Monitoring of prestressed concrete structures in a nuclear power plant with fibre Bragg grating systems

Monitoring the prestress level of nuclear prestressed concrete containments and prestressed concrete pressure vessels is an important maintenance activity on a nuclear site. Adequate prestress level ensures the containment or vessel is able to withstand internal pressure and contain radioactive material in the unlikely event of an accident. Prestress monitoring is currently limited to manual measurements of the total prestress force at the prestressing tendon ends. It would be beneficial to develop an online monitoring system which can measure prestress levels at several locations along a prestressing tendon. A system like this could replace existing prestress monitoring, ensure safe operation through early warning of prestress loss,and provide additional data to make informed decisions about maintenance, lifetime extension and future reactor design.This project developed a fibre optic system capable of monitoring prestress levels. Standard 7-wire prestressing strands were instrumented with fibre Bragg grating temperature and strain sensors, while load cells measured total force at the strand ends. This provided measurement of all prestress loss effects. Sensor fabrication employed brazing to create an all-metal sensor package to reduce radiation effects in the fibre, and withstand the high stress involved in this harsh environment application. For the first time the sensors and instrumented strands were thoroughly tested in the laboratory and in two field trials representing similar conditions to the real application. Strain sensors were shown to measure strain up to 7.2 mƐ, equivalent to approximately 1300 MPa, consistently for three cycles in the laboratory. Instrumented prestressing strands were tested, during the first field trial, in two concrete beams for 170 days where sensors measured prestress levels in ambient conditions. In the second field trial several instrumented strands were stressed on a multi strain calibration rig. The knowledge and experience gained during the project will facilitate the development of a fibre optic system capable of monitoring prestress levels in prestressed concrete containments and prestressed concrete pressure vessels.Monitoring the prestress level of nuclear prestressed concrete containments and prestressed concrete pressure vessels is an important maintenance activity on a nuclear site. Adequate prestress level ensures the containment or vessel is able to withstand internal pressure and contain radioactive material in the unlikely event of an accident. Prestress monitoring is currently limited to manual measurements of the total prestress force at the prestressing tendon ends. It would be beneficial to develop an online monitoring system which can measure prestress levels at several locations along a prestressing tendon. A system like this could replace existing prestress monitoring, ensure safe operation through early warning of prestress loss,and provide additional data to make informed decisions about maintenance, lifetime extension and future reactor design.This project developed a fibre optic system capable of monitoring prestress levels. Standard 7-wire prestressing strands were instrumented with fibre Bragg grating temperature and strain sensors, while load cells measured total force at the strand ends. This provided measurement of all prestress loss effects. Sensor fabrication employed brazing to create an all-metal sensor package to reduce radiation effects in the fibre, and withstand the high stress involved in this harsh environment application. For the first time the sensors and instrumented strands were thoroughly tested in the laboratory and in two field trials representing similar conditions to the real application. Strain sensors were shown to measure strain up to 7.2 mƐ, equivalent to approximately 1300 MPa, consistently for three cycles in the laboratory. Instrumented prestressing strands were tested, during the first field trial, in two concrete beams for 170 days where sensors measured prestress levels in ambient conditions. In the second field trial several instrumented strands were stressed on a multi strain calibration rig. The knowledge and experience gained during the project will facilitate the development of a fibre optic system capable of monitoring prestress levels in prestressed concrete containments and prestressed concrete pressure vessels