Nuclear Waste Disposal and Safety:
Introduction – Nuclear Waste Disposal and Safety from nuclear power plants activities, defense related nuclear weapon activities or other applications of nuclear technology is hazardous to many life forms and the environment. It poses a problem of handling and protecting the environment to be safe to the present and future generations.
Waste is defined as any material that has been discarded as being of no further use. This concept also holds for radio active waste. In other words, waste that emits nuclear radiation is called radio-active waste.
Since radioactivity decays in course of time naturally, radio active waste has to be isolated and confined in appropriate Nuclear Waste Disposal facilities for a sufficient period of time until it no longer poses a threat. The type of waste and radio active isotopes determine the time period of storage of waste which may vary from few days (for short living isotopes) to millions of years (for long living nuclear fuel).
The radioactive wastes are of three forms, namely liquid, solid or gaseous form, each of which are treated differently.
Types of Radio active waste
Radio active wastes are generally classified into following types.
- Low level radio active waste.
- Intermediate level radio active waste.
- High level radio active waste.
1. Low level radio active waste:
It is larger than the high level radio-active waste in volume wise. Radio-activities are contained in the low level. Radio active waste is significantly less and made up of isotopes with shorter lives than the isotopes in high level radio active waste. Large amounts of waste such as protective clothing, cleaning rags, glove boxes, air filters, shielding material, etc. are contaminated with small amount of radio activities.
If low level radio active waste is stored for a period of 10 to 50 years to decay, then it can be well disposed off in normal way.
However, it may be useful for mammals for greater reproductive capacity, better disease resistance, increased growth rate, greater resistance to higher radiation doses, better neurological function, better wound healing.
It also beneficial for plants, like accelerated growth and increased harvest.
2. Intermediate level waste
Higher amounts of radioactivity is present in intermediate level waste. It requires shielding but not cooling. It typically comprises resins, chemical sludges, metal fuel cladding and contaminated materials from reactor decommissioning. Intermediate level waste may be solidified in concrete or bitumen for disposal.
3. High level radio-active waste
High level radio active waste consists of the spent nuclear fuel, from generally material from the core of a nuclear reactor (or) from Nuclear weapons. This waste includes uranium, plutonium and other highly radio-active elements created during fission. High level radio-active waste is made up of fission fragments and transuranic elements.
These two components have different time periods to decay. The radioactive fission fragments decay to different stable elements via different nuclear reaction chains involving α, β & γ emissions to dispose off normally. It may take 1000 years. On the other hand, transuranics take nearly 500,000 years to reach such levels. Most of high level radio-active waste isotopes emits large amount of radiation and have long half lives – some longer than 100,000 years.
Collection, storage and disposal of nuclear materials
- Low activity, solid nuclear wastes are collected in paper (or) plastic lined containers and disposed by incineration (or) land disposal method.
- High activity solid nuclear wastes are collected in shielded containers.
- Low activity liquid and gaseous wastes are usually diluted (or) dispersed before disposal.
- Special glass containers are used to store ‘high activity liquid and solid wastes’ and are maintained at 100 – 150°C so that these wastes will become glassy materials.
Waste disposal process from nuclear reactor
The withdrawn spent fuel with cladding from the nuclear reactor is placed in a waste pond where heat is removed and short living radio nuclides decay. To remove activity due to release of fuel from defective cladding, treatment of the water pond is needed continuously. Then, the spent fuel is transferred to the reprocessing plant where the fuel cladding is removed. The spent fuel is dissolved in nitric acid and U235 and Pu239 are separated having the solution with 99% non-volatile fission products known as Highly Active liquid waste. The removed U235 and PU239 are purified and stored for future use as fresh fuel.
The waste from the cooling pond is transferred to an intermediate storage and kept there for about 30 to 100 years. Here most of the heat is removed and radioactive nature is reduced to a low level. Then the waste is permanently buried in the earth or sea.
Solid wastes disposal
Scrap materials or discarded objects contaminated with radioactive materials are called solid wastes. Non-combustible solid wastes are buried deep in the ground. But if the wastes are combustible, they are burnt to ashes and then the radioactive matter is mixed with concrete, poured as cubic block and buried.
Liquid wastes disposal
The Disposal of liquid wastes is done in two methods.
- Dilute and disperse method: In this method, the liquid wastes are diluted with large quantities of water and then released into the ground. The drawback of this method is that there is a chance of contamination of underground water if the dilution factor is not adequate.
- Concentrate and contain method: Sometimes due to the amount or nature of the isotopes, the dilution of radioactive liquid water is not desirable. In such cases, the liquid wastes are concentrated to small volumes and stored in underground tanks. As leakage from these tanks will lead to significant water contamination, the tanks must be of assured long term strength and leakage proof.
Gaseous wastes disposal
The gaseous wastes from the ruptures in the fuel element cladding release gaseous fission products. Gaseous wastes released into air are absorbed by plants and inturn affect the health of humans. Many methods have been developed to remove gases like cypton, iodine, tritium and CO2. Generally, gaseous wastes are diluted with air, passed through filters and then released to atmosphere through large stacks.
Approaches to radio-active waste disposal
Nuclear Waste Disposal is discarding waste with no intention of retrieval. Waste management means that operations starting with generation of waste and ending with disposal.
The following methods are carried out to dispose radio-active waste.
- Deep geological repositories.
- Ocean dumping.
- Sea bed burial.
- Sub-sea bed disposal.
- Subductive waste disposal method.
- Transforming radio active waste to non-radioactive stable waste.
- Dispatching to the sun.
This method was started in 1957 to handle long living waste. The deep geological sites provides natural isolation system over hundreds of thousands of years to contain long-living radio active wastes. Low level radio active waste is generally disposed in near surface facilities. High level radio-active waste is disposed in rocks that are crystalline (granite, etc.,).
Radio-active waste material is dropped through 4000 m of water and it will be embedded itself 60 – 80 m into sea-beds clay sediments. These wastes are expected to survive for 700 to 1500 years. Thereafter the waste will diffuse through the sediments.
High level radio active waste material can be dumped 5 km below the sea surface, covered by hundreds of meters of thick sedimentary soft clay. These mud flats drilled up to hundreds of meters. The high level radio active waste would be lowered into these holes and stacked vertically one above the other interspersed by 20 m (or) more of mud pumped in. Therefore these potential waste repositories are geologically stable over periods of the order of 107 years and likely do not have human activities such as resources of fishes (or) hydrocarbons (or) minerals.
Subductive waste-disposal method
Subduction is a process in which one tectonic plate slides beneath another and it is reabsorbed into the mantle. The high level radio active waste will be deposited in a subducting plate, so that the waste will be carried beneath the earth’s crust where it will be diluted and dispersed through the mantle.
Transmutation of high-level radio active waste
Transmutational devices are consisting of a hybrid of sub-critical nuclear reactor and an accelerator of charged particles to destroy radio activity of neutrons.
It is proposed that plutonium and other highly concentrated waste can be placed in the earth orbit and then accelerated so that waste would drop into the sun. It involves vast technical development and extremely high cost compared to other means of Nuclear Waste Disposal. It should be assured of that no waste would be released in the event of failure of space transport system.