Comprehensive Cfd Analyses concerning the Serious Incident Occurred in the Paks

On 10-11 April, 2003, a serious incident occurred in a special fuel assembly cleaning tank, which was installed into the service shaft of the 2 unit of the Paks NPP of Hungary. As a consequence of the incident, most of the 30 fuel assemblies put into the cleaning tank have seriously damaged. At the Institute of Nuclear Techniques, Budapest University of Technology and Economics, several CFD investigations were performed concerning the course of the incident, the post incidental conditions and the recovery work. The main reason of the incident can be originated from the defective design of the cleaning tank which resulted in the insufficient cooling of the system. First, the CFD calculations, with which the thermal-hydraulic deficiencies of the design were revealed, are presented in this paper. This investigation clearly showed how as strong temperature stratification could develop inside the cleaning tank that it was able to block the coolant flow through the fuel assemblies. After the blocking of the flow, the coolant started to boil, and the assemblies became uncovered. The temperature of the surfaces of the fuel assemblies rose above 1000 oC. With the aid of the radiative heat transfer model of the CFX-5.6 code, the surface temperatures and also the heat transfer between the shrouds of the assemblies and the inner surface of the cleaning tank mantle were analyzed. When the cleaning instrument got opened the fuel assemblies suffered a serious thermal shock and the assemblies highly damaged. For better understanding of the post-incident state inside the cleaning vessel and for serving as a basis for the safety analyses of the cooling systems, other detailed 3D thermal-hydraulic calculations were performed. These calculations also serve as a basis for the recovery work. The recovery work is planned to be started in the close future. During this work, the service shaft will be operated in a low-coolant-level operational mode. The operators of the damaged fuel removing equipments will work standing on a platform which will be placed into the service shaft just above the surface of the coolant. Therefore protecting the workers against unnecessary personal doses is a very important task. Therefore, the vertical distribution of the contamination in the service shaft was estimated for different operational and incidental scenarios with a wide parameter-study performed with a 3D CFD model. All calculations were performed with different types of CFX codes.