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.
CANDU reactors 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 reactors’ secondary loops is not radioactive, so in the event that a heat release is required, the secondary side could be safely vented to the atmosphere and new plain water introduced once the water inventories are depleted.