Study of the RBMK core behavior during a single/multiple pressure tube rupture accident by coupled 3D neutron kinetic / thermal-hydraulic codes

Full text of publication follows: The study of the RBMK reactor core is a great challenge for the analyses carried out by Best-Estimate coupled codes because of the lack of the huge amount of information generally available for the other reactor types and because of the high level of core heterogeneity. The high number of core channels and of pipe collectors, the core asymmetry, the complex neutronic interaction between the graphite moderator, the coolant and the performance of the Control and Protection System are only an example of the RBMK complexity. The DIMNP of Pisa University, in the framework of the TACIS project collaboration with the Software Development Center 'NIKIET' of the Russian MINATOM, has developed and tested a detailed thermal-hydraulic and neutronic nodalization of the Smolensk 3 NPP, simulating all Group Distribution Headers as well as all Drum Separators. The code used for the overall analyses was RELAP5C3D. After a qualification process of the modeling, it was conducted a series of transients postulating the single and the multiple core channel flow blockage. The consequent high temperature increase of the fuel clad and of the pressure tube resulted in the pressure tube rupture itself and conduced to CCFL phenomena. Then the possibility of the pressure tube rupture propagation to other core channels has been also investigated utilizing conservative safety criteria. (authors)