시차주사 열량측정법을 통한 지르코늄 합금의 수소 고용도한계 분석

Abstract

지르코늄 합금은 낮은 중성자 흡수 단면적과 고온에서의 우수한 내식성 및 기계적 특성을 가지고 있기 때문에 원자력발전소에서 구조재료로 사용되어 왔다. 원자력발전은 고연소도 장주기 운전으로 전환됨에 따라 핵연료 피복관은 더욱 가혹한 환경을 겪게 되었다. 원자력발전소 운전조건뿐만 아니라 건식저장 시에도 지르코늄 합금 내 수소 침투(hydrogen pick-up)는 주요 이슈이다. 핵연료 피복관내 수소 농도가 고용도한계(TSS)를 초과하게 되면 취성(brittle)의 수소화물이 석출하게 된다. 원자력발전소 운전조건에서는 상대적으로 낮은 고용도를 나타내는 피복관 바깥쪽으로 수소가 집중되고, 이로 인해 취성의 수소화물이 석출되어 피복관 건전성을 위협할 수 있다. 건식저장 조건에서는 진공건조(vacuum drying) 과정에서 고용되어 있던 상당량의 수소가 냉각이 진행됨에 따라 석출되고, 수소화물 재배열(hydride re-orientation)과 지연수소균열(delayed hydride cracking) 등과 결부되어 피복관 건전성을 위협할 수 있다. 또 지르코늄 합금의 경우 용해 시(TSSD)와 석출 시(TSSP)에 따라 수소 고용도 한계(TSS) 결과가 상당한 이력현상(hysteresis)을 나타내는 것으로 알려져 있다. 다시 말해, 승온 시와 냉각 시에 수소 고용도한계가 상당한 차이를 보인다는 것이다. 그럼에도 불구하고, Zircaloy-2와 Zr-Nb 합금을 제외한 다른 금속의 데이터는 부족한 실정이고, 석출 시보다는 용해 시의 고용도에 대한 연구결과가 많았다. 심지어 ZIRLO와 HANA 피복관의 고용도한계 데이터는 찾기 어려웠다. 본 연구에서는 시차주사 열량측정법(differential scanning calorimetry)을 통하여 Zircaloy-4, Optimized ZIRLO, HANA-6 피복관의 수소 고용도한계를 측정하였다. 피복관 종류에 따라 고용도한계에 차이가 나타나긴 했지만 그 정도는 미미하게 나타났고 TSSD와는 달리 TSSP의 경우는 합금조성에 민감한 경향을 나타냈다. 또 TSSP의 경우 최대온도(Tmax)가 증가함에 따라 수소화물이 석출되기 시작하는 온도가 감소하는 경향을 나타냈다. TSSP는 최대온도에 따라 차이가 나타나지만 TSSP(max)와 TSSP(min)의 범위에 존재할 것으로 예상되었고, TSSD와 TSSP 간의 이력현상의 정도는 최대온도가 높아질수록 증가하고, 수소농도가 증가할수록 감소하는 경향을 나타냈다.| Due to low neutron absorption cross section, good mechanical properties at high temperature and proper corrosion resistance, zirconium-based materials have been used for nuclear core structural components. Hydrogen pick-up in zirconium alloy is one of the most key issues in reactor operation and dry storage with the trend of high burn-up/extended fuel cycle for improved fuel utilization in pressurized water reactor. If the cladding experiences sufficiently high temperature conditions, most of the hydrogen can dissolved. However, the hydrogen precipitates out as brittle hydride phase when the terminal solid solubility (TSS) of hydrogen is exceeded. In dry storage conditions, the spent fuel cladding temperature increases during the vacuum drying process, and the temperature then slowly cools down to room temperature. During the cool-down, the brittle hydrides can be precipitated. These precipitated hydrides can reduce the ductility of the cladding, and threaten the integrity of cladding including hydride re-orientation and delayed hydride cracking. The current database for TSS in zirconium alloys shows that there is a significant hysteresis depending on whether hydrides are dissolving or precipitating. In other words, the TSS does not have a unique value according to direction of approach to temperature. However, these studies mainly devoted to the TSS of Zircaloy-2 or Zr-Nb alloys in low hydrogen content range, and were about the TSS of hydrogen for dissolution (TSSD) rather than precipitation (TSSP). In the present study, data of the TSS of hydrogen in Zircaloy-4, Optimized ZIRLO and HANA-6 were derived by differential scanning calorimetry(DSC). Regarding the influence of alloying on TSS, there was only a subtle difference in TSS behaviors among Zircaloy-4, Optimized ZIRLO and HANA-6. The TSSP seems to be sensitive to alloying effect compared with the TSSD. In addition, the TSSP decreased with increasing maximum temperature. The magnitude of hysteresis of hydrogen solubility increased with increasing maximum temperature.; Due to low neutron absorption cross section, good mechanical properties at high temperature and proper corrosion resistance, zirconium-based materials have been used for nuclear core structural components. Hydrogen pick-up in zirconium alloy is one of the most key issues in reactor operation and dry storage with the trend of high burn-up/extended fuel cycle for improved fuel utilization in pressurized water reactor. If the cladding experiences sufficiently high temperature conditions, most of the hydrogen can dissolved. However, the hydrogen precipitates out as brittle hydride phase when the terminal solid solubility (TSS) of hydrogen is exceeded. In dry storage conditions, the spent fuel cladding temperature increases during the vacuum drying process, and the temperature then slowly cools down to room temperature. During the cool-down, the brittle hydrides can be precipitated. These precipitated hydrides can reduce the ductility of the cladding, and threaten the integrity of cladding including hydride re-orientation and delayed hydride cracking. The current database for TSS in zirconium alloys shows that there is a significant hysteresis depending on whether hydrides are dissolving or precipitating. In other words, the TSS does not have a unique value according to direction of approach to temperature. However, these studies mainly devoted to the TSS of Zircaloy-2 or Zr-Nb alloys in low hydrogen content range, and were about the TSS of hydrogen for dissolution (TSSD) rather than precipitation (TSSP). In the present study, data of the TSS of hydrogen in Zircaloy-4, Optimized ZIRLO and HANA-6 were derived by differential scanning calorimetry(DSC). Regarding the influence of alloying on TSS, there was only a subtle difference in TSS behaviors among Zircaloy-4, Optimized ZIRLO and HANA-6. The TSSP seems to be sensitive to alloying effect compared with the TSSD. In addition, the TSSP decreased with increasing maximum temperature. The magnitude of hysteresis of hydrogen solubility increased with increasing maximum temperature.