Tritium Permeability of Incoloy 800H and Inconel 617
Tritium Permeability of Incoloy 800H and Inconel 617
The design of the NGNP reactor and high-temperature components must consider the permeation of fission-generated tritium through the high-temperature components such as the heat exchanger. Loss of fission-generated tritium represents an environmental release and hydrogen contamination of the helium coolant. To support engineering design, the tritium permeability of Incoloy 800H and Inconel 617 has been measured, and compared to test values established for these materials in hydrogen permeation.
A system was designed, fabricated, and tested for measuring the permeability of metal alloys at high temperatures. The system uses counter-flow gas loops with calibrated mass flow and pressure controllers, an induction heater to heat the tubular sample to temperatures in the range 650 to 950°C, an ion chamber for measuring tritium concentration in the loops, an internal calibrated thermocouple for controlling the sample temperature, and an infrared thermal imaging system for measuring the temperature distribution in the heated sample. The operation of the system was tested and validated using hydrogen, and tritium testing was completed in FY 2011.
When operating with tritium in helium produced from mixing source gas from the Tritium Storage and Assay system, there is uncertainty regarding the concentration and uniformity of the mixture. There is an indicated offset in the permeability constant based on the values shown on the ion chamber monitor system. This offset is nominally two orders of magnitude lower than that expected from the calculated concentration when compared to the previously established hydrogen permeation values. The offset is reduced when adjustment is made for the difference between the indicated values for the secondary ion chamber and the summed activity from primary and secondary measured by the ion chamber located in the tritium recovery hood.
A significant unknown in the system response is the background concentration of hydrogen in the system. Although a nominal 15 ppm H2 was reported for the system during hydrogen testing, definite measurement of a low concentration of hydrogen and tritium in a helium mixture is generally beyond conventional mass spectroscopy or thermal conductivity analytical techniques.
There is a possibility that calibration of the ion chambers using electronic inputs and testing with a cesium-137 gamma ray source does not reflect the operating conditions. It may be necessary to acquire a helium-tritium calibration source gas to validate the ion chamber response. It may also be possible to use a gas mixture that was produced for the TMIST test program to minimize uncertainty of mixing.
Rearrangement of the current system to include an ion chamber to monitor primary loop tritium concentrations is also recommended.
The system was used to measure the permeability of Incoloy 800H and Inconel 617 and at primary tritium partial pressures as low as 0.15 Pa, significantly below the hydrogen pressures used in the published data. Upper and lower bound fits were obtained for both Incoloy 800H and Inconel 617, which made different assumptions about the measured primary tritium concentration and the background hydrogen concentration, both of which influence tritium permeability. The temperature-independent permeability constant and the activation energy were determined to be as 0.01 cm3 hydrogen (STP)/cm·sec·atm1/2 and 25.1 kcal/mol (lower), and 0.10 cm3 hydrogen (STP)/cm·sec·atm1/2 and 27.1 kcal/mol (upper), for Incoloy 800H. For Inconel 617, values of 0.15 cm3 hydrogen (STP)/cm·sec·atm1/2 and 29.8 kcal/mol (lower), and 6.6 cm3 hydrogen (STP)/cm·sec·atm1/2 and 35.6 kcal/mol (upper) were obtained. The upper values for both alloys give permeability that are still considerably lower than published values in the temperature range of interest. Due to uncertainties in the measurements, it is not presently known whether this represents a real decrease in permeability at low tritium pressures.
As a result, the permeation measurements made in hydrogen should be considered the valid data set for use in any assessment until the issues associated with the tritium measurements can be resolved. If funding is available in the future the cause of the anomalous tritium permeability will be investigated.