Probability of double nuclear bursts in the Chernobyl reactivity accident

Discussions are made for potentially existing double nuclear bursts in a reactivity accident in reactors having positive void reactivity coefficients, with a typical example being taken from the Chernobyl accident case. A brief discussion is first made on the physics backgrounds of reactivity coefficient consisting of Doppler, void, and moderator temperature effects. Three dimensional spatial kinetic analyses are then made to study the case of the Chernobyl reactivity accident, where the emphasis is on the effect of uniform and non-uniform void generation in the core during the accident. The result with uniform void generation indicated that the reactor could have become quite unstable to induce an uncontrolled power surge. The initial local void, depending on its spatial area, would also result in an uncontrolled power surge with an initial local void expanding into the global core. In this case, some parts of the core quickly attained the fuel melting temperature, while the rest of the core remained well below it. Under such circumstances, there would be a good reasoning for the local initial core damage, leaving other part of the core uninfected and still ready for the second power surge. The speculating discussions have been tried to explain the background situations for the potential double nuclear bursts, and also for the two witnessed explosions given in the accident report(1). (author)