Hydrogen uptake by oxidized zirconium alloys

Zirconium alloys are used in CANDU (Canadian deuterium uranium) reactor pressure tubes and nuclear reactor fuel claddings. Although the alloy is usually prefilmed with a protective thin oxide, undesirable hydrogen builds up in the bulk alloy over years of operation. Deuterium transported through zirconium oxide into the bulk metal has been determined by exposing thin (similar to 1 mm thickness) prefilmed samples to a controlled gaseous environment at the relatively high temperature of 380 degrees C for 15 days. The exposures were followed by bulk alloy hydrogen analysis using differential scanning calorimetry, which is insensitive to hydrogen in the oxide. Amounts of deuterium transported through oxides grown on different substrates of zirconium and its alloys have been determined for comparison. They range from 1 to 4 mu g cm(-2). Measurable amounts of deuterium were also transported through the oxide at the lower temperature range of 200 to 330 degrees C (which encompasses the pressure tube operating range of similar to 250 to 315 degrees C) by exposure to a relatively high pressure of deuterium gas (similar to 230 to 780 kPa) for 10 days. Measured fluxes of deuterium after such exposures ranged from 1 to 14 mu g cm(-2). Through-oxide-thickness deuterium concentration profiles of the samples were obtained using secondary ion mass spectrometry. The profiles show features that may be correlated to changes in the nature of the oxide and its function as a barrier against deuterium uptake.