First, pressure thrust for nozzles is the cross sectional area of the nozzle(in^2) X the pressure (psi) to give a load in lbs. This is the loading that gives you longitudinal stress in the nozzle neck (or the vessel itself). Does this load translate into an "external radial load"? I think not...
I have experience with design of 3 bolt flanges at both Idaho and Hanford facilities. I do not have any experience with evaluating leak rates or potential leakage rates. Call 949-349-3559