Crack Surface Pressure Effect on Leak-Before-Break Evaluation

Leak-Before-Break (LBB) analysis allows nuclear power plants to eliminate consideration of the dynamic effects of pipe rupture from the plant design basis for the affected Class I piping system, and remove protective hardware such as pipe whip restraints, jet impingement barriers, etc. Accurately calculating leakage rate of the postulated circumferential flaw in the pipe line system is critical for the LBB analysis. To obtain the final leakage rate, two calculations have to be performed: critical circumferential flaw size and then, the corresponding leakage rate through the flaw. If present, including crack face pressure in the evaluation will help enlarge the crack opening distance and hence, increase the leakage flow rate per unit flaw length. However, it creates larger force and bending moment on the crack, which reduces the critical flaw size and therefore, limits the final leakage amount. The opposite effect of crack face pressure on critical flaw size and leakage rate per unit flaw size needs to be evaluated ascertain its overall effect on LBB evaluation. In this paper, leakage rates are calculated and compared for cases with or without consideration of crack face pressure for various piping systems with typical dimensions and loadings. The crack face pressure effect on the final leakage rate of LBB evaluation is discussed.