High-level waste is sufficiently radioactive for its decay heat (>2kW/m3) to increase its temperature and the temperature of its surroundings significantly. As a result, high-level waste requires cooling and sufficient shielding. Most of the heat, at least after short-lived nuclides have decayed, is from the medium-lived fission products cesium-137 and strontium-90, which have half-lives on the order of 30 years.
HLW accounts for over 95 percent of the total radioactivity produced in the process of nuclear electricity generation. In other words, while most nuclear waste is low-level and intermediate-level waste, most of the radioactivity produced from the nuclear power generation process comes from high-level waste. HLW contains both long-lived and short-lived components, depending on the length of time it will take for the radioactivity of particular radionuclides to decrease to levels considered non-hazardous for people and the surrounding environment. If short-lived fission products are generally separated from long-lived actinides, this distinction becomes important in the management and disposal of HLW.
Spent Nuclear Fuel as HLW
Spent nuclear fuel, also called the used nuclear fuel, is a nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant or an experimental reactor), and a fresh fuel must replace that due to its insufficient reactivity. The reduction of reactivity is a combinative effect of:
- the net reduction of fissile nuclides,
- the production of neutron-absorbing nuclides (non-fissile actinides and fission products)
Due to the presence of a high amount of radioactive fission fragments and transuranic elements, spent nuclear fuel is very hot and very radioactive. Reactor operators have to manage the heat and radioactivity that remains in the “spent fuel” after it’s taken out of the reactor. In nuclear power plants, spent nuclear fuel is stored underwater in the spent fuel pool on the plant, and plant personnel moves the spent fuel underwater from the reactor to the pool. Over time, as the spent fuel is stored in the pool, it becomes cooler as the radioactivity decays away. After several years (> 5 years), decay heat decreases under specified limits so that spent fuel may be reprocessed or interim storage.
For the once-through cycle, interim storage can be either at the power plant site or at a centralized location that stores the fuel from more than one power plant. After a minimum period of 50 to 100 years of interim storage, spent nuclear fuel must be transferred to a final disposal facility. The preferred option is a deep geological repository, an underground emplacement in stable geological formations. The once-through cycle considers the spent nuclear fuel to be high-level waste (HLW) and, consequently, it is directly disposed of in a storage facility without being put through any chemical processes, where it will be safely stored for millions of years until its radiotoxicity reaches natural uranium levels or another safe reference level.
This strategy is favored by several countries: the United States, Canada, Sweden, Finland, Spain, and South Africa. Some countries, notably Finland, Sweden, and Canada, have designed repositories to permit future material recovery should the need arise. In contrast, others plan for permanent sequestration in a geological repository like the Yucca Mountain nuclear waste repository in the United States.
Annual Production of SNF
Consumption of a 3000MWth (~1000MWe) reactor (12-month fuel cycle)
It is an illustrative example, and the following data do not correspond to any reactor design.
- A typical reactor may contain about 165 tonnes of fuel (including structural material)
- A typical reactor may contain about 100 tonnes of enriched uranium (i.e., about 113 tonnes of uranium dioxide).
- This fuel is loaded within, for example, 157 fuel assemblies composed of over 45,000 fuel rods.
- A common fuel assembly contains energy for approximately 4 years of operation at full power.
- Therefore about one-quarter of the core is yearly removed from the spent fuel pool (i.e., about 40 fuel assemblies). At the same time, the remainder is rearranged to a location in the core better suited to its remaining level of enrichment (see Power Distribution).
- The removed fuel (spent nuclear fuel) still contains about 96% of reusable material (must be removed due to decreasing kinf of an assembly).
- The annual spent nuclear fuel production of this reactor is about 25 tonnes. If the fuel were reprocessed and vitrified, the waste volume would be only about three cubic meters per year, but the decay heat would be almost the same.
Typical Long-lived Radionuclides – Spent Fuel
- 239Pu. 239Pu belongs to the group of fissile isotopes. 239Pu decays via alpha decay to 235U with a half-life of 24100 years. This isotope is the principal fissile isotope in use.
- 240Pu. 240Pu belongs to the group of fertile isotopes. 240Pu decays via alpha decay to 236U with a half-life of 6560 years. 240Pu has a very high rate of spontaneous fission and a high radiative capture cross-section for thermal and resonance neutrons.
- 236U. 236U is neither a fissile isotope nor a fertile isotope. 236U is fissionable only by fast neutrons. Isotope 236U is formed in a nuclear reactor from fissile isotope 235U. 236U decays via alpha decay to 232Th with a half-life of ~2.3×107 years. 236U occasionally decays by spontaneous fission with a very low probability of 0.00000009%. Its specific activity is ~6.5×10-5 Ci/g.
- 135Cs. 135Cs is a fission product (yield: 6.911%) with a half-life of ~2.3×106 years.
- 99Tc. 99Tc is a fission product (yield: 6.139%) with a half-life of ~0.211×106 years.
- 93Zr. 93Zr is a fission product (yield: 5.458%) with a half-life of ~1.53×106 years.
- 129I. 129I is a fission product (yield: 0.841%) with a half-life of ~15.7×106 years.
Typical Medium-lived Radionuclides – Spent Fuel
- 241Pu. 241Pu belongs to the group of fissile isotopes. 241Pu decays via negative beta decay to 241Am with a half-life of 14.3 years. This fissile isotope decays to a non-fissile isotope with a high radiative capture cross-section for thermal neutrons. An impact on the reactivity of nuclear fuel is obvious.
- 232U. 232U belongs to the group of fertile isotopes. 232U is a side product in the thorium fuel cycle, and this isotope is a decay product of 236Pu in the uranium fuel. 232U decays via alpha decay to 228Th with a half-life of 68.9 years. 232U very rarely decays by spontaneous fission. Its specific activity is very high ~22 Ci/g and its decay chain produce very penetrating gamma rays.
- 137Cs. 137Cs is a fission product (yield: 6.337%) with a half-life of ~ 30.23 years.
- 90Sr. 90Sr is a fission product (yield: 4.505%) with a half-life of ~ 28.9 years.
- 151Sm. 151Sm is a fission product (yield: 0.531%) with a half-life of ~ 88.8 years.
- 85Kr. 85Kr is a fission product (yield: 0.218%) with a half-life of ~ 10.76 years.
High-level Waste – Final Disposal
Final disposal, or permanent disposal, is a final stage of the back end of the nuclear fuel cycle. Final disposal is unavoidable and common for all the strategies of nuclear fuel cycles, despite the reduction in waste volume and radiotoxicity with current or future reprocessing techniques. This last step is the final disposal of the waste and whether it is untreated spent nuclear fuel or vitrified high-level waste arising from fuel reprocessing, it is still necessary to safely store them for the long term until its radioactivity reaches safe levels. However, the period they require safe storage depends on reprocessing technologies. For example, the once-through cycle comprises two main back-end stages:
- interim storage
- final disposal.
In these cases, the fuel assemblies are first after irradiation stored in spent fuel pools at the reactor site for an initial cooling period. Over time, as the spent fuel is stored in the pool, it becomes cooler as the radioactivity decays away. After several years (> 5 years), decay heat decreases under specified limits so that spent fuel may be interim storage. Interim storage can be either at the power plant site or at a centralized location that stores the fuel from more than one power plant. After a minimum period of 50 to 100 years of interim storage, spent nuclear fuel must be transferred to a final disposal facility. The preferred option is a deep geological repository (DGR), an underground emplacement in stable geological formations. Crystalline rock (granite, welded tuff, and basalt), salt, and clay are the most suitable formations for geological disposal. The once-through cycle considers the spent nuclear fuel to be high-level waste (HLW) and, consequently, it is directly disposed of in a storage facility without being put through any chemical processes, where it will be safely stored for millions of years until its radiotoxicity reaches natural uranium levels or another safe reference level.
The danger of Radioactive Waste
Many factors determine radioactive waste dangers since it is important to note that there are many types of radiation. Dangers are usually determined by:
- Type of Radiation. Unstable isotopes decay spontaneously through various radioactive decay pathways, most commonly alpha decay, beta decay, gamma decay, or electron capture. We must consider which type of radiation (and its energy) you are exposed to. For example, alpha radiation tends to travel only a short distance and does not penetrate very far into tissue if at all. Therefore, alpha radiation is sometimes treated as non-hazardous, since it cannot penetrate the surface layers of human skin. This is naturally true, but this is not valid for internal exposure to alpha radionuclides. When inhaled or ingested, alpha radiation is much more dangerous than other types of radiation. In short, the biological damage from high-LET radiation (alpha particles, protons, or neutrons) is much greater than that from low-LET radiation (gamma rays).
- Intensity. The intensity of ionizing radiation is a key factor determining the health effects of exposure to radiation. It is similar to being exposed to heat radiation from a fire (in fact, it is also transferred by photons). If you are too close to a fire, the intensity of thermal radiation is high, and you can get burned. If you are at the right distance, you can withstand it without any problems, and it is comfortable. If you are too far from a heat source, the insufficiency of heat can also hurt you. In a certain sense, this analogy can be applied to radiation from ionizing radiation sources. We must note that radiation is all around us. We are continually exposed to natural background radiation, which seems to be without any problem.
- Chemical properties of the radioactive material. Chemical properties are the key factor for possible internal exposure. For example, Sr-90, Ra-226, and Pu-239 are radionuclides known as bone-seeking radionuclides. These radionuclides have long biological half-lives and are serious internal hazards. Once deposited in bone, they remain unchanged in amount during the individual’s lifetime. These radioactive chemical elements must be isolated from the environment as long as they are radioactive. On the other hand, in the case of artificial tritium ingestion or inhalation, a biological half-time of tritium is 10 days for HTO and 40 days for OBT (organically bound tritium) formed from HTO in the body of adults.
As a result, radioactive waste management assumes different approaches for different types of radioactive waste.
See also: Fear of Radiation – Is it rational?