몬테카를로 중성자수송 전산모사에 있어서 새로운 구형입자 샘플링 방법 및 핵분열선원 수렴진단 방법 개발

Abstract

A new implicit modeling method of spherical particles and diagnostics of fission source convergence are developed for the Monte Carlo simulation of the pebble reactor core. Firstly, an implicit method for the geometry modeling of the spherical particles, which are randomly distributed in the mediums, is proposed using the geometrical cross sections. Two correction methods which are occurred in using the proposed method are developed. For the verification of the proposed modeling method, the packing fractions in infinite and finite mediums are estimated and compared with those of the other studies. Analysis shows that the results with the proposed method give good agreements than those of the other methods within 0.5 % relative error. In addition, the local packing fraction near the wall boundary and sampled distance probability with the proposed method are evaluated. And, the results are compared with those of the explicit modeling. Analysis shows that the modeling accuracy in the low packing fraction is considerably improved; however, the proposed method shows low modeling accuracy for the high packing fraction medium. Secondly, a method to confirm and diagnose the fission source convergence is proposed that are based on the averages and standard deviations of the fission source sites. The verifications of the diagnostic method are pursued by estimating OECD/NEA benchmark problems with modification of MCNP5 code. Also, for the verification of the diagnostic accuracy, the fission source distributions using the Shannon entropy and the nine center distance sum (NCDS) methods are evaluated and compared. The results show that the proposed method gives best diagnostic ability of the fission source convergence than the other methods. Finally, for the application of the proposed methods into the Monte Carlo simulation, a Monte Carlo simulation algorithm is constructed. The proposed methods in this study can be directly used to model the spherical geometries and to diagnosis the fission source convergence for the pebble bed reactor analysis. Also, it is expected that the proposed methods can be applied into the following Monte Carlo analysis fields: (i) modeling of the spherical particles such as the pebble bed blanket of the fusion reactor, TRISO particles in the VHTR prismatic reactor, and shielding materials containing spherical particles, (ii) direct application into the Monte Carlo simulation codes for the advanced diagnostics of the fission source convergence in any fission source iteration problems.