## 2 A Brief Introduction to Laser-Scanning 3D Model Reconstruction Technology  
The process of reconstructing a 3D model of an object into a mathematical model that can be suitable for computer processing is commonly known as 3D reconstruction. Currently, two primary approaches exist for acquiring 3D models of objects. The first method entails the direct generation of a 3D model using specialized 3D modeling software [24]. The second method involves the acquisition of 3D information about the object's surface through dedicated equipment, followed by the reconstruction of the object's 3D model [25]. While the first method is well established, its application to nuclear facilities, characterized by complex systems and numerous pieces of equipment, presents significant challenges and demands substantial labor. Moreover, utilizing outdated paper drawings for modeling may exhibit certain deviations between the created models and the actual objects. In the second method, laser-scanning 3D model reconstruction technology is the prevailing technique, which entails utilizing a laser scanner to capture surface data of the object, encompassing spatial coordinates, reflection intensity, and color information. The precise point cloud data acquired through this method prove instrumental in 3D modeling. The comparisons of laser-scanning 3D model reconstruction and 3D software modeling technology are shown in Table 1.  
The above table compares the traditional 3D software modeling technology with laser-scanning 3D model reconstruction technology, and the results show that the laser-scanning 3D model reconstruction technology requires relatively high economic costs and skilled manpower, but it spares us the time cost of acquiring the HWRR plant model, improving the accuracy of the model. From a long-term perspective, this provides us with the equipment base and human resources to subsequently undertake other nuclear facility model reconstruction projects.  
In addition, the drawings retained during the construction of the nuclear facilities in the 1960s were incomplete, and the overhaul and replacement of equipment during their operation caused changes in the paper drawings, and the parts involved in the entire nuclear facility amounted to at least tens of thousands, so it is difficult to complete the model construction of the huge nuclear facility project by relying on the traditional modeling method alone. The use of laser-scanning 3D model reconstruction technology is suitable for the construction of complex models, which can effectively establish a complete model and compensate for the shortcomings of traditional modeling methods.  
Laser-scanning 3D model reconstruction technology facilitates the precise and comprehensive acquisition of point cloud data for decommissioned nuclear facility components in a non-contact manner, thereby preserving digital assets. The heightened automation in the subsequent stages of the modeling process ensures efficiency. Given the intricate and radioactive nature of decommissioning environments within nuclear facilities, laser-scanning 3D model reconstruction technology assumes a pivotal role as an essential auxiliary method for the reconstruction of decommissioning models. Its benefits include rapid data collection, elevated measurement accuracy, and non-contact capabilities [26]. The subsequent section offers a concise overview of the operational principles of a 3D laser scanner and the process involved in laser-scanning 3D model reconstruction.