Radioactivity Evaluation Methods for Decommissioning and Calculation of Neutron Spectrum for Main Steam Pipe Room of Fugen

The main systems and design parameters of the first prototype ATR （Advanced Thermal Reactor） Fugen of Japan are introduced in this paper. It is also shows the development of decommissioning policy in Japan and the decommissioning technology for Fugen . Based on the calculation for neutron flux distribution and neutron energy spectrum under ¹⁷N（β,n） reaction in main steam pipe room of Fugen by MCNP code and the analysis of the effect of steam density and steam pipe diameter on calculation results, it concluded that the power transient would not effect the neutron distribution in main steam pipe room and the enlargement of the diameter of steam pipe would help to reduce the radiation in cover materials on steam pipe.