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GK SPECIAL TOPIC : RADIOACTIVE WASTE
Saturday, 17 May 2014 06:50

 

RADIOACTIVE WASTE

Nuclear power is characterized by the very large amount of energy available from a very small amount of fuel. The amount of waste is also relatively small. However, much of the waste is radioactive and therefore need to be carefully managed.

Types of radioactive waste

Low-level Waste is generated from hospitals, laboratories and industry, as well as the nuclear fuel cycle. It comprises paper, rags, tools, clothing, filters etc. which contain small amounts of mostly short-lived radioactivity. It is not dangerous to handle, but must be disposed off more carefully than normal garbage.

Intermediate-level Waste contains higher amounts of radioactivity and may require special shielding. It typically comprises resins, chemical sludges and reactor components, as well as contaminated materials from reactor decommissioning. It may be solidified in concrete or bitumen for disposal.

High-level Waste may be the used fuel itself, or the principal waste separated from reprocessing this. It contains the highly-radioactive fission products and some heavy elements with long-lived radioactivity. It generates a considerable amount of heat and requires cooling, as well as special shielding during handling and transport. If the used fuel is reprocessed, the separated waste is vitrified by incorporating it into borosilicate (Pyrex) glass which is sealed inside stainless steel canisters for eventual disposal deep underground. On the other hand, if used reactor fuel is not reprocessed, all the highly-radioactive isotopes remain in it, and so the whole fuel assemblies are treated as high-level waste. Both high-level waste and used fuel are very radioactive and people handling them must be shielded from their radiation. Such materials are shipped in special containers which prevent the radiation leaking out and which will not rupture in an accident.

Radioactive wastes management

As the Low-level radioactive waste is generally produced by government facilities, nuclear power plants, various industries, and institutional facilities (e.g., hospitals and universities). And have short half-life tens to hundreds of years.

Thus most low-level radioactive wastes are solidified, put into drums, and buried in 20-foot-deep trenches, which are then backfilled and covered in clay.

Whereas high level radioactive wastes have long half-life thus disposal process is used for management.

Disposal process for the management of radioactive waste involves confinement or isolation of these wastes from biosphere in the repositories. Based on the longevity and concentration of the radionuclide present in the waste, the repository could be either near-surface or in deep geological formation.

• Near Surface Method includes Dry Cask Storage. It is a method of storing high-level radioactive waste, such as spent nuclear fuel that has already been cooled in the spent fuel pool for at least one year. These casks are typically steel cylinders that are either welded or bolted closed and the fuel rods are surrounded by inert gas. Ideally, the steel cylinder provides leak-tight containment of the spent fuel. Each cylinder is surrounded by additional steel, concrete, or other material to provide radiation shielding to workers and members of the public. Some of the cask designs can be used for both storage and transportation.. It is a relatively inexpensive method which can be done at a central facility or adjacent to the source reactor. The waste can be easily retrieved for reprocessing. But strong radiation from wastes might lead to corrosion of the tanks and a consequent leakage of radioactive wastes.

• Deep ocean disposal: In this method, containers made of borosilicate glass are filled up with nuclear waste. This glass has the capacity to prevent any nuclear radiation from leaking out. The container is enclosed in yet another water-tight metal container and dumped into the ocean. Though these containers are said to be leak-proof, there is a speculation that a minor quantity of radiation does manage to escape from these containers which affects the marine biodiversity.

• Deep geological burial: The containers used in this method are similar to the ones used in the deep ocean disposal system. But in this case, the containers are buried deep underground, in less-populated areas. This method depends on the natural decaying ability of the radioactive material for its success. The materials are buried deep under the earth for thousands of years and allowed to settle into a safe level of radioactivity.

• Nuclear waste recycling is a new waste disposal method being looked into, in which the uranium, plutonium and other fission products are separated into different streams using chemical processes. The advantage of doing this is that these products can be re-used or disposed of more easily. However, the recycling process is not feasible at present because many countries do not have proper facilities to implement this method, and it is also a costly process.

India has extensive & varied experience in the operation of near surface disposal facilities (NSDFs) in widely different geo-hydrological and climatological conditions.

Disposal of long - lived and high level waste in deep underground geological formation is under evaluation.