The investigation of reactor pressure vessel (RPV) material from the decommissioned Greifswald nuclear power plant (NPP) representing the first generation of Russian type WWER-440/V-230 reactors offers the opportunity to evaluate the real toughness response. The Greifswald RPVs represent different material conditions, viz., irradiated, annealed and irradiated, and annealed and re-irradiated. This paper presents test results from measurements of the trepan taken from forged base metal ring 0.3.1. located in the reactor core region of the unit 4 RPV. This unit was shut down after 11 years of operation and represents the irradiated condition. The key part of the testing is focused on determination of the reference temperature T₀ following ASTM E1921-10. The T₀ of 11 thickness locations from the inner to the outer RPV wall varies between −112°C and −130°C, with a mean value of −121°C. These are very low values for WWER RPV steel irradiated with fluences between 5.4 and 1.2 × 10¹⁹ n/cm² (E > 0.5 MeV) from the inner to the outer RPV wall. The fracture toughness values at cleavage failure K_Jc measured on LS oriented pre-cracked and side-grooved Charpy size SE(B) specimens from defined thickness locations of the forged ring show strong scatter. More than the allowed 2 % of the specimen size adjusted K_Jc-1T values lie below the fracture toughness curve for 2 % fracture probability. The application of modified master curve (MC)-based evaluation methods indicates that the material is non-homogeneous. Because of very low K_Jc values, the Structural Integrity Assessment Procedures for European Industry step 3 evaluation gave a maximum T₀^SINTAP of −40°C. The multimodal MC evaluation of K_Jc values from all thickness locations gives a T₀^MM of −118°C and a standard deviation of 25 K. The course of the fracture toughness curves evaluated via a multimodal approach also does not represent the measured K_Jc values, as more than 2 % lie below the fracture toughness curve for 2 % fracture probability. The reason for the occurrence of very low K_Jc values is intergranular planes detected on the fractured surfaces of the specimens.