casks are needed. Moreover, a better method of spent fuel transfer should be developed to reduce both the risk during transfer and the radiation dose imposed to the workers. Besides, regulatory agencies should upgrade existing policies, thereby enhancing the security requirements for dry cask storage. Last but not least, researchers should improve the lifetime of dry casks, and make them less vulnerable to environmental condition. Some thorough studies on cask aging have been done, and protection against local environmental conditions, temperature variation, or other degrading mechanisms are called for implementation and further studies. 
Spent nuclear fuel discharged from nuclear reactors has to cool down and store securely before being sent for any long-term disposal plan, due to large amount of decay heat of fissile products. Spent fuel pool is a good temporary waste storage method designed for low inventories; nonetheless, it is considered a risky option with the increased spent fuel inventories because it is more likely resulting in catastrophe, owing to ineffective air-cooling for dense-packed fuels in the emergency of loss in water coolant. Dry cask storage can be adopted to alleviate the increasing pressure on inventory of spent fuel pools, especially when the old spent fuels are stuck in pools for the lack of a viable long-term disposal program in the United States. The spent fuel transferred from spent fuel pools can be stored in steel casks with additional outer layer of radiation shielding wall. The decay heat is taken off by self-driven natural convection that is not vulnerable to loss of coolant. The main drags for moving the old spent fuels from pools to casks are the high cost and low availability of casks. Ongoing researches on lowering the cost and improving the performance of casks are needed for this good alternative to temporary waste storage.
© Hoi Ng. 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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