Can a similar event happen here?
Given our geologic location, we don’t face the same risks of earthquakes or tsunamis like those that hit the Fukushima Daiichi plant in March, 2011. Our stations are built in an area of very low seismic activity. That’s in sharp contrast to Japan, which sits on the Pacific Ring of Fire, where about 90% of the world’s earthquakes occur. And, unlike the Pacific Ocean, inland lakes like Lake Huron are highly unlikely to produce a tsunami or damaging water swell.
Even still, our stations are seismically qualified to national and international standards and have multiple back-up cooling and safety systems to ensure reactor control, cooling and radiological containment.
How are our reactors different from those in Japan?
In Boiling Water Reactors, like those at Fukushima, the heat produced by nuclear fission in the reactor core causes circulating water to boil, producing steam. The steam, which is radioactive, drives a turbine directly, after which it is cooled in a condenser (essentially a heat exchanger cooled by sea water, lake water, a large river or cooling tower) and converted back to liquid, which circulates back through the reactor. CANDUs don’t permit boiling of the primary loop water. This very hot water, which is also radioactive, circulates in a closed loop entirely within containment. It passes through steam generators (heat exchangers), which transfer heat to secondary loops to power steam turbines and associated electrical generators. The residual low-pressure steam is routed back through a condenser, where it becomes water again and is returned back to the steam generators.
The steam and water in CANDU’s secondary loops is not radioactive, so in the event that a heat release is required, the secondary (non-nuclear) side could be safely vented to the atmosphere and new plain water introduced once the water inventories are depleted.
What if the electrical power supply was lost?
In the unlikely event of a loss of all power, our plant’s emergency power supply (EPS) would power nuclear safety-related systems needed to control, cool and contain the fuel. The EPS is seismically and environmentally qualified and has sufficient fuel stores to operate unaided for a seven-day period. Additional fuel is kept on site as a further back-up and we could secure more fuel from offsite if necessary.
The EPS is similar to other standby generators but is remotely located from them to reduce chances of it being disabled by the same incident. Cables and control equipment involved in switching the EPS into service are routed through areas that are considered to be at lowest risk of damage.
In the event more unlikely event that EPS systems also become unavailable and all power is lost, our reactors would safely shut down and stabilize using CANDU’s convection cooling system design. Then, all units would be placed in guaranteed shutdown state until power was restored.
Bruce Power has also purchased five new fire trucks which can pump cold water into the nuclear reactor to cool the fuel, and nine new standby generators which can be deployed anywhere on site from their offsite storage unit, which is on high ground.