Fast evaluation of three-dimensional gamma dose rate fields on non-equispaced grids for complex atmospheric radionuclide distributions.

The gamma dose rate caused by airborne radionuclides is a major concern in the mitigation of nuclear accidents. Unfortunately, there is no fast method for calculating the three-dimensional (3D) gamma dose rate field near the source, because the corresponding airborne radionuclide distribution is usually calculated on non-equispaced grids and existing fast methods are only suitable for equispaced grids. This paper presents a method that accurately calculates the 3D dose rate field on non-equispaced grids, accelerating the computation by around two orders of magnitude. This method splits the time-consuming 3D integral in the dose rate model into a large convolution with a regularized smooth function and a small correction term. A nonuniform fast Fourier transform (NFFT) is used to rapidly calculate the convolution, which significantly enhances the computational speed. Our approach is applied to different grids and is compared with the FFT-based convolution method in two complex air dispersion simulations and a field experiment. The results show that the proposed method is in good agreement with the original 3D integral method and avoids grid-dependent interpolation errors in the FFT-based convolution method. This method enables a coupled analysis of wind, radioactivity, and dose rate on arbitrary grids, which is important for simplifying the emergency response in the case of small modular reactors.