Criticality safety analysis of the incident occurred at Paks NPP in 2003

On 11 April, 2003 a serious incident occurred in the service pit of Unit 2 of the Paks NPP, during which 30 fuel assemblies being in a chemical cleaning tank were damaged. In order to determine the most important reactor physics properties of the newly formed neutron-multiplying system, a detailed 3D computational model was developed in MCNP. We used the data from neutron flux and pulsed-source reactivity measurements to validate and improve our calculations. Since further spontaneous and deliberate rearrangement of the fuel was expected, the model was also used to analyze the possible new configurations. It turned out that system could not be considered to be inherently subcritical, albeit we managed to give a set of conditions under which the fuel could be safely removed from the vessel. We have proven the possibility of using the detector signals to indicate potentially dangerous processes. (authors)