Regular Article

A simplified analysis of the Chernobyl accident

¹ CEA/INSTN, Gif sur Yvette 91191, France
² Institut Franco-Chinois de l'Energie Nucléaire, Sun Yat-sen University, 519082 Zhuhai Campus, Guangdong Province, P.R. China

^* e-mail: Bertrand.mercier@cea.fr

Received: 13 July 2020
Received in final form: 3 December 2020
Accepted: 21 December 2020
Published online: 20 January 2021

Abstract

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