Discuss the safety considerations necessary for the dry storage of irradiated fuel.
Solution
Dry storage of irradiated fuel requires careful consideration of numerous safety factors. The following key considerations must be addressed in the design and operation of a dry storage facility:
1. Radiation Shielding (2 marks)
The irradiated fuel emits intense gamma radiation and also neutron radiation from spontaneous fission and (, n) reactions. The storage facility must provide sufficient shielding to reduce dose rates to acceptable levels for workers and the public. Concrete is the most commonly used shielding material for the outer structure (silo), while the fuel cask itself (cast iron in a stainless steel jacket) provides primary shielding. Special attention must be paid to shine paths --- gaps, penetrations, or joints through which radiation can stream without being attenuated. Scattering of gamma radiation must be considered when fuel is being moved or inspected.
2. Decay Heat Removal (1.5 marks)
The fission product decay heat must be continuously removed to prevent fuel overheating, which could lead to cladding failure and release of radioactivity. Passive air cooling is the preferred method because it requires no active systems (pumps, fans, power supplies) and is therefore inherently reliable. The fuel cask is fitted with cooling fins to improve heat transfer. The temperature of the stored fuel must be monitored.
3. Criticality Safety (1.5 marks)
The fuel still contains a significant proportion of fissile material. The storage configuration must prevent any criticality event by:
- Careful geometry control (spacing of fuel elements)
- Exclusion of water and other moderating materials (water flooding from fire-fighting operations must be considered in the design)
- Appropriate sealing of storage units
The design must account for any credible redistribution of fuel within the store.
4. Containment (1 mark)
The fuel must be securely contained to prevent any release of fission products to the environment. The containment boundary includes the fuel cladding, the cask, and the silo structure.
5. Monitoring (1 mark)
The facility must be instrumented with:
- Dose rate monitors and alarms around the store perimeter
- Airborne contamination monitors --- the air removed from the storage area must be monitored (and filtered) before discharge to the environment
- Temperature monitoring of the stored fuel
6. Protection Against External Hazards (1 mark)
The concrete silo must be designed to survive:
- Mechanical and seismic shocks (earthquake)
- Fire and explosion --- explosive gases such as hydrogen must be excluded from the store
- Corrosive chemicals
7. Security and Record-Keeping (1 mark)
The fuel must be protected from damage and theft (safeguards). Batch control and record-keeping must be maintained. The fuel must remain accessible for later removal, transport, or inspection.