The interest on scenarios dealing with the boron dilution events in PWR-NPP has been increased among the scientific community in the last decades so that research programs have been performed to investigate the boron issue. The reason is due to the safety concerns risen by the potential of local re-criticality and power excursion associated to undue low borated water entering into the core. In the context of an international cooperation project named SETH, set up by the OECD, a test campaign addressed to investigate inadvertent boron dilution events in PWR has been performed on the integral PKL test facility. PKL III is a PWR simulator installed in Erlangen, Germany. The facility replicates the entire primary system and most of the secondary system of a nuclear steam supply system of 4 loops, 1300MWe nuclear power plant, on a scale of 1:145. The present paper is focused on the revisiting of the post-test calculation of the experiment E22, restart of natural circulation and displacement of low borated water after a small break LOCA, carried out with different Relap code versions adopting an updated nodalization relevant for the prediction of simultaneity of natural circulation restart and boron distribution in the primary circuit. The calculation results obtained using different models of the steam generator U-tubes that are represented respectively with one, three and seven equivalent pipes, are compared and the Fast Fourier Transform Methodology is applied to the calculations to quantify the discrepancies with respect to the recorded experimental data.