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# Nuclear Waste Disposal Methods

The disposal of nuclear waste needs a perfect prediction of the actions of human kind along with optimal conditioning of nature which must be made for at the least thousands of years. When we look backword in our time scale we acknowledge that the radioactive rocks that made the basis of natural reactors at Oklo went critical almost 2 billion years ago and produced 100 billion kilo watt hour of equivalent energy. This lasted about five hundred thousand years.

The nuclear wastes of that era remained localized in the surroundings even today. The wastes of Uranium and Thorium decay have remained in sandstones overlying the area.

The ice age remained active for almost 90 thousands years and the cave paintings drawn 200 thousands years back definitely gives an idea that our ancestors had seen, experienced and made definitive approach against these nuclear decays. The idea of dilution methods of cleaning up the wastes with clean water still needs a lot of approval.

Mineral geology disposal methods of disposing by burying waste canisters within deep rock formation also came from the same idea of the natural fission that took place millions of years ago. Metal canisters of wastes would be placed in drill holes made larger than the canisters itself and the extra space around the canisters would be filled with buffer materials like Bentonite or a mixture or rock and Bentonite. Future plans of retrieval of these canisters could be made by providing steel sleeve to pull out the canisters.

These waste packages which are literally a collective term for waste canister with sleeve create a barrier against the leaching or leakage of nuclear wastes might need a time period of thousands of years before the canisters corrode and give away.

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## Types of Nuclear Waste Disposal Methods

The various nuclear waste disposal methods that are carried out at the moment derives from the point of how slowly the waste would decay into the surrounding instead of a completely fool proof method. There are many drawbacks and most of these are partly technical and partly due to physiological barriers.

The idea of recovering nuclear wastes from disposal areas has some specific reasons. These may be carried out due to economic and safety reasons as spent fuels do have some recoverable Uranium and Plutonium as well as the unacceptable radionuclide leaching into the surrounding.
• Rock melting disposal
This method involves packing hot liquid or solid wastes inside the cavities of mines or after making drill holes in rock deep inside the earth layer of 3 Km. The heat from the waste is expected to melt the rock and allow the two materials to mix while the liquid after cooling would solidify into a rock waste mass.

• Ice sheet disposal
Another suggestion is to pack the nuclear wastes into thick layer of ice sheets close to the poles where the canisters would be lowered into shallow drill holes which will eventually sink downward till the bedrock by melting the ice. But this has some future climatic conditioning questions as well as natural ice movements in permafrost region.

• Sea bed disposal
It was also suggested that drill holes could be made in the deepest regions of ocean bed within the fine sediments of sea bottom but as there are certain amount of danger involving marine lives it has been kept under wraps. Although the idea has some plus points as well. The canisters if ever gets damaged would get diluted in large amount of salt water and as the water would move slowly there is a chance that these might get absorbed efficiently.

• Outer space disposal
The idea of disposing nuclear wastes in outer space by propelling rockets is under consideration for several years but due the volume of wastes that gets generated is seen as unviable economically. These could be carried out for very dangerous nuclear wastes of small quantities. But since the consequences of malfunctioning and technical snags that might arise during the operation is now kept aside as unacceptable.

## Low Level Nuclear Waste Disposal Methods

The radioactive nuclear wastes fall under three specific categories of low, intermediate and high level wastes.

 Low level waste Up to about $10^{8}$ Bq per cubic meter Intermediate level waste From 108 to $10^{13}$ Bq per cubic meter High level waste More than $10^{13}$ Bq per cubic meter

The low and intermediate level wastes are usually buried in nuclear reactor waste sites which are under constant monitoring in order to ensure no leakage into water beds or supplies.

These low level wastes are mostly the Hospital and research labs wastes where these are used as nuclear medicines. These are usually short span life and do not pose long term threat to public health.

The low level radioactive waste (LLRW) under the purview of National Research Council Board as well as Radiation effects research evaluates the impact of these LLRW on bio medical research especially in University and medical centres.

These include counting vials, tissue culture cells and animal carcass and other materials like glass and plastic containers, apart from other absorbent materials.
• Materials are stored safely as they have short decay period and disposed of later as non-radioactive materials
• Other materials are compacted, super compaction, or incineration
• Finally these products are disposed of as LLRW in local landfills with authorised license conditions

## Current Nuclear Waste Disposal Methods

Almost all of the radioactivity in Department of Energy’s high level waste originates from radionuclides with half-lives of about 30 years and less. About 98% of radioactivity of high level wastes comes from four radionuclides Ba 137 m, Caesium 137, Strontium 90, and Yttrium 90 of which Caesium 137 has the longest life with a half-life of 30.17 years.

The current steps in the process of nuclear waste managements are as follows:

1. Characterization

Determination of specific physical chemical and radiological components of the wastes in each tank. This step is important as some of these tanks contain a complex mixture of unknown waste constituents and the detailed knowledge of such tank contents is necessary to determine how it would best to retrieve, pre-treat and finally treat the wastes.

2. Retrieval

Removal of stored wastes from tanks by pumping and transferring these to treatment facilities as the waste might exist in liquid, solid or other forms and definitive steps are required to turn these into a form which will facilitate pumping.

3. Pre-treatment

Separation of high level portion of wastes from low activity portions and from other non-radioactive elements such as aluminium, organic compounds, and soluble salts. The method of evaporation is used during this step in order to reduce the volume of the contaminated water in the waste. This is necessary to decrease the amount in order treat it proper before being sent to high level repository.

4. Treatment

The immobilization of waste is carried out by mixing with glass forming material and melting into a glass. The molten glass is then poured in stainless steel canisters and allowed to harden. Remaining low activity portion wastes are mixed with cement and allowed to harden into a grout.

5. Disposal

The final placement of immobilized waste in order to ensure isolation from surrounding environment till these are relatively less dangerously radioactive. These canisters are stored on site until these are disposed of permanently at designated near surface designated sites.