"Canadian nuclear safety regulators say they have underestimated the seriousness of a design feature at the country's electricity-producing reactors that would cause them to experience dangerous power pulses during a major accident.
If reactors are not shut down quickly, their ability to keep radioactivity from escaping would be put to the test, according to an internal commission document.
The document says Canada's seven nuclear stations, which all use Candu technology, have a feature known as 'positive reactivity feedback,' in which their atomic chain reactions automatically speed up if the water pumped into the reactors to cool them leaks, one of the worst accidents possible at a nuclear station. If reactors aren't immediately shut down during this type of incident, positive reactivity leads to a quick snowballing in the pace of nuclear reactions, which in turn could cause potentially damaging overheating.
The fear is that with a large loss of coolant, such overheating could put the nuclear facilities' containment features -- the concrete domes and other protective mechanisms around reactors that are the last-ditch defences to stop the spread of radioactivity into the environment -- to a dangerous test."