Thermal Creep of Irradiated Zircaloy Cladding

Abstract

As part of an effort to investigate spent-fuel behavior during dry-cask storage, thermal creep tests are being performed with defueled Zircaloy-4 cladding segments from two pressurized water reactors — Surry at ≈36 GWd/MTU burnup and H. B. Robinson at ≈67 GWd/MTU burnup, with corresponding fast (E > 1 MeV) fluence levels of 7×1025 and 14×1025 n/m2. The Surry rods are particularly relevant because they were stored in an inert-atmosphere (He) cask for 15 years. The Robinson rods were received after reactor discharge and pool storage. Commensurate with their high burnup, the Robinson cladding has significant waterside corrosion and hydrogen uptake. Test results to-date indicate good creep ductility for both claddings in the 360–400°C and 160–250 MPa (hoop-stress) regime. Partial recovery of radiation hardening may have occurred during the long tests at 400°C, which led to improved creep ductility. Creep-rate sensitivity is significant for stress and even more so for temperature. The higher hydrogen content in the Robinson material appears to have no detrimental effect on creep behavior at the test temperature. One Robinson sample, which ruptured in the weld region at 205°C during cooling from 400°C under stress (190 MPa), precipitated all visible hydrides in the radial direction.

Results of Thermal Creep Test on Highly Irradiated Zirlo

“…A comparison of these results with the equivalent data from the Zr-4 H.B. Robinson rod [6], with similar burnup and hydrogen content as the ZIRLO rod, may suggest similar creep behaviour of Zr-4 and ZIRLO.…”

“…Although according to existing research [6], significant annealing should be observed after 72 hours at 420ºC, this effect is not discernible after 330 hours at 400ºC or an even longer period, 1000 hours, at 380ºC. Therefore, it may be concluded that the performed microhardness evaluation would support the idea that the acceleration on the measured creep is not due to the thermal annealing of the irradiation damage.…”

“…Furthermore, the selected conditions allow a direct comparison with some of the results obtained by Tsai et al [6] on the CWSR Zr-4 H.B. Robinson rod with a similar burnup.…”

“…Thus, this problem has been actively investigated [2][3][4][5][6][7] and considered in the operational limits established for the transportation and storage of spent fuel [8,9].…”

Review on Spent Nuclear Fuel Performance and Degradation Mechanisms under Long-term Dry Storage

“…사용후핵연료의 건전성에 영향을 미칠 수 있는 요인 들로는 크립(creep) [5][6][7]과 수소화물재배열 [8][9][10], 지연수 소화균열(Delayed Hydride Cracking, DHC) [11][12][13], 응력 부식균열 [14,15], 산화 및 부식에 의한 파괴 등을 들 수 있 다 [16]. 미국 원자력안전규제위원회(U.S. Nuclear Regula- …”