How Does Nuclear Waste Disposal Work?
31 countries currently use some form of nuclear power, with the 455 currently operational reactors generating some 393,000 Megawatts of electricity – nearly 20% of the world’s total energy production. Despite high-profile disasters such as Chernobyl, Three-Mile-Island, and Fukushima, nuclear power is actually among the safest and cleaner forms of electricity generation, placing dead-last in terms of deaths per kilowatt-hour generated – yes, even behind solar and wind power. Unlike coal and oil nuclear power emits no greenhouse gases, and unlike solar, wind, and hydroelectricity is not dependent on geographic location or local weather conditions. Energy experts thus predict that if the public stigma can be lifted, especially with more modernly designed nuclear reactors, nuclear power may be poised to make a comeback as the green energy source of the future.
But despite its many advantages, nuclear power has one glaring Achilles’ heel: radioactive waste. Over the past four decades the global nuclear industry has generated some 62,500 metric tons of waste, with a further 2,300 tons added every year – much of which will remain dangerously radioactive for thousands of years. Unlike other kinds of industrial waste, nuclear waste cannot feasibly be converted to a less dangerous form, nor – given the current political climate – can much of it be reprocessed and recycled. So, then, how to we dispose of it?
While commercial nuclear power is nearly 70 years old, the concept of permanent nuclear waste disposal is surprisingly a recent one. In the early days of nuclear power, engineers assumed that reactors would operate on a more sophisticated fuel cycle, with spent fuel being reprocessed to produce new fuel elements and to remove radioactive isotopes for use in medicine, scientific research, and nuclear weapons. Much research was also performed on breeder reactors, which convert non-fissile isotopes into fissile ones, thus generating more fuel than they consume. Combined, these two technologies would produce relatively little high-level radioactive waste. However, this plan was based on the belief that earth’s uranium reserves were extremely limited, and when Uranium was found to be far more abundant than initially assumed, a once-through fuel cycle without reprocessing became much more economical. At the same time, companies such as Westinghouse and General Electric found that it was much easier and cheaper to scale up existing reactors they had designed for US Navy submarines than to develop civilian reactors from scratch. As the development costs had already been subsidized by the US Government, these reactors could be sold at much more competitive prices. As a result, reprocessing and breeder reactors fell by the wayside, and the nuclear waste soon began to pile up.
Nuclear waste can be divided into three basic categories: low, intermediate, and high-level. Low and intermediate-level waste is defined as material that will remain dangerously radioactive for less than 300 years, and consists mainly of contaminated materials such as solvents, tools, laboratory glassware, and clothing used in the processing of nuclear fuel. Due to the low activity and short decay timeline of this waste, it is relatively straightforward to deal with, typically being kept in shielded, monitored storage containers on-site at nuclear power plants or processing facilities. Once the radioactivity decays to safe levels, the material can be disposed of like any other industrial waste.
High-level waste consists mainly of spent fuel from nuclear reactors and is a different beast entirely, for many of the radionuclides in spent fuel can remain dangerously radioactive for tens of thousands of years. As this is longer than human civilization – at least in its current form – is expected to survive, any feasible disposal scheme must with a high degree of certainty prevent these nuclides from leaking into the environment without any human supervision or interference. Over the years a number of disposal schemes have been proposed, including dumping the waste in deep-sea trenches, placing it in tectonic subduction zones so it gets drawn into the earth’s mantle, or launching it into outer space. However, all these proposals have been rejected for one reason or another – for example, the failure rate of modern rockets, while low, is significant enough that a launch failure causing widespread radioactive contamination is a very real – and unacceptable – possibility. Furthermore, the London Convention of 1972 and the Basel Convention of 1992 explicitly ban the disposal of radioactive waste in the oceans. Consequently, all currently waste disposal schemes are based on deep geological burial. The most advanced of these projects currently are Finland’s Onkalo and Sweden’s Forsmark repositories, and it is these disposal models that will be discussed now.
To understand how deep geological disposal works, it is first necessary to understand the composition and behaviour of the radionuclides found in spent nuclear fuel. Spent fuel contains four basic types of nuclides, the first of which is the remains of the fuel itself. Typical reactor fuel is enriched to contain around 3% of the fissile isotope Uranium 235, the rest consisting of the non-fissile isotope Uranium-238. As the fuel is consumed in a reactor, the U-235 content will drop to around 0.8%. These are among the most long-lived nuclides in the spent fuel, U-235 having a half-life of 704 million years and U-238 4.5 billion years.
The next components in spent fuel are the fission products. These are light elements formed when Uranium atoms split apart during nuclear fission, and have half-lives ranging from 8 hours for Xenon-135 to 15 million years for Iodine-129. The shortest-lived fission products are intensely radioactive and produce a large amount of decay heat – so much, in fact, that even after a reactor is shut down, if the cooling system fails this decay heat can build up and cause the core to melt down. Therefore, upon being removed from a reactor, spent fuel elements are immediately placed in actively-cooled storage pool. The water in the pool carries away the decay heat and shields plant operators from the ionizing radiation, allowing the fuel to be stored for up to six years when most of the short-lived fission products will have decayed away. Despite the routine nature of pool storage it is actually one of the most vulnerable and dangerous steps in the fuel-disposal process, as a failure of the pool’s cooling system can lead to serious consequences, as happened during the 2011 Fukushima Daiichi nuclear disaster. When the earthquake and subsequent tsunami knocked out emergency power to the plant, the lack of cooling caused spent fuel in the storage pools to overheat, generating hydrogen gas that accumulated and ignited, blowing the roof off the building.
Yet another category of radionuclides found in spent fuel are the actinides or transuranic elements. These are heavy elements produced when U-238 absorbs neutrons from the nuclear reaction. These tend to have relatively long half-lives, ranging from 432 years for Americium-241 to 379,000 years for Plutonium-242. And finally there are the activation products, produced via the neutron activation of non-radioactive structural materials like the zirconium cladding on the fuel bundles. The most common and long-lived of these is Chlorine-36, with a half-life of 300,000 years.
It is important to note here that half-lives alone do not give an accurate picture of just how long a given isotope will remain dangerously radioactive. Many nuclides do not immediately decay into stable isotopes, instead undergoing a long decay chain whereby one radioactive isotope decays into another radioactive isotope and so on. For example, Neptunium-237 has a decay chain 13 steps long ending in the stable isotope Thallium-205, with the half-lives of the intermediate daughter nuclides ranging from four microseconds to 160,000 years. Many of these daughter nuclides are significantly more radioactive than Neptunium which, combined with Neptunium’s long half-life of 2-million years, means that the whole decay series will remain environmentally hazardous for more than 10,000 years.
Currently, once nuclear fuel is removed from cooling pools, it is placed in shielded dry casks and kept in monitored storage on-site at the nuclear power plant. This is how radioactive waste is handled in almost every major nuclear nation. However, as reliable monitoring cannot be counted on for more than 100 years or so due to political and climatic changes, nations such as Sweden, Finland, and South Korea have turned to deep geological repositories to safely store their waste without requiring human intervention.
As previously mentioned, the ultimate goal of permanent disposal is to prevent dangerous radionuclides from leaking into the environment until all the most dangerous isotopes have decayed to harmless forms. In deep geological disposal this is accomplished by burying the waste deep inside monolithic geological formations through which groundwater migration is minimal. For example, for many years Atomic Energy of Canada Limited studied the feasibility of burying nuclear waste in the Canadian Shield, a massive formation of dense, 2.5-billion-year-old granite. Similarly, the cancelled Yucca Mountain repository in Nevada and the current repositories at Forsmark in Sweden and Onkalo in Finland are all dug into deep, dense igneous bedrock. However, deep burial on its own is not enough; as any single barrier against leakage can be expected to fail at some point, all current disposal schemes include multiple redundant barriers, enveloping the waste in concentric protective layers like a Russian nesting doll.
Surprisingly, the first and most effective of these barriers is the fuel itself. The Uranium used in nuclear reactors is in the form of Uranium Oxide, a hard, black ceramic-like material that is pressed into small cylindrical pellets. These pellets are stacked and sealed into tubes of zirconium cladding, which are then bound together to form the fuel elements. Uranium Oxide is extremely insoluble in water and traps most of the radionuclides tightly within its crystal lattice, meaning that even if bare fuel pellets were simply dumped in an open aquifer, even after 10,000 years very few of these nuclides would actually escape into the environment. However, certain nuclides such Chlorine-36 and Caesium-137 are more mobile than others and could possibly leak out of the fuel pellets if the fuel cladding is breached. These are collectively known as the “instant release group,” of which only around 10%, the “instant release fraction” are likely to leak out. This, however, is nominally prevented by the next barrier – the zirconium fuel cladding, which due to its natural corrosion resistance is only expected to be breached after 400,000 years. Further protection is provided by encasing the fuel bundles in storage casks made of stainless steel, titanium, or copper, the latter of which is so corrosion-resistant it is expected to last up to one million years.
In the extremely unlikely event both the storage cask and fuel cladding are breached, the casks are packed in a dense clay-like substance known as Bentonite, which both seals the casks against the ingress of water and acts as an ionic buffer that traps escaping radionuclides before they can reach the groundwater. However, this buffering action is only effective against positively-charged atoms like Caesium, Strontium, and other heavy metals; negatively-charged ions like Iodide and Chloride can still potentially get through. But in order to reach aquifers used by humans and other animals, these nuclides must travel through hundreds of metres of bedrock via the groundwater, which in the areas where nuclear repositories are built moves at a positively glacial pace of around one centimetre per year. Combined, these various barriers and safeguards ensure that by the time any radionuclides leak into the environment, they will already have decayed into harmless isotopes or be so dilute as to present little to no danger to the environment. Indeed, the sheer degree of redundancy in this system can be considered overkill and then some, a fact borne out by a remarkable discovery made in 1972 in a Uranium mine at Oklo, Gabon. Some 2 Billion years ago, when the proportion of fissile U-235 was higher than today, rainwater seeping into the ground set off a nuclear chain reaction in Uranium ore deposits, effectively creating a natural nuclear reactor. This process carried on for thousands of years until the Uranium was depleted, leaving behind the same collection of fission products found in spent manmade nuclear fuel. Yet despite the Oklo deposits being located in porous sandstone through which rain and groundwater constantly percolate – in other words, the worst-case scenario for a nuclear waste repository – over the next 2 billion years these fission products migrated no more than a metre or so from their original source. Thus it is expected that modern repositories, with their multiple layers of protective metal, clay and dense granite, should easily be able to contain our nuclear waste for at least 100,000 years. And for more on the Oklo reactors, please check out our video The 2 Billion Year Old Earth-Based Nuclear Reactor.
But while scientists have confidence in the passive effectiveness of deep geological repositories, there is another, major factor that can’t be as easily accounted for: future humans. As nuclear repositories are expected to last hundreds of thousands of years, it is more than likely that all knowledge and records of their existence will be lost over the millennia. Provided human civilization lasts that long, how then do we prevent our distant descendants from stumbling upon our nuclear waste and unleashing an ecological disaster? This question, explored in depth in the 2010 Danish documentary Into Eternity, has generated considerable controversy, with some experts arguing that once sealed, all traces and records of the repositories should be erased so that humanity eventually forgets their existence and locations. Others, however, argue that this would increase the risk of the repositories being found by accident and that some form of warning – perhaps in the form of an engraved stone monument – should be left behind to discourage people from disturbing the site. Still others argue that such a warning would only entice future explorers. After all, inscriptions warning of curses has done little to discourage people from entering and looting ancient Egyptian tombs.
Much closer to our own time, many proposed nuclear repository projects have fallen prey to another human impulse: NIMBYism, with many citizens vehemently opposing the storage or transport of nuclear waste near their communities. In the late 1970s, Atomic Energy of Canada Limited began a series of studies to evaluate the feasibility of storing nuclear waste in the Ontario section of the Canadian Shield, drilling experimental boreholes to monitor groundwater migration. The locals, believing that nuclear waste was already being stored in the area, immediately had their groundwater tested and discovered to their horror that it was radioactive. The resulting public outcry ultimately forced AECL to abandon the project. However, had anyone bothered to test the water before the researchers’ arrival, they would have discovered that it had been mildly radioactive all along, the granite of the Canadian Shield being naturally full of Uranium and other radioisotopes. Similar political forces also lead to the cancellation of the Yucca Mountain repository project in 2011, after more than three decades of research had been conducted on the site.
Currently, only four deep geological repositories are in use or under construction in Finland, Sweden, South Korea, and Germany, with the vast majority of the world’s nuclear waste still being kept above ground in dry-cask storage. Several other repositories are in various stages of discussion and planning, but given the sharp global drop in interest in nuclear power following the Fukushima disaster, it remains to be seen whether any of them will be brought to completion. Recent years have also seen renewed interest in the concept of transmutation, whereby nuclear waste is converted into a less dangerous form by bombarding it with neutrons from a nuclear reactor or particle accelerator. Unfortunately this process is not yet feasible with today’s technology, as transmuting waste on a large scale would require such vast amounts of energy as to be uneconomical. However, it may be possible to carry out limited transmutation, whereby the longest-lived nuclides such as Neptunium are separated out of spent fuel and converted into lighter, more short-lived isotopes, greatly reducing the total time the fuel remains dangerously radioactive. While promising, the process of separating out these nuclides is itself prohibitively expensive, and research is ongoing to develop methods for selectively transmuting long-lived nuclides within the fuel itself. Whatever the ultimate solution, it is clear that a long-term solution for dealing with radioactive waste will be the key to achieving the long-awaited nuclear renaissance.
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