석출된 수소화물이 지르칼로이-4 피복관의 기계적 물성에 미치는 영향

Abstract

A spent nuclear fuel cladding discharged from a pressurized water reactor contains hydrogen around 150-800 wppm which is dependent on the burn-up and power history. During long-term dry storage, the cladding temperature slowly decreases with decay heat. As the temperature decreases, the hydrogen is precipitated in Zr-matrix below the terminal solid solubility (TSS). Precipitated hydride can threat the fuel integrity due to embrittlement. Therefore, in this paper, hydride reorientation and its effects on the mechanical property were studied. The TSS of hydrogen in zirconium alloy has a hysteresis. In order to understand the hydride reorientation, the TSS of hydrogen in Zircaloy-4 during cooling and heating were measured by the differential scanning calorimetry (DSC) with a hydrogen content of 40-731 wppm. A significant hysteresis gap was observed between the TSS for dissolution (TSSD) and precipitation (TSSP). It was confirmed that the hydrogen dissolution temperature was unaffected by previous thermal history with respect to hydride precipitation temperature. The TSSP temperature was increased with decreasing maximum temperature but a temperature gap still existed even when the maximum temperature was equal to the TSSD temperature. The terminal solid solubility can be represented by the following equations. TSSD: C= 2.255×105 exp(-39101/RT), TSSP: C= 4.722×104 exp(-26843/RT), TSSP2: C= 8.612×104 exp(-30583/RT) In addition, it can be concluded that hydride can be precipitated without a temperature gap on cooling when hydrides are present in the matrix, similarly, hydride can also dissolve quasi-immediately on reheating. However, it should be noted that hydrogen dissolution and precipitation curves tend to follow the intrinsic dissolution and precipitation solvus independent of thermal history. The study of the radial hydride formation was performed in cold worked and stress relieved (CWSR) Zircaloy-4 cladding with the help of the ring tensile and finite element method. Un-irradiated Zircaloy-4 cladding ring specimens charged with hydrogen were tested at various temperatures and loading conditions. The morphology of hydride revealed that reorientation of the hydride occurred where an applied hoop stress is higher than 60±5 MPa precipitated at 400 ºC. The threshold stress for hydride reorientation tends to decrease with increasing temperature. The threshold stress line from this study is close to the lower bound of irradiated cladding results, shown in literatures. It is also important to note that hydride reorientation did not occur between the terminal solid solubility for dissolution and precipitation temperature. After extended long-term dry storage, hydrides may significantly reduce cladding ductility and impact resistance especially when the radial hydrides are present in the material. In this study, the ring compression tests (RCTs) were conducted in hoop direction with circumferential and radial hydrided Zircaloy-4 cladding at the 25, 150, and 300 C. The RCT with circumferential hydrided specimen is very brittle at room temperature but Zircaloy-4 cladding can deform significantly at 150 C. It is concluded that circumferential hydride did not affect significantly above 150 C, however, the RCT with radial hydrided specimen at 150 C clearly shows that radial hydrides can act as a crack initiation path. The fracture energy/area of RCTs with radial hydrided Zircaloy-4 clearly shows that the brittle to ductile transition occurs between 200 and 250 C.