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Nuclear reactors



A computer Scientist at Stanford, John McCarthy has a good
nuclear energy page at his website:

http://www-formal.stanford.edu/jmc/progress/nuclear-faq.html

He wrote:
... The Chernobyl accident depended on the specific
characteristics of the RBMK reactors, of which the Soviets
built 16 before switching to designs more like those used in
the rest of the world. (It may be that the North Korean reactors
are similar). The relevant features of RBMK reactors include:

"positive void co-efficient of reactivity". This means that if
the reactor gets too hot and some of the water turns to steam,
the rate of the nuclear reaction increases. In most other power
reactors, the void coefficient is negative. If some water boils
the reactor tends to stop. ...

1) Can somebody elaborate on the concept of "void coefficient
of reactivity"? Comparing it to the negative-versus-positive
feedback in electronics is only an analogy. What makes one
boiled water reactor more stable than another? How can this
be answered without becoming too technical?

2) I remember reading (a year or two ago?) that boiled water
reactors available today are designed to be safer than those
built twenty years ago. What new features do they have?
Ludwik Kowalski