# A Wireless Monitoring System for Cracks on the Surface of Reactor Containment Buildings
## 1 Introduction  
Civil infrastructure usually plays an important role in society's prosperity [1]. The safety of these structures during their normal operations cannot be neglected as any health problem as a result from them may cause severe life and property loss, which has happened many times in the past. Due to aging, loadings, environmental conditions and extreme events like earthquakes, structures may deteriorate in their performance slowly, or be damaged severely. Then, structural health monitoring (SHM) [2], which is defined as detecting changes to the material or geometric properties of structures, becomes essential to the safety of these structures. With proper tools and techniques, continuous, real-time and automated monitoring systems could evaluate reliability of a structure, identify potential harmful factors and provide assessment for maintenance decisions during its service life. Traditional wired monitoring systems have advantages in acquiring high-fidelity data and robustness, but they still have some drawbacks. Such systems require electronic cables to deliver data sampled by limit sensors to a central device, which usually could be time consuming and expensive for installation and maintenance. In addition, installing these cabled sensors on large civil structures could sometimes interfere with their everyday normal operations. Fortunately, with developments in Micro-Electro-Mechanical System (MEMS) technology, wireless communications and digital electronics, the advent of wireless sensor network (WSN) [3] technology brings new opportunities for structural health monitoring. Compared with wired systems, monitoring systems using WSNs could benefit from their obvious advantages like cable-free and plug and play deployment, which allows engineers to reduce time and cost forinstallation. Furthermore, these tiny sensor nodes make it possible for dense network deployments, thus allowing better understanding of the monitored structure. Currently, many SHM systems based on WSNs have been implemented on various civil structures including bridges, tunnels, and tall buildings and so on. Utilizing 64 Micaz nodes with two different accuracy accelerometers that measure ambient vibrations, [PERSON] and [PERSON] _et al._[4; 5] implemented an up to 46-hop WSN monitoring system on the Golden Gate Bridge. [PERSON], [PERSON] and [PERSON] _et al._[6; 7] deployed and evaluated a wireless smart sensor networks monitoring system consisting of 70 Imote2 nodes with custom-designed SHM sensor boards on the Jindo Bridge. [PERSON] and [PERSON] _et al._[8; 9; 10] presented an SHM system based on WSNs that addressed challenges for long-term deployments and was used in deployments for monitoring a cable stayed bridge and a timber footbridge. [PERSON] _et al._[11] proposed an event-based monitoring application using eight Tmote Sky nodes with resistance strain gages on a steel railway bridge. Meanwhile, an integrated SHM system for highway bridges based on customized WSN platforms was developed by [PERSON] _et al._[12] As for tunnels, [PERSON] _et al._[13] designed a WSN based underground monitoring system using Micaz nodes with crackmeters and inclinometers, and the SHM system was tested in the Prague Metro and the London Underground. [PERSON] _et al._[14] utilized 16 3 MATE! nodes with accelerometers, deformation gauges and thermometers to monitor the health status of a medieval tower. [PERSON] _et al._[15] installed a WSN monitoring system on a tall building called Diwang Tower with eight sensor nodes and comparisons were made with the cable-based system. In this paper, we implement a wireless monitoring system for nuclear power plants, especially for monitoring changes in crack width on the surface of reactor containment buildings during pressure tests. Customized WSN platforms are developed and the sensors used include crackmeters and thermometers. Issues like route discovery, time synchronization, data collection and duty-cycling schemes are considered in software design.