## 1 Introduction  
As of July 2022, there are currently 194 nuclear power units globally in the decommissioning process or awaiting it, with projections indicating that the global nuclear decommissioning market will continue to expand in the upcoming decade [1, 2]. Notably, at least 105 nuclear power units worldwide have been operational for over 40 years. Due to immature developments in computer technology at the time, these reactors lack detailed electronic drawings, and even comprehensive paper drawings are challenging to preserve. Furthermore, over the decades of their operation, activities such as equipment replacements, plant expansions, and process circuits modifications have inevitably led to discrepancies between the paper drawings of nuclear facilities and the actual conditions [3]. For the final phase of a nuclear facility's lifecycle, the decommissioning process, an accurate assessment of the decommissioning site is crucial, especially considering the complexity of underground process circuit systems and the industrial and radiation risks involved [4]. Given the irreversibility, safety, and economic considerations of the decommissioning process, virtual simulation technology, empowered by the advancements in computer technology, serves as a robust auxiliary tool for nuclear facility decommissioning. It provides a supportive platform for tasks such as assisting in decommissioning plan designs, validating and confirming implementation plans, process planning and optimization,as well as personnel training [5; 6; 7]. Given the pivotal role that virtual simulation technology plays in the process of decommissioning nuclear facilities, the acquisition of highly accurate 3D reactor models becomes the primary task for simulating the decommissioning of nuclear facilities [8; 9]. The heavy water research reactor (HWRR) is the first reactor of its kind in China; it has a tank-type structure with heavy water as the moderator and coolant and graphite as the reflector layer, and the original design power is 7 MW. It achieved criticality for the first time in 1958 and operated at full power in the same year. The HWRR was upgraded to a rated power of 10 MW and an enhanced power of 15 MW after a major overhaul and modification in 1980. In December 2007, the HWRR was permanently shut down and entered a phase of secure closure. By the end of 2019, the HWRR had officially commenced its decommissioning process after a successful operational span of approximately 50 years, during which it released a total cumulative energy of 50,500 MW-d [10; 11]. The early reactor construction date and the complexity of its process system equipment and pipelines, as well as the major modifications and reconstruction conducted during the operational lifespan of the HWRR, have resulted in the preservation of incomplete paper drawings, with no digitized drawings available. Furthermore, the absence of detailed digitized drawings or 3D models has presented challenges to decommissioning the HWRR, necessitating the adoption of innovative technologies for the acquisition of its 3D model [12]. Presently, laser-scanning 3D model reconstruction technology has achieved maturity in applications within the digital preservation of cultural heritage [13; 14; 15], building deformation monitoring [16; 17], surveying and mapping engineering [18; 19], and related domains. However, in the field of reconstructing decommissioning models for nuclear facilities, there are relatively few applications that deal with the design and integration of scanning platforms and the optimization of point cloud data processing methods, and detailed practical applications in nuclear facility decommissioning projects are lacking [20; 21; 22; 23]. This study utilizes 3D laser-scanning technology for non-contact measurements in radioactive environments, acquiring a significant amount of point cloud data, which subsequently undergo denoising and registration processes.