The separation of coolant and moderator has created a positive void coefficient
of reactivity — an undesirable characteristic from a reactor safety point of
view, unique to pressure-tube reactor designs such as the CANDU and the
RBMK, the Russian design used at Chernobyl.
A positive void coefficient of reactivity means that the nuclear chain
reaction speeds up whenever there is a loss of coolant accident. This is
undesirable because under adverse circumstances, a loss of coolant accident
may be compounded by a loss of regulation as well (a power surge).
When the coolant is lost in a CANDU reactor, the moderator remains
intact, so the nuclear reaction increases because there is an increased
availability of thermal neutrons when steam forms in the primary cooling
circuit.
So, under accident conditions, the heat increases in the nuclear core
creating more power, not less. Thus a positive void coefficient of reactivity
is a much more dangerous design, like having a car engine accelerate at the
same time one is trying to apply the breaks.
-excerpt from ANALYSIS OF THE RELICENSING APPLICATION FOR
PICKERING NUCLEAR GENERATING STATION prepared by Fairewinds Associates Ltd. for Durham Nuclear Awareness.