The extensive experimental program performed consisted in the characterisation of the ductile to brittle transition region with standard and reconstituted specimens using subsized compact tension and Pre-Cracked Charpy V-notch specimens, under non-irradiated and irradiated conditions. Experimental results validated the reconstitution technique down to inserts of 10 mm which allows specimen reorientation and therefore, the comparison of LT and TL material orientations. The 110 specimens tested in this program allowed the Master Curve approach to be validated for the base steel of Santa María de Garoña nuclear power plant. By comparing the results for the compact and Pre-Cracked Charpy V-notch specimens, the existence of a systematic bias between these two geometries has been analysed. Although the neutron irradiation effect on the condition studied is predicted to be very limited (due to the small fluence), the results did allow the irradiation-induced shift of the ductile to brittle transition temperature to be detected. Comparison between the directly measured fracture toughness and the conventional semi-empirical approach proposed by the ASME Code reveals the overconservatism of the latter approach.