The direct simulation of the γ radiation field in a large space has a very low calculation efficiency.

This study aims to apply the global variance reduction (GVR) method to the calculation of the γ radiation field in a large space.

Firstly, the volume correction factor was introduced for modifying the lower limit of the weight window wth to address the over-splitting problem caused by the volume difference between the counting cells/grids. The global quality factor (FOM_G factor) calculated by the flux-based GVR method using the volume correction was 39 times higher than that obtained by direct simulation. Then, a non-counting area correction method was proposed to address the time-consuming problem encountered in non-counting area calculation while the FOM_G factor was further improved by 40%. Finally, based on both the volume correction and non-counting area correction, the calculation of the γ radiation field were compared with that of seven GVR methods based on the particle error, weight, track, number, energy, collision and flux, respectively. The smoothing factor SI was introduced into the flux-based GVR method for results further analysis. [Results and Conclusions] The results show that the FOM_G factor calculated by the seven GVR methods is about 2~3 orders of magnitude higher than that obtained by direct simulation, and the standard deviation σ is reduced by 2~3 orders of magnitude. The FOM_G factor calculated by the weight-based GVR method is 2 304 times higher than that obtained by direct simulation; this value yields the best variance reduction effect among all GVR methods. As SI increases, the lower limit of the weight window w_th of the simulation decreases, and the FOM_G factor first increases and then decreases. When SI=0.8, the calculated FOM_G factor has the largest value, which is 3 246 times higher than that obtained by direct simulation.