## 2 Materials and Methods
### Simulation Method  
Because of the large-scale size of the experimental terminal with heavy shielding, particle fluence is sharply reduced when passing through the shielding materials. Therefore, two variance reduction methods were used to enhance the statistics of the simulation results.  
Method 1 was realized by the user subroutine usimbs.f in FLUKA, with an approach similar to that employed in Ref. [19]. The main program calls this routine at each step of the particle transport and assigns the relative weight according to customized conditions, which are based on the attenuation length and the distance between the track hit and the source point. Only neutrons and photons were accounted for in the biasing due to their importance in deep shielding penetration. The relative importance was incremented gradually with the distance between the particle hit and the \({}^{100}\)Mo target.  
Figure 4: (**a–c**) Cross-sectional view of the experimental terminal geometry. (**d**) Three-dimensional model of the target system in Flair. Distances are given in cm.  
Figure 3: Target geometry: (**a**) side view section and (**b**) top view section. Distances are given in cm.  
Method 2 sets incrementing importance values for the regions along the approximate transport direction of particles [9]. The importance-setting map of the experimental terminal area is displayed in Figure 5.  
The effects of the different variance reduction methods were compared, as shown in Figure 6. Both the methods could significantly improve the statistics of events after penetrating the shielding body. Method 2 provided a better result according to the comparison.  
The CPU chip used in the calculation was the Intel(R) Xeon(R) Gold 6240 CPU @ 2.60 GHz. The average time per primary in Method 1 (6.4 \(\times\) 10\({}^{-5}\) s/primary) and Method 2 (7.8 \(\times\) 10\({}^{-5}\) s/primary) are, respectively, 6.3 and 7.7 times full-simulation (1.0 \(\times\) 10\({}^{-5}\) s/primary). If the geometry buildup time is considered, the total time consumption for Method 2 is much longer. Thus, when encountering shielding issues during management of large-scale nuclear facilities based on FLUKA, weighing the accuracy of simulation results and the progress of the project at the same time, the following simulation strategy should be considered:  
1. Carry out a full simulation based on construction drawings to record the source-term energy spectrum and then select appropriate shielding materials based on physical models;
2. Utilize Method 1 to improve the statistics of simulation results and iterate the shielding design within a relatively short time, and then evaluate and optimize the design of key parts such as mazes and ventilation penetrations. Method 1 can also be better customized by creating more layers or adjusting the biasing factor, which may finally provide a satisfactory result, but with more time.
3. If necessary and/or feasible, perform regional segmentation of the geometric model and assign importance to achieve more accurate simulation results. Method 2 is not applicable if the geometry is too complicated.  
Figure 5: Region importance biasing (10\({}^{-4}\)-10\({}^{5}\)). Distances are given in cm.