Implementation of the Leak-Before-Break
concept at NPP power units
On the basis of the calculation and experimental substantiation for pipelines with a diameter of more than 150 mm, the Leak-Before-Break (LBB) concept was introduced on the majority of Ukrainian NPPs.
For the main coolant pipeline and the pressurizer surge line, the Leak-Before-Break concept is introduced.
The list of tasks for the calculation justification of the LBB concept included:
input data preparation, which included the configuration of piping systems, geometrical and physical characteristics of the pipeline elements, load factors for normal operation, violations of normal operation, transient proceses, emergency situations, hydraulic tests and maximum design earthquake;
estimation of nominal limit stresses according to the actual normative documents;
stress-strain state calculation and determination of pipelines critical zones and taking into account stresses from water hummer, stratification and other specific loads;
structural strength estimation with hypothetical cracks by two-criterion approach of fracture mechanics for static and seismic loads;
calculation of surface cracks growth and corrosion cracks growth;
calculation of the opening area of through-wall cracks for stationary loads corresponding to the normal operation of the power unit;
leakage coolant calculation through a through-wall defect and its estimation according to the LBB concept.
The Leak-Before-Break concept was implemented at 9 power units of Ukrainian NPPs (unit 1,2,3 Zaporizhzhya NPP, unit 1,2,3 Rivne NPP, unit 1,2 South-Ukraine NPP, unit 1 Khmelnytska NPP) for main coolant pipeline and the pressurizer surge line.