For radioactive waste, this means isolating or diluting it such that the rate or concentration of any radionuclides returned to the biosphere is harmless. A time in this case plays a very important role, since radioactivity naturally decays over time. The radioactive decay of certain number of atoms (mass) is exponential in time. The rate of nuclear decay is also measured in terms of half-lives. The half-life is the amount of time it takes for a given isotope to lose half of its radioactivity. If a radioisotope has a half-life of 14 days, half of its atoms will have decayed within 14 days. In 14 more days, half of that remaining half will decay, and so on. Half lives range from millionths of a second for highly radioactive fission products to billions of years for long-lived materials (such as naturally occurring uranium). Notice that short half lives go with large decay constants. Radioactive material with a short half life is much more radioactive (at the time of production) but will obviously lose its radioactivity rapidly. No matter how long or short the half life is, after seven half lives have passed, there is less than 1 percent of the initial activity remaining.
To achieve this, practically all radioactive waste is contained and managed, with some clearly needing deep and permanent disposal. Current approaches to managing radioactive waste have been segregation and storage for short-lived waste, near-surface disposal for low and some intermediate level waste, and deep burial or partitioning / transmutation for the high-level waste. From nuclear power generation, unlike all other forms of thermal electricity generation, all waste is regulated – none is allowed to cause pollution.
Danger of Radioactive Waste
Radioactive waste dangers are determined by many factors, since it is very 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 have to take into account to which type of radiation (and its energy) are you exposed. For example, alpha radiation tend to travel only a short distance and do not penetrate very far into tissue if at all. Therefore, alpha radiation is sometimes treated as non-hazardous, since it cannot penetrate surface layers of human skin. This is naturally true, but this is not valid for internal exposure by 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. Intensity of ionizing radiation is a key factor, which determines health effects from being exposed to any radiation. It is similar as 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 there without any problems and moreover it is comfortable. If you are too far from heat source, the insufficiency of heat can also hurt you. This analogy, in a certain sense, can be applied to radiation also from ionizing radiation sources. We must note that, radiation is all around us. We are continually exposed to natural background radiation and it seems to be without any problem.
- Chemical properties of the radioactive material. Chemical properties are 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 there essentially unchanged in amount during the lifetime of the individual. These radioactive chemical elements have to be isolated from the environment as long as they are radioactive. On the other hand, in 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?
Sources of Radioactive Waste
Radioactive waste comes from a number of sources. Radioactive (or nuclear) waste is a byproduct from nuclear reactors, fuel processing plants, hospitals, various industrial applications and research facilities. Radioactive waste are also naturally occurring radioactive materials (NORM) that can be concentrated as a result of the processing or consumption of coal, oil and gas, and some minerals.
- Nuclear Fuel Cycle. The nuclear fuel cycle is a process chain consisting of a series of differing stages. The nuclear fuel cycle starts with the mining of uranium and ends with the disposal of nuclear waste. Radioactive waste from the front end of the nuclear fuel cycle is usually alpha-emitting waste from the extraction of uranium. It often contains radium and its decay products. The back end of the nuclear fuel cycle involves managing the spent fuel after irradiation. Spent nuclear fuel is due to presence of high amount of radioactive fission fragments and transuranic elements very hot and very radioactive.
- Medical Waste. Radiation is used in variety of medical examinations and treatments. Doses from medical radiation sources are determined, whether a person underwent a treatment or not. In general, radiation exposures from medical diagnostic examinations are low (especially in diagnostic uses). Doses may be also high (only for therapeutic uses), but in each case, they must be always justified by the benefits of accurate diagnosis of possible disease conditions or by benefits of accurate treatment. These doses include contributions from medical and dental diagnostic radiology (diagnostic X-rays), clinical nuclear medicine and radiation therapy. Radioactive medical waste tends to contain beta particle and gamma ray emitters. It can be divided into two main classes. In diagnostic nuclear medicine a number of short-lived gamma emitters such as technetium-99m are used. Many of these can be disposed of by leaving it to decay for a short time before disposal as normal waste.
- Industrial Waste. Industrial waste can contain alpha, beta, neutron or gamma emitters. Gamma emitters are used in many industrial uses such as radiography, which is a method of non-destructive testing where many types of manufactured components can be examined to verify the internal structure and integrity of the specimen. Neutron emitting sources are used in a range of applications, such as oil well logging.
- NORM. Naturally occurring radioactive materials (NORM) and Technologically Enhanced Naturally Occurring Radioactive Materials (TENORM) consist of materials, usually industrial wastes or by-products enriched with radioactive elements found in the environment, such as uranium, thorium and potassium and any of their decay products, such as radium and radon. For example, coal, a combustible black or brownish-black sedimentary rock, contains a substantial amount of the radioactive elements uranium and thorium. According to the UNSCEAR, the average specific activity of both uranium-238 and thorium-232 in coal is generally around 20 Bq/kg (range 5-300 Bq/kg). Coal mines in Freital, Germany, which have uranium concentrations of 15000 Bq/kg, are an exception. Burning coal gasifies its organic materials, concentrating its inorganic components into the remaining waste, called fly ash. Around 10% of coal is fly ash. Fly ash is hazardous and toxic to human beings and some other living things, and fly ash also contains the radioactive elements uranium and thorium, which are concentrated by a factor of 10.
Types of Radioactive Waste
As was written, radioactive waste is any waste that contains radioactive material. This material is either intrinsically radioactive, or has been contaminated by radioactive material, and that is deemed to have no further use. Basic classification is based on the intensity (specific activity) of a material. Therefore, there are three broad categories of radioactive waste.:
- Low-level Waste. Low-level waste, LLW, comes from reactor operations and from medical, academic, industrial, and other commercial uses of radioactive materials. Low-level wastes include paper, rags, tools, clothing, filters, and other materials which contain small amounts of mostly short-lived radioactivity. Low-level waste (LLW) has a radioactive content not exceeding 4 giga-becquerels per tonne (GBq/t) of alpha activity or 12 GBq/t beta-gamma activity. LLW usually does not require shielding during handling and transport, most LLW is suitable for shallow land burial. To reduce its volume, it is often compacted or incinerated before disposal.
- Intermediate-level Waste. Intermediate-level waste (ILW) contains higher amounts of radioactivity and it generally require shielding, but the heat it generates (<2 kW/m3) is not sufficient to be taken into account in the design or selection of storage and disposal facilities. Intermediate-level wastes includes ion-exchange resins, chemical sludge, contaminated materials from reactor decommissioning and some radioactive sources used in radiation therapy. Intermediate-level radioactive waste that requires long-term management. The owners and the producers of intermediate-level radioactive waste are responsible for managing the waste they produce. It may be solidified in concrete or bitumen for disposal. As a general rule, short-lived waste (mainly non-fuel materials from reactors) is buried in shallow repositories, while long-lived waste (from fuel and fuel reprocessing) is deposited in geological repository.
- High-level Waste. High-level waste, HLW, is primarily spent fuel removed from reactors after producing electricity. HLW is also a type of nuclear waste created by the reprocessing of spent nuclear fuel (e.g., waste formed by vitrification of liquid high-level waste). 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. HLW accounts for over 95 percent of the total radioactivity produced in the process of nuclear electricity generation. 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 that are considered non-hazardous for people and the surrounding environment. If generally short-lived fission products can be separated from long-lived actinides, this distinction becomes important in management and disposal of HLW.