Application of the Leak Flow Rate Calculation Program FLORA toReal Cracks in Piping

摘要

Detecting the presence of subcritical cracks in reactor coolant pressure boundaries is essential for the Leak-Before-Breakconcept. The selection and the optimal design of the leakage monitoring system for this task requires the pre-calculation ofthe leakage rates to be anticipated. In the case of fluid release the failure analysis based on a post-calculation of the leakagerates is of importance, too.Framatome ANP, formerly Siemens Nuclear Power, uses the program FLORA (FLOw RAte) to calculate the mass fluxthrough the crack by thermal-hydraulic analysis. This program which is based on experimental, analytical and numericalinvestigations enables flow rate calculations covering? discharge rates over the entire thermodynamic range,? discharge opening geometries from finest cracks up to complete pipe breaks, and? leak flow rates from single droplets up to critical discharge (sonic velocity through cracks/pipe breaks).The applicability for laminar and turbulent conditions in cracks both in the cases of continuous leakages and of droppingleakages has been validated with the aid of measured leakages through real cracks in power plant piping. Due to safe designand intensive non-destructive testing activities, only few cases of through-wall cracking failures causing leakage in Germannuclear power plant systems have occurred in the past. From the Siemens data base, which collects failure data of nuclearpower systems, five cases which are characteristic for the most frequent occurring failure mechanisms were chosen. Twocases led to significant leakage rates, the other three cases resulted in the discharge of only droplets. In this paper theevaluation concept for leakage rates is introduced, and the features of the current version of FLORA are demonstrated.