The inadvertent withdrawal of the regulating control rod group during full-power operation leads to an increase
in the reactor power and thus to an increase in the primary circuit temperature and pressure. The fuel assemblies with the
highest power peaking factors reach the state of boiling crisis and may be damaged. The present analysis of the processes
in the containment is performed using the COCOSYS computer code. The calculation is part of a complex analysis of the
accident that includes calculations with three other computer codes – determination of the isotopic inventory of the core at
the end of the fuel campaign, evaluation of the number of damaged fuel assemblies and the mass and energy release into the
containment, analyses of the radiological consequences of the accident. The analysis with the COCOSYS code demonstrated
that a significant amount of the radioactive products released from the primary circuit are retained in the containment and
only a small part of them reaches the environment through the leakages. It was confirmed that the acceptance criteria for
the containment pressure and temperature are met.
Keywords: ATWS, COCOSYS, containment phenomena, fission products behaviour.