A local method of characteristics for dose assessment

Absorbed dose from gamma radiation is one of the most important safety considerations when planning a nuclear facility decommissioning. In this paper, a local method of characteristics (LMOC) is presented for gamma dose assessment in three-dimensional space. The LMOC method – an improvement on the method of characteristics (MOC) - performs local area-based ray tracking to improve the computation efficiency, and the Voronoi diagram is utilized to calculate angular weights. In order to attenuate the ray effect, the non-collision flux of the radiation source is used to construct the constant scattering source. Then the scattering term is iteratively solved to obtain the flux under each energy group. Finally, the flux-dose conversion factor is used to obtain the absorbed dose in the corresponding medium. As a variant of the MOC, LMOC inherits the advantages of MOC in processing complex geometries and natural parallelism, and can also deal with the dose assessment problem where the detecting position is far away from the radiation source. Experimental study of the proposed LMOC is performed by computing the dose rate on three different cases: Backscattering with shielding, backscattering without shielding and a general case. The result from the proposed method is then compared with that from the MCNP code and Point-Kernel method. The result shows the effectiveness of the LMOC in calculating gamma radiation, and it has better performance in the general case (<5%) than the backscattering tests (<10% excluding the area near the source).