Programming of high speed calculation method for gamma-ray sky-shine

In the safety evaluation of pressurized water reactors in Japan, the gamma-ray sky-shine is calculated as a radiation source from the reactor containment vessel. It is known that the once-scattered point kernel attenuation method used in safety evaluation predicts a different tendency from the actual attenuation depending on distance generally, because this method do not appropriately consider the effect of scattering of gamma-ray penetrating the ceiling of the vessel. In addition, the prediction error in this method increased in distance from the radiation source even if a correction is employed using the buildup factor of air. In this study, a high speed calculation method (FSKY4C) was programmed for nuclear emergency preparedness to calculate the gamma-ray sky-shine as a radiation source from the reactor containment vessel. Calculation was carried out using the FSKY4C and the G33-IE program with once-scattered point kernel attenuation method, and the prediction by these methods were compared with the gamma-ray sky-shine experiment done at Kansas State University. The calculation results by FSKY4C were a little larger than measurements made at Kansas State University. Prediction accuracy were within a factor of 2, however attenuation tendency with distance corresponded well. For the reactor containment vessel of the commercial plant, the calculation results by FSKY4C and G33-IE shows good agreement when a steel containment with cylindrical shield was assumed. When the steel containment with a dome shield and concrete containment were assumed, results by G33-IE were about one order of magnitude bigger than those by FSKY4C except near the source point. (author)