According to a guideline by the regulatory body, materials, which irradiated by neutron less than 6.25μSv/h, can be treated as "Non-Radioactive Waste (NR)" (Advisory Committee 2007). And we call a boundary of radioactive waste and Non-radioactive waste "NR Boundary". We improved an estimation procedure of NR boundary. In our previous works (Tanaka 2014), we had estimated NR Boundary around Main Steam line (MS) of the BWR using "Sn and Sn Coupling technique". In the case of Boiling Water Reactor (BWR) MARK I type power plant, NR Boundary is in the vicinity of an area where MS is situated on. This Coupling technique provides a means of coupling fluxes calculated with two-dimensional (2D) Sn code "DORT" (ORNL 1998) to three-dimensional (3D) Sn code "TORT" (ORNL 1998) through construction of external sources for TORT. We had set neutron-flux on the inner surface of Biological Shielding Wall (BSW), which had been calculated using DORT in the previous works, as external neutron source for this TORT calculation. We had also measured neutron-flux using activation foils at 9 locations around MS. Comparing the calculation results and the measured flux at ten locations, we had found that underestimation had occurred at some locations around MS. We supposed that one of cause of this underestimation is that Sn method tends to calculate neutron diffusion more excessive under the local phenomena in neutron-flux distribution like streaming occurs. To improve this underestimation, we decided to apply MCNP-5 (Monte Carlo N-Particle transport code) (LANL 2003) to the calculation around MS because MCNP-5 simulates the local phenomena such as streaming better than 3D Sn code. We also performed TORT calculation with alternative directional mesh such as biased directional mesh, which have mesh arrangement that leaned along the streaming. We have implemented NR boundary estimation around MS in TORT calculation and MCNP-5 calculation. We compared both calculation results with the measured fluxes.
