The safety approach used at CANDU nuclear power plants – which includes all Canadian plants – limits both the probability of an accident occurring and the potential effects of such an accident. This approach is called “defense in depth,” and it has five main components:
High-quality station equipment – All suppliers of components to CANDU nuclear power plants, such as manufacturers of pumps, valves, piping, and electrical systems, must meet stringent qualifications. Control components that are critical to the plant’s operation are duplicated – which means that, if one component malfunctions, another will take over and the safety of the station will not be jeopardized.
Operator training – Control-room operators at each power station are carefully screened and spend approximately eight years in training. They must be authorized by the Canadian Nuclear Safety Commission (CNSC), which sets the examinations and reviews the qualifications of the applicants. Part of the training and testing is conducted on simulators that replicate the control room and are capable of simulating normal and emergency operating conditions.
Fault detection and correction – Each Canadian nuclear power station is continuously monitored through testing and inspection of its components and safety systems. Constant and prompt detection ensures that nuclear power stations operate within limits prescribed by the CNSC and set out in their operating licences. The CNSC has inspectors onsite at these stations and can withdraw an operating licence at any time, if regulations or licence conditions are not met.
Special safety systems – All Canadian nuclear reactors are equipped with special safety systems that automatically shut the reactor down in the event of any major equipment malfunction, but maintain cooling of the fuel in the event of a failure of the reactor-cooling system. These systems are:
Containment systems – Each reactor at Canadian nuclear power plants sits in a containment system that consists of an airtight building with reinforced concrete walls of up to 1.8 metres thick. At stations with several reactors, such as at the Pickering, Bruce, and Darlington stations, each reactor building connects to a common vacuum building, which acts as containment. In the event of a release of radioactive steam into the reactor building, this steam would be vented to the vacuum building and prevented from escaping into the environment. Once in the vacuum building, the radioactive steam is condensed into liquid and contained. This safety feature is unique to CANDU design.