• pipe break
• hepb
• pipe whip restraint

NUCLEAR PLANT HIGH ENERGY PIPING:
PIPE BREAK AND PIPE WHIP RESTRAINT DESIGN AND VERIFICATION METHODOLOGICAL APPROACH

Summary

Restraints design is conceived in order to protect structures, systems and components from the anticipated rupture and whipping phenomena of high energy lines in nuclear power plants and guarantee the safe shutdown itself. It is therefore compelling for the plant safety to investigate these topics accurately. Various research programs have been conducted in many countries to develop analytical methods and to verify the validity of these methods.
In this thesis work a nonlinear dynamic Finite Element Analysis (FEA) is conducted using ANSYS-LS DYNA program in order to simulate the smashing of a whipping pipe against its own restraint. Break locations will be identified, the thrust force will be computed, plastic hinge formation evaluated, stress-strain curve outlined.
The task reproduces two JAERI tests performed between 1979 and 1982 under BWR LOCA conditions for a 6” and 4” pipe.
Modelling High Energy Pipe Break phenomena covers deep accuracy in sizing components and structures involved; their stiffness and mass, thicknesses and mostly material properties. The study analyzes different material behaviors under impact starting from ANS 58.2 recommendations, and furtherly exploiting different properties embedded within ANSYS LS-DYNA such as the bilinear kinematic stress-strain trend and finally the multi-linear plasticity law, suggested as the most suitable.
The other important aspect is that of inputting to the Code a well-defined thrust force trend; this has been done following the regulations and later on with a transient blowdown analysis using RELAP5 Code. The two are proven to be consonant but mainly too conservative specially in the rise time of the force. Anyway a procedure to deal with HEPB phenomena has been built and tested and has been practiced for a real case analysis.