Zircaloy in boiling water reactor (BWR) systems often exhibits, besides slight uniform corrosion, a nodular type of corrosion. Highest nodular corrosion was found with materials never β-quenched during their fabrication, whereas β-quenched materials generally showed an improved behavior. Results of nodule thickness measurements after increasing burnup were normalized by using a correlation w ∼ (BU)^0.7 (w = average nodule oxide thickness, BU = burnup). The scatterband of the normalized data was different for different reactors. The statistical distribution was generally Gaussian with a tail at high values.

To find reasons for the material-inherent scatter of the in-pile corrosion behavior, archive samples were examined by electron microscopy and high-pressure steam tests. The latter led to a comparable ranking if the specimens were fully recrystallized before testing. The microstructural examinations indicate a correlation of the in-pile corrosion with the size distribution of the intermetallic precipitates and the spacing between the precipitates.

High-pressure steam tests have been used to identify the fabrication steps which influence nodular corrosion. According to these tests it is mainly the β-quenching treatment and all annealing or working steps in the upper α-phase range that have an influence.

Low nodular corrosion was achieved using quenching rates >5 K/s if the subsequent annealing stays in the low α-range. In the cold-worked condition, nodular corrosion in general was less pronounced. However, with respect to the in-reactor performance, no marked difference was found between stress-relieved and fully recrystallized cladding tubes.

For actual practice it is concluded that an appropriate β-treatment is needed and that temperature treatments of the material after its last β-quenching should stay within a range in which the distribution of the alloying elements is not affected unduly. Several in-pile programs are under way in cooperation with utilities to confirm the progress achieved.