In the CANDU 6 design, there are special safety systems incorporated into every plant that perform no active function during the normal operation of the plant—they are there only for safety. The reactor cannot be operated without all of the special safety systems being available. There are four special safety systems in a CANDU plant:
- Shutdown System Number 1 (SDS1)
- Shutdown System Number 2 (SDS2)
- Emergency Core Cooling (ECC) System
- Containment Building
SDS1 and SDS2
SDS1 and SDS2 are examples of the defence-in-depth protection philosophy. In SDS1, solid mechanical shutdown rods drop in from the top of the reactor core and stop the chain reaction by absorbing neutrons.
SDS2 injects a concentrated solution of neutron-absorbing liquid directly into the low-pressure moderator, which also stops the chain reaction.
Both SDS1 and SDS2 are independent, fully-capable, passive shutdown systems. They are designed to be "fail-safe". This means that if a component of the shutdown system fails, the rest of the system is either capable of performing its function, or is automatically activated to shut down the reactor.
Either shutdown system reduces the heat being generated from 100% to 10% in just two seconds. Residual heat can be taken away by either normal or emergency heat removal systems.
Emergency Core Cooling (ECC) System
The ECC system is an example of the mitigation philosophy. It is designed to continue or re-establish fuel cooling in the unlikely event of a loss-of-coolant accident. The system is available to supply cooling water, if required, during the loss of coolant accident, at any time during reactor operation. When called upon, the system injects water for fuel cooling into the heat transport system.
The ECC system incorporates three stages: high pressure injection, medium pressure injection, and low pressure recirculation. The high-pressure injection stage uses pressurized tanks to inject water into the heat transport system. The medium pressure stage supplies water by pumping it from a storage tank. The long-term, recirculation stage recovers water that has been collected in the basement of the reactor building and pumps it back into the heat transport system through heat exchangers.
Containment Building
This is an example of the accommodation philosophy—a strong, thick, seismically-qualified, building surrounds the reactor. The building acts as a sealed vessel that contains the radioactive material in the unlikely event of a serious accident.