지르칼로이-4 피복관 내에서 응력에 의한 수소의 확산거동 연구

Abstract

우리나라의 원자력발전은 1978년 고리 1호기에서 처음 상업운전을 시작하였으며 현재 24기의 원자력발전소가 운영 중이다. 하지만 원자력 발전소의 설비가 증대됨에 따라 사용후핵연료의 보관 및 처리가 사회적 문제로 떠오르고 있다. 이에 따라 국내에서는 사용후핵연료의 보관방법으로 건식저장을 고려하고 있다. 사용후핵연료의 건식저장은 미국, 캐나다, 독일, 러시아 등 많은 나라에서 이미 시행 중이며 국내에서도 중수로(Pressurized Heavy Water Reactor, PHWR)의 사용후핵연료를 건식저장 방법으로 보관하고 있다. 하지만 원자로 운전이 장주기, 고연소 운전으로 바뀜에 따라 핵연료 피복관에 함유된 수소의 농도가 증가하게 되었다. 피복관 내에 고용된 수소는 사용후핵연료 건식저장 중 피복관의 건전성에 영향을 주는 주요 인자로 고려되고 있다. 따라서 많은 연구자들이 수소고용도(terminal solid solubility, TSS), 수소화물 재배열(hydride re-orientation), 지연수소균열 (delayed hydride cracking, DHC) 등 수소에 의해 발생하는 사용후핵연료의 열화기구에 대해 연구하였다. 특히 Puls와 Shi, Sagat, Shmakov, McRae, Kim 등이 지르코늄 합금 내의 수소의 거동에 관한 이론적인 연구를 수행하였다. Puls 외 다수의 연구자들은 수소의 확산이 온도 구배와 농도 구배, 그리고 응력 구배에 의해 발생된다고 주장하였다. 온도와 농도가 일정하다면 수소가 응력 구배가 높은 곳으로 확산하며, 확산된 수소가 TSSP(TSS for precipitation)를 넘게 되면 수소화물로 석출된다고 설명하였다. 반면 Kim 은 수소의 확산은 온도 구배와 농도 구배에 의해서만 발생되며 등온 상태에서 응력 구배에 의해서는 수소의 확산이 발생하지 않는다고 주장하였다. 단지 균열선단에서 발생하는 응력 집중이 지르코늄 합금의 내에 존재하는 과포화 상태로 있는 수소의 석출을 야기시키며, 이로 인해 발생한 농도 구배에 의해 수소의 확산이 발생한다고 설명하였다. 이에 Kammenzind 외 여러 연구자들은 등온 상태에서 응력구배에 의한 수소의 확산을 확인하기 위한 실험을 진행하였으나 뚜렷한 수소의 확산을 관찰하지 못하였다. 본 연구에서는 응력에 의한 수소의 확산여부를 확인하기 위해 링 인장시험을 수행하였으며 등온조건, 예열조건 등 다양한 열적 조건에서 Zircaloy-4 합금 내의 수소의 확산을 관찰하였다. 등온조건에서는 수소의 확산이 관찰되지 않았지만 예열조건에서는 일부 시편에서 수소의 확산이 관찰되었다. 이를 통하여, 고용된 수소의 농도, 실험 온도 및 온도 이력, 응력 구배 등에 의해 수소의 확산이 발생하는 문턱조건(threshold)이 존재하며, 이를 만족할 때 등온에서도 응력 구배에 의해 수소가 확산할 수 있음을 확인하였다. 본 연구의 결과를 통하여 응력이 수소의 거동에 미치는 영향을 이해하는데 도움이 될 것으로 판단된다. 특히 사용후핵연료 2차 손상기구인 DHC 현상의 예측과 정밀한 모델 개발에 응용할 수 있을 것이다.|The first commercial nuclear power plant (NPP), Kori #1, has been operating since 1978 and 24 of NPP is operating in recent. However, due to increase of NPP, interim storage and disposal of spent nuclear fuel (SNF) remains to be a social issue. Many countries such as Unite State, Canada, Germany, and Russia already have implemented dry storage of SNF and Korea also has stored SNF of pressurized heavy water reactor (PHWR) using dry storage. Unfortunately, fuel cladding contains much more hydrogen than before because reactor operating changed from low burn-up to high burn-up. Absorbed hydrogen is considered as one of major factors which degrade the fuel cladding integrity. Therefore, many researchers had conducted hydrogen related SNF degradation mechanism such as terminal solid solubility (TSS), hydride re-orientation and delayed hydride cracking (DHC). In particular, Puls, Shi, Sagat, Shmakov, McRae and Kim conducted research on hydrogen behavior in zirconium alloys. Many researchers including Puls claimed hydrogen diffusion is driven by temperature gradient, concentration gradient and stress gradient. Hydrogen is diffused by chemical potential gradient which constituted temperature, concentration and stress then hydrogen is precipitated as a hydride form when hydrogen concentration reaches TSS for precipitation. In contrast, Kim claimed hydrogen diffusion is driven by not stress gradient but temperature gradient and concentration gradient. He also claimed stress causes the precipitation of supersaturated hydrogen near crack tip and driving force of hydrogen migration is the concentration gradient between crack tip and bulk region. Hence many researchers including Kammenzind conducted experiments to investigate the relationship between stress gradient and hydrogen migration. But they did not find the relationship clearly. In this study, ring tensile tests were conducted under various thermal conditions such as isothermal conditions and thermal cycled conditions to investigate the relationship between stress gradient and hydrogen diffusion in Zircaloy-4 cladding. No hydrogen diffusion was observed under isothermal conditions, but hydride accumulation was observed under some of thermal cycled conditions. According to the results, it is estimated that there is some threshold condition to cause hydride accumulation and hydrogen is diffused by stress gradient when threshold condition is satisfied. This results is useful to understand the relationship between stress and hydrogen behavior. In particular, this result useful to predict DHC phenomenon and develop accurate model.; The first commercial nuclear power plant (NPP), Kori #1, has been operating since 1978 and 24 of NPP is operating in recent. However, due to increase of NPP, interim storage and disposal of spent nuclear fuel (SNF) remains to be a social issue. Many countries such as Unite State, Canada, Germany, and Russia already have implemented dry storage of SNF and Korea also has stored SNF of pressurized heavy water reactor (PHWR) using dry storage. Unfortunately, fuel cladding contains much more hydrogen than before because reactor operating changed from low burn-up to high burn-up. Absorbed hydrogen is considered as one of major factors which degrade the fuel cladding integrity. Therefore, many researchers had conducted hydrogen related SNF degradation mechanism such as terminal solid solubility (TSS), hydride re-orientation and delayed hydride cracking (DHC). In particular, Puls, Shi, Sagat, Shmakov, McRae and Kim conducted research on hydrogen behavior in zirconium alloys. Many researchers including Puls claimed hydrogen diffusion is driven by temperature gradient, concentration gradient and stress gradient. Hydrogen is diffused by chemical potential gradient which constituted temperature, concentration and stress then hydrogen is precipitated as a hydride form when hydrogen concentration reaches TSS for precipitation. In contrast, Kim claimed hydrogen diffusion is driven by not stress gradient but temperature gradient and concentration gradient. He also claimed stress causes the precipitation of supersaturated hydrogen near crack tip and driving force of hydrogen migration is the concentration gradient between crack tip and bulk region. Hence many researchers including Kammenzind conducted experiments to investigate the relationship between stress gradient and hydrogen migration. But they did not find the relationship clearly. In this study, ring tensile tests were conducted under various thermal conditions such as isothermal conditions and thermal cycled conditions to investigate the relationship between stress gradient and hydrogen diffusion in Zircaloy-4 cladding. No hydrogen diffusion was observed under isothermal conditions, but hydride accumulation was observed under some of thermal cycled conditions. According to the results, it is estimated that there is some threshold condition to cause hydride accumulation and hydrogen is diffused by stress gradient when threshold condition is satisfied. This results is useful to understand the relationship between stress and hydrogen behavior. In particular, this result useful to predict DHC phenomenon and develop accurate model.