The high-level nuclear waste (HLW) repository is a 500-1,000 m deep underground structure to dispose high-level nuclear waste. The waste has a very long half-time and is exposed to a number of stresses, including high temperatures, high humidity, high pressure These stresses cause the structure to deteriorate and create cracks. Therefore, structural health monitoring with monitoring sensors is required for safety. However, sensors could also fail due to the stresses, especially high temperature. Given that the sensors are installed in the bentonite buffer and the backfill tunnel, it is impossible to replace them if they fail. That’s why it is necessary to assess the sensors’ durability under the repository’s environmental conditions before installing them. Accelerated life test (ALT) can be used to assess durability or life of the sensors, and it is important to obtain the same failure mode for reliability tests including ALT. Before conducting the test, the proper stress level must be designed first to get reliable data in a short time. After that, acceleration of life reduction with increasing temperature and temperature-life model should be determined with some statistical methods. In this study, a methodology for designing stress levels and predicting the life of the sensor were described.

The high-level nuclear waste (HLW) repository is a 500-1,000 m deep geological disposal system with a very long life expectancy for disposing of high-level waste, which is known to have a half-life of several thousand years. This repository is subject to harsh environmental conditions, such as high temperature and radiation from high-level waste, that can cause deterioration and crack. When radiation escapes through cracks, it can injure persons on the ground. Therefore, it is essential to install a sensor that can detect problems such as cracks. But, since the high-level nuclear waste (HLW) repository is sealed with bentonite and backfill, the sensor cannot be removed or replaced once it has been installed. Therefore, it is necessary to develop a highly durable monitoring sensor that can withstand harsh environmental conditions. Before attempting to improve durability, it is first required to assess durability quantitatively. And an accelerated life test is a widely used method for assessing durability. However, it is important to obtain the same failure mode when conducting a reliability test, such as an accelerated life test. If the accelerated life test is conducted using different failure modes, the dependability of the results is inevitably diminished. Therefore, in this study, a representative failure mode for the piezoelectric sensor used in the accelerated life test was derived through experiments and literature research.

The high-level waste disposal system is an underground structure exposed to complex environmental conditions such as high temperature, radiation, and groundwater. The high-level waste disposal causes structural cracks and deterioration over time. However, since the high-level waste disposal system is a structure that should be operated for a very long time, developing a high-durability monitoring sensor to detect cracks and deterioration is essential. The durability of the sensor can be evaluated by predicting the expected life through the accelerated life test, one of the reliability qualification tests. The most important factor in the accelerated life test design is setting the harsh stress level. This study figured out the harsh stress level of the piezoelectric sensor, which is commonly used for underground structure monitoring. It is possible to determine the appropriate stress level for the accelerated life test by investigating the harsh stress level for the temperature factor. It will contribute to more accurate life expectancy prediction.

본 논문에서는 사회기반시설의 손상탐지를 위한 경 방향모드 압전 오실레이터의 가능성을 연구하였다. 경 방향 모드 오실레이터 센서는 구조물의 주요부에 부착된 경 방향 모드 압전소자와 피드백 오실레이터 회로로 구성되어있다. 구조물의 손상은 구조물의 임피던스를 변화를 야기 시키며, 그 결과로 구조물의 공진 주파수가 변화하게 된다. 오실레이터 센서는 간단한 방법으로 이 공진 주파수 변화를 연속적으로 관측할 수 있다. 본 연구에서는 알루미늄 시편에 크랙의 크기와 개수를 인위적으로 변화 시키면서, FEM해석과 실험을 통해 경 방향 모드 압전 오실레이터 센서의 적용 가능성을 증명 하였다.

막 여과는 막을 통해 물을 통과시켜 수중의 오염 물질이나 불순물을 제거하는 기술이며 고도수처리 및 하수 처리와 같은 다양한 분야에서 사용된다. 하지만, 우발적인 수질 오염이나 막 손상의 경우에는 대응하기 어렵다는 단점이 있다. 따라서 여과 과정을 거친 후 식수의 오염을 막기 위해 막의 손상을 모니터링 해야 한다. 본 연구에서는, 압전 센서를 사용하여 고주파 기반 전기역학적 임피던스(EMI)를 측정하여 막 완결성 시험을 수행하였다. 외부 배관에 부착된 압전 센서는 배관 내부 압력 변화로 인한 막의 물리적 특성의 변화를 검출 할 수 있다. 압력 감쇠 시험을 진행하는 동안, 부착된 압전 센서를 통해 EMI 를 측정함으로써 막 손상으로 인한 압력 강하를 측정 할 수 있다. 본 기술을 검증하기 위하여 실제 수원지에서 현장 테스트가 수행되었다. 신뢰성을 높이기 위해 각 측정 단계에서 20 회 실험을 수행 하였고 그 결과, 미세한 압력 강하를 검출함으로써 막의 손상을 모니터링 할 수 있음을 확인하였다.

The evolution of the electro-mechanical impedance (EMI) of piezoelectricity (PZT) sensor was investigated to determine the setting times of fiber reinforced cementitious materials in this study. Penetration resistance test was also conducted to validate the EMI sensing technique. As a result, the setting times of fiber reinforced cementitious materials can be effectively monitored through the EMI sensing technique using an embedded PZT sensor.

As most infrastructure have low natural frequency for vibration, an energy harvester to operate wireless sensors for them should be aimed to low frequency and have high output efficiency. This study proposes the energy harvester with parallel-connected single crystal ceramic for low frequency in order to gain enhanced efficiency. The performance is confirmed by the experiment using the acceleration data of hangers in Yeongjong Bridge.