Aegis: an Advanced Lattice Physics Code for Light Water Reactor Analyses

In the current light water reactor analysis, the neutronics analysis procedure is divided into two stages – the lattice physics and the core calculations. The former is to make a tabulated cross section set for the latter, commonly on the basis of homogenized fuel assemblies or fuel pincells. The latter performs simulations to obtain the neutronics characteristics of a core. Since the overall performance of a core analysis system is dominated by a weak link in its cascade, the lattice physics and the core analysis codes equally share the responsibility for prediction accuracy. Improvements in fuel design are continuously performed in order to reduce fuel cycle cost and to increase plant performance. Though such fuel design changes are aimed at improving neutron economy, mechanical behavior and/or thermal hydraulics performance, they may pose challenging situations for a nuclear design code, e.g., when advanced burnable poison containing various burnout speeds, offset water rods of complicated shape and highly heterogeneous fuel such as MOX are involved. In order to handle these issues and to maintain prediction accuracy for neutronics design, continuous efforts are being undertaken by many researchers and engineers. Development of the AEGIS code is one of these ongoing efforts to improve accuracy and to increase the capability of lattice physics computation. The AEGIS code is a lattice physics code developed by Nuclear Engineering Ltd. in cooperation with Nagoya University and Nuclear Fuel Industries, Ltd. The primary purpose of the AEGIS code is the preparation of a cross section set for the SCOPE2 code, which is a threedimensional pin-by-pin core analysis simulator developed by Nuclear Fuel Industries, Ltd. [1]. At this stage of development, goals for the AEGIS code are as follows: 1) Incorporate the latest developments in lattice physics computation. 2) Incorporate calculation models as rigorously as possible, considering practical computation time. 3)Eliminate the conventional "pin-cell" calculation for spatial homogenization and energy condensation in order to reduce uncertainty. 4)Develop and incorporate innovative numerical algorithms for higher calculation efficiency. 5)Enable not only single fuel assembly calculation, but also large scale calculations, such as for a whole core, that can be carried out with a consistent lattice physics computation model. In Section 2, various calculation models and numerical algorithms to achieve the above objectives are described. Verification and validation results of the calculation models adopted in the AEGIS code are presented in Section 3. Finally, concluding remarks are presented in Section 4. AEGIS is a lattice physics code incorporating the latest advances in lattice physics computation, innovative calculation models and efficient numerical algorithms and is mainly used for light water reactor analyses. Though the primary objective of the AEGIS code is the preparation of a cross section set for SCOPE2 that is a three-dimensional pin-by-pin core analysis code, the AEGIS code can handle not only a fuel assembly but also multi-assemblies and a whole core geometry in twodimensional geometry. The present paper summarizes the major calculation models and part of the verification/validation efforts related to the AEGIS code.