The objective of this work is to develop an adequate procedure to determine mechanical properties of nuclear reactor components by means of small specimen test technique. Atomic reactor power plant is a huge structure that the safety factor due to its special function is momentous. The irradiation can change the mechanical properties of the steel used in nuclear power plant structure. The radioactive effect on the vessel of nuclear power reactor causes radiation damage by altering the material properties of nuclear pressure vessel. The effects of the irradiation are mainly important on the toughness parameters (DBTT, KIC). In this work, instrumented Charpy tests was used to study and analyze the effect of specimen size and geometry on the dynamic fracture toughness of 15H2MFA steel used in nuclear pressure vessels of Paks atomic reactor in Hungary. Different concepts (function of the pressure vessel, fission phenomena, neutron interactions, swelling and radiation Creep) are discussed briefly to elaborate the concept of embrittlement and changes in DBTT of the steel in the nuclear reactor pressure vessel. Results show different factors affecting the DBTT curve of the tested steel.