Analysis of transport-activation internal coupling method implemented in Monte Carlo code cosRMC

Abstract

When fusion nuclear reactor is in operation, neutron activation causes a large number of radioisotopes. Activation calculation is a very important step in both of reactor shielding calculation and radiation safety analysis. The capability of transport-activation coupling calculation was developed in Monte Carlo code cosRMC though the built-in burnup solver Depth. In the process of neutron transport, the one-group cross section of activation-related was calculated by internal coupling method and the manipulation and transfer of input file for energy spectrum to external activation code was no longer necessary, which is more efficient and flexible than traditional external coupling. A cuboid iron model is established, and the activation calculation is performed by cosRMC and ALARA respectively. In the constant energy spectrum mode, the relative error of 56Fe atomic density between two codes is 0.0019 %, which verified the accuracy of capability of activation internal coupling calculation in the cosRMC code. The relative errors of 57Fe and 58Fe atomic density are 17.97 % and 36.75 % between constant and variable energy spectrum. This showed that whether energy spectrum remains constant has a significant impact on the results. Moreover, the one-group reaction rate with continuous energy/ multi-group cross section were different, which is because the pre-generated multi-group cross section libraries are independent of the geometry and energy spectrum of the specific problem. The resonance shielding effect of the specific problem cannot be accurately considered when using the multi-group cross section, resulted in relative errors of 60 % for the one-group cross section of 56Fe(n,γ) reaction. cosRMC can directly use the continuous energy method to calculate the problem dependent one-group section, which will give more accurate results.