|Fig. 1: The nuclear fuel cycle.  The post-reactor process is particularly important. (Courtesy of the NRC)|
Nuclear waste refers to the materials derived from nuclear processes that are either innately radioactive themselves or have been contaminated by other radioactive elements. There is much debate over how this waste should be disposed of and this is especially true in the case of high level waste (HLW). The section below gives more detail on classifying nuclear waste.
High level waste can be further classified into two categories: the transuranics which result when uranium absorbs one neutron but does not fission and turns into mostly plutonium, americium and curium, and the actual products of fission.  The actual fission products tend to decay rapidly after creation and those products that decay slowly do not usually pose much of a hazard. Therefore, the challenge in waste disposal is a result of the creation of the transuranics.
Low Level Waste is the type of waste generated by hospitals and industry and is comprised of clothing and tools with short-lived radioactivity. Because of the low levels of radiation, special shielding is not required during handling and it can be disposed of in near-surface sites. 
High Level Waste contains products of fission and transuranic elements generated by the reactor. The heat it produces is significant and so both cooling and shielding are required for its storage. Fission products such as Cs-137 and Sr-90 (with half-lives of roughly 30 years) account for most of the heat and penetrating radiation initially produced. Transuranics, which have longer half lives (24,000 years for Pu- 239), account for most heat produced after 1000 years. Initial cooling is usually achieved by under-water storage on-site at the reactor facility in spent fuel pools with a typical water depth of 40 ft. 
|Fig. 2: Deep Borehole.  (Courtesy of the DOE)|
When nuclear fuel is removed from a reactor, it has usually undergone only ~6.5% burn-up meaning that only 6.5% of the atomic fuel is 'burned' or converted to fissile plutonium. The reason for the low burn-up rate is that there comes a point when the levels of fission fragments and heavy metals do not allow the reaction to take place as easily (reduced nuclear cross section) as before and it may no longer be economical to continue extracting energy from those fuel rods.  At this point energy is still being emitted from the fuel rods both in the form of alpha,beta, and gamma radiation and in the form of heat. Due to the heat produced, the fuel is usually stored temporarily under large water ponds which are cooled by heat exchangers and act to absorb radiation. As spent fuel pools near capacity, older spent fuel is moved into dry cask storage. The industry norm storage time is about 10 years althought the NRC has authorized transfer as early as three years. 
After being stored, the fuel can either be reprocessed which aims to recover and recycle the usable portion of the fuel or it can be stored in a facility designated for long- term disposal. This process is visualized in Fig. 1 in the post-reactor stage of the cycle. 
Used fuel is typically comprised of Uranium, long-lived actinides such as plutonium, and unusable waste. Reprocessing is used to recover much of the useful portion of the spent fuel by separating fission products from residual uranium and plutonium which can be used again as fuel. 
The issue of disposal stems from the 4% waste mentioned above which, after being dissolved in acid is now in liquid form. It can then undergo vitrification where it is heated strongly to produce a powder which can be mixed with Pyrex glass, allowing it to be more safely stored in the event of water intrusion into the repository.  However, there are currently no facilities where this waste can be disposed of. It is simply stored in reinforced concrete casks with the hope that it will one day be reprocessed. The need for disposal has also not been very dire yet since only relatively small volumes of waste have been produced.
In the USA, one potential site for long-term storage has been identified as a geological repository by the DOE since the 1980's. Yucca Mountain lies on federal land in Nevada, 90 miles northwest of Las Vegas.  Currently, a total of 70,000 tons of HLW are scattered across 39 states in cooling ponds, some of which are located near to rivers or on water tables. However, moving it all to a central location at Yucca Mountain has been controversial and met with much opposition from the state. This opposition stems from the fact that between 1 and 7 shipments would be required across the nation's highway/rail system and directly through Las Vegas for the next 24 years. 
Space - This idea has been ruled out of the possible solutions for disposal because of the risks involved with an exploding shuttle full of radioactive waste, if that were to take place. The other practical issue relates to the economics of using an expensive shuttle launch to ship just a tiny fraction of existing nuclear waste at a time. 
Deep Boreholes - This disposal method can fall under the broad umbrella of geological disposal, along with the sub-seabed storage and subduction zone methods below. In the case of deep boreholes which have diameters large enough to fit the concrete casks of spent fuel rods (as shown in Fig. 2), they could be dug up to a few miles deep and sited near to the reactor facility. It would still be challenging to retrieve that waste in the future if we ever wanted to. 
Sub-Seabed Storage - Would be similar to drilling deep boreholes, but it would be done under the ocean. There are many legal and technical reasons which make this impossible. For example, it is a violation of international convention to dispose of nuclear waste at sea. 
Subduction Zone - Using the natural plate tectonic features of the earth is a very interesting idea but also is a violation of international treaties. Additionally, extensive long-term studies would be required to determine how fuel would be transported once it is inserted into the Earth's tectonic conveyor belt to ensure that the nuclear waste would not resurface in the distant future in the form of a volcanic eruption. 
The methods above focus specifically on storing and disposing of waste products of nuclear reactors. However, there has also been significant investment in finding ways of reducing the amount of waste created in the first place.
There are currently 55 nuclear startups with $1.6 billion in funding. The nuclear sector is very restrictive and presents great barriers to new players because of the history of the NRC (Nuclear Regulatory Comission) as an entity intended to thwart nuclear arms proliferation and not one that is focused on engaging with innovative entrepreneurs.  The two companies below have received significant publicity for their novel approaches to producing less waste.
Transatomic Power - Founded in 2011, this company aims to use novel designs and materials to improve the molten salt reactor in order to use nuclear waste as a power source.  In March 2014, the company published a white paper claiming that their design could generate up to 75 times more electricity per ton of mined uranium than typical LWRs. This claim prompted an analysis by MIT nuclear science professor Kord Smith in which it was later found that the reactor design would improve efficiency by more than a factor of two, which would still be a great accomplishment. Even this would reduce waste by 53% compared to today's LWRs. Other questions have arisen surrounding the technology's ability to sustain a fission chain reaction using only spent fuel but the company has made its technical analysis public information to invite further analysis. 
Terrapower - is pursuing a novel type of reactor the travelling wave reactor which uses nuclear waste as a power source. Molten chloride is used as both the coolant and medium for the fuel. The nuclear reaction moves like a standing wave through the fuel core converting uranium to plutonium. The company found attractiveness in the use of molten salt reactors, such as chloride, due to their innate safety and economic advantages over conventional reactor designs. If a meltdown were to occur, the molten salt fuel could be moved to underground storage without any need for pumping equipment, where it would cool down. Other advantages of chloride salt reactors outlined by Terrapower's Innovation Director include high power density and efficiency, high solubility of uranium in the chloride solution, significantly less waste, and no longer needing ongoing uranium enrichment after startup which reduces concern over proliferation. 
© Gregory Tuayev-Deane. 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.
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