When a pressurised pipe in a reactor coolant system breaks, it results in hydraulic loads on the reactor containment system and escaping fluid exerts a thrust force on the pipe. The double-ended guillotine break (DEGB) is generally the most onerous loss of coolant accident (LOCA) in design of a reactor coolant system [1]. In addition to the hydraulic loads, the continuing thrust force on the broken end of the pipe generates a rapidly accelerating rotational displacement of the pipe section on the break side of the plastic hinge, the phenomenon called pipe whip. The whipping pipe has the potential to damage objects within the hazard zone therefore must be assessed. Currently a 20bar threshold is used in nuclear power plant (NPP) design of light water reactors (LWRs) to delineate high energy lines (HEL) and moderate energy lines (MEL). The threshold is used as a process to establish the requirement for additional pipe whip assessments to be performed as part of HEL guillotine break analysis. It is currently argued that no such studies are required for MEL. However the basis of using the 20bar threshold for not carrying out pipe whip assessment of MELs is not well understood. The work presented here provides details of the finite element (FE) analyses undertaken to substantiate the 20 bar threshold used for the differentiation of HEL and MEL. Using the FE analysis method, a range of pipe characteristics, and pressures in the range of 10-50bar have been studied to determine whether a plastic hinge will occur, and whether pipe whip effects will be seen for that case. The FE results have also been used to assess the equivalent plastic strain criteria generally used to define the formation of a plastic hinge and initiation of pipe whip.
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