Evaluation of Hydride Reorientation Behavior and Mechanical Properties for High-Burnup Fuel-Cladding Tubes in Interim Dry Storage

Abstract

The hydride stress reorientation behavior and the mechanical properties of irradiated cladding tubes were investigated to evaluate the high-burnup fuel-cladding tube properties in interim dry storage. As for the boiling water reactor ͑BWR͒ Zircaloy-2 ͑Zry-2͒ cladding, the hydride reorientation to the radial direction occurred at relatively low hoop stresses during the hydride reorientation treatment ͑HRT͒, such as less than 70 MPa. The increase of reorientation with hoop stress was not monotonic for the specimens in which a part of the hydrides remained precipitated at the HRT temperature, such as the case for 50GWd/t type cladding at a 300°C HRT. The degree of reorientation depended on the HRT solution temperature rather than on the estimated temperature at which the hydride precipitation occurred under the relatively moderate HRT conditions. In the relatively low cooling rate HRT, the hydride preferential precipitation in the Zr liner increased for Zr lined cladding compared to that in a relatively high cooling rate. The ductility of the specimens after the 300°C HRT showed relatively good correlation to the Polymax index which reflects the length or continuity of the hydrides regardless of their orientation. The ductility of the specimens after the 400°C, 0 MPa, 30°C/h HRT increased in ring compression testing at room temperature compared to no HRT ͑as-irradiated͒ specimens, and it indicated recovery of irradiation damage occurred at the 400°C annealing temperature and affected the ductility of the irradiated Zry-2 cladding. As for the pressurized water reactor Zircaloy-4 cladding, little increase in the radial hydride ratio occurred in a 100 MPa, 340°C or less HRT. On the other hand, the amount and the length of the hydride in the midwall area of the cladding depended on the temperature and the cooling rate from the HRT due to hydrogen migration from the hydride rim area. It is deduced that the ductility in ring compression deformation was affected by the orientation, amount, and length of hydride in the midwall area.

Hydride Reorientation and Delayed Hydride Cracking of Spent Fuel Rods in Dry Storage

“…At the peak temperature of 400°C, many large radial hydrides were observed in the unirradiated and irradiated Zircaloy-2 cladding tubes, as shown in Figures 9(c) and (f), but lowering the peak temperature to 300°C suppressed precipitation of the radial hydrides, lowering the number and size of the radial hydrides in both tubes, as shown in Figures 9(b) and (e). [19] Given that creep deformation at 400°C is larger than that at 300°C, even under the same stress of 70 MPa, it is clear that the enhanced precipitation of large radial hydrides at 400°C in the HRT, as shown in Figures 9(c) and (f), is due to considerably higher creep deformation. Critical evidence for the stress effect on hydride reorientation was obtained from the HRT, where all the test conditions were the same except for the stress changing from 40 to 70 MPa, as shown in Figure 10.…”

“…To prove the experimental fact, as shown in Figure 5, that the prior creep deformation effect promoting precipitation of the reoriented hydrides in the radial direction can occur not only in the Zr-2.5Nb alloy but also in Zircaloys, Aomi's work [19] that investigated hydride reorientation behaviors of the irradiated and unirradiated cladding tubes of Zircaloys under stress was cited in this study. More details of the cladding tubes used in this test are given elsewhere.…”

“…It should be noted that the enhanced reorientation of hydrides at 400°C has nothing to do with the effect of crack tip blunting, because the tube used has the smooth surface without a deep notch or a crack. Especially for the unirradiated cladding tube, its hydrogen concentration must be lower than 25 wppm, which is the maximum allowable concentration, although the exact hydrogen concentration was not given by Aomi et al [19] Thus, it is clear that the hydrogen concentration of the unirradiated tube is so low and constant that all the hydrogen must be dissolved completely at a peak temperature of either 300°C or 400°C [19] given the fact that the hydrogen solubility for hydride dissolution at as low a temperature as 300°C is 64 ppm. [15] Thus, the hydrogen concentration effect, if any, on hydride reorientation can be disregarded.…”

“…More details of the cladding tubes used in this test are given elsewhere. [19] In the hydride reorientation test (HRT), the irradiated and unirradiated Zircaloy-2 cladding tubes that were loaded to the hoop stress of 70 MPa from the beginning of the thermal cycle (as with point A in Figure 2) were heated to the peak temperature changing from 300°C to 400°C, held there for 1 hour, and then cooled to room temperature with a faster cooling rate of 30°C/h. Thus, Aomi's HRT was conducted in the same way as in the case of Figures 5(a) and 6 as far as the loading time is concerned.…”

An evaluation of the segmented expanding cone-mandrel test to assess hydride re-orientation and ductility reduction for Zircaloy-2 cladding tubes

“…[7,8,24,30,32,39]. When re-orientation tests are done by pressuring the tubes, the stress in the tube is generally kept constant [23,25,27]. The applied load was between 1.125 kN and 2 kN, the target hold temperature was between 400 C and 500 C. Typical threshold values for re-orientation are 50e70 MPa [23e25], so the applied load should be sufficient to induce hydride reorientation at the target temperatures.…”