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| Fig. 1: Spent fuel rods are composed of uranium oxide and placed in a spent fuel assembly, which is then placed into the dry casks. Rods placed in dry casks typically have spent time in spent fuel pools. (Courtesy of the NRC) |
An especially problematic issue regarding Nuclear energy production is the containment of hazardous and radioactive waste byproducts of fission. This paper will focus on the containment of spent fuel rods in dry cask storage.
Fuel rods are metal tubes that hold a ceramic pellet consisting of pressed uranium oxide which is sintered at a high temperature (over 1400°C). These fuel rods are placed in a fuel assembly and then put into the reactor to act as the reactor fuel for the fission process. The fission process creates an incredible amount of heat that produces steam to power a turbine. Eventually, (around 18 months) a third of the fuel rods must be replaced with fresh fuel rods to maintain efficiency. Typically these "spent fuel rods" are placed in pools of water which cool the fuel rods and shield radiation. [1] However, with limited space in spent fuel pools in the early 1980s, dry cask storage systems became a means to store spent fuel safely. [2]
Dry casks are typically metal cylinders containing the spent fuel rods with a concrete or metal outer shell. Fig. 1 shows how the spent fuel rods are placed in a dry cask. [2] Concrete, lead, and steel protect from gamma radiation escape, while polyethylene and concrete protect from neutron escape. Other safeguards include "boron-doped" materials that absorb neutrons to prevent both neutron escape and catalyzed fission. [1]
This is called "dry" storage, because inert gas (typically helium) is pumped throughout the system, rather than water used in fuel pools. [3] Cooling is accomplished through passive heat conduction through solid materials and natural convection or thermal radiation. [4]
Some advantages of the dry cask storage system include the mobility of the casks and its safety against earthquakes. The casks are not fixed to the ground, so if it needs to be relocated, it can be easily moved, making the cask a dual-purpose system (transportation and storage).
Also, because of this mobility, it is more difficult to "crack" during an earthquake. [3] Not only are dry casks less prone to natural disasters such as earthquakes, but they are also less prone to terrorist attacks, because far more casks would have to be targeted to produce the same amount of damage as a targeted spent fuel pool.
Because of the inert gas and natural cooling through convection and thermal radiation, the dry cask storage system does not require the complex radiation monitoring system that spent fuel pools use. The use of inert gas rather than liquid also contributes to far less waste than the spent fuel pools. [3] Finally, the dry cask storage system efficiently uses space.
Although dry casks systems have many advantages, they also have limitations that have prevented their use in current nuclear waste containment. First, although dry casks may be less prone to terrorist attacks, they are not constructed with potential attacks in mind. So, under an attack, dry casks are more prone to failure and exposure of radioactive material to the surrounding environment.
Another limitation is the type of spent fuel rods that the dry cask system can hold. Passive cooling methods such as convection and thermal radiation are only possible with older spent fuel rods, where "decay heat" is much lower. Thus, spent fuel rods typically spend time in the spent fuel pools before being placed in the dry casks. [4] Finally, production of dry casks is very expensive. [3]
The first dry cask storage system was established at the North Anna nuclear power plant site in Virginia in 1986, and since then dry cask storage systems have become more prevalent. Due to the lack of spent fuel pools, the number of dry cask storage systems is expected to rise, [1]
Current legislation includes the Dry Cask Storage Act of 2015, which has been brought to the Senate and referred to the Committee on Environment and Public Works. The bill would set up a system for approval of dry cask storage transfers by the NRC (Nuclear Regulatory Commission). [5]
© Franklin Huang. 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.
[1] "Dry Cask Storage of Spent Nuclear Fuel," U.S. Nuclear Regulatory Commission, October 2016.
[2] Safety and Security of Commercial Spent Nuclear Fuel Storage: Public Report (National Academies Press, 2006), Ch. 4.
[3] H. Ng, "Dry Cask Storage," Physics 241, Stanford University, Winter 2014.
[4] L. S. Romanato, "Advantages of Dry Hardened Cask Storage Over Wet Storage For Spent Nuclear Fuel," Centro Tecnológico da Marinha em São Paulo, 24 Oct 11.
[5] J. D. Werner, "U.S. Spent Nuclear Fuel Storage," Congressional Research Service, R42513, May 2012.