## 3 Application in the Reconstruction of the HWRR Decommissioning Model
### Accuracy Verification  
The raw point cloud data serve as the foundation, and all subsequent post-processing tasks revolve around the point cloud data obtained from the initial scans. Ensuring the accuracy of the raw point cloud data acquisition is crucial. Therefore, precision calibration of the scanner is necessary to confirm the accuracy of the point cloud data obtained from the scanning equipment [36; 37].  
When calibrating the accuracy of the FARO Laser Scanner Focus\({}^{\lx@sectionsign}\) 350, the calibrated instrument used was a 35 mm dual-frequency laser interferometer standard device with higher accuracy than the scanner, which was verified through metrological calibration. The FARO Laser Scanner Focus\({}^{\lx@sectionsign}\) 350 scanned six target spheres evenly distributed at approximately 5, 10, 15, 20, 25, and 30 m under the highest resolution, as illustrated in Figure 8. The calibration test was conducted under environmental conditions with a temperature of 19.9 \({}^{\circ}\)C and a relative humidity of 59%. The calibration results for the FARO Laser Scanner Focus\({}^{\lx@sectionsign}\) 350 are presented in Table 4. Comparing the measured values from the scanner with the reference values from the laser interferometer, the error is  
Figure 7: The point cloud model for the HWRR plant.  
within \(\pm\)0.1 mm for scanning distances less than 15 m, and the maximum error is 0.3 mm for scanning distances exceeding 15 m. Furthermore, the calculated uncertainty for this scanner is 0.1 mm. From the results of the calibration, the scanner meets the requirement of an error less than 0.5 mm for single-station point cloud data acquisition in the HWRR decommissioning area.  
After verifying that the acquired point cloud data are reliable, it is imperative to register and stitch the raw point cloud data due to the numerous scanning stations within the HWRR plant. To guarantee that the point cloud registration accuracy meets the requirements for subsequent 3D model reconstruction, a thorough analysis of the point cloud registration error should be conducted.  
The integrated point cloud, derived from stitching, lays the groundwork for subsequent 3D model reconstruction. Theoretically, a smaller point cloud stitching accuracy corresponds to a higher quality in the reconstructed 3D model. The outcome of the point cloud registration from the 572 scanning stations within the HWRR plant is illustrated in Figure 9, with the average point distance error being 1.9 mm and a substantial 91.6% of the 1186 target spheres being of high quality, thus meeting the precision criteria for subsequent 3D model reconstruction of the HWRR plant.  
\begin{table}
\begin{tabular}{c c c c} \hline
**No.** & **Measured Value/mm** & **Reference Value/mm** & **Error/mm** \\ \hline
1 & 4999.8 & 4999.9 & \(-\)0.1 \\
2 & 9999.7 & 9999.8 & \(-\)0.1 \\
3 & 15,000.5 & 15,000.2 & 0.3 \\
4 & 20,000.0 & 19,999.9 & 0.1 \\
5 & 25,000.5 & 25,000.3 & 0.2 \\
6 & 30,000.5 & 30,000.2 & 0.3 \\ \hline \end{tabular}
\end{table}
Table 4: Result of the FARO Laser Scanner FocusS 350 calibration test.  
Figure 8: Diagram of the FARO Laser Scanner FocusS 350 calibration test.