The fuel (whether from stripped Magnox or leached oxide) is dissolved in concentrated nitric acid. The uranium is oxidised to the 6+ valence state and nitrogen oxide gases are released:
Insoluble fission products (noble metals such as molybdenum, technetium, rhodium, and palladium, plus fine particles of stainless steel or Zircaloy) are removed by centrifuging.
Dissolver off-gases must be treated. Volatile fission products released during dissolution include:
- Tritium (H-3)
- Carbon-14 (as CO and CO₂)
- Krypton-85
- Iodine-129
- Xenon
These gases are scrubbed with nitric acid (to remove NOₓ), sparged with air (to remove iodine), and scrubbed with caustic soda (to remove carbon-14). The nitrogen oxides can be treated and recycled as nitric acid.
Key Point: Care must be taken to ensure criticality safety during dissolution, especially when using enriched fuel. Favourable geometry vessels and gadolinium neutron poison are used as criticality controls.