As we discussed in a previous post, one of the standard solutions for getting rid of things that we do not want is to bury them. While nuclear waste depositories for the most part utilize existing exhausted mines, an alternative is to drill extremely deep holes into the earth to dispose of nuclear waste.

          First, a hole is drilled more than 3 miles crystalline basement rock like granite deep in the crust of the Earth using modified technology from the oil and gas industry. Some designs call for lining the storage segment of the hole with cement or a type of aluminum phyllosilicate clay called bentonite to help prevent leakage of waste out of the borehole into the surrounding rock. Next high-level nuclear waste such as spent nuclear fuel and transuranics, is sealed into strong steel cylinders. The cylinders are lowered into the borehole to fill up about a mile of the hole. Once the storage segment is filled, then the hole is sealed. Some of the materials that might be used to seal the hole include clay, cement, crushed rock or asphalt.

          In some disposal scenarios, young and very hot nuclear waste is lowered into the hole and radiates enough heat into the surrounding rock to melt it. As the waste ages and cools, the rock solidifies and forms a hard shell around the waste, isolating it.

         Any amount of waste could be disposed of with the borehole solution. Unlike a underground repository which has a maximum capacity and a huge investment, drilling additional boreholes would increase capacity and each borehole would cost far less than a repository. Another advantage of the borehole approach is the fact that the boreholes can be drilled in many places where geology favorable layers of crystalline basement rock are located. This makes them especially attractive for countries which do not have a lot of nuclear waste and do not want to invest the enormous cost for a repository that would not be filled. Because the boreholes could be placed near nuclear installation that were generating the waste, all of the problems and dangers of transportation of nuclear waste could be avoided.

         The amount of land needed for borehole disposal is quite small and the environmental impact would be minimal. The well head, waste handling installation and a security zone could all be contained in one square mile. After the repository is filled and the borehole sealed, the land could be returned something close to its original condition.

         In 1957, the United States Academy of Sciences first considered the possibility of deep borehole disposal. Since then, much more attention has been focused on repositories but the attractiveness for the deep borehole alternative has steadily increases. Periodically, studies are made about the possibility of deep borehole disposal. Current estimates suggest that about one thousand deep boreholes would be required to contain all current high-level nuclear waste and projected waste to be generated in the United States. The cost would be roughly the same as the projected cost of a repository such as the one at Yucca Mountain. The deep borehole would be preferable because their drilling could be spread out in time and geography, reducing concentrated costs and transportation problems. Deep borehole disposal is a very viable alternative to all other suggested methods of high-level nuclear waste disposal.

          Another option for burial of high-level radioactive waste such as spent nuclear fuel rods is burial at sea where it would not be disturbed by geological processes or human activity.

          The appeal of ocean floor disposal consists of several different feature. For one thing, in the sediment of the ocean floor, while there is water, it does not flow. If wastes leaked and dissolved, they could take ten thousand of years to migrate through a meter of the dense clay of the ocean floor. Any waste that made it to the open ocean would be very diluted and decayed. In addition, large areas of the ocean floor are geologically stable and unlikely to be disturbed by human activity.

          On the other side of the equation is the fact that it would be very difficult to recover the nuclear waste from the ocean floor should it ever become desirable or necessary. And, it would be very difficult to establish an international authority to effectively monitor such activity and enforce adherence to agreed upon rules.

           Any waste being disposed of would be encased in shielded canisters and covered with thick shells of concrete. Once the waste has been packaged, it could then be placed in torpedoes which would increase penetration of the ocean floor. An alternative would be to drop the containers in shafts that could be drilled with existing underwater drilling techniques.

           Another solution would be to dump containers of waste into subduction zones which are places where one tectonic plate slides under another tectonic plate. The theory is that the containers would be carried deep into the crust of the Earth’s mantle away from the ocean floor and all human activity.

         All of these solutions would be vulnerable to natural disasters at sea as well as terrorists during transportation. .

         In 1972, the Convention on the Prevention of Marine by Dumping of Wastes and Other Matter was drafted. It is commonly referred to as the London Convention and was put into force in 1975. Currently, over eighty countries are signatory of the London Convention. The Convention is an agreement to control pollution of the sea by dumping and to encourage regional agreements supplementary to the Convention. The Convention only covers deliberate disposal at sea of waste from ships, aircraft and platforms. It does not cover discharges from sources on land from pipes and outfalls, wastes generated from the normal operation of vessels or placement of materials in the ocean for purposes other than disposal. The Convention is currently set to be in force until 2018. If signatory countries want to dispose of nuclear wastes at sea, they will have to pull out of the Convention, lobby for major revision of the Convention or wait for the Convention to expire in 2018.

         However attractive disposal at sea might appear for nuclear waste, there are a number of technical and political difficulties that must be overcome before such disposal is a viable option.

Diagram of a design for a nuclear waste disposal torpedo from wattsupwiththat.com:

         There are hundreds of thousands of tons of depleted uranium hexafluoride in temporary storage in the United States. A very small amount has been used in the manufacture of such things as tank armor, armor piercing artillery, shields for industrial radioactive materials and other applications.

         Research supported by the U.S. Department of Energy was begun in 1993 into the use of depleted uranium in heavy concrete. There was a dual goal in the research. They were looking for a way to dispose of large quantities of depleted uranium and also a new type of material for the creation of dry casks (covered in last post) used for storage and transportation of spent nuclear fuel. Scientists at Idaho National Engineering and Environmental Laboratory conceived of the new type of concrete and were awarded patents for it in 1998 and 200

          Ducrete is a special type of concrete that was invented to help solve the radioactive waste problem. The name is derived from depleted uranium concrete. Concrete is made from a gravel aggregate bound together with Portland cement, water and sand. In ducrete, the gravel aggregate is replaced by granular depleted uranium left over from the process of creating enriched uranium for nuclear reactor fuel or nuclear weapons. The depleted uranium is mostly U-238 with a much lower proportion of U-235 than natural uranium ore. Ducrete is a very efficient shielding material. The depleted uranium shields against gamma rays and the water and sand absorbs neutrons.

          There were a series of technical problems that had to be overcome in order to develop ducrete. Depleted uranium in the form of uranium hexafluoride is too reactive to be used in ducrete. It has to be oxidized to form depleted uranium trioxide (DUO3) or depleted triuranium octaoxide (DU3O8). Hydrogen gas is then used to produce uranium dioxide (DUO2) in a powdered form. The second issue is the fact that concrete requires aggregate or gravel. The powder that comes out of the chemical processing has to be turned into a granular form. The powder is heated but does not reach the melting point of the material. Individual atoms diffuse across the boundaries between the powder grains and form larger chunks called depleted uranium aggregate that are suitable for use in concrete.

          The ducrete can be used to build dry casks and containment storage building to store radioactive waste. Ducrete casks are smaller and lighter dry casks than containment buildings made of conventional concrete. Because of the additional shielding properties of ducrete, it is actually cheaper to produce ducrete dry casks than using steel, lead and depleted uranium metal for shielding in addition to concrete. While ducrete may be a good cheap alternative for temporary storage, the mild radioactivity of the ducrete casks makes them more expensive to dispose of than conventional dry casks.

          However superior ducrete may be for constructing dry casks and containment buildings, it has not actually been utilized for storage of spent nuclear fuel in the United States. No facilities utilizing ducrete have been licensed in the U.S.

           When nuclear fuel has been burned in a reactor and there is no permanent disposal site, there is an alternative to storing the spent fuel rods after they have cooled off in a pool of water for at least a year. Spent fuel rods can be stored in steel cylinders in what is called ‘dry cask’ storage.

          Nuclear fuel rods are tubes of uranium oxide pellets. About two hundred of the tubes are packed together in an ‘assembly.” These assemblies are then inserted into the core of a reactor.

          When they are spent, they are removed to a spent fuel pool. After a year, the assemblies can be removed from the pools and sealed into steel cylinders called canisters that are around 16 feet long and 6 feet in diameter. The canisters are either welded or bolted shut.  Then the atmosphere in the canister is exhausted and replaced with an inert gas which is typically helium. Finally the steel canister is placed inside a ‘cask.’

          The cask containing a welded canister consists of a concrete cylinder with walls over two feet thick. The cask containing the bolted canister is a six inch shell of boron doped metal that acts as a neutron shield .

          The casks are stored in what are referred to as ‘Independent Spent Fuel Storage Installation’ which are licensed by the United States Nuclear Regulatory Commission. The first storage facility licensed by the NRC was the Surry Nuclear Power Plant in Virginia.

         The casks may be stood upright in concrete or steel buildings. Alternatively, they may be placed horizontally in chambers about the size of a one-car garage. As of the end of 2010, there were forty eight dry cask storage facilities licensed at reactor sites and fifteen dry cask storage facilities not located at reactor sites. About sixty two metric tons of spent nuclear fuel currently exist in the United States. Of that, about twenty two percent, or fourteen tons, is stored in dry casks.

         Dry casks can also be used to transport spent nuclear fuel to sites for permanent disposal when such a site is available. A typical design for truck transport is similar to the dry casks intended for storage but there are some differences. The diameter of the cask diameter is four feet which is expanded to six feet on the ends by disk shaped ‘impact limiters.” The overall length including the impact limiter disks is twenty feet. There is an inner steel canister surrounded by a lead gamma shield which in turn is enclosed in a second steel shell. The second steel shell is encased in a neutron shield and a final shell. Average weight is twenty five tons.

         Rail transport casks very similar in terms of shells and shields but are much bigger. Cask diameter is eight feet with overall diameter at eleven feet. The overall length is twenty five feet. The average weight of such an cask is about one hundred and twenty five  tons so it can transport five times as much as a truck cask.

         These casks should last at least a hundred years but they were only intended as temporary storage. The existing dry cask storage facilities may be targets for terrorists or vulnerable to natural disasters. Unfortunately with the cancellation of the Yucca Mountain Repository, the United States currently has no permanent spent fuel storage site.

          While the United States has been struggling with the Yucca Mountain site for nuclear waste disposal, other countries have been operating or planning repositories.

          Sweden has been operating a repository since 1988. Finland has one that has been in operation since 1992 and another that opened in 1998. Germany operated one that closed in 1995 and another that closed in 1998. The US has been operating a repository for transuranic wastes since 1999.

          There are repositories under construction in Finland, Germany and Korea. Repository projects are being discussed in Argentina, Belgium, Canada, China, Japan, and the United Kingdom. Germany has a proposed repository on hold, France and Switzerland are working on siting repositories and the United States has cancelled the only major repository for spent nuclear fuel that was being worked on.

          Radioactive waste was placed in the Asse II repository, a old salt mine in Germany between 1967 and 1978. The repository was officially closed in 1995. In 2008, it was reported that brine contaminated with cesium-137, plutonium and strontium had been leaking from the mine since 1988. This highlights one of the greatest worries about underground storage of nuclear wastes; contamination of the ground water around the repository.

          The other closed repository in Germany is the Morsleben repository in the Bartensleben rock salt mine that accepted waste from 1972 to 1998. The salt dome is collapsing and almost half a million cubic meters of concrete has been pumped into the pit to stabilize it temporarily. This highlights another great concern about underground repositories; geological stability of the geology around the repository.

          There was discussion of creating an international nuclear waste repository in Australia but the public backlash was so severe and long that the project was cancelled. The debate over the use of nuclear power and the disposal of nuclear waste arouses strong passions on both side of the issue. In Germany, where there is strong nuclear opposition, the recent disaster at Fukushima has generated such a public outcry that Germany has announced that it will phase out nuclear power generation completely. This is third great problem with geological repositories. While many people approve of the concept of underground spent nuclear fuel repositories in the abstract a lot fewer want one in their neighborhood.

           Of all the alternatives suggested for disposal of spent nuclear fuel, the deep geological repositories would seem to be the simplest and most easily manageable. The technical issues can certainly be addressed. Geological science can find places where the ground water will not be threatened and where the geology is stable. The biggest problem is not technical but political. The recent Fukushima disaster has caused a world-wide cooling of public enthusiasm. Given that the temporary storage for the world’s spent nuclear fuel may be completely full within five years, solution of both the technical and political problems around permanent disposal of spent nuclear waste must be answered as soon as possible.

Asse II salt mine in Germany:

          In 1957, the National Academy of Sciences of the United States recommended long term burial as the best solution for permanent disposal of nuclear wastes. Starting in 1978, the U.S. Department of Energy (DOE) has been studying Yucca Mountain in Nevada as a possible site for the first long term geological repository for U.S. The U.S. Congress passed the Nuclear Waste Policy Act in 1982 which gave the DOE the responsibility for locating a site, building an underground repository for nuclear waste  and operating the site on an ongoing basis. In 1984, the DOE selected ten sites in six states as possible locations for a repository.

           After the report on the sites was delivered in 1985, President Reagan selected three sites from the ten studied to go through an intensive review known as site characterization. Yucca Mountain was one of the three sites selected by Reagan. Congress amended the Nuclear Waste Policy Act in 1987 to focus exclusively on Yucca Mountain. The DOE contracted with utilities to begin accepting spent nuclear fuels at the Yucca Mountain Repository.

           In 2002, the United States Congress finally approved the Yucca Mountain Nuclear Waste Repository as a long term repository for spent nuclear fuel and other high-level radioactive wastes. In 2006, the DOE suggested if the project was fully funded that the Yucca Mountain repository could be opened and accepting nuclear waste by 2017.

          The plan called for forty miles of tunnels under Yucca Mountain which could be used to store a maximum of 77,000 metric tons of waste, include up to a maximum of 63,000 metric tons of spent nuclear fuel. Current estimates are that U.S. reactors will have produced that amount of spent fuel by 2014.

           Since the 1987 decision to drop two other proposed sites, the Yucca Mountain Project has been fiercely opposed by critics including two thirds of the people who live in Nevada. There have been protests, complains, laws suits, opposition by elected officials as the debate raged. The studies of possible ground water infiltration at the site were challenged and ultimately validated. Studies indicated that the cost of not moving forward with the repository would be very high.

          Democratic Nevada Senator Harry Reid became the Senate Majority leader in the 2006 mid-term elections when the Democratic Party took control of the Senate. He had been a long term opponent of locating a nuclear repository in his state and he used the resources of his new position to fight against it. The 2008 budget cut the Yucca Mountain repository budget to around four hundred million but the project managers did their best to rearrange their budget to continue work.

        In the 2008 presidential campaign, Barack Obama said that he would cancel the Yucca Mountain project. After he was elected, he was told by the Nuclear Regulatory Agency that the president did not have the authority to cancel the project. But in 2010, President Obama and Energy Secretary Chu did managed to cancel the Yucca Mountain Repository project.

         At this time, the U.S. has no long term storage for spent nuclear fuel. Spent fuel is being stored at reactor facilities in temporary water pools or dry cask storage. It is estimated that the temporary storage facilities at U.S. reactors will be full within 5 years. This is a critical problem facing the nuclear power industry in the U.S.

NRC Yucca Mountain Project diagram:

1. Canisters of waste, sealed in special casks, are shipped to the site by truck or train.
2. Shipping casks are removed, and the inner tube with the waste is placed in a steel, multilayered storage container.
3. An automated system sends storage containers underground to the tunnels.
4. Containers are stored along the tunnels, on their side.

          A standard human way of dealing with something that you want to get rid of is to dig a hole and bury it. This has been a popular proposal of disposing of spent nuclear fuel and many countries that use nuclear power either have or are working on such repositories. The authorities in these countries claim that such repositories can be safe, economical and protect the environment but a large part of the public remains highly skeptical.

          Proposed repositories are general one thousand feet deep or deeper. The rock should be solid and dense. Stability is one of the most important factors. That means that there should not be any nearby volcanism. There should not be any geological faults in the area. A second very important factor is that there should be very limited movement of ground water through the rock strata to prevent threats to the water table. Finally, the repository should be as far from human habitation and land use as possible.

          Once the repository site has been selected and the tunnels dug, there will likely be additional preparation in the form of additional sealing of the walls of the tunnels. Salt mines have been a popular choice for conversion to nuclear fuel repositories because many tunnels and large areas have been excavated. Locations with a granite matrix are also popular choices.

            Concrete cylinders containing spent nuclear fuel rods would be transported to the repository site from the spent fuel pools, a process that has its own problems. The cylinders would be lowered into the repositories and arranged in a configuration that leaves room between the cylinders for heat dissipation. Eventually, when the repository is full, the access tunnel would be sealed. There have been discussions about how to leave a maker that would be comprehensible for thousands of years so that future societies would know that the site was dangerous.

          Belgium, Canada, Finland, France, Japan, Korea, Sweden, Switzerland, and the USA have created major underground laboratories to test the feasibility of deep geological repositories for spent nuclear fuel. Argentina, Belgium, Canada, China, Finland, France, Germany, Japan, Korea, Sweden, Switzerland, United Kingdom and the USA all have repositories filled and closed, currently operating, being planned or canceled.

         Whenever such a repository is proposed, after the serious effort to verify that all the technical details have been reviewed and all technical concerns have been satisfied, the hard work of selling the location to the public begins. Politics becomes the most important factor. On the positive side, there may be commercial interests who play down or even try to hide any potential problems. On the negative side, there may be environmental groups who over emphasize potential problems.

         Unfortunately, we do not have the luxury of kicking this can down the road. Current estimates are that the world’s spent nuclear fuel pools will all be full of spent fuel within 5 years at the current rate of nuclear fuel use. Deep repositories are the most realistic of the proposals for permanent disposal, despite technical and political problems. Time is running out to implement a disposal solution.

Planned repository in Finland:

          There are a number of different proposals for permanent disposal of spent nuclear fuel. Here are summaries of some possible techniques.

          Deep geological repository: This proposal consists of digging a deep hole in a geologically stable formation and burying sealed containers that would not leak into the water table. A depository in Yucca Mountain in Nevada was approved in 2002 and was under development until 2010 when it was defunded by an act of Congress.  

          Dry cask storage: In this technique, fuel rods are sealed into steel cylinders filled with an inert gas. The cylinders are welded or bolted shut and some are encased in a concrete, steel or a shell of another material to block radiation. These casks can be used for both transportation and storage. They can be stood on end or stacked horizontally in concrete and steel enclosures. This is considered to be only a temporary storage option pending development of a long term option.

          Ducrete: Ducrete is a special form of high density concrete. The name is derived from “Depleted Uranium Concrete”. Crushed ceramic pellets of depleted uranium dioxide is used in place of gravel in a Portland cement binder. The depleted uranium absorbs gamma rays and the water used in the cement absorbs neutrons. The Ducrete was developed as a means of shielding other nuclear waste and disposing of depleted fuel pellets.

          Ocean floor disposal: This alternative consists of burying spent nuclear fuel in the ocean floor where it won’t be disturbed by either geological or human activity. Some options discussed included dropping concrete encased waste into holes drilled in the ocean floor or using torpedoes to send the encased waste into the ocean floor. It would be difficult to recover the waste at a future time if that was desirable. There is an international treaty in force to prevent dumping radioactive materials in the oceans that will be in force until 2018 at which time such a disposal method could be considered.

          Deep borehole disposal: In this type of disposal, extremely deep holes would be bored into the earth as much as five miles. Some proposals call for lining the bore hole. Other proposals would rely on hot waste to melt the borehole and its contents which would then solidify. The borehole above the waste would be filled with clay, cement, crushed rock, asphalt and/or other materials to further isolate the waste.

          Nuclear transmutation: In this form of waste treatment, spent nuclear fuel is irradiated with fast neutrons in a reactor. Dangerous and long-lived transuranic radioisotopes such as plutonium, neptunium americium and curium fission and are transmuted into other elements which have shorter half-lives or are not radioactive.

          Other techniques of spent nuclear fuel disposal are being discussed and developed. However, none of the techniques covered above or other proposals has been selected as being the definitive safe economical method of permanent spent nuclear fuel disposal.

Underground storage diagram from geo.arizona.edu

          Nuclear reactors burn nuclear fuel to generate electricity. Most reactors burn uranium oxide in the form of ceramic pellets in long tubes. The tubes comprise the core of the reactor where the fission reaction takes place. The zirconium cladding of the fuel rod tends to migrate into the center of the pellets while the lower boiling point fission products move to the edge of the pellet. Small bubbles form in the pellet which fill with cesium-137 from decaying xenon.

         Three percent of the mass of spent nuclear fuel consists of the fission products of U-235 and Pu-239. Fission products include every element in the periodic table from zinc up to the lanthanide series. Some of the fission products are either non-radioactive or have short half-lives. Other fission products are long-lived radioisotopes including strontium-90, cesium-137, technetium-99, and iodine-129.

         One percent of the mass of spent nuclear fuel is Pu-239 and Pu-240 which results from conversion of U-238. Pu-239 is a concern because it might be used in the creation of nuclear weapons. If the reactor is run for a long time, the amount of Pu-240 is greater than twenty percent and the proportion of Pu-239 is much less useful for weapons development. If the fuel is only irradiated for a short time, the proportion of Pu-239 is suitable for weapons production.

         Ninety six percent of the mass of spent nuclear fuel is uranium, with just under ninety five percent being the original U-338. About eight tenths of a percent is unburned U-235 and four tenths of a percent is U-236.

        Minor actinides including neptunium americium and curium will be present in minute amounts in spent nuclear fuel. The exact proportions will depend on the exact type of nuclear fuel being burned in the reactor. MOX or Uranium/Thorium fuels will produce slightly different rations of the actinides.

        When natural uranium nuclear fuel is burned in a reactor, it starts out with about seven tenths of a percent of U-235. When it is considered spent and removed from the reactor, it still have about one quarter of one percent of U-235 and one quarter of one percent of Pu-239. It is spent not because there is no fissile material left in the fuel but because the buildup of fission products has begun to absorb so many neutrons that the ability of the fuel to sustain a chain reaction has dropped to low to be useful.

        When spent fuel is removed from the reactor, it continues to generate fading amounts of heat due to beta emission of the fission products. Spent nuclear fuel that has been removed from the reactor core is stored in spent fuel pools of water. Most such pools circulate water around the fuel and then through a heat exchanger in order to dissipate the heat. The water in the pool also provides protection against the radiation emitted by the spent fuel. Current estimates are that the exiting spent fuel pools will be full within five years at current use of nuclear fuel.

Spent nuclear fuel pool – picture from radioactivechat.blogspot.com:

         I have already touched on radioactive waste in several earlier posts but I wanted to treat the subject in a more systematic and thorough way. I am going to start with a breakdown of radioactive wastes into six categories.

         Spent nuclear waste from nuclear reactors: Most of the fuel for nuclear reactors is in the form of ceramic pellets of uranium oxide which are packed into long metal rods. A few thousand of these rods are joined together into what is called a “fuel assembly.” When the fuel in the assembly is exhausted and can no long power the reactor, the assembly is removed. Because there is no permanent storage or disposal solution for spent nuclear fuel, the spent fuel assemblies are usually stored in pools of water which dissipates heat from the assembly and blocks the radiation emitted by the rods.

          High-level waste from spent fuel reprocessing: Some spent fuel is dissolved in corrosive chemicals and then unfissioned uranium and plutonium are recovered from the resulting liquid. Ninety nine percent of the waste from reprocessing is generated by defense related nuclear weapons research and development. There are plans to solidify this waste but for the time being the waste is stored in liquid form in underground steel tanks in facilities in Idaho, Washington, South Carolina and New York.

         Transuranic Radioactive Waste: Transuranic elements are man-made elements that lie above uranium in the periodic table. Transuranic wastes are mainly generated by the production of nuclear weapons. Plutonium is the most prominent transuranic in such wastes. Also included in this category are contaminated clothes, tools, lab equipment, and containers.

          Uranium mill tailings: When uranium is mined and milled, huge piles of waste are generated which generally remain in the area of the mill. Radium is the most important radioactive material in the mill tailings. It decays into radioactive radon gas which diffuses into the atmosphere. Mill tailings also include selenium, molybdenum, uranium and thorium.

          Low-level radioactive waste: Low-level radioactive waste includes contaminated industrial or research waste. This includes clothing, packaging materials, organic fluids, and other waste products that do not fall into the other categories in this list. This waste is generated by DOE research facilities, private industry, nuclear plants, universities and other institutions.

         Naturally occurring and accelerator produced radioactive waste: There are naturally occurring radioactive materials that can be concentrated by human activity and pose a hazard. More diffuse naturally occurring wastes come from multiple sources including metal mining and processing waste, coal ash, phosphate waste, oil and gas production waste and water treatment residue and soil removed from above a deposit of uranium ore. Particle accelerators use magnetic fields to accelerate nuclear particles to high velocities for research, medical and industrial purposes. Short lived radioactive waste can be produced by such accelerators.

        In future posts, I will go into more detail on each of these types of waste including the problems and dangers they pose and methods of storage and disposal.

Ionizing radiation warning symbol created in 2007 by the International Atomic Energy Agency and the International Organization for Standardization: