Nuclear Waste Disposal

Nuclear Waste Disposal - Issues and Impact on Industry and Society

Sarah M. Don

Physics ERT Semester 4, 2008 Mr Maud

October 24, 2008

© Sarah Don, Australia, 2008

Introduction The issue of nuclear power sparks debate worldwide due to its controversial waste disposal dilemma. The use of nuclear fuel to generate power is becoming more prevalent in many countries around the world as the price of fossil fuels continues to rise; however, its many benefits are often overshadowed by the complications of nuclear waste disposal. New technologies are constantly being developed to allow for more effective reprocessing and safer storage and disposal of nuclear waste. Sometimes environmental issues are not the only governing factors in whether a country decides to adopt waste reprocessing strategies. However controversial it is, nuclear energy seems to be the cleaner power production method for present and future generations. Nuclear Power Generation Different designs of nuclear power plants vary in the way they use nuclear reactions to heat water. However, the process of producing electricity from heated water is no different in a nuclear power plant than it is in a coal or geothermal power plant. A circuit of water is heated to produce steam, which turns turbines attached to a dynamo, which in turn produces electricity which is supplied to the city grid. In a nuclear power plant, the heat for creating steam is generated by nuclear fissions. The fuel assembly inside a reactor can be made up of several different actinide compositions, but commonly the most essential isotope is fissile U235. Only 0.7% of naturally occurring uranium is U235, while the rest is U238 so U235 has to be enriched in the fuel assembly in order to have enough fissile material to keep a steady nuclear reaction going. The burnup (irradiation) process is started by firing several neutrons into the core of the reactor. Some of these neutrons will collide with U235 atoms and be absorbed. The nucleus of U236 is very unstable and begins to wobble, eventually causing the atom to split (fission) into two or three parts. As shown in Figure 1, when the U236 atom fissions, several neutrons are liberated. These neutrons then collide with other U235 atoms causing a chain reaction of nuclear fissions. This chain reaction must be controlled, however, by neutron absorbing materials called neutron poisons. The neutron flux and thus rate of fission in the core of a reactor can be manually and electronically controlled by control rods which slide in and out of the core between fuel assemblies, absorbing neutrons. The wall of the reactor core (steel and concrete) and any water or gas inside the core also help to absorb neutrons. The most ideal situation is that only one neutron from every fission is allowed to cause another fission, while the others are absorbed.

Figure 1 – Nuclear fission (NSDL, 2008)

Fast vs. Thermal Reactors The difference between a fast and a slow (“thermal”) reactor is the temperature at which it runs. In a fast reactor, the core runs at a very high temperature compared to that of a thermal reactor. It is called a fast reactor because as the core temperature increases, the speed of the liberated neutrons increases. The more water a reactor contains, the more “thermal” it is considered to be. As the amount of water between the fuel rods in the reactor core decreases, the neutrons that are produced as a result of nuclear fission are able to maintain a higher amount of energy since there are fewer water

© Sarah Don, Australia, 2008

molecules to slow them down. This causes the relative quantities of the different actinides in the waste to be altered since the actinides’ ability to capture neutrons varies with neutron energies. This in turn, alters the ratio of the waste constituents. As shown in Figure 2, when a neutron is travelling very fast, the cross-section of a target atom appears small. This means that when the majority of neutrons in the core are travelling at high speeds, the ratio of liberated neutrons to absorbed neutrons is quite large. However, when the neutrons have less energy, Figure 2 – Atom cross-sections for neutrons of the cross-section of target atoms becomes different energies much larger, making it easier for a neutron to be captured by an atom. This reduces the ratio between liberated and absorbed neutrons in the core which requires less moderation by the control rods in order to keep controlled burnup inside the core. Thermal reactors use moderators such as light water (H2O) or heavy water (D2O), with which fast neutrons from fission collide. This way the efficiency of the neutron captures is higher. Typically, thermal reactors burn fuel for 50-60 MWd/kg (approximately 3-5 years depending on the type of thermal reactor), staggered between three fuel batches inside the core. Fast reactors avoid moderation. This increases the ratio of fission to capture and also facilitates the breeding of new fissile isotopes such as Pu239 from U238 in certain designs (see Equation 1). 𝟐𝟑𝟖 1 𝟗𝟐𝐔 + 0n

→

239 92U

239 92U

→

239 0 93Np + −1e

239 93Np

→

𝟐𝟑𝟗 0 𝟗𝟒𝐏𝐮 + −1e

Equation 1

This allows extended burnup to 140 MWd/kg (approximately 7-9 years) which means that the fuel is spent more efficiently and more electricity can be generated from the same volume of fuel. Such fast and breeder reactors can also house a blanket, which is a mass of actinide waste from previous burnup cycles that is placed inside the core of a fast reactor for the purpose of transmuting the isotopes into less radiotoxic isotopes for further recycling or safer final disposal. Nuclear Waste Constituents Uranium, neptunium, plutonium, curium, californium, and americium are the most prevalent actinides produced by nuclear fission. These elements are problematic because of their radioactivity and extremely long half-lives. Am241, in particular, decays to Np237 (as shown in Equation 2), which has a half-life of 2.144×106 years, (Berthou, 2003) and thus determines the long-term radiotoxicity of the nuclear waste. If the amount of Am241 in the spent fuel is reduced, then the part of the spent fuel that cannot be recycled can be more easily stored and disposed. 241 95Am

→

237 4 93Np + 2He

© Sarah Don, Australia, 2008

Equation 2

Nuclear Waste Disposal When a fuel assembly has reached maximum burnup, it is removed and stored in a cooling pond (see Figure 3) for several months to years. After 40 years of storage in a cooling pond, only about one thousandth of the spent fuel’s radiation remains, compared to immediately after the spent fuel assembly was removed from the reactor. The water in the cooling pond lowers the temperature of the spent fuel assembly and also shields it so that further nuclear decay can occur to reduce the radiotoxicity for safer disposal. (WNA, 2008)

New fuel assemblies are “burnt” in nuclear reactor to produce electricity

Figure 3 – Cooling pond for spent fuel assemblies (WNA, 2007)

Spent fuel assembly from nuclear reactor

Spent fuel assembly is stored in a cooling pond

Recyclable isotopes are reprocessed into MOX or UOX fuel assemblies or breeder reactor blankets

Unrecyclable high level waste is stored in dry casks

By-products of reprocessing are disposed of

Disposal

Figure 4 – Nuclear waste storage and disposal options

After cooling in the storage pond, there are two options for the waste (see Figure 4) – one option is that the waste is reprocessed and burned again and the other option is that it is moved to dry cask storage (see Figure 5). Spent fuel assemblies are encapsulated in dry casks made of metal or concrete to shield the spent fuel’s radiation so that it can be stored before final disposal. After the inner cylinder is filled with approximately 10-80 fuel assemblies (depending on their size), the canister is filled with an inert gas and sealed. The casks are then stored in concrete bunkers. So far, all nuclear waste generated that has to eventually be disposed of is stored in dry storage casks (or cooling ponds) until the final disposal facilities become available.

© Sarah Don, Australia, 2008

Figure 5 – Dry storage cask (U.S.NRC, 2007)

Spent nuclear fuel can be reprocessed in several different ways if the country has access to the reprocessing facilities. As shown in Figure 6, one approach to reducing the long-term radiotoxicity of spent nuclear fuel involves recycling some of the more radiotoxic nuclides by adding them to a MOX (heavy metal oxide) fuel assembly. MOX fuel consists of the Plutonium vector from spent thermal reactor fuel added to depleted or natural uranium and can be used in thermal reactor cores designed to run on uranium oxide (UO2 or “UOX”) fuel. The transuranics part of thermal reactor spent fuel can be added to the MOX fuel to create a modified MOX assembly. As these minor actinides are exposed to the neutron and thermal flux of a thermal reactor, they capture neutrons or undergo fission, transmuting into less radiotoxic isotopes. This in turn makes disposal of the spent fuel much easier. However some of the nuclides that are reprocessed and added to such modified MOX fuel assemblies are also neutron poisons and absorb more neutrons than required in the reactor. This reduces the fuel’s overall efficiency and increases the cost of running a nuclear reactor to generate power. Some countries choose not to reprocess spent fuel to recycle the transuranics due to the high cost of reprocessing and the cost of diminished efficiency of the fuel over the burnup cycle.

Figure 6 – Creating a modified MOX fuel assembly to partially recycle spent nuclear fuel

Not all the nuclear waste can be reused in a MOX fuel assembly, however, so the remaining high level nuclear waste has to be stored (in dry storage casks) and eventually disposed of. At this point in time, the final disposal stage of non-recyclable waste has not been established yet. However, there is worldwide research currently being conducted in order to design a facility that meets the expectations of the U.S. Nuclear Regulatory Commission (U.S.NRC) and local governments of the areas where such disposal facilities would be constructed. The U.S.NRC has declared that any permanent storage facilities to be built must be able to guarantee that the waste will be immobilised for at least 1,000,000 years – a regulation that is near impossible to comply with. Currently at the Massachusetts Institute of Technology, one of the biggest studies into terminal nuclear waste disposal looks at methods that could be used to develop a nuclear waste repository in Yucca Mountain, Navada, USA. The latest design of the repository constists of several 5cm-wide, 5km-deep bore-holes into granitic rock, incorporating a millimeter wave drilling technique. (Lai, 2008) The bore-holes would be filled in with sand around the waste canisters as silicon is one of the most effective neutron poisons, preventing any further nuclear fissions from developing into an uncontrolled nuclear chain reaction (see Figure 7). To further this study, researchers at MIT are looking into the drilling process to line the bore-holes with melted granite glass which would offer further shielding as well as sealing off the bore-hole to the surrounding granite.

© Sarah Don, Australia, 2008

This seems to be a promising design for the first nuclear waste repository in the world as well as any future repositories in other countries. Just one of these bore-holes can hold 3.32 years worth of US minor actinide waste and only costs about 0.5% that of current US storage methods. If nuclear waste can be stored in geologically stable areas in such a sealed and controlled way as this, there would be less cost involved in the nuclear fuel cycle. One particular downside to providing such costeffective disposal of nuclear waste is that for some countries, it may be more cost effective to simply dispose of all the waste rather than reprocess and recycle. However, it is likely that most countries will continue to use reprocessing strategies for social and political reasons (to appear “green”) if not just for the safety and health of the environment. Figure 7 – Schematic of the canister, the sand gap and the surrounding rock.

Social Impact and Opinion of Nuclear Power and Waste Disposal Many people around the world have the attitude of “not in my backyard”. It’s because of this attitude that a lot of politicians world-wide win votes by declaring that they will not implement nuclear waste programs. Most of the fear and anxiety concerning nuclear waste storage and disposal has to do with misleading information or a lack of education. After the accident at Chernobyl’s reactor in 1986, a lot of people believed that it is easy for a meltdown to occur. (Dean, 2006) This is however untrue – it is actually more difficult to keep a nuclear reactor running than it is to cause it to run out of control due to the reactor’s sensitivity to neutron flux. Also, there are many electronic as well as mechanical mechanisms in place to drop the control rods in case of an unexpected rise in neutron flux to shutdown the reactor even before turning in the direction of a meltdown. So people have little to worry about when it comes to the production of nuclear powered electricity. Nuclear waste disposal is also a controversial issue because of radiation’s mysteriousness to the uneducated and the lack of answers about the disposal issue. The reason why there are no nuclear repositories as of yet is because the world is only just beginning to accumulate enough nuclear waste to require a final disposal stage in the nuclear fuel cycle. At least the nuclear waste can be contained and stored or immobilised and disposed of in sealed canisters (or other materials such as certain types of glass) that are buried deep in the ground. This is much unlike current coal or oil power plants that release billions of tonnes of CO2 into the atmosphere every year that immediately affects people all over the world. The damage that CO2 has done to Earth is almost irreversible, however, if a nuclear leak occurred, the damage would be very localised, and in much smaller quantities than certain other types of pollution such as CO2. Summary Although nuclear power generation and waste storage has its disadvantages, its advantages far outweigh those of coal or oil power generation. Many people find it disconcerting that there is currently no nuclear waste disposal program, and consequently believe that nuclear waste will become a serious problem. However CO2 pollution in the atmosphere is already causing a much larger problem than the nuclear waste issue could. There are very promising designs of different types of waste repositories that will be constructed in the near future. Recycling will still play a large roll in nuclear waste reduction, and this process, along with terminal storage methods will continue to be improved in the future due to the extensive current and future world-wide research. People shouldn’t be concerned about nuclear power as electricity will be predominantly generated by nuclear fuel within the next few decades and disposal strategies will soon be well established.

© Sarah Don, Australia, 2008

Bibliography ABC (2008) “Carbon Conundrum”, ABC – the lab, Australia, http://www.abc.net.au/science/features/carbon/default.htm (23/10/08) Dean, T. (2006) “New Age Nuclear”, COSMOS Magazine, Australia. Don, S. (2008) “Optimisation of the Nuclear Reactor Neutron Spectrum for the Transmutation of Am241 and Np237”, Research Science Institute Paper, Massachusetts Institute of Technology, USA. Lai, A. C. (2008) “Thermal Optimization of Deep Boreholes for Minor Actinide Waste Disposal”, Research Science Institute Paper, Massachusetts Institute of Technology, USA. National Science Digital Library (2008) “Nuclear Fission: Basics”, AtomicArchive.com, http://www.atomicarchive.com/Fission/Fission1.shtml (23/10/08) U.S.NRC (2007) Dry Cask Storage, U.S. Nuclear Regulatory Commission, USA, http://www.nrc.gov/waste/spent-fuel-storage/dry-cask-storage.html (22/10/08) V. Berthou, C. Degueldre, and J. Magill (2003) “Transmutation Characteristics in Thermal and Fast Neutron Spectra: Applications to Americium,” Journal of Nuclear Materials, 320, 156-162. WNA (2008) Used Fuel Management, World Nuclear Association Information Papers, http://www.world-nuclear.org/how/usedfuelmanag.html (20/10/08)

© Sarah Don, Australia, 2008