High resolution gas adsorption (N2 and Kr) at 77 K will be used to characterize the structural and energetic homogeneity of pore surfaces in three types of graphite specimens irradiated before and after turnaround. Prior NSUF-supported research showed that irradiation after turnaround induces significant development of mesopores (2-50 nm) simultaneously with an increase of internal BET surface and open porosity. However, the mechanism of these changes is still not completely understood. The goal is to determine whether or not new the new pores are limited by homogeneous surfaces (basal planes in graphite crystallites) or by rough, disordered surfaces (edge sites, possibly in the binder). To that end we will use high resolution gas adsorption (N2 and Kr) at 77 K to obtain information on structural and energetic uniformity of graphite surfaces. It is known that sub-monolayer adsorption of N2 and Kr is very sensitive to the underlying surface structure. Adsorption isotherms at 77 K show distinct phase changes at surface corresponding to completion of ordered (commensurate) layers of Kr or N2 on the ordered graphite basal planes. Moreover, Kr adsorption in multilayers continues with distinct steps characteristic to layer-by-layer adsorption, which is a clear indication for presence of structurally homogeneous surface sites. These features will be used to estimate the fraction of basal planes in the total (BET) surface area. By comparing with the features of the non-irradiated graphite it will be possible to determine whether or not porosity changes observed for graphite irradiated after turnaround are caused by delamination of graphite crystallites and exposure of new basal planes. In addition to clarifying some of the microstructural changes induced by irradiation, the success of this project will demonstrate the viability of a new method for quantification of basal plane and edge sites surfaces in graphite materials. This is important for normalization of other graphite properties that depend on microstructure, such as intrinsic oxidation reactivity, which preponderantly involve edge (prismatic) sites of graphite crystallites. The work proposed consists of collecting a series of N2 and Kr adsorption isotherms (with at least 150 point each) on 8 graphite specimens irradiated at various doses, before and after turnaround. The graphite irradiated from the HTV and Deep Burn campaigns at ORNL is stored in LAMDA. We propose to measure 2 irradiated specimens of fine grain, iso-pressed graphite 2114 (Mersen, France), 4 irradiated specimens of medium grain, extruded graphite PCEA (Graftech, USA) and 2 irradiated specimens of large grain, vibro-molded graphite NBG-18 (SGL, Germany). Three more un-irradiated specimens will serve as reference. The total time for completing the project, including measurements and data analysis, is estimated at 4 months. At the end of the project the results will be prepared for publication.

High purity graphite is used as moderator and structural component in several types of advanced nuclear reactors such as high-temperature gas-cooled reactors (HTGR) and thermal molten salt reactors (MSR). It is well known that irradiation by high energy neutrons induces severe changes in the graphite structure and its physical properties, including dimensional changes and volume variations. The mechanism of these transformations is still not well understood, and intense research continues through national research organizations in several countries and through international collaboration. The end goal is to develop reliable models for predicting long-term effects of in-core irradiation of nuclear graphite. These models are needed for design and certification of advanced nuclear reactors with improved safety features, higher reliability and extended lifetime, in line with the DOE-NE mission. The proposed project will provide new information about the mechanism of structural changes in irradiated graphite at the mesoscale (2 to 50 nm range). Specifically, the project aims at demonstrating a simple method (high resolution gas adsorption of Kr and N2 at 77 K) for characterization and quantification of energetically uniform surfaces (such as basal planes in graphite) that might be produced by cracking and exfoliation of graphite crystallites exposed to high doses of neutron irradiation. The work proposed here continues and extends the scope of a 2016 project funded by NSUF-RTE on gas-adsorption characterization of irradiation effects on nuclear graphite. Sustained research effort supported by DOE-NE Office (Nuclear Technology R&D Program, NE-4) is currently pursued in the US universities and national laboratories. Specific actions comprise Advanced Reactor Technologies (ART) program and its precursor Next Generation Nuclear Plant (NGNP) programs. Simultaneously, US scientists participate in nuclear graphite research projects through international collaboration within the Graphite International Forum (GIF) and the International Atomic Energy Agency (IAEA) projects. In September 2017 DOE has sponsored the organization in the US of the International Nuclear Graphite Specialists Meeting (INGSM) where US scientists and their international partners find an opportunity to share ideas and exchange knowledge on critical aspects related to manufacture, characterization, monitoring irradiation behavior, and safe use of nuclear graphite in advanced reactors. The successful completion of this project will contribute not only to filling a critical knowledge gap related to the DOE mission, but will also demonstrate US engagement as a strong partner with other international organizations active in research and development of critical nuclear materials.