At Paks NPP, Hungary, a major project was launched to investigate the possibility of lifetime extension up to about 60 years. As a part of this project, detailed calculational and experimental investigation of the pressure vessel neutron radiation load having been performed. A computational procedure was developed earlier for evaluating the fast flux in the pressure vessel. The neutron source is taken from core calculation performed by the code and it is given in the core boundaries. The neutron transport from the source to and through the pressure vessel is investigated by . This procedure has now been tested against a new set of activation foil measurements. Good agreement was found between the measured and calculated reaction rates for fast reactions. In the case of thermal reactions, the agreement was weaker. The effect of error in nuclear data on fast reaction was studied.