Dose coefficients for external photon radiation are widely used for assessment of radiation dose to public and workers due to ground surface contamination and air immersion of radionuclides released in nuclear accidents. Dose coefficients based on Chinese reference voxel phantom were presented. Photons that incident into the cylinder, which is slightly larger than phantom, is simulated using Geant4, including distributions of angle, height and energy of photons; Secondly, photons are sampled on surface of cylinder around Chinese Reference Male/Female Voxel Phantom with MCNPX 2.4.k from the above source, dose coefficients of 20 initial gamma ray energies, from 15keV to 10 MeV, are calculated. Thirdly, dose coefficients of 68 important nuclides are evaluated by combining of decay data from ICRP 107 and photon coefficients using cubic-spline fitting. GB/T 17982-2000 shows an overestimation for ground contamination while an underestimation of high Z nuclides for air immersion compared to our results.

To solve the three-dimensional transient problems,the Improved Quasi Static Method(IQS) is adopted to deal with the temporal problem,and an improvement of synthesis method(YN) is also introduced.By factorizing the neutron flux,the time step of this method is enlarged so as to decrease the number of spatial calculations.For spatial calculations,Nodal Green's Function Method(NGFM) is used to determine the distribution of shape function,so the spatial mesh of IQSYN/NGFM can be about twenty times as large as that of finite difference method.