Describe the instrumentation that should be installed at a pond storage facility for irradiated fuel.
Solution
Three types of instrumentation are essential at a cooling pond facility:
1. Installed Gamma Monitors (2 marks)
At least three gamma-sensitive dose rate monitors must be installed around the pond. These must be carefully positioned so that they cannot become inadvertently shielded from the pond surface (e.g. by equipment or structural features). A voting logic system (e.g. 2-out-of-3) is used to reduce the likelihood of spurious (false) alarms while maintaining safety.
The alarm system should have two trigger levels:
- Local alarm at approximately 100 mGy/h --- alerts operators to investigate
- Evacuation alarm at approximately 100 mSv/h --- triggers immediate evacuation of all personnel
2. Pond Water Beta Counter (1 mark)
A beta activity monitor immersed in or sampling the pond water provides early warning of fission product release from damaged fuel. An increase in beta activity in the water indicates that the cladding of one or more fuel elements has been breached, allowing fission products to leach into the water.
3. Air Monitor (1 mark)
An air monitor is installed above the pond to detect airborne contamination (gaseous fission products such as Kr and Xe, and volatile species such as I) that may escape from the pond water surface, particularly if fuel cladding has been damaged. This monitor provides warning to operators and supports decisions about ventilation and evacuation.