Lesson 5 ◆ 5.6 Vitrification of High Level Waste

  1. Feed preparation: The highly active liquid (HAL) is fully characterised by chemical and radiochemical analysis. Reagents are added to prevent caking.

  2. Calcination: The HAL is fed into a sloping, rotating kiln in an electrically heated furnace and heated to approximately 850 degrees C in a reducing atmosphere. The liquid is progressively evaporated and de-nitrated. The nitrates are converted to oxides, producing a finely divided powder.

  3. Glass making: The HA calcine powder is fused with borosilicate glass granules (0.5—2 mm diameter) in a mix of roughly 75% glass and 25% waste. The mixture is melted in a directly heated induction furnace at 1,100 degrees C for approximately 8 hours.

  4. Pouring: The molten glass/waste mixture is poured into stainless steel product containers (42 cm diameter, 1.3 m tall). Each container accommodates two pours, equivalent to approximately 150 litres or 350 kg of material. This corresponds to the total HLW from two tonnes of LWR fuel.

  5. Sealing: After cooling and solidification, the container lids are welded on and the containers are decontaminated by high-pressure water jets.

  6. Off-gas treatment: Gases from the calciner and furnace (water vapour, entrained solids, nitrogen oxides, volatilised fission products such as Ru and Cs) are treated by dust scrubbing, condensation, acid scrubbing, electrostatic precipitation, and HEPA filtration, removing 99.99% of radioactivity.