Tritium Permeability of Incoloy 800H and Inconel 617

Abstract

Design of the Next Generation Nuclear Plant (NGNP) reactor and its hightemperature components requires information regarding the permeation of fission generated tritium and hydrogen product through candidate heat exchanger alloys. Release of fission-generated tritium to the environment and the potential contamination of the helium coolant by permeation of product hydrogen into the coolant system represent safety basis and product contamination issues. Of the three potential candidates for high-temperature components of the NGNP reactor design, only permeability for Incoloy 800H has been well documented.To support engineering design of the NGNP reactor components, tritium permeability values for Inconel 617 and Incoloy 800H were determined in Fiscal Year 2011 using a measurement system designed and fabricated at Idaho National Laboratory. The measured tritium permeability of Incoloy 800H and Inconel 617 deviated substantially from the values measured for hydrogen. The experiments were repeated in FY-12 with Incoloy 800H using reconfigured tritium detection and in-process hydrogen measurement to reduce uncertainties from the first series of tests. The tritium permeability of Incoloy 800H was measured in the temperature range 700 to 950°C and at primary concentrations of 0.5 to 125 (atom) parts per million tritium in helium (partial pressures of 10 -7 atm) -four orders of magnitude lower partial pressure than used in the hydrogen permeation testing. The new data provide some evidence that permeation has become surface-limited, as expected for sufficiently low pressures. In this case, the resultant tritium flux is linearly related to the pressure, unlike the diffusionlimited (high pressure) regime, in which it is proportional to the square root of pressure. In the surface-limited regime, the proportionality constant is not the permeability K, but the dissociation rate constant K d . Since the present measurements appear to span at least a part of the transition region between these two extremes, a quantitative estimate of K d could not be obtained, but the theory and experimental requirements for doing so are outlined. vi vii