Bhabha Atomic Research Center’s scientists Mr AP Srivastava, Mr SK Sharma, Mr S Saini, Mr S Neogy, Mr SK Ghosh, Mr D Kabiraj & Mr R Tewari in recently published research paper “Understanding the effect of irradiation temperature on microstructural evolution of 20MnMoNi55 steel” in Nature have highlighted the effect of irradiation temperature on microstructural evolution of Reactor pressure vessel steel in nuclear applications.Reactor pressure vessel, made up of special grade low alloy steel, acts as a pressure boundary in light water reactors. During reactor operation, reactor pressure vessels are known to get exposed to modest doses of neutron irradiation in their lifetime, typically 0.05-0.1 dpa after 40 years of operation. This seemingly small dose, however, is sufficient to reduce the fracture toughness of reactor pressure vessel significantly. Hence, a study on the performance of RPV steel under irradiation assumes a lot of significance. Under reactor operating conditions, when a high energy neutron enters in a structural material, it usually dislodges a host atom from its lattice position with high kinetic energy. This dislodged atom, which is commonly known as primary knock on atom, displaces other host atoms from their lattice positions creating collision cascade which generates large number of point defects such as vacancies, self-interstitial atoms and their clusters. The concentration, configuration and distribution of these irradiation induced defects in the microstructure finally govern the structural integrity of any in-core reactor structural materialIrradiation experiments were performed with varying ion energies to achieve nearly uniform irradiation damage of 0.05, 0.2 and 3 dpa in a 300nm wide region. Irradiated samples were characterized using GIXRD, PAS, TEM and nanoindentation. Unirradiated samples showed predominant presence of a combination of di- and tri-vacancy type of defects. Most of the dislocations present in unirradiated samples were screw dislocations, while mixed type was noticed upon irradiation irrespective of the irradiation temperature. PAS study showed formation of distinct defect types at different irradiation temperatures. TEM study confirmed formation of dislocation loops and defect clusters on irradiation. Higher irradiation temperatures resulted in the extension of the width of the damage region owing to increased migration of defects.In the present study, indigenously developed RPV steel16 of SA 508 Grade 3 Class 1 type has been irradiated with helium ion to achieve various levels of damage including that which is usually experienced by such steels in nuclear reactors. The primary aim of this study has been to assess the microstructural stability of the RPV steel against irradiation damage to 0.05, 0.2 and 3 dpa imparted under varying irradiation temperature and to identify the underlying mechanisms leading to various responses with respect to dose and temperature variations. Irradiation temperature of 573 K has been selected as it is close to reactor operating temperature and a low irradiation temperature of 77 K has been selected as at this temperature, primary defect formation will be slightly enhanced whereas their diffusion will be substantially reduced compared to high temperature irradiation which in turn may dictate the observed difference in radiation damage responses, while an increase in temperature shall reflect the recombination and annihilation of defects due to enhancement in mobility. The microstructural stability has been assessed in terms of changes in domain size, microstrain, dislocation density, S-parameter (open volume defect density) and hardness. The defect microstructure developed under irradiation has been characterized using transmission electron microscopy.