Pool storage is the most common method for on-site storage of irradiated fuel.
- Pool depth: 10—15 metres
- Fuel stack height: 4—5 metres
- Minimum water depth above fuel: At least 2.5 metres (for adequate shielding)
- Fuel support structure: Rigid aluminium or stainless steel lattice/rack
- Water chemistry: Demineralised; pH controlled; biocide added to prevent algae
- Criticality control: Neutron absorbers (boron, cadmium) in storage racks; boronated water
How it works:
The water serves three functions simultaneously:
- Shielding --- absorbs gamma and neutron radiation
- Cooling --- removes decay heat by convection (with heat exchangers)
- Visibility --- water is transparent, allowing visual identification and manoeuvring of fuel assemblies
Pond water is continuously circulated, monitored, and purified (by filtration and ion exchange) because it can become contaminated with Co (from neutron activation of steel) and Cs (from failed fuel cladding). A trace of NaOH may be added to inhibit corrosion, particularly for Magnox fuel whose cladding is corrodible.
The ponds normally include an import/export facility allowing fuel to be transferred into or out of transport flasks while underwater, so the fuel is never unshielded.