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The DEFEX Project started on 1 April 1993. It is an undertaking made in international co-operation, aiming at the study of secondary defect formation as a consequence of primary defects. The experiments were made in the R2 test reactor using a special simulation technique for the primary defects. Test rodlets typical of 8x8 BWR fuel with three different types of Zircaloy-2 cladding were investigated in a total of eleven tests. Standard cladding was compared to zirconium liner cladding as well as to rifled cladding, which has been proposed as a remedy against secondary failure by hydriding. Linear Heat Rate, pellet - clad gap and irradiation time were the principal test parameters. The rodlets were previously unirradiated.
The DEFEX Project was preceded by two exploratory test series, financed by Swedish sponsors. Defect Fuel Experiment No. 1 was performed in December 1990 and March 1991. Defect Fuel Experiment No. 2 was performed between June 1992 and January 1993.
The non-destructive measurements that were made include on-line elongation measurement, profilometry measurement, eddy current scanning, neutron radiography, gamma scanning, visual inspection and gap squeeze measurement.
Destructive examinations included metallography, fuel grain size measurements, determination of fuel porosity distribution, cladding hydrogen determination by SEM, Electron Probe Micro Analysis in one case, mass spectrometry of residual gas and UO2 oxidation determination by X-ray powder diffraction.
Four of the DEFEX test rodlets, all with 0.15 micron gap but with different types of cladding, failed in conjunction with extensive hydriding at Linear Heat Rates of 40 and 50 kW/m. For three of these rodlets, with standard and rifled cladding, time to failure varied between 76 and 102 hours. The fourth rodlet, with liner cladding, failed after 216 hours. The hydriding degradation process could be followed by studying the elongation curve. It was observed that those rodlets that failed early exhibited a steeper increase after the initial features than those that did not fail. This fact is ascribed to intensive hydriding of the cladding. For the liner rodlet, which remained intact for the longest time, the elongation curve had a similar shape in the beginning, but after two days' irradiation it levelled off. The ensuing elongation is ascribed to the influence of irradiation growth.
The oxidation behavior of the rodlets with liner cladding and those with standard cladding was not very different at the low burnups of the DEFEX Project. However, the liner cladding showed less hydrogen pick-up than the standard cladding.
The Final Report gives diagrams of actual rods and dimensions.
A summary of the experiments is given in Table 4.1 of DEFEX16 report.
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Keywords: boiling water reactor, defect, elongation, zircaloy.