Radioactive Waste Management and Disposal

Introduction• Sources of radioactive wastes

– Different stages of nuclear fuel cycle• Mining, fuel fabrication, operation, spent fuel handling..

– Decommissioning of NPPs– Medical facilities

• X-ray labs, radiological medicines..– Industrial sources

• Radioisotopes

• Underlying objective that governs the management of such waste: protection of man and environment, now as well as in the future.

• Focus: Radioactive waste management and disposal practices in India

Introduction to RadioactivityRadioactivity: process by which a nucleus of an unstable atom loses energy by emitting particles or ionizing radiation

Introduction to Radioactivity

Level of hazard of radioactive waste diminishes with time.

Half lives may be of the order of few seconds to millions of years.

Unit of radioactivity: Becquerel (Bq)

Why radioactivity is of concern?

Radioactive Waste Management Policy in India• Discharge as low as reasonably achievable (technically, economically & socially)• Low and intermediate level solid/solidified waste in near surface shallow land

repository.• High-level and alpha contaminated liquid waste immobilized in suitable matrix &

stored in an interim storage facility with cooling and surveillance. Thereafter, these solidified waste products will be emplaced in a suitably engineered deep geological repository.

• Waste not qualifying for near surface disposal is provided suitable interim storage pending its disposal in a deep geological repository.

• Spent radiation sources are either returned to the original supplier or handed over to a radioactive waste management agency identified by the regulatory body.

• Co-location of near surface disposal facility with the nuclear installations.• The regulatory body determines the period for which active control of the shallow

land repository should be maintained by the waste management agency (~100yrs). Thereafter, the passive control will be passed on to the Central Government (~300yrs).

Classification of Radioactive wastesClassification of liquid / gas waste based on the activity level.

Category Activity (Bq/L)

Exempt neglegible

Low level 37–3.7×106 Bq/L

High level >3.7×1011 Bq/L

Solid wastes classified as:- combustible-non-combustible-compressible-non-compressible

Further classification based on activity level as low, high etc.

1.K. Raj, K.K. Prasad, N.K. Bansal, “Radioactive waste management practices in India,” Nuclear Engineering and Design 236 (2006) pp: 914–930.

Basic steps in radioactive waste mgmt.

Radioactive waste mgmt facilities

• Efficient gas cleaning techniques– Wet scrubbers like venturi, dust, packed bed– Cyclone separators– Low pressure drop demisters– Chillers, HEPA filters– Charcoal filters….

• Choice depends on activity, partculate density etc…

• Filter beds treated as solid radioactive waste

Gaseous waste management

1.P.K. Dey, N.K. Bansal, “Spent fuel reprocessing: A vital link in Indian nuclear power program,” Nuclear Engineering and Design 236 (2006) pp:723–729.

• Chemical treatment: Liquid wastes with low levels of activity treated by co-precipitation using chemicals like barium chloride. Resultant sludge is concentrated by decantation, filtration and centrifugation. Resulting solids are immobilized in cement matrix before disposal.

Low and intermediate liquid waste

• Ion exchange: Synthetic organic ion exchange resins such as synthetic zeolites, the clay mineral vermiculite and ammonium molybdophosphate. Selective ion exchange is used for the management of waste streams in Tarapur. A unique transportable shielded ion exchange facility is also operational at Trombay.

• As a result of this treatment, the intermediate level waste is split into two streams, viz., a small volume of high-level waste and a large volume of low-level waste, which is treated and discharged to the environment.

Low and intermediate liquid waste

• Evaporation: Widely used for concentrating the liquid waste as it gives very high volume reduction factor as well as high decontamination factor.

• Both steam and solar evaporation methods are employed.

• Solar evaporation(zero release operation), is preferred for larger volumes of waste with low activity at sites such as Rajasthan.

Low and intermediate liquid waste

• Membrane process: Membrane based processes like reverse osmosis and ultra filtration are used in combination with other treatment methods like chemical treatment or ion-exchange process to further improve the decontamination. A reverse osmosis plant of capacity 100m3/day is in operation for treatment of low-level waste at Trombay.

Low and intermediate liquid waste

• Cementation: Cement and cement composites are used for immobilization of low-level radioactive concentrates.

• Cementation process also used for in situ immobilization of intermediate level waste in specific cases. In situ cementation results in large waste processing rate with extremely low exposure to the radiation workers.

• At Trombay, cone mixers located in hot cells with easy remotized operational amenability are used for conditioning of intermediate level waste in cement matrix.

Conditioning

• Polymerization: Polyester styrene used for immobilization of ILW concentrates and spent ion exchange resin from nuclear power stations and other facilities, and for in situ solidification of low-heat generating liquid waste from reprocessing plant.

• Radioactive spent resins polymerization:– Resins are hydro pneumatically transferred to resin storage tank. – Excess water is removed by vacuum de-watering system. – Mixing assembly is mounted on this product drum. – Requisite amount of polyester styrene polymer is premixed with

optimized concentration of accelerator (dimethyl aniline) and catalyst (benzoyl peroxide).

– This polymer is then gradually poured into a product drum with constant stirring.

• These resin fixation facilities exist at nuclear power plant sites in Narora (Uttar Pradesh), Kakrapar (Gujarat) and Tarapur (Maharashtra).

Conditioning

• Matrix development: Borosilicate glass matrix is adopted for vitrification of HLW.

• Conditioned products are evaluated using advanced analytical instruments like scanning electron microscope, electron microprobe analyzer, X-ray diffractometer, inductively coupled plasma spectrometer and thermal analysis system.

Mgmt of high level liquid waste

• Vitrification : Consists of metering of preconcentrated waste and glass forming additives in the form of slurry into the process vessel located in a multi-zone furnace.

• Calcinated mass is fused into glass at about 950 ◦C and is soaked at 950–1000 ◦C for 8 h to achieve homogenization.

• The molten mass is then drained into stainless steel canisters and allowed to cool slowly in an insulated assembly. This is then welded remotely.

Mgmt of high level liquid waste

• Vitrified waste canisters are further enclosed in secondary stainless steel containers called overpacks.

• India has vitrification plants at Tarapur, Trombay and Kalpakkam.

• Wastes classified and categorized• Low level wastes are burned in special incinerators• Hydraulically operated baling press is used to compress

and pelletize low active non-combustible waste.• Pelletized waste stored in near surface bays.

Mgmt of solid waste

• Storage of radioactive waste containing mainly shortlived radionuclides enables their decay and subsequent release within authorized limits.

• Storage of high-level radioactive waste, like spent fuel, is done to dissipate the decay heat prior to its reprocessing or disposal.

Storage

• Near surface disposal facilities (NSDF): Seven in India– stone-lined earth trenches (SLT)– reinforced concrete trenches (RCT)– tile holes (TH)

Storage

Stone-lined earth trenches: Shallow excavations in soil, 1–4m deep, provided with stone lining, employed for very low active waste. On completion of the disposal operation, trenches are backfilled and closed by providing a soil cover of nearly one meter thick. Vermiculite, bentonite and native soil having good sorption properties are used as backfill materials.

• Reinforced concrete trenches (RCT):Storage

4.8m deep, 2.5m wide and 15m long. Outer containment wall thickness varies from 350mm at the top to 750mm at the bottom. Each zone of trenches is serviced by either a gantry or a mobile crane. Adequate waterproofing is provided. On completion of the filling operation, these trenches are closed by pre-cast concrete slabs, which also provide necessary shielding. Adequate sealing and water proofing is provided subsequent to closure of the battery.

• Tile Holes (TH):Storage

Waste packages in 200 L standard packing containing conditioned waste with more than 4000 Bq/g of alpha activity are retrievably stored in tile holes. These are circular vaults, nearly 4m below ground level having an average inside diameter of 710 mm. These are made of 6mm thick carbon steel shell with 25mm thick concrete lining on both sides and provided with adequate waterproofing.

• Interim storage of vitrified wastes:Storage

• High-level vitrified wastes characterized by decay heat.• Storage facility at Tarapur, with capacity for storing nearly 1700

overpacks with an inventory of nearly 80,000,000 TBq of radioactivity.

• Underground outer vault of dimensions 74.5m×31.5m×5.2m and two inner vaults (thermal) of sizes 34.0m×25.2m×2.5m each.

• The over packs are suspended vertically from the top slab. A concrete roof of 1.2m thickness provides adequate radiation shielding.

Disposal• Disposal in deep geological repository envisages emplacement of

vitrified wastes at depths of about 500–600m in appropriate host rocks, e.g., granite, granite gneisses, charnockite, basalt and other geological set ups.

• The Indian programme on geological repository commenced in the early eighties with underground experiments in an abandoned section of a gold mine in Kolar at a depth of 1000 m.

• A layout for an underground research laboratory based on the site-specific investigations. The layout, extends over an area of about 4 km2 and considers emplacement of around 10,000 high-level waste overpacks on a pit mode.

Summary

• Among large scale power generation facilities, nuclear power plants are the only ones which take full responsibility of the wastes produced by them.

• Waste management facilities at various nuclear installation sites are operating safely and successfully for more than four decades.

• By suitable treatment and conditioning of waste it is ensured that prime objective of safety of environment is fully achieved.

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