|Fig. 1: Yucca Proposed Design (Source: Wikimedia Commons)|
Nuclear energy was discovered in the middle of the twentieth century; since then it has become one of the most popular forms of energy. Shortly after nuclear energy and all of its capabilities become readily know, it was also discovered that the waste created from this product could be harmful. The radioactive substance that is created after nuclear fuel is used is called nuclear waste, and thus waste remains toxic for over a thousand years. The issue of nuclear waste in the United States is important because the US is the world leader in electricity generation from nuclear energy with 104 reactors currently in use.  The issue now is to properly store this nuclear waste, so that it does not cause harm to humans or the environment.
There are three types of nuclear waste; low level, intermediate level, and high level. These three levels of nuclear waste vary in terms of the amount of waste volume they account for and actual radioactivity. Low level nuclear waste makes up roughly 90% of the volume of nuclear waste but only contributes 1% of the radioactivity.  This is because it is made up of items such as clothing and tools used during the operation of the nuclear power plant. The intermediate level of waste is comprised of 7% of the volume of waste and about 4% of the radioactivity.  Intermediate waste is comprised of parts of the reactor that have been used. The most significant type of waste is high level waste, which contributes only 3% of the volume but is responsible for 95% of the radioactivity.  High level waste is comprised of the spent nuclear fuel (SNF). This waste is a product of the reactions occurring inside of the nuclear reactor. The high level waste or SNF can no longer be used to create electricity, yet it is still highly radioactive and potential harmful if not stored in the proper manner. 
When it comes to storing nuclear waste there is either temporary storage or permanent storage. However, the U.S. follows a "once- through" nuclear fuel cycle strategy which creates a constantly increasing inventory of spent nuclear fuel in wet storage at reactor sites.  Yet, it appears as though storage pools are the current and permanent location for nuclear waste as time ticks away and no permanent options are available. Now, older spent nuclear fuel is being dry stored in casks in independent spent fuel storage installations.  Dry storage casks can be vertical or horizontal in alignment, with the vertical casks more commonly used because they are standalone units that come with their own shielding. Horizontal casks do not have the same amount of protection and must be placed in steel lined concrete bunkers for proper safety measures. 
Ensuring geological isolation of nuclear waste over tens of millennia or longer poses questions that Earth scientist continue to study. With the apparent end of plans to use Yucca Mountain (Fig 1), U.S. nuclear waste repository planning is at a crossroads.  While interim storage of SNF at one or more secure locations is an option, permanent underground disposal will likely be necessary.  The U.S. will continue to research for a sustainable way to dispose of nuclear waste.
© Michaael Humphrey. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
 A. McDonald, "Nuclear Power Global Status," IAEA Bull. 49-2, 45 (2008).
 C. Kumar, "Commercial Nuclear Energy Production and Nuclear Waste," Physics 241, Stanford University, Winter 2016.
 E. D. Federovich, "Technical Issues of Wet and Dry Storage Facilities for Spent Nuclear Fuel," in Safety Related Issues of Spent Nuclear Fuel Storage, ed. by J. D. B. Lambert and K. K. Kadyrzhanov (Springer, 2007), p. 189.
 C. E. Neuzil, "Can Shale Safely Host U.S. Nuclear Waste?" EOS Trans. Am. Geophys. Union 94, 261 (2013).