Radioactive waste streams
In effect all radioactive materials resulting from the operation of a nuclear power plants is waste, for it cannot be recycled, except plutonium. The generation of human-made radioactivity is irriversible. Several different radioactive waste streams can be discerned originating from the nuclear energy system:
• Mining waste
• Operational waste
• Routine releases
• Spent fuel
• Decommissioning and dismantling waste
Uranium ore always contains a number of other radioactive elements, a part of which are alpha-emitters. In the extraction process the uranium is separated from all other elements, which are disposed of in the mining waste, called mill tailings, a mud of finely ground ore, chemicals and water. The mud is stored in large ponds. The water from the ponds, containing the radioactive elements plus a number of toxic chemicals, seep into the ground, contaminating the groundwater table over large areas. When the water of the storage ponds has evaporated, after the mine has been depleted and abandoned, the fine powder is blown away by the wind over large distances. Most uranium mines are located in arid areas. Hundreds of thouands square kilometers are contaminated with dangerous alpha emitters. This waste stream is not classified as 'nuclear waste' by the nuclear industry. The consequences of this practice are not investigated by the nuclear industry. Evidence from local residents point to seriously adverse health effects of the chronic exposure to the low concentrations of the dangerous radionuclides in the dust and groundwater [more i18].
All activities and industrial processes involving radioactive materials generate radioactive waste. Usually this waste is packed in drums or concrete casks and stored in temporary storage facilities. It may contain all kinds of hazardous radionuclides, fission products, actinides and activation products, in relatively low concentrations, but nonetheless hazardous.
A part of the man-made radioactivity is released into the environment routinely, because complete confinement is practically not feasible or is too costly. Nominally operating nuclear power plants discharge large amounts of tritium and carbon-14 into the environment, for these radionuclides are difficult to retain. Officially these radionuclides are rated low-risk, because of their low-energy beta emission. Evidence points otherwise: in the vicinity of nuclear power stations a significantly higher incidence of cancer and leukemia with young childern is proved to occur [more i22, i23, i24, i25].
Reprocessing plants (La Hague, France and Sellafield, UK) discharge huge amounts of radioactive wastes into the environment (air and sea), comprising all gaseous radionuclides plus the radionuclides which are difficult to retain by fixation in stable chemical compounds. In addition the unavoidable waste streams of the separation processes, containing all kinds of radionuclides from spent fuel, are partly discharged into the sea [more i31]. This practice may be dictated by economic arguments [more i26].
Spent nuclear fuel
In principle the nuclear industry does not consider spent nuclear fuel to be waste, because of the plutonium it contains. Plutonium could be used in closed-cycle modes of the nuclear energy system. The closed-cycle mode implies reprocessing of the spent fuel, a costly process with large=volume waste streams and accompanied by massive discharges of radioactivity into the environment. In practice only rcycling plutonium into a small number of conventional light-water reactors (LWRs) occurs. The gain does not balance the extra energy investments and costs of reprocessing [more i31]. The ultimate goal of reprocessing, establishment of the breeder reactor, proved to be unfeasible. For that reasons spent nuclear fuel has to be classified nuclear waste.
Spent nuclear fuel contains more than 90% of the man-made radioactivity generated during the operation of a nuclear reactor and produces considerable amounts of residual heat, due to the decay of its radioactive contents. For decades after removal from the reactor spent fuel has to be cooled to prevent meltdown. For that reason spent fuel is stored in cooling ponds adjacent to the reactors or at reprocessing plants. After a long cooling period storage in air-cooled dry casks is also practicized. Classified as most dangerous, usually nuclear industry has only spent fuel in mind when talking about nuclear waste. Although the radioactivity of spent nuclear fuel decreases with time, it is still dangerously high after tens of millions of years [more i10].
Decommissioning and dismantling waste
As a result of the intense neutron radiation during the fission process a nuclear reactor and its associated appendages become strongly radioactive. After closedown of the power plants the so-called nuclear island has to be decommissioned and dismantled. These activities generate very large volumes and masses of radioactive waste: tens of thousands of tonnes and tens of thousands of cubic meters [more i20]. The specific radioactivity of these wastes (measured per tonne waste), containing not only activation products, but also fission products and actinides as a result of contamination during the operational lifetime, is lower than that of spent nuclear fuel, but the total content of radioactivity is very large and long-lived. The waste is dangerous to man even after millions of years.
Isolation from the human environment
Radioactivity cannot be destroyed nor made harmless to humans. To prevent large populations, for example Europe or the USA, from being exposed to ever-increasing amounts of radioactivity, all radioactive wastes have to be isolated from the biosphere forever. That means that the waste has to be stored in such way that it will take millions of years before the radioactivity can re-enter the human environment by natural processes, for example via groundwater flows. The chances of re-entering radioactivity should be reduced to any conceivable minimum. Even then enough unknowns will remain which will enhance those chances and reduce the safety of the permanent nuclear waste storage. Economic priorities might pose a serious threat [more i26].
The definitive storage facility for radioactive waste is envisioned as a mine deep (500 meter or more) in a very stable geologic formation. Which geologic formations are best suited to accomodate a geologic repository? Each country seems to answer this question on its own way, dependent on the geologic options present and the political situation of the moment. For example, in the Netherlands and in Germany salt domes are discussed, in Belgium and France old clay formations, in the USA a volcanic formation (recently cancelled, without naming a new option) and in Sweden, Finland and Switzerland granitic formations.
After placing the radioactive waste in the mine, the galeries and caverns have be filled up with bentonite and other materials to prevent migration of radionuclides dissolved in groundwater entering the storage facility. Water ingression will almost certainly happen, sooner or later.
At present all nuclear wastes are stored in provisional and temporary storage facilities.The costs of the construction and operation of one geologic repository will be very high, likely counted in tens of billions of euros [more i16]. To store the yearly production of radioactive waste of the world, every two years a new geologic repository has to be opened. None exists in the world today.
The weakly radioactive but nevertheless dangerous mining wastes have to be isolated from the biosphere (atmosphere, groundwater) too. This process, called mine rehabilitation, is still not practicized in the world [more i18].
Figure 11-1. Waste streams of the nuclear system.
The nuclear energy system generates non-radioactive waste streams, which are not discussed here except CO2, and radioactive waste streams. The man-made radioactivity of 1000 nuclear bomb equivalents per year is distributed over very large volumes and masses of solids, liquids and gases. Considerable amounts of radioactive waste are discharged into the human environment. Not a single kilogram of radioactive waste ever generated has been safely isolated from the biosphere.
Figure 11-2. Composition of fresh and spent nuclear fuel.
Fresh nuclear fuel consists of very pue uranium, enriched in the fissile uranium-235 isotope. During operation of the reactor a part of the U-235 nuclides are fissioned. a part is converted into non-fissile U-236 and a part has not fissioned when the fission process is no longer sustainable and the fuel has to be removed from the reactor. By neutron capture - the neutrons coming from fissioning nuclides - a small part of the non-fissile U-238 isotope are converted into fissile and non-fissile plutonium isotopes. A part of the formed plutonium is fissioned and so contributes to the energy production. Another part of the plutonium is converted into the minor actinides: nuclides with a higher atomic number than plutonium, having nasty properties. The radioactivity of 1 kg spent fuel is a billion times higher than of 1 kg fresh fuel. In reality the constituents of fresh and spent nuclear fuel are mixed on atomic scale, so it is not possible to cut out a few pieces from spent fuel to obtain all fission products, plutonium and minor actinides in separate blocks. Separation of these materials requires a complicated sequence of processes, called reprocessing [more i31, i42].
Figure 11-3. Geologic repository
Symbolic presentation of a geologic repository for radioactive waste. The purpose is to isolate the radionuclides from the biosphere for geologically long periods.
Figure 11-4. Geologic repository
The Swedish KBS concept for deep geological disposal of spent fuel. According to this concept the spent fuel would be packed in durable canisters, with thick outer layre of very pure copper, which are to be placed in holes in the floor of galleries of a deep mine in granite. The holes and the galleries are to be filled up with bentonite, a mineral which would retard the migration of radionuclides from leaking canisters by groundwater. In the long run, how long is unknown, all canisters will go leaking. Radioactive wastes with lower specific radioactivity would be packed in concrete canisters and stored in large caverns, which would be also filled up with bentonite (not pictured here).