Radioactive Waste Management
Tairabi Khanadal, Savita Patil, Arfa S. Bargir, Shireen Chattarki and Rajeshwari
Akkamahadevi Women’s University, Vijayapura
, INDIA.
(Received on: July 3, Accepted: July 12, 2017)
ABSTRACT
Concern for the environment and establishment of radiation protection goals is the major priority in planning India’s nuclear energy programme. This programme is based on closed fuel cycle where reprocessed uranium and plutonium are recycled. Radioactive waste is generated at each stage of fuel cycle which includes mining and milling of uranium ore, fuel fabrication, reactor operation, spent fuel reprocessing, decontamination and decommissioning of aged nuclear facilities.
Keywords: Low level waste Nuclear wastes, solid waste, liquid waste, gaseous waste, polymerization, vitrification, resin fixation. Resorcinol Formaldehyde Poly condensate Resin (RFPR).
INTRODUCTION
Radioactive is a waste product containing radioactive decay material. It is a product of nuclear fission. The industries are not directly connected to the nuclear power industry which produce radioactive waste. Radioactivity diminishes over time, so it needs to be isolated for a period of time till there is no hazard. The majority of nuclear waste is “low level waste”, meaning it has low levels of radioactivity per mass or volume. The low-level wastes can include radioactively contaminated protective clothing, tools, filters, rags, medicinal tubes.
OBJECTIVES
The main objective in managing and disposing of radioactive waste are to protect people and the environment. This is achieved mainly by isolating the waste into environmentally compatible inert matrix and discharging remaining into the environment following as low as reasonable achievable (ALARA) principle.
PROCEDURE
Radioactive waste is categorized into three types1) solid waste 2) liquid waste 3) gaseous waste. Basic steps in Radioactive Waste Management Characterization, storage, treatment, conditioning, disposal, surveillance/ monitoring, etc are important basic steps involved in the management of radioactive waste: These are briefly described here.
Characterization
The waste is characterized to determine its physical, chemical and radiological properties and to facilitate documentation. It helps to segregate radioactive materials for exemption, reuse and disposal methods or to assure compliance of waste packages with requirements for storage and disposal.
Storage
Storage of radioactive waste helps in monitoring to check it’s isolation from man and environment, determine the suitable treatment, conditioning and disposal method and allows decay of short-lived radionuclides which subsequently helps in treatment/disposal.The processes that are employed for treatment of low level waste are filtration, chemical treatment, ion-exchange, steam evaporation, solar evaporation and membrane processes.
Chemical Treatment
Strontium (90Sr) and Caesium (137Cs) are two predominant radionuclides present in low levels liquid waste. They are separated by co-precipitation process using chemicals like bariumchloride, sodiumsulphate, potassium Ferrocyanide, coppersulphate, etc. Subsequent to precipitation, the resultant sludge from clarifloculator is further concentrated by decantation, filtration and centrifuging. The resulting solids contain bulk of the radioactivity originally present in the liquid waste and are immobilized in cement matrix before disposal.
Ion-Exchange
Evaporation
Evaporation is used for concentrating the liquid waste as it gives high volume reduction and decontamination factor. Selection of the process is decided by radioactivity and volume of waste as well as the climatic conditions of the site. Evaporation based on steam heating is used for waste of low volume and high activity. Solar evaporation, is a preferred mode of evaporation for large volume of waste with low activity. This process is used at Rajasthan Atomic Power Plant for treatment of low level liquid waste generated in house.
Membrane Processes
Membrane processes like reverse osmosis and ultrafiltration are used essentially for treatment of low level liquid waste. These are generally employed in combination with other treatment methods like chemical treatment or ion-exchange process.
Conditioning
Conditioning of radioactive waste involves operations that transform radioactive waste into inert solid form suitable for handling, transportation, storage and disposal. The conditioning process depends on the compatibility of the matrix with waste, chemical and mechanical durability of solidified product, cost of processing, and disposal. In India cementation and polymerisation are the methods adopted conditioning of low and intermediate level waste.
Cementation
Cement and cement composites are extensively used for immobilization of low and intermediate level radioactive waste concentrates, chemical sludges etc.
Vitrification
This process consists of metering of pre-concentrated waste and glass forming additives in the form of slurry into the process vessel which is made of high Ni-Cr alloy (Inconel-690) to withstand high temperature, oxidizing and corrosive conditions. It is located in a multi-zone induction furnace where concentration, calcination and glass formation take place.
Solid Waste
Polymerization
In India Polyester styrene polymer has been used for immobilization of ILW concentrates, spent ion-exchange resin from nuclear power stations and other facilities. In resin fixation plant, radioactive spent resins are transferred to resin storage tank. Excess water is removed by vacuum de-watering system. Mixing assembly is then mounted on this product drum. Requisite amount of polyester styrene polymer premixed with optimized concentration of accelerator (Dimethyl Aniline) and catalyst (Benzoyl Peroxide) is poured into the product drum with constant stirring. The process is exothermic and peak temperature is in the range of 70-85 degree C. These resin fixation facilities exist at nuclear power plant sites. Chemicals used for fixation of spent resin : Polyester styrene resin (Isophthalic grade), Benzoyl peroxide (Catalyst),N-N Dimethyl aniline (Accelerator).
Process Description and Operational Steps: 1. Testing of empty CS drum by vacuum dewatering. 2.Loading of tested empty drum into drum handling cask. 3.Spent resin transfer from PHT/SFSB hopper to resin receiver column (RRC) 4.Draining of resin water slurry from resin receiver column to drum. 5. Vacuum dewatering of spent resins with water6.Chemical addition and mixing 7.Matrix solidification 8.Shifting of polymerized drum from shielding flask to High integrity container for disposal. 9.Mix the de-watered resin for 10 minutes. 10. Add pre accelerated polyester styrene into drum while mixer is on by opening the pinch valve.12. After mixing of pre-accelerated polymer with resin add pre-catalysed polymer while mixer is on by opening pinch valve.
The philosophy of radioactive waste management embodies three basic principles:Dilute and disperse (for very low active waste) to the environment well below the nationally accepted level.Delay and decay of short lived radionuclides and Concentration and containment of intermediate and high level waste.
Disposal is the final step in the radioactive waste management system. The safety, mainly achieved by isolation, is attained by placing barriers around the radioactive waste in order to restrict the release of radionuclides into the environment. The barriers can be either natural (natural clay or rock having high sorption capacity of radionuclides) or engineered (stone concrete or metal) and an isolation system can consist of one or more barriers. During the period when the radioactive waste is contained by the system of barriers, the radionuclides in the waste are undergoing decay thereby reducing hazard with time.
National Policy: The national policy for radioactive waste management is as follows:
4. Alpha contaminated waste not qualifying for near surface disposal is provided suitable interim storage pending its disposal in deep geological repository.
5. Spent radiation sources are either returned to the original supplier or handed over to a radioactive waste management agency identified by the Regulatory Body. 6. Spent fuel is a resource material and needs to be processed for recovery and recycle of fissile material. 7. Regulatory Body determines the period for which active control of the shallow land repository should be maintained.
Waste from Outside Nuclear Power Activities: Radioactive wastes are also generated in R&D activities using research reactors, accelerators and radioactive isotopes. Similarly radioactive wastes are generated in radio isotopes production, radioisotopes applications and decontamination as well as in industrial activities where raw materials containing naturally occurring radionuclides are processed on a large scale as in the case of production of artificial fertilizers and the extraction of oils and gas. The concentration of radionuclides in such waste streams may exceed the levels for exempt waste due to the effect of concentration taking place during the processing though the ores themselves may contain only natural radionuclides at low concentrations.
CONCLUSION
India has decades of experience and expertise for safe management of radioactive waste. It is recognized that the technologies currently adopted are adequate. Development and induction of cross-cutting technology have to be adopted not only to meet the challenges posed by approach to near zero discharges but also to address recycle and recovery of valuable resource from these wastes leading to a positive impact on the environment. Besides, Waste Management in India has to meet the requirements of Advanced Fuel Cycles for Fast Breeder Reactors and Advanced Heavy Water Reactors. Because of high volumes and low activity of low and intermediate level waste attention is focused on the technology development that can address downsizing of equipment, effective decontamination and minimization of secondary wastes.
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