• Nuclear Engineering and Technology
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• 제27권2호
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• pp.203-215
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• 1995

소형냉각재 상실사고시 원자로냉각재펌프( RCP )의 지속적인 운전은 원자로냉각재의 불필요한 누출을 초래하여 심각한 노심노출 및 이에따른 핵연료 손상을 야기시킬 수 있다. TMI 사고 후 미국 NRC의 요구에 따라 CE형 발전소 사용자 단체에서는 “T2/L2”라는 RCP 트립전략을 개발하여 CE형 발전소에 적용 가능토록 일반비상운전지침서에 반영하였다. 상기 T2/L2 RCP 트립전략은 사고후 원자로 냉각재 계통의 압력이 감소하여 RCP 트립설정치에 도달하면 처음 두대의 RCP를 우선 정지시키고, 사고가 LOCA임이 확인되면 나머지 두대의 RCP를 정지시키는 방식을 채택하고 있다. 본 논문에서는 영광3, 4호기의 RCP 트립설정치를 분석, 선정하고 T2/L2 전략의 안전운전양상을 입증하였다 분석결과, 최악의 파단크기로 밝혀진 0.15 ft$^2$의 고온관 파단 LOCA 영광3, 4호기 RCP 트립설정치는 가압기 압력 1775 psia로 나타났으며, 운전원이 마지막 두대의 RCP를 트립시키지 못하였을 경우 혹은 최악의 시점에서 정지시켰을 경우에도 영광3, 4호기의 노심냉각능력은 확보될 수 있음이 확인되었다. 또한 영광3, 4호기의 RCP 트립전략은 미국 NRC가 요구하는 최대 핵연료피복재온도 관점에서의 10 CFR 50.46 요구조건과 운전원 조치시간 관점에서의 ANSI 58.8 요구조건도 충분히 만족함이 판명되었다 따라서, 1775 psia의 RCP 트립설정치를 사용한 영광3, 4호기의 T2/L2 RCP 트립전략은 사고시 운전원에게 향상된 운전지침을 제공할 수 있을 것으로 판단된다.

• Nuclear Engineering and Technology
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• 제45권4호
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• pp.513-522
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• 2013

The ODSCC detected in the TSP position of Ulchin 3&4 SGs are typical ODSCC of Alloy 600MA tubes. The causative chemical environment is formed by concentration of impurities inside the occluded region formed by the tube surface, egg crate strips, and sludge deposit there. Most cracks are detected at or near the line contacts between the tube surface and the egg crate strips. The region of dense crack population, as defined as between $4^{th}$ and $9^{th}$ TSPs, and near the center of hot leg hemisphere plane, coincided well with the region of preferential sludge deposition as defined by thermal hydraulics calculation using SGAP computer code. The cracks developed homogeneously in a wide range of SGs, so that the number of cracks detected each outage increased very rapidly since the first detection in the $8^{th}$ refueling outage. The root cause assessment focused on investigation of the difference in microstructure and manufacturing residual stress in order to reveal the cause of different susceptibilities to ODSCC among identical six units. The manufacturing residual stress as measured by XRD on OD surface and by split tube method indicated that the high residual stress of Alloy 600MA tube played a critical role in developing ODSCC. The level of residual stress showed substantial variations among the six units depending on details of straightening and OD grinding processes. Youngwang 3&4 tubes are less susceptible to ODSCC than U3 and U4 tubes because semi-continuous coarse chromium carbides are formed along the grain boundary of Y3&4 tubes, while there are finer less continuous chromium carbides in U3 and U4. The different carbide morphology is caused by the difference in cooling rate after mill anneal. There is a possibility that high chromium content in the Y3&4 tubes, still within the allowable range of Alloy 600, has made some contribution to the improved resistance to ODSCC. It is anticipated that ODSCC in Y5&6 SGs will be retarded more considerably than U3 SGs since the manufacturing residual stress in Y5&6 tubes is substantially lower than in U3 tubes, while the microstructure is similar with each other.