Radioactive Waste Disposal

The radioactive waste management approach is to consider the nature of radioactive elements involved on terms of their half-lives and then choose the appropriate method of handling.

1. If the concentrations of radioactive elements are largely short-lived, then 'delay and decay' approach is preferred; that is, to hold on to such a waste for a sufficiently long time that the radioactivity will die in the meanwhile.

2. A second approach is to 'dilute and disperse' so that the hazard in the environment is minimized. But when the radioactivity is long-lived, the only approach that is possible is to 'concentrate and certain' the activity. In order to carry out concentrating the waste (generally the sludge), chemical precipitation, ion exchange, reverse osmosis and natural or stream evaporation, centrifugal, etc. are resorted to. The resulting solids are highly concentrated in radioactivity.

Low - level radioactive is often buried in shallow landfills. High-level nuclear waste from both commercial reactors and defense industry presents a difficult. There are many proposals for disposing high-level nuclear wastes. The solution for the disposal of these wastes is isolating radioactive waste from man and biosphere for a period of time such that any possible subsequent release of radio-nuclides from the waste repository will not result in undue radiation exposure.

The following options are,

a. Deep geological repositories

b. Ocean dumping

c. Seabed burial

d. Sub-seabed disposal

e. Subductive waste disposal method

f. Transforming radioactive waste to non-radioactive stable waste.

g. Dispatching to the sun.

Geologic Disposal

Geological disposal in deep geological formations-whether under continental crust or under seabed-as a means of radioactive waste disposal for handling long-lived waste. The deep geological sites provide a natural isolation system that is stable over hundreds of thousands of years to contain long-lived radioactive waste. In practice it is noted that low-level radioactive waste is generally disposed in near-surface facilities or old mines.

High-level radioactive waste is disposed in host rocks that are crystalline (granitic, gneiss) or argillaceous (clays) or salty or tuff.

Ocean Dumping

For many years the industrialized countries of the world (e.g. USA, France, Great Britain, etc) opted for the least expensive method for disposal of the wastes by dumping them into the oceans. Essentially, waste-filled missiles, which when dropped through 4000 m of water, will embed themselves 60-80 m into the seabed's clay sediments. These penetrators are expected as survive for 700 to 1500 years. Thereafter the waste will diffuse through the sediments. Penetrator disposal is potentially both feasible and safe.

Sub-seabed Disposal

Seabed disposal is different from sea-dumping which does not involve isolation of low-level radioactive waste within geological strata. The floor of deep oceans is a part of a large tectonic plate situated some 5km below the sea surface, covered by hundreds of meter of thick sedimentary soft clay. These regions are desert like, supporting virtually no life. The seabed fluvial proposal envisages drilling these ‘mud-flats’ to depths of the order of hundreds of meter, such boreholes being spaced apart several hundreds of meter.

The high-level radioactive waste contained in canisters, would be lowered into these holes and stacked vertically one above like other interspersed by 20 m or more of mud pumped in.

Subductive Waste Disposal Method

This method is the state-of-the-art in nuclear waste disposal technology. It is the single viable means of disposing radioactive waste that ensures non return of the relegated material to the biosphere. At the same time, it affords inaccessibility to eliminated weapons material.

The principle involved is the removal of the material from the biosphere faster than it can return. It is considered that ‘the safest, the most sensible, the most economical, the most stable long-term, the most environmentally benign, the most utterly obvious places to get rid of nuclear waste, high-level waste or low level waste is in the deep oceans that cover 70% of the planet’.

Subduction is a process whereby one tectonic plate slides beneath another is eventually reabsorbed into the mantle. The subductive waste disposal method forms a high-level radioactive waste repository in a sub ducting plate, so that the waste will be carried beneath the Earth’s crust where it will be diluted and dispersed through the mantle. The rate of subduction of a plate in one of the world’s slowest subduction zones is 2.1 cm annually. This is faster than the rate (1 mm annually) of diffusion of radionuclides through the turbidite sediments that would overlay a repository constructed in accordance with this method. The subducting plate is naturally predestined for consumption in the Earth’s mantle.

Transmutation of High-Level Radioactive Waste

This route of high-level radioactive waste envisages that one may use trans mutational devices, consisting of a hybrid of a sub-critical nuclear reactor and an accelerator of charged particles to ‘destroy’ radioactivity by neutrons. The fission fragments can be transmuted by neutron capture and beta decay, to produce stable nuclides. Transmutation of antinodes involves several competing processes, namely neutron-induced fission, neutron capture and radioactive decay. The large number of neutrons produced in the spaitation reaction by the accelerator is used for ‘destroying’ the radioactive material kept in the sub-critical reactor.

Solar Option

It is proposed that ‘surplus weapons’ plutonium and other highly concentrated waste might be placed in the Earth orbit and then accelerated so that waste would drop into the sun. Although theoretically possible, it involves vast technical development and extremely high cost compared to other means of waste disposal. Robust containment would be required to ensure that no waste would be released in the event of failure of the ‘space transport system’.