Comparison of gamma-ray skyshine dose evaluation performance among Monte Carlo codes; EGS, MVP, MCNP

In applying the Monte Carlo method to skyshine calculations, examinations are required on the validity of the calculation methods such as variance reductions and the accuracy of the calculation. In this study, using typical codes of the continuous energy Monte Carlo method, i.e. EGS5, MVP2, and MCNP5, analyses were conducted of Kansas State University skyshine benchmark experiment. And the calculation results of the gamma-ray skyshine doses by these codes were compared with each other. The following results were obtained: (1) For the gamma-ray skyshine dose calculations using Monte Carlo codes, the method of using mass energy absorption coefficients as conversion factors was effective. And under this experimental condition, dose calculations were possible for about 50,000,000 histories without variation reduction techniques. (2) The photon spectrum calculation results by EGS5, MVP2, and MCNP5 were in good agreement. (3) The dose calculation results by EGS5 and MCNP5 were in good agreement and agreed well with the experimental values. (4) In the case of MVP2 dose calculations, if dose conversion was performed in multi-group form, about twenty percent error was observed. However, if dose conversion was performed using dosimetry files, the calculation results by MVP2 were significantly improved and agreed well with those by EGS5 and MCNP5, and experimental values. From these results, it was ascertained that EGS5, MVP2, and MCNP5 can evaluate gamma-ray skyshine doses with sufficient accuracy to the same extent. In the case of MVP2, however, dose calculations using dosimetry files which are transformed from dose conversion factors are recommended. (author)