초록 ▼

○ 연구개발 목표
고연소 핵연료, 출력증강, 계속운전, 공격적인 수화학 도입에 따른 핵연료 건전성 확보와 건식중간저장 및 장기 저장 사용후핵연료의 건전성 확보를 위해 손상핵연료 수소화열화성능 모델 개발, 재료열화 모델, 실시간 핵연료 손상기구 예측기술 및 핵연료 열화속도 예측기술 개발

○ 연구 개발 내용 및 결과 (2013.04 - 2015.03)
- 노외 수소화/재료물성 열화 시험 및 시험 자료 분석
- 핵연료 피복관 2차 수소화손상 기구 종합 모델 개발 및 적용
- 고연소 핵연료 재료물성 열화

○ 연구개발 목표
고연소 핵연료, 출력증강, 계속운전, 공격적인 수화학 도입에 따른 핵연료 건전성 확보와 건식중간저장 및 장기 저장 사용후핵연료의 건전성 확보를 위해 손상핵연료 수소화열화성능 모델 개발, 재료열화 모델, 실시간 핵연료 손상기구 예측기술 및 핵연료 열화속도 예측기술 개발

○ 연구 개발 내용 및 결과 (2013.04 - 2015.03)
- 노외 수소화/재료물성 열화 시험 및 시험 자료 분석
- 핵연료 피복관 2차 수소화손상 기구 종합 모델 개발 및 적용
- 고연소 핵연료 재료물성 열화 성능 종합 모델 정립
- 지르코늄 합금 피복관 내 수소화물 재배열 실험 및 모델 개발
- 시차주사 열량측정법(DSC)을 통한 지르코늄 합금의 수소 고용도 한계 데이터 구축
- 하나로 조사 피복관 재료의 기계적 특성 시험 및 결과 분석

Abstract ▼

Ⅳ. Results of Project
○ Study on secondary hydriding phenomenon of zirconium alloy fuel cladding
Basically, secondary hydriding phenomenon of zirconium alloy fuel cladding is directly related to amount of both hydrogen and steam. Thus, experiments are conducted in the temperature range of 400-

Ⅳ. Results of Project
○ Study on secondary hydriding phenomenon of zirconium alloy fuel cladding
Basically, secondary hydriding phenomenon of zirconium alloy fuel cladding is directly related to amount of both hydrogen and steam. Thus, experiments are conducted in the temperature range of 400-500 ℃ under atmospheric pressure with varying hydrogen to steam pressure ratio. According to the results, in hydrogen-rich atmosphere, weight gain increases linearly after incubation time in reaction of zirconium alloy.Reaction at P(H₂)/P(H₂O) of 5×10² or 10³, meanwhile, can be divided into three stages.At the beginning of the stage, weight gain increases linearly due to direct reaction of bare cladding surface with gaseous hydrogen. After that, oxides which play a protective role for hydrogen permeation are formed and oxidation which is diffusion controlled process becomes a dominant reaction combined with slow hydriding distinct from the commonly-observed ‘pickup’. Finally, a transition to linear kinetics occurs at certain specific weight gain as well as weight gain increases rapidly, linearly with reaction time. In other words, when oxide film reaches a certain thickness regardless of temperature, localized mechanical cracks are made and these act as shortcuts for direct permeation of hydrogen through the bare substrate. Thus, initiation of secondary hydriding depends on whether loss of integrity of oxide occurs or not, rather than whether value of P(H₂)/P(H₂O) exceed the critical value or not.

○ Hydride reorientation test and material property degradation evaluation of cladding
In this research, hydride reorientation test of Zircaloy-4 cladding was conducted using ring test method. Hydrogen was inserted into Zircaloy-4 tube by gaseous diffusion method and its hydrogen concentrations were 181-381 wppm. Zircaloy-4 tube was cut into 5 mm that is ring specimen length. A hydride ring specimen and grips were set up on a lever arm type creep machine. Test temperature was 400 ℃, 335 ℃, 300 ℃.After heating a furnace until test temperature, once the temperature stabilized, the specimen was slowly cooled down to room temperature with a cooling rate of ~ 0.5 ℃/min. Then constant load was applied. As a result, hydride reorientation occurred in cooling condition, but didn’t occur in temperature between TSSP and TSSD. Threshold stress analysis was conducted on the samples which showed hydride reorientation. A contour of hoop stress distribution through FEM analysis compared with a surface morphology through optical microscopy. Consequently, in this research, hydride reorientation occurred on a less hoop stress than other experimental data. In addition, ring compression test was performed to estimate cladding material property degradation with radial and circumferential Zircaloy-4 in RT-400 ℃. The ductility of hydride embedded Zircaloy-4 is strongly dependent upon the hydride morphology rather than hydrogen content and DBTT is function of hydride morphology and hydrogen content.

○ Measurement on the terminal solid solubility of hydrogen in zirconium alloys
The terminal solid solubility of hydrogen in Zircaloy-4, Optimized ZIRLO^TM andHANA-6 were investigated with a differential scanning calorimetry(DSC). The present TSS for zirconium alloys are similar to the reported results and there was almost no difference in TSS behaviors among Zircaloy-4, Optimized ZIRLO^TM and HANA-6 alloys.The TSSP seems to be more sensitive to chemical composition than the TSSD.Additional experiments were performed to evaluate the maximum temperature effect on the TSSP. When the maximum temperature is less than the TSSD temperature, that means some hydrides are not dissolved, the peak can be detected but the curve does not show a clear peak. There was no observable change in the TSSP temperature when the maximum temperature is higher than 500 ℃. On that basis, TSSP1 and TSSP2 of Zircaloy-4, Optimized ZIRLO^TM and HANA-6 were derived. The TSSP decreased with increasing maximum temperature and the magnitude of hysteresis of hydrogen solubility increased with increasing maximum temperature.

○ HANARO Irradiation Test & Data Analysis
Post-irradiation tensile tests were conducted using degraded ring and plate specimen to simulate spent nuclear fuel which was corroded and hydrided.
Experimental results showed ultimate tensile stresngth(UTS) decrease, on the other hand, ductility of material significantly increase, with increase in temperature from room temperature to 400 ℃. Dimpled fracture surface which was generally obsereved in case of ductile fracture was obsereved in specimens which irradiated at high temperature condition. In specimens which irradiated at room temperature, there are lots of irradiation defects such as vacancies, dislocations, and dislocation loops. These defects blocked the dislocation motion so that higher stress was necessary for slip process and UTS increased. At higher temperature, irradiation defects was recovered due to active diffusion of vacancies and interstitial atoms so that ductility was significantly increased.