As part of PWR life-time assessment project, a large R&D program, PV-CT50, has been carried out from 1995 to 2004, to assess pressure vessel (PV) steel's toughness curve using very large compact tension (CT) specimens (50 × 125 × 120 mm3) allowing measurements in the ductile to brittle transition range. A new high capacity irradiation rig was designed to meet the specific CT50 specimens loading requirements in the 70 MWth OSIRIS material testing reactor operated by CEA's Nuclear Energy Division at the Saclay Research Center. This paper presents the measurement and calculation developments required in these particular conditions. Spectrum characteristics and neutron flux levels were determined by a comprehensive dosimetry on mock-ups featuring activation foils and damage monitors. At the same time, a new calculation methodology based on the TRIPOLI-4.3 Monte Carlo code and IRDF-2002 dosimetry files was validated against calorimetric and neutron activation measurements.