Comprehensive analysis of the Chernobyl accident

Standard computational methods have been used to make a preliminary analysis of the neutronics and thermal-hydraulics of the first two phases of the Chernobyl accident (initial positive reactivity generation and first Doppler transient). The most important results are discussed. These include the fact that it is impossible to define only one value of the reactor void coefficient. In the central channels (20% of the reactor volume) the effective coefficient is three times higher than the average, while the value is practically zero in the peripheral channels (40% of the reactor volume). The difference is due not only to the statically larger neutron worth of the central zone, but also to the dynamic instability of the radial flux distribution. The void increase in the central regions induces an increase of power and of the void in these regions, thus increasing the reactivity in an unexpected way. A design error in the scram rods (too short a graphite follower) can generate an undesired positive reactivity insertion while the first 1.25 m of the rods is being inserted. This effect can introduce from 0.5 to 1.2$ reactivity. The Doppler effect is the only one introducing negative feedback during a transient. The current model for the calculation of the effective fuel temperature must be reconsidered. A more sophisticated approach in needed to assess the rate of thermal and mechanical energy delivered during the explosion. For a self-consistent evaluation of the accident it is necessary to use two dimensional (R-Z) multigroup kinetics and dynamics models developed in the last ten years for the analysis of reactivity accidents in fast reactors. It is important to reach a proper understanding of the various mechanisms that caused the Chernobyl accident and for this purpose an interdepartmental group in ENEA is working on a comprehensive analysis using classical computational models and codes for thermal reactors. A new tool is being implemented: the code NADYP-Water, a new version of the two dimensional space-time code NADYP. International co-operation on the subject is desirable. (author). 4 refs, 2 figs