[ARROW] Design Flaw
John-Thor Dahlburg in an article from the Los Angeles Times, Sept. 2, 3, 4, 1992 said that the GRAPHite moderated RBMK reactor which blew up in the spring of 1986 was so poorly designed that the "scram button" that was supposed to smother the reaction by dropping control rods into the atomic pile, actually accelerated the neutron storm inside.

Diagram-4 Effect of shutdown rods.
[Effect of shutdown rods]

Dahlburg notes that Friedrich Niehaus, a specialist with the International Atomic Energy Agency said, "It's like hitting the brake pedal on the truck and feeding gas to the engine instead." [33] He also writes that when asked to rate models of Soviet built reactors that were made for generating electricity, over 200 nuclear-energy specialists and physicists gave the units only a "C" rating for reliability and sturdiness and the Chernob yl-type RBMK reactors a grade of "D". Right now the No. 4 reactor is encased in a concrete sarcophagus, which the experts say has deteriorated badly and the reactor itself is still in danger.

"Indeed it is a very serious error in this reactor design to try to run with all the control rods out. The main reason is that some of these rods are used for emergency shutdown, and if they are all pulled out well above the core, it takes too long for them to fall back into the high power part of the reactor in an emergency, and the sh utdown is very slow." [34]

When comparing reactors, the USA reactors are pressure vessel reactors where an extra supply of ordinary water cools the fuel, whereas the RBMK reactor contains a solid called graphite. In USA reactors, the moderator heat is taken away in the steam to the boilers. In the RBMK design the heat from the graphite, which operates at 700 degrees Centigrade, flows from the graphite back through the pressure tubes and is taken away by the boiling water. "However, the problem with graphite at high temperatures is that if it is exposed to air, it will burn slowly. Thus it is very important to keep air away from graphite." [35]

Diagram-5 Comparison of containment of US and Soviet reactors.
[Comparison of containment of US and Soviet reactors]

It is important to note that in 1979 there was an accident at the USA Three Mile Island plants which resulted in a complete general survey of safety of reactors in all countries including the USSR. It was found that, "Neither the RBMK reactors nor the USA style pressure-vessel reactors had containment. But, the RBMK is a huge reactor - about 71 meters high and the Soviets felt that to put all this in containment would be difficult and costly." [36] And so, the Soviets put only the bottom pipes in containment because this was easier. When the Chernobyl Unit 4 reactor was shut down and emergency core cooling was initiated, it had only a partial containment.

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