A Study on Hydrogen Induced Integrity Degradation Mechanism of Zirconium Alloy Cladding

(1) ????????? ?????? ??? ???????????? ?????? ???????????? ???????????????????????? ?????? ????????? ?????? ????????? ?????? ??? grid-to-rod fretting, debris fretting ?????? ????????? ??????, ?????? ?????????-????????? ????????? ???????????? (PCMI) ?????? ???????????? ????????? ???????????? ?????? ????????? ???????????? ??????????????? ????????? ??? ??????. ????????? ???????????? ??? ????????? ?????? ????????? ????????? ???????????? ?????? ???????????? ???????????????, ???????????? ????????? ????????? ????????? ?????? (gap) ????????? ????????? ????????? ??????????????? ????????????. ??? ???????????? ???????????? ??????????????? ???????????? ????????? ?????? ???????????? ?????? ????????? ????????????. ??? ??? ??????/????????? ???????????? ?????? ?????? ??? ????????? ?????? ?????????-??????????????? ?????? ???????????? ?????? ?????? ????????? ???????????? ????????? ???????????????????????????????????? ????????? ??????. ?????? ?????? ????????? ????????? ?????? ????????? ?????? ?????? ???????????? 90???????????? ??????????????? ???????????? ??????. ?????? ????????? ????????? ???????????????????? ???????????? ?????? ???????????? ?????????, ??? ????????? ?????? ????????? ????????? ???????????? ???????????? ?????? ????????? ???????????? ???????????? ??????. ????????? ????????? ?????? ??????????????? ????????? ????????? ????????? ????????? ??????????????? ???????????????????????? ?????? ????????? ??????????????? ????????? ???????????? ??????. ????????? ?????? ??????????????? ?????? ????????? ???????????? ?????? ?????? ????????? ???????????? ??????/????????? ???????????? ???????????????. ????????? ????????? ????????? ??????????????????, ???????????? ???????????? ????????? ??????????????? ???????????? ?????? ????????? ??????????????? ???????????? ????????? ????????? ????????? ??????????????? ?????? ?????? ????????? ??????????????? ???????????????????????? ???????????? ????????? ??????????????? ???????????? ?????????. ???, ?????? ?????? ?????????-?????? ????????? ????????? ?????? ??????????????? ???????????? ?????? ????????? ?????? ???????????? ????????????. ??? ???????????????, ??? ????????? ???????????? ?????? ??????????????? ???????????????????????? ???????????? ????????? ????????? ?????? ????????? ???????????????. ????????? ????????? TGA ????????? ?????????????????? ??????????????? ?????? ????????? ??????????????? 1 ??g??? ???????????? ????????? ????????? ??? ????????? ??????????????????, ??? ??????????????? ??????/????????? ??????????????? ???????????? ????????? ????????? ?????? ?????? Zircaloy-4 ???????????? ????????? ???????????????. ??????/????????? ???????????? 104 ????????? ?????????-?????? ???????????????, ?????? ??? ????????? ?????? ????????? ????????? ????????????, ?????? ???????????? ?????? ??? ????????? ????????? ??????????????? ???????????? ????????? ??????????????? ??????????????? ???????????????. ??????, ??????/????????? ???????????? 5??102 ?????? 103??? ????????????, ????????? ????????? ?????? ????????? ????????? ??????????????? ?????????????????? (1) ?????? ????????? ??????, (2) ??????-?????? ???????????? ?????? ??????, (3) ?????? ????????? ????????? ??????????????? ???????????? 3?????? ???????????? ???????????? ???????????????. ????????? ????????? ????????? ????????? ?????? ???????????? ????????? ??? ??????????????? ?????? ?????? ??????, ???????????? ?????? ??????, ????????? ????????? ?????? ?????? ?????? ?????? ?????? ?????? ????????????. ?????? ??????????????? ??????, ?????? ???????????? ???????????? ?????? ????????? ????????? ????????? ????????? ??????????????? ?????? ???????????? ?????? ????????? ??????????????? ?????????. ????????? ????????? ??????/????????? ???????????? ??????????????? ???????????? ?????? ?????????????????? ?????? ?????? ?????????, ???????????? ?????? ????????? ????????? ????????? ????????? ?????????????????? ????????? ????????? ????????? ????????? ?????? ????????? ???????????? ??????????????? ?????? ????????????. ???, ??? ???????????????, ?????? ????????? ?????? ??????/????????? ?????? ???????????? ???????????????????????? ??????????????? ????????? ???????????? ??? ?????????, ???????????? ????????? ?????????-?????? ????????? ?????? ????????????????????? ????????? ??????????????? ???, ???????????? ??????????????? ???????????? ????????? ???????????? ????????? ?????? ???????????? ???????????????. ??????????????? ??? ??????????????? ????????? ????????? ?????? ??????/????????? ???????????? ?????? ????????? ????????? ???????????? ??? ?????? ????????? ?????? ????????? ???????????????, ?????? ????????? ?????? ?????? ?????? ???????????????????????? ?????????????????? ????????? ????????? ?????? ?????? ???????????? ???????????? ????????? ???????????????. ???????????????, ?????? ?????????????????? ?????? ????????? ???????????????. (2) ?????????????????? ???????????? ??? ???????????? ????????? ??????????????? ?????? ?????? ????????? ?????? ????????? ?????? ??????????????? ?????? ????????? ????????????????????? ?????? ?????? ??? ?????? ???????????? ?????? ???????????? ??????. ?????? ?????????????????? ???????????? ????????? ?????? ????????? ??????, ????????? ?????? ?????? ?????? ??? ??????????????? ?????? ????????? ??????????????????, ??? ????????? ????????? 2024??? ?????? ?????? ????????????. ?????? ????????? ????????? ?????? ????????? ?????? ????????? ????????? ???????????? ?????????????????? ?????? ?????? ?????? ????????? ?????? ????????? ??????????????? ?????? ?????? ?????? ???????????? ????????? ???????????????. ??? ???????????? ????????? ???????????? ???????????? ???????????? ??????, ??????, ?????? ??? ??????????????? ?????? ????????? ???????????? ??????. ????????? ???????????????, ?????? ???????????? ????????? ?????? ????????? ?????? ???????????? ?????? ???????????? ???????????? ????????? ?????? ?????? ?????? ????????? ???????????? ??? ?????????????????? ???????????? ?????? ??? ???????????? ???????????? ?????? ????????? ?????????. ???????????? ?????? ?????? ????????? ?????? ?????? ?????? ??? ?????? (decay heat)??? ?????? ???????????? ????????? ??? 400 ??? ??????????????? ??? ??? ????????? ??? ?????????????????? ???????????? ?????? ??????????????? ?????? ????????? ?????? ???????????? ???????????? ??? ?????? ???????????? ????????? ???????????? ???????????? ????????????. ????????? ???????????? ????????? ????????? ?????? ???????????? ?????? ???????????? ???????????? ???????????? ???????????? ???????????? (hoop stress)??? ?????????. ?????? ??????????????? ???????????? ???????????? ????????? ????????? ???????????? ?????? ?????? ????????? ?????? ???????????? ??????????????? ??????????????? ????????????. ????????? ??????-????????? ???????????? ?????? ???????????? (threshold stress) ????????? ??????????????? ???????????? ???????????? ??????????????? ????????? ?????????????????? ??????????????? ??????????????? ????????? ????????? ??????????????? ????????????????????? ??????. ????????? ????????? ???????????? ??????????????? ????????? ????????? ????????? ????????? ????????? ???????????? ????????? ????????? ????????? ??????????????? ?????????, ???????????? ???????????? ?????????????????? ???????????? ?????? ?????? ???????????? ???????????? ???????????? ????????? ?????? ?????? ?????????????????? ????????????. ??? ???????????????, ???????????? ???????????? ?????? ?????? ?????? ?????? ??????????????? ?????? ?????? ????????? ?????? ????????? ????????? ???????????? ?????????, ???????????? ????????? ????????? ?????? ????????? ???????????? ?????? Zircaloy-4??? ?????? ??? ?????? ????????? ???????????????. ?????? ??????????????? ???????????? ?????? ????????? ???????????? ????????????, ????????? ????????? ?????? (?????? ??????????????? ???????????? ????????? ??????????????????????????? ????????? ?????? ?????? ??? ????????? ???????????? ???????????? ???????????? ????????? ??????)??? ???????????? ??? ????????? ???????????????. ???????????????, ???????????? ??????????????? ????????? ????????? ????????? ?????? ????????? ??????????????? ?????? ????????????, ????????? ????????? ???????????? ??????????????? ????????? ????????? ?????? radial hydride continuity factor (RHCF)??? ???????????? ??? ?????? ?????? ????????? ???????????????. ?????? ????????? ????????? ?????? ????????? ???????????? ?????? ????????? ????????? ????????? ????????? ??????, RHCF??? ??????????????? ?????? ???????????? ???????????? ????????????. ???, ?????? ???????????? ???????????? ?????? ??? ?????? ?????? ?????? ?????? ???????????????, ?????? ?????? ????????? ?????? RHCF??? ???????????? ??????????????? ???????????????. ??????, ?????? ????????? ?????? ?????? ?????? ?????? ???????????????, ?????? ?????? ????????? ?????? RHCF??? ???????????? ??????????????? ??????????????? ?????? ???????????? ????????? ?????????. ??????, ?????? ?????? ?????? ??????????????? ?????? ????????? ?????? ???????????? ?????? ????????? ?????? ?????????, ?????? ???????????? ???????????? ??? ????????? ????????? ???????????? ?????? ????????? ???????????? ??????????????? ????????????. ??? ???????????????, ????????? ????????? ?????? ????????? ??? ????????? ????????? ?????? ????????? ?????? ????????? ????????? ??????????????? ????????????, ?????? ??????????????? ???????????? ?????? ???????????? ???????????? ??????????????? ???????????? ?????? ??? ???????????? ???????????? ????????? ???????????? ????????? ????????? ??????????????? ???????????????. ?????? ??? ???????????? ????????? ??????????????? ?????? ?????? ??? ?????? ?????? ???????????? ???????????? ??? ????????????????????? ???????????? ????????? ????????? ???????????? ????????? ???????????? ??? ????????? ???????????????. ???????????????, ?????? ????????? ?????? ????????? ????????? ?????? ??? ?????? ????????? ?????????.(1) Fundamental understanding on secondary hydriding mechanism of zirconium alloy cladding under in-reactor operating conditions In order to fundamentally understand the secondary hydriding mechanism of zirconium alloy cladding, the reaction of commercial Zircaloy-4 tubes with hydrogen and steam mixture was studied using a thermo-gravimetric analyzer with two variables, H2/H2O ratio and temperature. Phenomenological analysis revealed that in the steam starvation condition, i.e., when the H2/H2O ratio is greater than 104, hydriding is the dominant reaction and the weight gain increases linearly after a short incubation time. On the other hand, when the gas ratio is 5??102 or 103, both hydriding and oxidation reactions take place simultaneously, leading to three distinct regimes: primary hydriding, enhanced oxidation, and massive hydriding. Microstructural changes of oxide demonstrate that when the weight gain exceeds a certain critical value, massive hydriding takes place due to the significant localized crack development within the oxide, which possibly simulates the secondary hydriding failure in a defective fuel operation. This study reveals that the steam starvation condition above the critical H2/H2O ratio is only a necessary condition for the secondary hydriding failure and, as a sufficient condition, oxide needs to grow sufficiently to reach the critical thickness that produces substantial crack development. In other words, in a real defective fuel operation incident, the secondary failure is initiated only when both steam starvation and oxide degradation conditions are simultaneously met. Therefore, it is concluded that the indispensable time for the critical oxide growth primarily determines the triggering time of massive hydriding failure. (2) Effects of hydrogen concentration and peak temperature on threshold stress for hydride reorientation under dry storage conditions of spent nuclear fuel In the case of spent nuclear fuel stored in a long-term dry storage operation, circumferentially precipitated hydrides can re-orient to the radial direction if the applied hoop stress in the cladding exceeds a certain limit, the so-called threshold stress. This hydride reorientation has been considered one of the limiting mechanisms that can deteriorate the cladding integrity during dry storage of spent nuclear fuel. In this study, the effects of the hydrogen content and peak temperature on the threshold stress were studied using ring tension tests with Zircaloy-4 cladding tube. To simulate the thermo-mechanical history of the cladding during interim dry storage, pre-hydrided specimens with hydrogen concentrations up to 585 wppm were tested following a transient temperature from a peak temperature, 350 ??? or 400 ???, to room temperature under constant tensile load. Recently suggested superimposing method, in which the hoop stress contour line calculated by finite element modeling is superimposed on the hydride morphology micrograph, was further improved to determine the threshold stress in this study. Results show that the threshold stress is the lowest when the hydrogen concentration in the specimen is the terminal solid solubility of dissolution at a given peak temperature. In the solid solution state regime, where the hydrogen content is lower than the dissolution solubility, the stress increases with decreasing hydrogen content. In contrast, it steadily increases with increasing hydrogen content and then tends to saturate to a certain value in the supersaturated state regime, where the hydrogen content is higher. It is also revealed that the threshold stress increases as the peak temperature decreases. That is, the threshold stress is found to depend on both the hydrogen content and peak temperature. Qualitative explanations, introducing the concept of stress gradient induced hydrogen migration, are presented. Lastly, future works in this area are suggested.Docto