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| Fig. 1: Containers used for storing the used solid radioactive waste. (Source: Wikimedia Commons) |
Nuclear power is economically competitive and a growing contribution of nuclear energy will continue to be necessary if the standard of living in industrialized nations is to be maintained and the energy needs of the developing nations to be met. [1]
However, as a result of operation of nuclear reactors, large volumes of radioactive wastes are produced, which must be processed. The waste can be divided into two parts. The more dangerous (from fuel rods) is high level waste (HLW). This very radioactive for 1000 years also has radiation from actinide decay daughers that takes hundreds of thousands of years to decay). The less dangerous is low level waste (LLW). This is a broader category that accounts for objects that have been contaminated by radiation. I will discuss how the less potent, low level radioactive waste is treated before it is contained or safe to be released. [2]
The nature and amount of wastes produced in the nuclear power plant depend on the type of reactor, its specific design features, its operating conditions and on fuel integrity. With time, the methods of treating and conditioning the waste has reached a high degree of effectivity and reliability, but at the same time are being further developed to improve safety and economy of the entire waste management system. [2]
In normal operation of nuclear power plants, some airborne radioactive wastes are generated either in particulate or aerosol gaseous form. Particulate gaseous aerosols can be generated in a wide range of particulate sizes in either liquid or solid form with possibly a combination with non-radioactive aerosols. [3]
Sources: the three main sources of aerosols are:
Generation by emission of activated corrosion products and fission products
Radioactive decay of gases to in-volatile elements
Adsorption of radionuclides (most common ones like halogens, tritium, noble gases and C-14) formed in the fission process on existing suspended material. [3]
The composition and radioactivity depend on the reactor type and the release pathway. [4]
It is common for contaminant gases and building ventilation air at nuclear power plants to be first passed through filters to remove particulates before being discharged into the atmosphere. The air cleaning system generally uses coarse pre-filters followed by high efficiency particulate air filters, which combined are able to be really quite efficient for small sized particles. [2]
Radioactive iodine arising from operation is continuously removed during the process via impregnated charcoal filters, that traps the iodine from the effluent, which then passes through particulate filters. [5]
In the case of the noble gas aerosols a decay storage mechanism is used whereby the gases are first pumped into storage tanks, which are then sealed and left for a while before the contents are released into the atmosphere via a monitored ventilation system. If the escaping gas is not as per the required standards, then the storage period is increasing accordingly. [2]
There are various types of dry solid wastes containing radioactive materials are generated during the operation of a nuclear power plant. The nature of the waste varies from facility to facility and include redundant items from the plant, ventilation system filters, etc. Another source is the accumulation of miscellaneous paper, plastic, rubber etc used during the operation and maintenance of the nuclear plant. [2]
The types of solids are segregated based on the physical nature of the materials into four different parts: combustible, non-combustible, compactible and non-compatible waste. [2]
One of the main aims in the treatment of solid waste is to reduce the waste volume to be stored and disposed of as much as possible and to contain and immobilize the radioactivity contained within the waste. Fig. 1 shows the containers used to store the radioactive solid waste. [6] As the solid radioactive waste contains a wide variety of materials in various forms, no single technique can adequately treat this waste and so a combination of techniques are used. [2]
Prior to the aforementioned techniques, steps such as shredding and crushing are used to reduce the physical size of the waste items. Paper, plastic, cloth, cardboard, wood and metals can be shredded into ribbon-like pieces, while brittle materials like glass or concrete blocks can be crushed into smaller fragments. The techniques can also be used as stand-alone processes for volume reduction of solid waste, but the techniques below are commonly used. [2]
Compaction is one of the basic and common techniques used for processing large volumes of solid waste. The method reduces storage and disposal volume by a large amount, but achieves little in terms of improvement of waste properties for longer term management. [1] Fig. 1: indicates an overview of the compaction process. [2]
As between 50-80% of solid radioactive waste produced at power plants is classified as burnable waste. Incineration of this waste represents a substantial improvement over simple compaction, as very high volume and mass reduction can be achieved. The final product is homogenous ash, which can be packaged without further conditioning into containers for storage and disposal. While incineration is suitable for combustible waste, it is also capable of destroying organic liquids like oils, greases or solvents, which are generally difficult to treat. [1,2]
Incineration of small quantities of solid waste are routinely carried out in relatively simple units. These units can be found in nuclear power plants in USA, Japan, Canada and other countries. More advanced incineration facilities that can incinerate waste with high specific activity are installed at centralized waste treatment facilities that can accept waste from many plants in the specific country and abroad. Such facilities currently operate in Sweden, Belgium, France and other countries. [2]
Most treatment processes have reached an advanced industrial scale. Although these processes and technologies are sufficient for effective management of gaseous and solid waste at nuclear power plants, further improvements are still possible and desirable. [2,7]
The increasing cost of waste disposal acts as an incentive to develop and adopt new processes and techniques to minimize waste quantities and volumes and also to develop new steps at the treatment stage.
There are many developments that have advanced over the years and some specific ones include; the vitrification of some intermediate-level waste to reduce volumes of waste to be disposed of, super-compaction of burnable waste, etc.
Not all new developments find broad implementation in waste management technologies in nuclear plants worldwide, but research and development reflects that the nuclear industry and facilities take great care to establish a safe and economic management of radioactive waste at nuclear power plants and that improvements in existing technology are foreseen. [2]
© Valmik Lakhlani. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. Okoshi and T. Momma, "Radioactive Waste Treatment Technologies," in Radioactive Waste Engineering and Management, ed. by S. Nagasaki and S. Nakayama (Springer, 2015).
[2] V. M. Efremenkov, "Radioactive Waste Treatment at Nuclear Power Plants," IAEA Bull. 31-4, 37 (1989).
[3] S. Werner, "Nuclear Waste Technologies," Physics 241, Stanford University, Winter 2017.
[4] S. Ali, "Nuclear Waste Disposal Methods," Physics 241, Stanford University, Winter 2011.
[5] S. Parekh, "Nuclear Waste Management," Phyisics 241, Stanford University, Winter 2014.
[7] M. De Graw, "The Future Nuclear Waste Management," Physics 241, Stanford University, Winter 2015.
