The CAESAR II Flange Module (found under the "Analysis" menu option) does use the BPV Code, Section VIII, Division 1, Appendix 2 - for flange stresses. The BPV also contains an equation for flange rigidity, which is a value related to leakage.

However, the flange leakage routine in the CAESAR II Flange Module is based on an FEA technique, discussed in the "October 1991 Newsletter", which can be found on this website, as mentioned in the post above.