The nuclear electricity plants in Japan are "boiling water" reactors, where the enriched uranium bundles are cooled by water (which boils to produce high pressure steam that drives the turbines), and the nuclear reaction is controlled by special rods that are inserted into the fuel stack to control or shut down the reaction when the reactors are shut down.
Unfortunately, if the "moderator" rods are not fully inserted, the nuclear reaction can continue, and regardless of that, the enriched uranium has high residual heat, and a "flywheel" effect after shut down that requires continued need for cooling water.
In the Japan incident, the nuclear reactors were shut down when the earthquake occurred, and the plants survived the earthquake in good shape, but the rush of water from the tsunami took out the external power supply to the generators that supplied electricity to the cooling pumps. (Pumps are normally run by electricity generated by the reactor, but with the reactors down, outside power was brought on line.)
Battery power was the final backup, but these soon ran down and could not be recharged. Thus the cooling was still an issue, resulting in the problems.
In Canada, the CANDU reactors are run using natural uranium, at a much lower concentration than enriched uranium. In addition, CANDU reactors are "moderated" (read: made to operate) with the use of heavy water (produced at Tiverton, ON). The heavy water slows down the neutrons to a speed that allows the nuclear reaction to proceed. That heavy water then goes through heat exchangers that transfer the heat to natural water, that produces steam to generate electricity.
If, in the CANDU case, the heavy water is drained from the reactor, the reaction simply quits (ie: the reactor ceases to run). While there is a slight flywheel effect, the remaining fuel is easy to cool down as it runs at a much lower temperature than the Japanese "boiling water reactors".
In summary, the Japanese reactors use a much higher concentrated uranium (and Plutonium in some cases), surrounded by very little water, while CANDU reactors use far less reactive uranium (ie: producing lower temperatures) surrounded by much more water.
Again - when coolant is lost (heavy water) in a CANDU system, the unit simply quits producing heat. In addition, CANDU reactors have a large "dousing tank" that can dump water (ie: not pump) huge amounts of water on the reactor vessel to cool it in an emergency. The Japanese units, i believe, don't have that feature,
The fact is, in a crisis situation such as occurred in Japan, a CANDU reactor is a far safer option than the BWR (boiling water reactor) that is used in many parts of the world.
We just don't have the same risks associated with the Canadian (CANDU) systems.
Hope this helps to clarify the confusing questions for those who aren't aware of the technical differences.