후프 응력이 사용후핵연료 피복관 내 수소화물 재배열에 미치는 영향

Abstract

국내 사용후핵연료의 경우 중수로 사용후핵연료를 제외한 거의 대부분의 경수로 사용후핵연료는 원자력발전소 내 습식 저장조에 보관되어 왔다. 그러나 습식저장조의 저장 능력 한계 때문에 점차 포화되고 있는 실정이다. 이에 따라 그 대안으로 장기 건식저장이 심도 있게 검토되고 있다. 이러한 장기 건식저장 시 수소화물 재배열(hydride reorientation)은 사용후핵연료의 피복관을 열화 시키는 주요 열화 기구로써 피복관의 반경방향 연성을 감소시켜 수송 시 발생할 수 있는 사고에 악영향을 미칠 수 있다. 따라서 사용후핵연료를 장기 건식저장하고 있는 국가들에서는 수소화물 재배열이 발생을 규제하고 있다. 그러나 국내에서는 아직 이 분야에 대한 본격적인 연구가 이루어지고 있지 않다. 따라서 본 연구에서는 국내 사용후핵연료의 주요 재료인 지르칼로이-4(Zircaloy-4) 피복관을 대상으로 링 인장 시험과 유한요소 해석을 통해 분석한 링 시편 내 응력 분포로 온도와 응력이 수소화물 재배열에 미치는 영향을 평가하였다. 연구 결과 일반적으로 알려진 바와 같이 후프 응력(hoop stress)이 발생하는 조건하에서는 원주방향의 수소화물(circumferential hydride)이 반경방향 수소화물(radial hydride)로 재배열되었으며 응력이 제거될 경우 반경방향의 수소화물 역시 원주방향으로 재배열되는 것을 확인하였다. 온도에 의한 영향을 확인 해 본 결과 온도가 낮아질수록 수소화물 재배열이 발생하기 시작하는 문턱응력(threshold stress)이 높아지는 것을 확인하였다. 또한 지르코늄합금은 일반적으로 알려진 고용 시와 석출 시의 고용도한계(terminal solid solubility) 차이가 수소화물 재배열에도 영향을 미치는 것으로 확인되었다. 고용도 한계 차이에 의해 고용된 수소의 과포화가 발생하는 구간에서는 응력이 발생하더라도 수소화물 재배열이 발생하지 않으며 수소화물 재배열이 발생하기 시작하는 시점은 석출 시의 고용도한계곡선(terminal solid solubility of precipitation, TSSP)과 만나는 지점으로 수소화물이 석출되기 시작하는 시점인 것으로 확인되었다. 유한요소법으로 분석한 후프 응력을 등고선 형태로 나타내어 온도에 따른 수소화물 재배열의 문턱응력을 확인한 결과 400 ℃기준 70±10 MPa 이었으며 300 ℃ 기준 80±10 MPa임을 확인할 수 있었다.|During last three decades, domestic spent nuclear fuel have been continuously stored in on-site wet storage. Recently it was reported that national on-site capacity would be exceeded in near future. Recently, the plan for long-term dry storage of spent fuel is being considered in-depth as a part of the public agreement for spent fuel management and application. Hydride reorientation have been considered as major degradation mechanism under dry storage condition. This mechanism critically reduce cladding ductility and make more susceptible to failure, especially during fuel transport. For this reason, most of nation implemented long-term dry storage regulate the occurrence of hydride reorientation. However study on the hydride reorientation is still in an inchoate stage in Korea. In this study, reorientation experiment using ring creep machine and Finite element method(FEM) analysis were performed to evaluate the effects of the hoop stress and temperature on the hydride reorientation under dry storage conditions. As a results, radial hydrides were occurred in the region where the high tensile hoop stresses was applied. The radial hydrides also reoriented in the circumferential direction when the hoop stress was removed. Threshold hoop stress of reorientation increased as the annealing temperature decreased. Also, the difference between the terminal solid solubility of a dissolution(TSSD) and the terminal solid solubility of a precipitation(TSSP) had influence on the hydride reorientation. This difference between TSSD and TSSP cause hydrogens in the cladding to supersaturate. Hydride reorientation was not occurred when the hydrogen existed in the form of super-saturated solid solution even if high tensile hoop stress applied. Therefore hydride reorientation begin to occur when the hydrogen in the form of solid solution is precipitated. The distribution of hoop stress obtained FEM analysis was shown by contour lines. And morphology of the specimen after the test was compared with results of FEM analysis. As a results, threshold hoop stress of hydride reorientation is 70±10 MPa at 400 ℃ and 80±10 MPa at 300 ℃.; During last three decades, domestic spent nuclear fuel have been continuously stored in on-site wet storage. Recently it was reported that national on-site capacity would be exceeded in near future. Recently, the plan for long-term dry storage of spent fuel is being considered in-depth as a part of the public agreement for spent fuel management and application. Hydride reorientation have been considered as major degradation mechanism under dry storage condition. This mechanism critically reduce cladding ductility and make more susceptible to failure, especially during fuel transport. For this reason, most of nation implemented long-term dry storage regulate the occurrence of hydride reorientation. However study on the hydride reorientation is still in an inchoate stage in Korea. In this study, reorientation experiment using ring creep machine and Finite element method(FEM) analysis were performed to evaluate the effects of the hoop stress and temperature on the hydride reorientation under dry storage conditions. As a results, radial hydrides were occurred in the region where the high tensile hoop stresses was applied. The radial hydrides also reoriented in the circumferential direction when the hoop stress was removed. Threshold hoop stress of reorientation increased as the annealing temperature decreased. Also, the difference between the terminal solid solubility of a dissolution(TSSD) and the terminal solid solubility of a precipitation(TSSP) had influence on the hydride reorientation. This difference between TSSD and TSSP cause hydrogens in the cladding to supersaturate. Hydride reorientation was not occurred when the hydrogen existed in the form of super-saturated solid solution even if high tensile hoop stress applied. Therefore hydride reorientation begin to occur when the hydrogen in the form of solid solution is precipitated. The distribution of hoop stress obtained FEM analysis was shown by contour lines. And morphology of the specimen after the test was compared with results of FEM analysis. As a results, threshold hoop stress of hydride reorientation is 70±10 MPa at 400 ℃ and 80±10 MPa at 300 ℃.