Waste treatment practices7-10

NPPs

Off-gas waste from NPPs contains mainly short-lived noble gas nuclides. Off-gas treatment is aimed at decay of short-lived nuclides and the removal of aerosol radionuclides. The off-gas treatment system consists of a hydro­gen recombiner unit, an activated charcoal retention unit and a filtration unit. The off-gas from the ventilation system is passed through the charcoal filter and the high efficiency particulates air (HEPA) filter to eliminate iodine and aerosol, respectively. The treated off-gas is discharged through a stack after verification that it is under the regulatory limit.

Treatment of the liquid waste from NPPs aims where possible at recycling in the plant system, removing the radioactivity in controlled liquid dis­charges and eliminating process effluents. The liquid waste treatment system in a BWR is composed of the low conductivity subsystem, the high conduc­tivity subsystem, the detergent waste subsystem, and the solidification sub­system. The liquid waste from processes in BWR is divided into the low conductivity effluent, which has relatively high purification, and the high conductivity effluent, which has relatively low purification. The low conductivity subsystem collects and processes liquid wastes typically from the equipment drains in the primary cooling system. This waste is filtered through a hollow fibre membrane for removal of insoluble material and demineralized by mixed ion exchange resin for removal of soluble chemi­cals, and then returned to condensate storage prior to further use as reactor coolant.

The high conductivity subsystem collects and processes liquid wastes from floor drains and effluents from regeneration of the resins. These wastes are concentrated by evaporation, and fed to the solidification subsystem. The distillate is demineralized on the mixed ion-exchange resin, and then returned to condensate storage or discharged to the ocean after verification under regulatory limit. The detergent subsystem collects and processes detergent waste from personnel hand-wash and laundry operations. These wastes are filtered, and then discharged to the ocean.

The solidification subsystem collects concentrated waste in a dedicated tank. This waste is mixed with cement or bitumen, and solidified in 200 L drums. The latest solidification subsystem adopts the dry-pelletizing method in which the concentrated waste is dried with a film evaporator, and dried powder mixed with binder is pelletized by a granulator. This method gives a high waste reduction volume compared to cementation/bituminization.

The liquid waste treatment system for PWR is similar to that for BWRs. PWR employs the recoverable effluent subsystem corresponding to the low conductivity subsystem of BWR. The recoverable effluent containing boric acid from the primary coolant system is demineralized, and then treated by evaporation to separate water and boric acid solutions for further use. Other subsystems are similar to those in BWRs.

Treatment of the solid waste from NPPs is aimed at stabilization and volume reduction for storage and conditioning prior to disposal. The solid waste treatment system is constructed typically of the pre-treatment sub­system, the incineration subsystem for combustible material, the compac­tion subsystem for incombustibles and the conditioning subsystem. The pre-treatment subsystem is composed usually of cutting and segregation. Large wastes are cut into small pieces appropriate for compaction/packing. Wastes are sorted into combustible, incombustible, compressible and incom­pressible wastes. Combustible wastes are burned typically in an excess air — type incinerator, and the incinerated ash is placed in a 200 L drum for storage. The compaction subsystem usually makes waste compacts of com­pressible and incombustible wastes with a compressing force between 50 kN and 3 MN. For higher reduction ratios, the super-compactor or the melting system is adopted in some NPPs. Waste compacts and incompressible wastes are placed in 200 L drums, and then filled with mortar in the conditioning subsystem for disposal. Figure 23.1 shows a typical treatment flow for BWR wastes.

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23.1 Typical waste treatment flow for BWR waste.

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