Three main radiological hazards are associated with cooling ponds:
(i) Loss of Shielding
A single typical irradiated fuel element produces dose rates of several grays per hour at 3 metres.
Loss of shielding can occur due to:
- Fuel handling errors --- raising fuel too near the pond surface
- Loss of pond water --- from structural damage or (more likely) unplanned pumping
Mitigation measures:
- Well-designed lifting tackle with mechanical interlocks
- Good operator training
- Installed gamma monitors (see instrumentation below)
- Careful plant design with locked valves to prevent accidental draining
(ii) Unplanned Criticality
Even after irradiation, the fuel retains over 90% of its original fissile content. If fuel elements were brought too close together, or if the spacing geometry were disrupted, a criticality excursion could occur.
Mitigation measures:
- Administrative controls on fuel movement
- Well-designed spacing of fuel elements in storage racks
- Neutron poisons (e.g. boron in the water, boron/cadmium in the racks)
- Seismic-resistant store design (to prevent geometry changes in an earthquake)
(iii) Release of Gaseous Fission Products
If fuel cladding is breached, gaseous fission products (primarily noble gases such as Kr and Xe) would be released into the pond water and then rapidly escape from the water surface into the air above the pond.
Mitigation measures:
- Air extraction system above the pond to capture and filter radioactive gases
- Continuous air monitoring
- Pond water monitoring for beta activity (early warning of cladding breach)