We show with simplified numerical models, that for the kind of RBMK operated in Chernobyl:
The core was unstable due to its large size and to its weak power counter-reaction coefficient, so that the power of the reactor was not easy to control even with an automatic system.
Xenon oscillations could easily be activated.
When there was xenon poisoning in the upper half of the core, the safety rods were designed in such a way that, at least initially, they were increasing (and not decreasing) the core reactivity.
This reactivity increase has been sufficient to lead to a very high pressure increase in a significant amount of liquid water in the fuel channels thus inducing a strong propagating shock wave leading to a failure of half the pressure tubes at their junction with the drum separators.
The depressurization phase (flash evaporation) following this failure has produced, after one second, a significant decrease of the water density in half the pressure tubes and then a strong reactivity accident due to the positive void effect reactivity coefficient.
We evaluate the fission energy released by the accident
Simulation of Anisotropic Deformation Behavior of Pressure Tubes During High Temperature Loading Conditions
