Andrew,Ok. Overstated a bit. A meltdown is possible in a Candu if the core loses light water coolant as well (ie. if rods are removed from the heavy water and not placed in light water). Although a Candu is designed so that this will not happen, in a catastrophic situation it conceivably could happen. The Candu has an emergency gravity system that will ensure the core is immersed in light water and this will ensure that fission ends without loss of cooling. Since Candus are built adjacent to some body of water, there is no lack of available coolant.
Since in a light water reactor you cannot immediately shut down the reactor by immersing the core in light water (it already is), you need to have some means of absorbing neutrons while in the water (such as control rods) or immerse it in some other coolant that is not a good moderator. If your control rods aren't working you have a big problem. In the Candu you just pour water on it.
Afraid not. Just having the core immersed is not sufficient. You need a method
of getting the heat out.
Actually, the CANDU is NOT safer than a US reactor; if fact a CANDU does NOT
meet the U.S. Nuclear Regulatory Commission standards to be licensed in the USA.
The CANDU NEEDS the extra shutdown mechanism of a D2O dump.
In a US reactor, the light water coolant temperature coefficient is negative. That is, if
the reactor coolant gets hot - it is a negative reactivity insertion.
A CANDU is precisely the opposite - if you lose coolant flow to the CANDU, and the
light water coolant temperature increases, and thus the light water gets less dense;
that is a POSITIVE reactivity insertion in a CANDU because all the moderation that
is needed is provided by the D2O.
U.S. reactors are "under-moderated"; which provides an inherent safety mechanism.
CANDUs are "over-moderated" - thus the D2O dump.
This is one of the reasons why the CANDU fails to meet U.S. safety requirements.
I wouldn't worry too much about losing control rods; they drop by gravity. All that's
required is to cut current to the electromagnets that hold them up and they drop.
Dr. Gregory Greenman