Background: During the process of Leak Before Break（LBB） design for the pipes in nuclear power plant

a through-wall crack is hypothesized in each selected location of the candidate pipe line

then the Crack-Opening Displacement（COD） and leak rate of the crack under normal operation condition is predicted

finally the applicability of the LBB design criterion is ascertained.Purpose: Discuss the disadvantages of the COD and leak rate calculation methodologies

develop a COD and leak rate calculation program.Methods: Through compare the results from the program developed by ourselves with PICEP

the program developed by ourselves is verified.Results: The results from the program developed by ourselves tally with PICEP.Conclusions: The disadvantages of each COD and leak rate methodologies are discussed;then a program was successfully developed.Both can provide some reference for the application of the LBB approach to the nuclear power stations of China.