Abstract
The hydrogen concentration in zirconium alloys used in nuclear fuel cladding can be determined quantitatively, rapidly and non-destructively with a spatial resolution of 25 μm by neutron radiography. Because the total neutron cross section of hydrogen is more than one order of magnitude higher than that of zirconium, low concentrations of hydrogen can be measured. Calibration measurements have been performed and confirm the linear dependence of the total macroscopic neutron cross section on the hydrogen to zirconium atomic ratio as expected from theoretical analysis. The fast and non-destructive character of the method provides the possibility of in situ investigations. Examples of in situ investigations of the diffusion of hydrogen into Zircaloy-4 and of hydrogen uptake during steam oxidation are given. For the steam oxidation experiments a fast hydrogen uptake was found during the first 10 s. After this first step, the hydrogen concentration in the sample decreases slightly. If breakaway oxidation occurs, additional hydrogen absorption takes place. The hydrogen concentration can then increase by nearly one order of magnitude by this process. The influence of variations in the total gas flow and the steam flow on the hydrogen uptake was investigated, too.
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