Nuclear Waste Disposal - A Comparison of Methods

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Nuclear power is generated by splitting atoms to release the energy held in the nuclei at the core of those atoms. The advancement of nuclear technology has given promising alternatives to produce highly efficient clean energy for relatively low cost. The first electricity production from atomic energy emerged in 1951 at Idaho’s Experimental Breeder Reactor I (Nunez 2019). However, radioactive nuclear waste, a byproduct from the operation of nuclear reactors, has been hindering the development and implementation of nuclear technology. Radioactive nuclear waste is very challenging to handle. Besides the highly hazardous characteristic of nuclear waste that raises health concerns and safety issues, the complication of radioactive nuclear waste management is escalated by the long-lived toxicity of the waste that can last for years to hundreds of thousands of years depending on the radionuclide half-live. Proper disposal of radioactive nuclear waste remains a highly contentious issue around the globe. 

Classification of Nuclear Waste

Nuclear technology has many applications and radioactive waste is generated in different kinds of facilities. From nuclear power plants to medical facilities, various types of radioactive waste exist in a variety of physical and chemical forms. The wide range of concentrations of radionuclides contribute to different levels of radioactivity of nuclear waste. To ensure the proper implementation of radioactive waste management, the International Atomic Energy Agency (IAEA) has developed a classification scheme for assessment purposes on radioactive waste. Classification in accordance with the IAEA's Classification of Radioactive Waste general safety guide (2009) is as follows:

• Exempt waste (EW): Waste that meets the criteria for clearance, exemption or exclusion from regulatory control for radiation protection purposes.

• Very short lived waste (VSLW): Waste that can be stored for decay over a limited period of up to a few years and subsequently cleared from regulatory control according to arrangements approved by the regulatory body, for uncontrolled disposal, use or discharge. This class includes waste containing primarily radionuclides with very short half-lives often used for research and medical purposes.

• Very low level waste (VLLW): Waste that does not necessarily meet the criteria of EW, but that does not need a high level of containment and isolation and, therefore, is suitable for disposal in near surface landfill type facilities with limited regulatory control. Such landfill type facilities may also contain other hazardous waste. Typical waste in this class includes soil and rubble with low levels of activity concentration. Concentrations of longer lived radionuclides in VLLW are generally very limited.

• Low level waste (LLW): Waste that is above clearance levels, but with limited amounts of long lived radionuclides. Such waste requires robust isolation and containment for periods of up to a few hundred years and is suitable for disposal in engineered near surface facilities. This class covers a very broad range of waste. LLW may include short lived radionuclides at higher levels of activity concentration, and also long lived radionuclides, but only at relatively low levels of activity concentration.

• Intermediate level waste (ILW): Waste that, because of its content, particularly of long lived radionuclides, requires a greater degree of containment and isolation than that provided by near surface disposal. However, ILW needs no provision, or only limited provision, for heat dissipation during its storage and disposal. ILW may contain long lived radionuclides, in particular, alpha emitting radionuclides that will not decay to a level of activity concentration acceptable for near surface disposal during the time for which institutional controls can be relied upon. Therefore, waste in this class requires disposal at greater depths, of the order of tens of meters to a few hundred meters

• High level waste (HLW): Waste with levels of activity concentration high enough to generate significant quantities of heat by the radioactive decay process or waste with large amounts of long lived radionuclides that need to be considered in the design of a disposal facility for such waste. Disposal in deep, stable geological formations usually several hundred meters or more below the surface is the generally recognized option for disposal of HLW.

Disposal Methods of Radioactive Nuclear Waste

Disposal methods for nuclear waste that are commonly accepted and widely used include near-surface disposal and deep geological disposal. The philosophy of these conventional methods is to safely contain the radioactive nuclear waste in order to avoid any chance of radiation exposure to humans and pollution to the environment. Near-surface disposal facilities can be at ground level or below the surface. Waste containers consisting of low-level radioactive waste or short-lived intermediate-level radioactive waste are stored in constructed vaults for above ground facilities and excavated caverns for below ground surface facilities. The near-surface disposal method is widely used in many countries including Spain, France, Japan, Sweden, Finland, Britain, and the US (World Nuclear Association 2020). For nuclear waste that remains radioactive for a long period of time, deep geological disposal provides multiple barriers to prevent radionuclides from reaching the environment and exposing humans to radiation. According to the World Nuclear Association, deep geological disposal is the preferred nuclear waste management method for Argentina, Australia, Belgium, Canada, Czech Republic, Finland, France, Japan, Netherlands, Republic of Korea, Russia, Spain, Sweden, Switzerland, Britain, and the US (World Nuclear Association 2020).  

In pursuit of alternative disposal strategies and techniques that would improve already established methods, novel methods and storage materials have been developed, including deep borehole storage and geopolymers. Despite these advancements and the potential viability of these propositions, these novel methods and materials still require extensive testing and field implementation. 

Although scientists and engineers specialized in nuclear radiation have put together detailed plans and protocols for storage of radioactive nuclear waste, concerns are continually expressed by the public and interest groups. Radioactivity of nuclear waste naturally decays and has a finite radioactive lifetime; however, for some long-lived radioactive waste, the process can last for hundreds of thousands of years. Many argue that conventional methods, which involve isolating radiotoxic substances and relying on them to naturally decay through a long period of time, are not real permanent solutions to the problem but merely passing the threat to future generations. In light of the concerns regarding the duration of burden associated with conventional disposal methods, numerous efforts have been contributed to explore other nuclear waste disposal solutions. Over the years, many conceptual ideas have been proposed such as disposal in outer space, disposal at subduction zones, disposal in ice sheets, and a rock-melting solution. 

In the following sections, different conventional and novel methods of radioactive waste disposal will be analyzed. The types of radioactive nuclear waste that are suitable for each method along with the corresponding advantages and disadvantages will be discussed. Furthermore, conceptual methods of nuclear waste disposal will be studied to explore the feasibilities and/or constraints related to each idea.   

As noted previously, conventional methods of nuclear waste storage typically involve near-surface or deep geological disposal. The optimal disposal method for a given situation relies upon the classification of the waste being disposed. Near-surface disposal lends itself well to the disposal of LLW and ILW that do not generate high heat and has primarily short-lived isotopes, while geological disposal is typically used to handle HLW, which may be in forms such as SNF (spent nuclear fuel). 

Near-Surface Disposal

Near-surface disposal, previously referred to as “shallow land” and “ground disposal” in scientific literature, is the primary method of disposal for low and intermediate level radioactive waste (LILW), typically with half lives of no more than 30 years (World Nuclear Association 2020).  In some cases, LILW with long-lived isotopes (with half lives greater than 30 years) are disposed via this method, given low enough concentrations (Tolentino and Oliveira de Tello 2013). The rationale behind storing LILW, which can still be harmful to human health but not nearly as much as HLW, in facilities that are near the surface is that “by the end of the institutional control period, the activity of the waste will have decayed to harmless levels with respect to likely future uses of the site and consequent potential exposure pathways, as shown by safety assessments” (IAEA 2005). LILW facilities are typically one of two forms: ground-level disposal facilities and below ground-level disposal facilities in caverns (otherwise known as mined cavities). Ground-level disposal facilities are conventionally some sort of excavated trench, with many meters of protective layering surrounding the area in which the radioactive waste is placed. The protective layering may include a concrete vault, an impermeable membrane cap with topsoil, and/or backfill. Some designs may also include a drainage system, as well as a gas-venting system (World Nuclear Association 2020). Figure 1 shows some common designs for ground-level disposal facilities: a) covered trench b) closed vault c) domed vault and d) open vault (Tolentino and Oliveira de Tello 2013). 

Mined cavities differ from ground-level disposal facilities in a few keyways, including the fact that excavation does not begin at the ground-level but rather in an underground cavern, and that the ultimate depth of the disposal facility is tens of meters below ground (up to 100 meters), a depth greater than that of ground-level facilities which are typically no more than 10 meters below ground level (Tolentino and Oliveira de Tello 2013). Rather than a standard trench like design, mined cavities may include “tunnels, vaults, vertical or horizontal caverns or some combination of these” (Tolentino and Oliveira de Tello 2013). The rationale behind building these subsurface facilities is that the location and depth of these facilities is typically “adequate to essentially eliminate concerns of intrusion by plants, animals and humans” (Tolentino and Oliveira de Tello 2013). 

Some of the challenges with near-surface disposal facilities is that there may be significant post-closure costs associated with institutional control and oversight of the facility. Surveillance and security, particularly for facilities that are near the surface, is necessary for a few decades to a few hundred years in order to restrict access to the facility to prevent human intrusion (IAEA 2014). Furthermore, although technology has rapidly evolved over the past few decades, “long term maintenance of records in a manner consistent with regulatory and other applicable requirements represents a significant technical challenge” for near-surface disposal facilities (IAEA 2002). Records must be readily accessible and duplicated in various locations and should detail “ the development, operation and closure of a near surface repository”. (IAEA 2002). As with any disposal facility, public fear and distrust, as well as the perception that disposal is only a temporary solution to the issue of nuclear waste disposal, presents challenges to the implementation of near surface disposal sites.

Deep Geological Disposal

Deep geological disposal is currently the primary method by which HLW and in some cases LILW and SNF are disposed of. Given the longevity and high heat generated from HLW and SNF, deep geological disposal aims to isolate the waste within “engineered and natural barriers (rock, salt, clay)” in order to protect the biosphere and people from the harmful effects of the radiation (World Nuclear Association 2020). Furthermore, storing waste hundreds of meters below the ground (typically 250-1000 meters) in mined repositories would require “no obligation to actively maintain the facility” as it is passed onto future generations, although recent designs have considered retrievability of the waste in the future event that nuclear waste can be fully reprocessed and rendered safe via technology (World Nuclear Association 2020, Birkholzer, et al 2012). 

The general principle by which deep geological disposal operates is known as the “multi-barrier concept” which entails the following: 1) Multiple engineered barrier layers will contain the waste until it decays 2) the natural geological formation (the natural barrier) surrounding the engineered barriers will isolate the waste from the biosphere and prevent human intrusion and 3) the depth of disposal site will ensure any leakage of radionuclides will face long transport times, such that it will likely decay before reaching the biosphere (Corkhill and Hyatt 2018). Through redundant systems of containment and protection, any one failure of the barriers put into place would not jeopardize the storage of the waste and can be compensated for via the other engineered or natural barriers without intervention. 

The same concept utilized in the overall design of deep geological disposal facilities also applies to the engineered barriers that directly surround the waste. In designing the waste package, a number of subcomponents that act as redundant layers of protection are considered: the waste form, the waste canister, backfill material, and tunnel seals/plugs. Selecting a waste form for the nuclear waste requires an understanding of the type of waste on hand (the chemical composition of the waste, concentration, phase, and etc.) and then the best material to use to contain that waste. Ultimately, the waste form is considered to “provide structural stability and resistance to waste dissolution, slowing the release of radionuclides” (Birkholzer, et al 2012). Common waste forms include the use of zircaloy or stainless-steel cladding for solid form wastes such as SNF, or stabilization and immobilization within borosilicate glass, ceramic, and glass ceramic for liquid wastes generated from fuel reprocessing (Birkholzer, et al 2012). For certain ILW wastes, wet cement paste (typically CEM I cement)  may also be used for waste immobilization given its cost-efficacy, thermal properties, and radiation shielding (Corkhill and Hyatt 2018). The waste canister’s primary purpose is to prevent waste contact with groundwater and to delay radionuclide transport for as long as possible (Birkholzer, et al 2012). Its design typically involves the implementation of thick, non-corrosion-resistant material such as carbon steel, and/or corrosion-resistant material such as copper or nickel-based alloys (Birkholzer, et al 2012). Backfill and tunnel seals or plugs, typically made of bentonite, bentonite-sand mixture, chemical buffering materials, excavated rock or in some cases, crushed salt, are used to retard nucleotide transport and protect the waste from potential geological damage (Birkholzer, et al 2012, IAEA 2009).

In regard to site selection, numerous geological factors are considered. Besides general good engineering properties of a particular site location (ease of construction, accessibility, etc.), the long-term stability of the host rock (measured in the millions of years) and the long-term low flow (or low proximity) to groundwater flow is also considered (Birkholzer, et al 2012). Through carefully selecting a site based on these parameters, deep geological storage can effectively prevent waste leakage and contamination of its surrounding environment. 

Overall deep geological disposal has the benefits of being a method in which nuclear wastes, particularly HLW, can be stored for the long-term (over 10,000 years with consideration of the natural barrier), and being comparable economically to other disposal methods at least LILW (Corkhill and Hyatt 2018, IAEA 2007). It is also currently the only internationally implemented method to dispose of HLW and in some cases SNF (note that SNF before reprocessing may alternatively be stored in spent fuel ponds or dry cask storage). Deep geological storage’s multi-barrier concept also ensures redundant protection and the minimization of chances of leakage.

Some challenges include maintaining “public confidence during the inevitably long process of site selection, site characterization, and repository development”, and accurately assessing the long-term viability and safety of any site, with account for uncertainties (Birkholzer, et al 2012). In fact, one proposed deep geological disposal site within Yucca Mountain in the state of Nevada has continually faced “technical and political challenges since 1987 and is yet to be constructed or licensed to accept waste” (Corkhill and Hyatt 2018). Furthermore, when a mined repository is built with future waste retrieval in mind, conventional and radiological safety may be negatively impacted, which may further concern the public when it comes to the implementation of this disposal method (IAEA 2009). Although deep geological facilities may be built to not require active monitoring, post closure costs may be high to maintain a level of institutional control (in some countries for a period 200 to 300 years) and surveillance for safety reasons (IAEA 2007).

As technology and material and chemical sciences continue to develop, novel methods of waste disposal and containment have been proposed and tested in controlled environments. Some of the most promising methods, including geopolymer storage and deep boreholes have been highlighted here. 

Geopolymer Storage

Although waste immobilization is a concept that is already actively practiced when preparing waste for storage or transport, novel materials and methods are currently being actively considered. As discussed in Section 2.2 Deep Geological Disposal about waste forms, waste immobilization aims to contain nuclear waste in a form such that there is greater structural stability and a minimization of dissolution. Geopolymer storage specifically is the novel method of storing certain types of waste within geopolymer material. In Dong, et al’s 2020 study, radioactive tributyl phosphate (TBP), a waste organic solvent that contains a variety of radionuclides including uranium and plutonium, and odorless kerosene (OK) organic liquids was solidified in a phosphate acid-based geopolymer (PAG). This was accomplished through the addition of tween 80 (T)80, phosphoric acid and metakaolin. Through the synthesis of the solidified TBP/OK form (SPT), and leaching tests, it was determined that only “a very limited amount of TBP/OK was released from the PAG-solidified TBP/OK sample”, indicating that this geopolymer could prove promising in waste immobilization (Dong, et al 2020). Other geopolymers have also emerged as promising means of immobilizing nuclear waste, including geopolymer cement. Geopolymer cement has two distinct advantages, including fast dewatering, the ability to remain stable up to temperatures of 1000°C, and being able to entrap radioactive elements within a “zeolitic geopolymeric framework, enhancing the innocuity of the containment” (Davidovits 1994). Although there is great promise in this field, there are still a number of significant challenges to overcome; namely, the fact that geopolymers may not be applicable in every scenario (i.e., PAG may not be able to withstand HLW in concentrations found in current disposal techniques), scaling of production, cost-efficacy, and the fact additional study is required to assess how geopolymer storage may interact with other components of a waste package. 

Deep Boreholes

An alternative method to mined repositories as described in Section 2.2 Deep Geological Disposal is the drilling and capping of deep boreholes. This method requires the drilling of a hole to up to a depth of about 5 kilometers and the emplacement of about 400 5-meter-long steel canisters containing SNF or vitrified wastes in the lower 2000 meters of the hole (Birkholzer, et al 2012, World Nuclear Association 2020). Subsequently, the upper 3000 meters of the hole would be sealed with durable materials including bentonite, asphalt and concrete (World Nuclear Association 2020). The benefit of this method is that boreholes could be drilled offshore, which would open large areas for nuclear waste disposal, and can be an effective way to dispose of smaller waste forms. However, the drawbacks of this method are that it has not been fully implemented outside of testing, and thus requires additional extensive study, and it may not be the most economically viable solution in all cases (Birkholzer, et al 2012).

Radioactive waste will contaminate the environment and pose a serious threat to the well-being of humans for a long period of time if treated improperly. Scientists from different nations are continuously exploring better ways to manage radioactive waste. Some of the conceptual methods that have been investigated extensively are discussed in this section.

Sub-seabed Disposal

For many years, sea disposal, which involves dumping packaged radioactive waste into the ocean, was one of the common methods to discard radioactive waste (Calmet 1989). Nonetheless, the potential dispersion of radionuclides in the sea that can possibly harm the marine environment has led to prohibition of sea disposal methods by international agreements. Subsequently, the sub-seabed disposal has been proposed as another option of underwater disposal method for radioactive waste. The sub-seabed is considered as a potential area to dispose of HLW because of its relatively stable and predictable geologic formations (Calmet 1989). Within this concept, packaged HLW in corrosion resistance containers would be embedded in the seafloor sediment. The burial of HLW could be achieved by either utilizing penetrators or drilling, similarly to the novel deep borehole storage disposal method. Depending on the burial technique, depth of embedment could vary from 7 to 800 meters deep (Bala 2014). In 1988, the disposal of HLW in deep ocean sediments was deemed to be technically feasible by the Organization for Economic Co-operation and Development (Calmet 1989).  

The objective of sub-seabed disposal of radioactive waste is the same as other storage methods, which is to isolate the waste long enough for any release of radiation into the environment and avoid exposure to humans. By burying radioactive waste under the seabed, the sediment beneath the ocean could serve as an extra containment of radionuclides compared to sea disposal and other storage methods. Furthermore, sub-seabed disposal could be executed far away from the coast of any countries such that NIMBY (not in my backyard) complaints would be avoided (Bala 2014). In terms of legality, the 1996 Protocol, which replaced the treaty that banned oceanic dumping of waste from the London Convention of 1972, extended the prohibition of any type of waste storage in the seabed to its signatory nations (Bala 2014). Although not all nations, including the US, are bound by the 1996 Protocol, further research and experimentation would be necessary to prove the sub-seabed method has insignificant impact on the marine environment in order to gain international acceptance.     

Ice Sheet Disposal

The concept of ice sheet disposal is to place heat generating HLW on stable ice sheets such that the surrounding ice would be melted by the heat and the waste would eventually sink into the ice sheet. When the water above the containers refreezes, the thick ice sheet would serve as a barrier to isolate the radioactive waste from the biosphere. In 1956, when this method was first proposed, B. Philberth suggested that the Antarctica and Greenland ice caps were suitable locations to implement ice sheet disposal (Philberth 1977). Three different approaches have been considered for ice sheet disposal including passive slow descent, anchor emplacement, and surface storage emplacement (DOE 1987). The radioactive waste containers would be placed in shallow holes in passive slow descent. The heat emitted from the radioactive decay process would melt the ice and the containers would sink into the bottom of the ice sheets. Anchor emplacement would be similar to passive emplacement, but with an anchor cable attached to the waste containers which would limit the embedment depth and permit retrieval of the waste. For surface storage emplacement large storage units would be built above the snow surface. The radioactive waste would be stored in the storage units and radioactive decay processes would generate heat, such that the storage units would slowly melt their way to the bottom of the ice sheet (DOE 1987).      

The ice caps at the polar regions are thousands of meters thick and are believed to remain stable for thousands of years. The thick stable ice caps could serve as giant barriers to prevent radiation from leaking into the environment. Another advantage is that polar regions, Antarctica for instance, are uninhabited remote areas far away from any community (DOE 1987). Similar to the sub-seabed disposal, NIMBY objections could be avoided. As one of the disadvantages, the uncertainties of global warming’s impacts and climate change could impact the stability and thickness of the ice sheets. In addition, the expected high operational costs due to the remoteness of ice sheet disposal locations could make this method unfavorable (DOE 1987).

Space Disposal

The idea of space disposal is to remove radioactive waste, particularly HLW with long-lived radionuclides, from the Earth permanently by launching it into outer space. The outer space disposal concept is not new and has been around for more than 40 years. The first investigation on the feasibility of transporting radioactive waste from nuclear power plants into space was done in 1973 by National Aeronautics and Space Administration (NASA) on behalf of the Atomic Energy Commission (ACE). The results of this exploratory research showed that outer space disposal of long-lived HLW was feasible in technical, economical, and safety standpoints (NASA 1973). Since the initial feasibility study by NASA, extensive research has been performed on the space disposal concept including selecting the ultimate destinations for the waste to be deployed to and the space shuttles that would be used to launch the waste. 

The major benefit of disposing radioactive waste into outer space is that it would permanently eliminate the responsibility and burden of dealing with such waste for the future generations. In addition, a secondary benefit as a derivative could emerge if space disposal for radioactive waste was implemented – any infrastructure that is built for launching the waste could provide an economical access to space for other purposes (Coopersmith 1999). On the negative side, the high cost of ejecting the waste to outer space relative to other disposal methods and the public concerns on the safety issue of launching a space shuttle loaded with radiotoxic substances have prevented this conceptual method to be implemented. However, with the accelerated advancement of today’s technology and the development of private aerospace companies such as SpaceX, the disadvantages associated with cost and safety would diminish over time and space disposal of radioactive waste could be implemented in the near future. 

In comparing the various methods by which nuclear waste is disposed of, it becomes clear that there is not one right solution. Different solutions occupy different niches, targeting different forms and levels of waste. Although a number of proposed solutions have yet to see implementation, evolving technologies and economics could see their implementation in the near future. The table below summarizes the advantages and disadvantages of each of the  methods that have been discussed in this paper.

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