As CANDUs use heavy water for coolant, they share with LWRs a negative coolant/moderator temperature coefficient. As the coolant heats up or boils, it reduces the power production. This is conservatively safe.
Chernobyl, on the other hand, used carbon for moderation and regular water for coolant. Since carbon is a better moderator, the coolant (water) acted as a poison. Ergo, lose cooling and the power goes UP! Bad design.
If CANDUs have a safety advantage it is in the lower reactivity of the natural uranium fuel and the lower burnup (fission product inventory). But both of these factors work against the economics of power production.
As a result, there is no significant advantage to either design in terms of public safety.
I didn’t realize modern LWR’s lost power production as the water boiled away - I knew that was the case with HWR’s because the heavy water is required for the reaction to occur at all (No reaction using LW in a CANDU design).
For boiling water reactors (BWRs), that’s exactly how power is controlled. Since boiling happens within the core, the more power, the more boiling and the less water molecules and so the less moderation. Without moderation, the power level decreases.
To control power we just cut the rate at which we flush the steam bubbles out of the core. That allows more “void fraction” and the power declines. To increase power, we increase the water recirculation rate through the core, flushing the bubbles out and reducing the void fraction, hence increasing the moderation and upping the power.
A pretty sweet system, I think.
In a loss of coolant accident (LOCA), water flashes to steam within the core and so with less moderator (ie water coolant), the chain reaction stops. Control rods go in in any case so that when the core is reflooded with cold water (and boric acid for PWRs), the reactor stays subcritical.
Sorry for the lectures but this stuff is my bread-and-butter!
Oh no it’s all cool - I knew about systems like thatbut I didn’t realize that current Nuclear reactors were using them - I thought it was all still based on similar technology to the Long Island and Cherynobyl reactors in the Light Water realm (just with better safeties in place)