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Old 06-19-2019, 09:33 AM
Frankenstein Monster is offline
Join Date: Apr 2004
Location: Europe
Posts: 706
Originally Posted by jjakucyk View Post
Was steam being generated at the bottom of the reactor rather than at the top (how?), causing the positive void coefficient to become apparent here but not yet in the rest of the reactor? Is it because the xenon poisoning was concentrated in the center of the reactor (thus the slight decrease in flux shown in graph "a" at center as compared to 80% top or bottom), so the little bit of extra reactivity caused by the water displacement caused runaway reactivity due to the xenon being burned off?
Basically yes, that's what the aforementioned IAEA report says.

Originally Posted by IAEA report
This situation in extreme conditions can be highly
unstable, because small spatial redistributions of reactivity can cause large spatial
redistributions of the power. One manifestation of this decoupling of the core is that
just prior to the accident the chain reactions in the upper and lower halves of the reactor
were proceeding almost independently, a situation that was exacerbated by heavy
xenon poisoning in the intervening central region. When control and safety rods were
inserted from fully withdrawn positions under these circumstances, the positive
scram effect discussed earlier could cause the lower part of the core to become
supercritical and the neutron distribution to shift quickly downwards irrespective of
the distribution just prior to rod insertion. Under the conditions of the accident, the
shift in power distribution resulting from the positive scram could be substantial.
Of course the flux does not "equalize" when there's a positive reactivity feedback loop.