Abstract
This document reports the progress and current status of activity during FY 2011 for procurement and installation at ORNL of state-of-the art equipment suitable for measurement of slow oxidation rates of nuclear graphite with low concentrations of oxidants (water, oxygen etc) in high purity helium. These measurements are needed for determination of kinetic parameters of oxidation of new grades of graphites selected for NGNP and development of predictive tools of their life-time in real exposure conditions. A thermogravimetric analyzer TAG 16/18 coupled with a mass spectrometer was procured from SETARAM and installed at ORNL. This equipment was integrated with auxiliary utilities, including a system for introduction of low concentrations of water (in the ppm range) to a stream of high purity helium and measurement of partial water vapor pressure. The operation of the gas introduction system was verified with graphite specimens of known oxidation behavior. The tests demonstrated that this equipment is very well suited for measurement of slow oxidation rates, as needed for determination of kinetic constants of oxidation of the new graphite grades considered for NGNP.
First oxidation experiments of PCEA nuclear graphite with low concentrations of water vapor in ultra-high purity helium were performed. Accelerated oxidation conditions were selected, which allow extraction of kinetic for the Langmuir-Hinshelwood mechanism of graphite oxidation with water from a reasonable number of tests, within a practical duration of experiments. These tests were designed to represent conditions of normal VHTR operation, where large graphite components will be exposed to low level of water vapor in the helium coolant, over many years of life time. The first results indicate that oxidation with low concentrations of water has a more complex kinetics than oxidation with air in excess. This fact demands continuation of experiments according to a well-structured plan. However, the performance of the new system and of the additional lines for introduction and monitoring low water concentrations in helium supports the conclusion that collection of accurate data with good statistical reproducibility is possible and should be continued in 2012 – 2013. This will allow completing the characterization of the new grades of graphite (such as PCEA and NBG-18) that are being considered as candidates for the NGNP project.
