The proposed project aims to quantify and compare porosity evolution in ion irradiated metal-1D/2D nanocomposites and gas-embrittled steels through positron annihilation spectroscopy (PAS) at NCSU-PULSTAR. The degradation of structural materials in nuclear plants under irradiation and/or environmental exposures has become major constraints to the safety and economy of nuclear energy. If the nuclear fuel cladding can tolerate a higher radiation exposure but simultaneously maintain the desirable thermomechanical properties, higher burn-up of the nuclear fuel hence higher efficiency and safety is achievable. This will also lead to a reduction of the fuel cost and the nuclear waste as well. However, the materials challenge in advanced nuclear reactor such as the generation IV needs to meet the requirement of tolerance from the high radiation flux and fuel temperature. To achieve higher DPA tolerance (upto 10^3 DPA), understanding of the mechanism of defects evolution at interfaces and materials’ damage tolerance provides important considerations to design self-healing interface. We have developed a self-healing approach by using the metal-carbon nanostructure composite, where percolating networks of 1D/2D nanodispersions outgas fission gases. In order to develop a generalized damage evolution theory that incorporates different mechanisms of damage that create and stabilize free volume in the interior of materials, we need to quantify porosity. However, the distribution of yet smaller pores/voids beyond TEM resolution is known to be very significant since density is higher. Positrons of various energies can be injected into materials, get trapped and annihilate with surrounding electron after some time, emitting gamma rays that are counted. At the NCSU PULSTAR facility, we will use Doppler Broadening Spectroscopy (DBS) to obtain the energy/momentum distribution of the annihilating positron-electron pairs. Positrons trapped in vacancies have reduced rate of annihilating with core electrons with higher momenta. DBS thus allows one to estimate the free volume distribution, by examining the S parameter which quantifies the sharpness of the 511 keV photon peak. In this RTE project, we will quantify the distribution of vacancies/pores in ion irradiated 1D/2D nanocomposites and gas-embrittled steels by DBS, and then compared with our previous TEM measurements and multiscale simulations. DBS will provide critical quantitative information on the accumulation of vacancies and pores, stabilized by fission gases or environmental hydrogen. PAS/DBS experiments are uniquely suitable for quantifying the smaller “invisible” cavities, allowing one to quantify the entire distribution. We anticipate that PAS allows designing of the 1D/2D dispersion to obtain a radiation tolerance at least one order of magnitude higher than existing materials. This project will provide insights on the role of interface in 1D/2D filler to improve radiation resistance, enabling better understanding of the irradiation mechanism at the nanoscale, which will further impact the development of new radiation-resistant materials. It will also validate a more general theory of damage mechanics, and provide verification for multi-scale modeling of damage evolution. Since we have already done the microstructure characterization in TEM, we can conduct the PAS/DBS experiment any time after the RTE award