Homogenized cross section generation considering axial heterogeneity for VHTR fuel block

The HELIOS/CAPP and McCARD/CAPP code system has been developed for analysis of a very high temperature gas cooled reactor. However, the fuel block of a PMR200 has an axial heterogeneity and few group homogenized cross sections generated from a 2D model provide significantly large errors in the case of using HELIOS, a 2D lattice code. Thus, the modified 2D model and three zone 2D model are presented for more accurate homogenized cross sections. First model is designed to conserve the total fuel and graphite number density and second model has three axial homogenized zones, one fuel zone in center and two axial graphite zones in both end sides. The accuracy of two models are evaluated by Monte Carlo simulation. As a result, the modified 2D model and three zone 2D model can similarly provide more accurate cross section which reduces the error from 2000 to 300 pcm. Additionally, the homogenized cross sections of a real 3D model are directly generated using McCARD which has the capability of generating multi-group cross sections. Therefore, it is confirmed that the two results show the difference under 150 pcm including the stochastic error, when the depletion calculation was carried out using CAPP with the cross sections.