Types of Nuclear Waste and Disposal Techniques

Nuclear Waste Disposal

Since the development and utilization of nuclear energy during World War II, countries have struggled with the issue of disposing of high-level nuclear waste . Nuclear waste is one of the biggest downsides to nuclear power, and can remain dangerous for hundreds of thousands of years. Geological disposal is often stated as the most preferable way of dealing with it, but what does it entail? In this report we will discuss problems that need to be overcome , ongoing research and try to choose a suitable location in India for disposal.

Classification of waste on basis of radioactivity

1.low level (90%)
2.intermediate level (7%)
3.high level(3%)

Low-level waste

Generated from hospitals and industry, as well as the nuclear  fuel cycle. Low-level wastes include paper, rags, tools, clothing, filters. Some high-activity LLW requires shielding during handling and transport but most LLW is suitable for shallow land burial

Intermediate-level waste

Low-level waste

• Intermediate-level waste (ILW) contains higher amounts of radioactivity and in some cases requires shielding.
• Includes resins, chemical sludge and metal reactor nuclear fuel cladding
• It may be solidified in concrete or bitumen for disposal
• Short-lived waste (mainly non-fuel materials from reactors) is buried in shallow repositories
• Long-lived waste (from fuel and fuel reprocessing) is deposited in geological repository

High-level waste

• It contains fission products and transuranic elements generated in the reactor core.
• Though it is only 3% of total volume but it is responsible for 95% of total radioactivity.
• It is highly radioactive and often hot.
• HLW is the most dangerous and the main candidate for geological disposal
• Certain radioactive elements (such as plutonium 239) in “spent” fuel will remain thousands of years
• Tc-99 (half-life 220,000 years)
• I-129 (half-life 15.7 million years)

Management of High-level wastes

Immobilization of high level liquid waste into vitrified borosilicate glasses. Engineered interim storage of vitrified waste for passive cooling in pools near power-plant and surveillance over a period of time qualifying it for ultimate disposal. Ultimate storage disposal of vitrified waste a deep geological depository.

Vitrified waste

Disposal of Waste

Management of High-level wastes

• Above ground disposal
• Geological disposal
• Deep borehole disposal
• Disposal at subduction zones
• Ocean disposal
• Disposal in outer space

Impractical methods

Methods	Main Reasons
Disposal in outer space	Very costlyHigh risk of space vehicle failure
Ocean Disposal	Declared illegal by international treaty
Disposal at subduction zones	High risk of earthquakes since located on plate boundariesRate of subduction is very slow

Above ground disposal

Above ground disposal

• Generally used for intermediate and high level waste
• Waste from a spent fuel pool is sealed (along with an inert gas) in a steel cylinder, which is placed in a concrete cylinder which acts as a radiation shield.
• Cheap , relatively easy to construct and monitor.
• Dry cask storage area

Deep borehole disposal

Disposing of high-level radioactive waste from nuclear reactors in extremely deep boreholes.  Placing  the waste as much as five kilometers beneath the surface of the   Earth. Waste is sealed in strong steel containers and lowered down the borehole, filling the bottom one or two kilometers of the hole. Borehole is then sealed with materials, including perhaps clay, cement, crushed rock backfill, and asphalt, to ensure a low-permeability

Advantages of Deep borehole disposal

A high-temperature scenario involves very young hot waste in the containers which releases enough heat to create a melt zone around the borehole. As the waste decays and cools, the melt zone re-solidifies, forming a solid       granite sarcophagus around the containers, entombing the waste forever. Environmental impact is small. Can be carried out near nuclear power-plant eliminating transportation risks.

Geological Repository

A deep geological repository is a nuclear waste repository excavated deep within a stable geologic environment (typically below 300 m).  Repositories include the radioactive waste, the containers enclosing the waste, other engineered barriers or seals around the containers, the tunnels housing the containers, and the geologic makeup of the surrounding area. Geological disposal can be safe, technologically feasible and environmentally sound

Management

Ongoing research

• Nuclear waste disposal is currently a matter of study. Study is undergoing in many countries related to geological disposal.
• European Countries like Finland , Sweden ,Germany ,Belgium have done considerable amount of research and constructing their repositories.
• Research was done in USA in Yucca mountains but constructing final repository there was cancelled.
• It was stated that cancellation took place due to political reasons ,not technical or safety reasons.
• Similarly in India research was done in Kolar gold mines in Mysore.

Geological constraints

Yucca mountain range

• Circulation of water
• Properties of Host Rock Depending on type.
• Erosion.
• Hazards like earthquake / volcanic eruption.

Water circulation

• Water circulation, as well as providing a path for soluble waste to escape to the surface, will increase the speed at which engineered barriers such as metal casing will degrade.
• Groundwater circulates in two distinct places within the bedrock, within the pores and within fractures.
• Crystalline rocks like granite ,basalt /tuff have very low permeability but are often highly fractured.
• In clays, such as those investigated in French and Belgian underground laboratories, the permeability is higher but fractures are much rarer.

Geo-hydrological study of area

Potential groundwater pathways defined by top soil, weathered rock, fracture networks, interflow porous layers should be identified.

Each rock type has different geomechanical property.

Moreover these property vary from site to site.

So in following slides we will look upon research done in different types of host rock.

Research in granitic rocks

In Finland research was done at onkalo . While in Sweden research was done at underground Äspö Hard Rock Laboratory.  Tunnels and other excavations in hard granitic rocks are stable over long period of time. But excavation results in stress release and opening of fractures.Permeability is very low unless water conducting joints or open fractures are present.

Research in Tuff

• Volcanic tuffs are also candidates for nuclear waste depositories, such as the Yucca Mountain site in the U.S.A.
• Welded tuffs can have low permeabilities, but the most attractive property of tuffs is their ability to trap some radionuclides through sorption.
• Yucca mountain range

Research in clay based rocks

In-situ underground experiments

In France Andra (French National Radioactive Waste Management Agency) is the public body responsible for the long-term management of all radioactive waste. Research was done at Meuse/Haute-Marne underground research laboratory which is in clay formation. Meuse/Haute-Marne is underground laboratory, located at a depth of 490 meters in the heart of a very stiff (indurated) clay formation (argillite). Over 10 years of research was done using:

• > 1,300 km of seismic profiles studied
• > 27 deep boreholes
• > 4.2 km of cored boreholes
• > 2.3 km of argillite cores

Properties of the rock formation

• the geological environment is stable; very low seismic risk.
• the clay layer is regular and homogeneous over a large surface area.It does not present any fault
• Argillite has excellent properties. It is a stiff (indurated) sedimentary rock with very low permeability
• Radioactive or nonradioactive elements dissolved in water move very slowly through this rock
• the rock can withstand mining excavation work.

Bentonite clay

• These layers are placed between host rock and waste to restrict groundwater flow and retard migration of radionuclides.
• Their swelling properties helps in sealing the fractures in host rock.
• Bentonite clay being used in sealing waste overpack

Indian context

India has extensive & varied experience in the operation of near surface disposal facilities (NSDFs) in widely different geo-hydrological and climatological conditions by BARC. There are seven NSDFs currently operational within the country. In India, the most promising formation is granitic rocks. A program for development of a geological repository for vitrified high level long lived wastes is being pursued actively, involving In-situ experiments, site selection, characterization and laboratory investigations

In-situ underground experiments

For assessment of the rock mass response to thermal load from disposed waste , an experiment of 8-years duration was carried out.

At a depth of 1000 m in an abandoned section of Kolar Gold mine.

Choosing a location

1. absence of seismic risks in the long term.
2. absence of significant water circulation inside the repository,
3. rock suitable to underground installations excavation.
4. confinement property for radioactive substances.
5. sufficient depth to keep the waste safe from potential aggressions.
6.  absence of nearby rare exploitable resources.

Conclusion

Based on above study these three regions in India can be used for disposal Western Rajasthan, Western Andhra Pradesh, Eastern Karnataka. Since this is very preliminary study , further research in these narrowed down regions can be carried out.

Further research required on:

• This sub-surface information by drilling and lithological studies.
• Geophysical surveys using Ground Penetration Radar (GPR).
• Water circulation study.
• Absence of exploitable natural resource.