The effect of radiation exposure on nano grain structured metals is of immense interest both from scientific and technological points of view. While in conventional metals, radiation produces various defects (point, line, surface and volume), it is not clear how these defects especially dislocations (line defects) and stacking faults (surface defects) can be accommodated in the relatively minute grains of nm-scale. The response of nano grain structured metals to neutron irradiation can be expected to be different from their large grained equivalents and this response could be in terms of the changes in mechanical properties post neutron irradiation or it could be an altered microstructure altogether. Very limited amount of research conducted in this area was a motivation to initiate the proposed study since the few attempts to study the effects of radiation on nano grain structured metals were only limited to observing the developed microstructure, and radiation effects on mechanical properties and the effect of processing routes were considered in only a few cases. This project is proposed to perform post irradiation examination (PIE) of ATR irradiated nc-Ni along with its conventional counterpart. The proposed PIE involves microhardness and tensile tests of 1 dpa samples along with microstructural characterization using X-ray diffraction and transmission and scanning electron microscopes at INL’s MFC and CAES. The expected period of performance is for 3 months starting January 2013. These results on nickel along with those obtained from PULSTAR reactor at NC State to relatively low dpa (~0.001 dpa) will enable us to investigate the influence of neutron irradiation on Hall-Petch relation. Earlier work on fcc Cu revealed decreased source hardening resulting in reduction of the slope of Hall-Petch plot and the current study will shed light on the plausible effect of crystal structure (carbon steel/bcc vs nickel and copper/fcc). Moreover, low dose irradiations of nc-Ni revealed some radiation softening similar to nc-Cu, but in contrast to ultrafine grained carbon steel which experienced insignificant radiation hardening. The softening in nc-Ni is suspected to be due to in-situ grain growth, but this contrasts with previous work which showed grain shrinkage after irradiation. Radiation effects at higher dose will be beneficial in examining these phenomena and providing evidence as to the superior radiation resistance of nano grain sized materials.

Advanced nuclear systems are designed to operate at far higher radiation fluences than the currently operating nuclear reactors and recent efforts have been on the development of new relatively more radiation resistant materials. Nanostructured materials have opened a new and fascinating path for research due to their unique and extraordinary properties, where the large grain boundary area may enhance resistance to radiation damage. It has been shown that the defect density reduces drastically for small grains below 50 nm and for the nuclear engineering field, it’s important to get more information about the behavior of nanomaterials exposed to radiation since these materials could be of application for reactors of the new generation such as Very-high-temperature reactor (VHTR), Gas-cooled fast reactor (GFR), etc.

Limited literature exists on the radiation effects on nanostructured materials especially neutron irradiation. The majority of currently available work was performed by ion irradiation, which is very different from neutron irradiation in a reactor. This work mostly involves microstructure characterization of irradiated nanostructured materials and evaluation of the mechanical properties of the nanostructure materials before and after irradiation. As part of one of the first NSUF/ATR grants, a number of nano and fine grain structured metals (Cu, Ni and carbon steel) were irradiated in ATR along with their conventional counterparts. While Cu and carbon steel have been studied in detail, Ni could not be tested to-date due to their relatively high induced radioactivity. These irradiated samples at Hot Fuel Examination Facility (HFEF) are now ready to be tested and we propose here to perform the microstructural and mechanical property characterization on nickel samples (nano crystalline and conventional grain sizes) irradiated to 1 dpa as a post irradiation examination study.