Dionissios D. Papadias

The Effect of Na + Impurities on the Conductivity and Water Uptake of Nafion 115 Polymer Electrolyte Fuel Cell Membranes

Water uptake and ionic conductivities are reported for Nafion 115 membranes as functions of water activity and percentage of sulfonic groups occupied by sodium impurities. Water content was determined gravimetrically under liquid hydration and at 100, 75.3, and 11.3% relative humidity (RH). Water content exponentially decreased from the H{sup +}-form membrane water uptake isotherm to the Na{sup +}-form isotherm when hydrated by water vapor. Ninety percent of this decrease is reached at a substitution level of 0.2Na{sup +}/SO{sub 3}{sup -}. Water uptake under liquid water hydration decreased more gradually, only 50% to completion at 0.2Na{sup +}/SO{sub 3}{sup -}. Four-probe conductivity testing of Nafion 115 membranes, normalized against dry dimensions, revealed that although hydration decreases immediately with the introduction of sodium impurities, ionic conductivity at 100% RH remains constant up to 0.15Na{sup +}/SO{sub 3}{sup -}. Above 0.15Na{sup +}/SO{sub 3}{sup -} an exponential decrease in ionic conductivity is observed with higher sodium content. The dependence of ionic conductivity on water content is also reported for sodium contents of 0, 0.27, 0.62 and 1Na{sup +}/SO{sub 3}{sup -}.

Overview and Current Status of Analyses of Potential LEU Design Concepts for TREAT

Neutronic and thermal-hydraulic analyses have been performed to evaluate the performance of different low-enriched uranium (LEU) fuel design concepts for the conversion of the Transient Reactor Test Facility (TREAT) from its current high-enriched uranium (HEU) fuel. TREAT is an experimental reactor developed to generate high neutron flux transients for the testing of nuclear fuels. The goal of this work was to identify an LEU design which can maintain the performance of the existing HEU core while continuing to operate safely. A wide variety of design options were considered, with a focus on minimizing peak fuel temperatures and optimizing the power coupling between the TREAT core and test samples. Designs were also evaluated to ensure that they provide sufficient reactivity and shutdown margin for each control rod bank. Analyses were performed using the core loading and experiment configuration of historic M8 Power Calibration experiments (M8CAL). The Monte Carlo code MCNP was utilized for steady-state analyses, and transient calculations were performed with the point kinetics code TREKIN. Thermal analyses were performed with the COMSOL multi-physics code. Using the results of this study, a new LEU Baseline design concept is being established, which will be evaluated in detail in a future report.

Thermal Analysis of a TREAT Fuel Assembly

The objective of this study was to explore options as to reduce peak cladding temperatures despite an increase in peak fuel temperatures. A 3D thermal-hydraulic model for a single TREAT fuel assembly was benchmarked to reproduce results obtained with previous thermal models developed for a TREAT HEU fuel assembly. In exercising this model, and variants thereof depending on the scope of analysis, various options were explored to reduce the peak cladding temperatures.