David J. Senor

Defect structure and evolution in silicon carbide irradiated to 1 dpa-SiC at 1100 °C

Transmission electron microscopy (TEM), swelling measurements, isochronal annealing, and thermal diffusivity testing were used to characterize the effects of radiation damage in SiC. Together, these techniques provided a comprehensive set of tools for observing and characterizing the structure and evolution of radiation-induced defects in SiC as a function of irradiation temperature and dose. In this study, two types of dense, crystalline, monolithic SiC were subjected to irradiation doses up to 1 dpa-SiC at a temperature of 1100 °C, as well as post-irradiation annealing up to 1500 °C. The microscopic defect structures observed by TEM were correlated to changes in the macroscopic dimensions, thermal diffusivity and thermal conductivity. The results demonstrated the value of using ultrapure β-SiC as an effective reference material to characterize the nature of expected radiation damage in other, more complex, SiC-based materials such as SiC/SiC composites.

Dimensional stability and tensile strength of irradiated Nicalon-CG and Hi-Nicalon fibers

Abstract Nicalon-CG and Hi-Nicalon fibers were characterized by measuring their density and tensile strength in the unirradiated, thermal annealed, and irradiated conditions. The results indicate the fibers that perform best after irradiation to 43 dpa SiC at 1000°C are those that approach stoichiometric and crystalline SiC. Hi-Nicalon fiber exhibited less than 1% densification, accompanied by a slight increase in tensile strength after irradiation. Nicalon-CG, in contrast, was significantly weakened in the annealed and irradiated conditions. In addition, Nicalon-CG exhibited substantial irradiation-induced shrinkage. Loss of fiber tensile strength after irradiation is shown to reduce the flexural strength of irradiated composites while fiber shrinkage, and resultant debonding from the matrix, are linked to a reduced composite elastic modulus.

A New Innovative Spherical Cermet Nuclear Fuel Element to Achieve an Ultra-Long Core Life for use in Grid-Appropriate LWRs

Spherical cermet fuel elements are proposed for use in the Atoms For Peace Reactor (AFPR-100) concept. AFPR-100 is a small-scale, inherently safe, proliferation-resistant reactor that would be ideal for deployment to nations with emerging economies that decide to select nuclear power for the generation of carbon-free electricity. The basic concept of the AFPR core is a water-cooled fixed particle bed, randomly packed with spherical fuel elements. The flow of coolant within the particle bed is at such a low rate that the bed does not fluidize. This report summarizes an approach to fuel fabrication, results associated with fuel performance modeling, core neutronics and thermal hydraulics analyses demonstrating a ~20 year core life, and a conclusion that the proliferation resistance of the AFPR reactor concept is high.

Predictive Bias and Sensitivity in NRC Fuel Performance Codes

The latest versions of the fuel performance codes, FRAPCON-3 and FRAPTRAN were examined to determine if the codes are intrinsically conservative. Each individual model and type of code prediction was examined and compared to the data that was used to develop the model. In addition, a brief literature search was performed to determine if more recent data have become available since the original model development for model comparison.

Modeling the Transverse Thermal Conductivity of 2-D SiCf/SiC Composites Made with Woven Fabric

Abstract The hierarchical two-layer (H2L) model describes the effective transverse thermal conductivity (keff ) of a two-dimensional (2-D) SiC f /SiC composite plate made from stacked and infiltrated woven fabric layers in terms of constituent properties and microstructural and architectural variables. The H2L model includes the effects of fiber-matrix interfacial conductance, high-fiber packing fractions within individual tows, and the nonuniform nature of 2-D fabric/matrix layers that usually include a significant amount of interlayer porosity. Previously, H2L model keff predictions were compared to measured values for two versions of 2-D Hi-NicalonTM/pyrocarbon (PyC)/isothermal chemical vapor infiltration (ICVI)-SiC composite, one with a “thin” (0.11-μm) and the other with a “thick” (1.04-μm) PyC fiber coating, and for a 2-D TyrannoTM SA/thin PyC/forced flow chemical vapor infiltration SiC composite. In this study, H2L model keff predictions were compared to measured values for a 2-D SiC f /SiC composite made using the ICVI process with Hi-Nicalon type S fabric and a thin PyC fiber coating. The values of keff determined for the latter composite were significantly greater than the keff values determined for the composites made with either the Hi-Nicalon or the Tyranno SA fabrics. Differences in keff values were expected for the different fiber types, but major differences also were due to observed microstructural and architectural variations between the composite systems, and as predicted by the H2L model.

Fuel Fabrication Capability Research and Development Plan

The purpose of this document is to provide a comprehensive review of the mission of the Fuel Fabrication Capability (FFC) within the Global Threat Reduction Initiative (GTRI) Convert Program, along with research and development (R&D) needs that have been identified as necessary to ensuring mission success. The design and fabrication of successful nuclear fuels must be closely linked endeavors.

SPHERICAL FUEL ELEMENT CONCEPT FOR SMALL REACTOR DESIGN

Abstract The Pacific Northwest National Laboratory (PNNL) is currently developing a novel spherical fuel element concept that offers low fuel temperatures, low stored energy, and long core life. Fuel performance modeling has been conducted using the PNNL-developed Atoms for Peace Reactor (AFPR)-100 as a platform for demonstrating the potential of the fuel element concept. The AFPR-100 is a small [100-MW(electric), 300-MW(thermal)], water-cooled reactor concept that is designed to use established technology, be passively safe, and be proliferation resistant. The fuel performance modeling has demonstrated that this fuel element has a short thermal time constant, has low fuel temperature, provides a barrier for retention of fission products, and will have long-term dimensional stability. A technique for manufacturing these fuel elements was developed. A fabrication demonstration was conducted in cooperation with a commercial vendor to evaluate the feasibility of manufacturing the fuel elements. In order to demonstrate the proposed technique, the proposed spherical elements were produced using existing processes that could be scaled to large batch sizes. Surrogate ZrO2 kernels were substituted for the fuel in this demonstration. Thorough characterization of the fuel elements was performed at various stages in the fabrication process. The metallographic characterization included electron microscopic analysis of coating microstructure, and particular attention was paid to interface regions to search for deleterious reaction zones, debonding, and porosity. Although this demonstration is not complete, early results are promising and will be discussed in this paper. This paper will describe the fuel element, show the results of fuel performance calculations for this element, describe the proposed fabrication process, and discuss the results of a fabrication demonstration to date that has been performed for this concept.

Project X Energy Station Workshop Report. Report by the Organizers and Co-Conveners of the Project X Energy Station Workshop

Project X Energy Station Workshop Report Report by the Organizers and Co-Conveners of the Project X Energy Station Workshop

Dimensional stability and tensile strength of irradiated Nicalon-CG and Hi-Nicalon SiC fibers

Nicalon-CG and Hi-Nicalon fibers were characterized by measuring their length, density, and tensile strength in the unirradiated, thermal annealed, and irradiated conditions. The irradiation was conducted in the EBR-II to a dose of 43 dpa-SiC at a nominal irradiation temperature of 1,000 C. The annealed specimens were held at 1,010 C for 165 days to approximately duplicate the thermal exposure of the irradiated specimens. The results indicate the fibers that perform best in an irradiation environment are those that approach stoichiometric and crystalline SiC. Hi-Nicalon exhibited negligible densification, accompanied by an increase in tensile strength after irradiation. Nicalon-CG possessed a higher tensile strength than hi-Nicalon in the unirradiated condition, but was significantly weakened in the annealed and irradiated conditions. In addition, Nicalon-CG exhibited unacceptable irradiation-induced shrinkage. Loss o fiber tensile strength after irradiation is shown to reduce the flexural strength of irradiated composites and Nicalon-CG fiber shrinkage observed in irradiated composites.