Deterministic Safety Technology for RBMK reactors

A European Community sponsored Project focused toward the setting-up of a chain of computational tools suitable for the deterministic safety analysis of transients expected in the RBMK NPP. The accident leading to the rupture of one fuel channel with fuel melting or high temperature damage, creep and brittie failure of the pressure tube and of graphite bricks with possibility of rupture propagation, constituted the reference scenario for the Project. Two chains of computational tools have been proposed and developed. The major effort was devoted to the development and the qualification of input decks or nodalisations for almost a dozen codes covering areas such as thermal-hydraulics, neutron kinetics, structural mechanics, nuclear fuel behaviour, fission product generation and transport. The Smolensk 3 was selected as reference NPP and non-licensing accident analyses were performed. A methodology was proposed for investigating the realism in the propagation of one pressure tube break to neighbouring pressure tubes, (i.e. addressing the MPTR issue). This consisted of three steps: a) identification of acceptable loads based on structure mechanics, b) calculation of applicable loads based on pressure differences and jet hydraulic loads acting on the graphite stacks, e) comparison between loads from the two steps. The completed study emphasized the importance of: • modelling the RBMK core channels one-by-one namely considering the geometric position within the array, the material composition, the initial stack temperature and the fluence, • the stiffness of the tank that constitutes the ultimate constraint preventing excessive channel deformation, again depending upon the position of the channel in the core