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

DOE-NE sets a mission to develop new nuclear generation technologies that are aimed at the development and deployment of next-generation advanced reactors and fuel cycles. Our target in this project is the development of affordable technology to fabricate the high-radiation resistant materials in next-generation reactors as stated in R&D objective 2: “Develop improvements in the affordability of new reactors to enable nuclear energy to help meet the Administration's energy security and climate change goals”. We focus on the development of the materials since they are very critical in the reactor design as the radiation can induce severe damages in materials, including swelling, hardening, creep, embrittlement and irradiation-assisted corrosion. The tolerance of radiation damage by structural materials plays a critical role in the safety and economy of the commercial nuclear power plant. 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. Nanostructuring has proven to be a key for improving the radiation resistance of materials. CNT and graphene are a well-known mechanically strong and flexible nanomaterial. If CNTs had been uniformly dispersed in a metal as 1D fillers, their high aspect ratio creates prolific internal interfaces with the metal matrix that may act as venues for the radiation defects to recombine (self-healing). The radiation tolerance from CNT incorporation is enhanced by at least five times, up to ~102 DPA. The technology is affordable to deploy in the new reactor as we have proven the industrial scalability: fabricating more than 500 kg of Al- carbon nanotube (CNT) composite with only 1.5× the cost of the original ingot. Especially in materials aspect, the new advanced reactors are still low levels of technical maturity.

However, next-generation nuclear fission reactor requires even higher DPA tolerance (10^3 DPA). To achieve this goal, understanding of the mechanism of defects evolution at interfaces and materials’ damage tolerance provides important considerations to design self-healing interface. 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 verify multi-scale modeling of damage evolution. Therefore, Revealing the basic irradiation physics of our-1D/2D nanocomposites materials, at this stage, guide us to assemble higher burn-up of fuel for nuclear fuel cladding and structural core, which would directly reduce the fueling costs and the amount of nuclear wastes, and improve the reliability and economy of the new nuclear reactor. The results will highly enhance the feasibility for future deployment.