HEPB Transient Dynamic Analysis. From a quasi-linear to a fully non-linear modelling

Federico Percivale, Alessio Tolu, Giovanni Di Giuseppe
Ansaldo Nucleare

Postulated pipe rupture loads are among the highest design-basis-accident loads in the design of Nuclear Power Plants. Piping breaks are postulated, usually, at terminal ends according to USNRC-SRP 3.6.2. After a sudden rupture, the pipe is subjected to an intensive fluid force. This force produces a transient dynamic event, which cause large displacements and intense plastic strain in the pipe sections. The primary objective, following each postulated rupture, is to demonstrate that the plant can be brought to and maintained in a safe shutdown conditions. To pursue this goal, when necessary, a Pipe Whip Restraint (PWR) shall be designed in order to prevent impacts between the whipping pipe and other structures considered essential for a safe shutdown of the plant. The present work concerns the study and the FE modelling of the transient dynamic phenomenon of a whipping pipe after a sudden rupture. Particular interest is dedicated to the investigation of resultant dynamic loads on supporting structures, located downstream the break point and to some modelling tricks used to simulate nonlinear localized equivalent behaviour of PWR, bumpers, gaps, supporting springs, annular elastic and plastic deformation of the pipe’ section and penetration point. These tricks mainly consist in developing rheological models able to represent the static and dynamic behaviour of the nonlinear devices.
Within the study of the rheological models, a series of simplified benchmark models have been developed and parametrically solved. Results obtained from the parametric simulation on the simplified models show the great importance of modelling all nonlinearities to get a reliable nonlinear dynamic behaviour. To perform all these dynamic simulation the ANSYS mechanical software has been used. In order to not have high computational time consuming, a special effort has been done to reduce the model dimension, using pipe beam elements and spring connections with advanced capabilities.
Finally , a portion of High Energy Pipe Line, of a Nuclear Power Plant, has been chosen and used to perform global studies on a real pipe line. A portion of line has been modelled using ELBOW290 elements. Pipe is sustained by 5 flexible nonlinear supporting structures and some fixed supporting points. Break location is localized inside a penetration. Thrust force is equal to 69KN, calculated according to ANSI/ANS 58.2. Five different models of the pipeline have been developed and dynamically solved, both considering the presence of the PWR and not. Models are distinguished by their “degrees of non linearity”. Forces time histories on support are evaluated and compared showing how nonlinearities greatly influence results and affect dynamic evolution of the system.