An improved on-the-fly global variance reduction technique by automatically updating weight window values for Monte Carlo shielding calculation

This paper presents an improved on-the-fly global variance reduction (GVR) method for Monte Carlo radiation shielding calculations. This method generates the weight window (WW) mesh internally in the Monte Carlo (MC) code, and updates automatically the WW value on-the-fly as the particle flux of a mesh element has a certain level of statistical error. This method has many advantages, such as, it does not depend on third party codes; it only needs one MC run to obtained the WW mesh together with the global flux map; and it has no limitation on the source definition and particle types. In addition, a method to deal with long history problem, which is caused by excessive splitting of tracks, is proposed in this paper. The on-the-fly GVR method has been implemented in the MCNP code and verified using a simple 2D model and the ITER benchmark model. The results show significant acceleration effects on the MC shielding calculation against analog simulations.