Nuclear Mathematical and Computational Sciences: A Century in Review, A Century Anew Gatlinburg, Tennessee, April 6-11, 2003, on CD-ROM, American Nuclear Society, LaGrange Park, IL (2003) CAPTURING THE EFFECTS OF UNLIKE NEIGHBORS IN SINGLE- ASSEMBLY CALCULATIONS Kevin T. Clarno and Marvin L. Adams Department of Nuclear Engineering Texas A&M University ktclarno@tamu.edu; mladams@tamu.edu ABSTRACT We present our recent progress on improving assembly-level calculations for reactor analysis, including modifications that support a larger quasi-diffusion core-level analysis effort. Our main focus is on accurately approximating the effects that neighboring assemblies have on the few- group cross sections, discontinuity factors, and other transport parameters of a given assembly. We also focus on tabulating these effects in an efficient way that allows accurate interpolation by the core-level algorithm. We describe our algorithms and present results from many difficult test problems containing MOX and UO 2 assemblies. Key Words: reactor analysis, albedo, lattice physics, assembly level, quasi-diffusion 1. INTRODUCTION – THE INTERFACE PROBLEM One of the main challenges that a reactor analysis methodology faces is obtaining the power distribution and averaged cross sections for an assembly whose neighboring assemblies are significantly different. If the neighbors are identical to the assembly in question, then an excellent approximation to the solution in the assembly can be obtained by solving a two- dimensional single-assembly problem with reflecting boundaries. However, if a neighboring assembly is significantly different, the reflecting boundary condition does not accurately model reality. Reactor analysts have tried many different approaches to approximating the effects of unlike neighbors on an assembly’s averaged cross sections. The most straightforward is to run multi- assembly calculations (“colorsets”), one for each four-assembly permutation that will appear in the core [1,2]. While straightforward in principle, this approach is computationally unwieldy, taxing to the user, and it does not eliminate the need to branch and interpolate on conditions in the neighboring assemblies. Thus, most analysis systems attempt to retain the single-assembly calculation and somehow account for the effects of different neighbors. In this paper we describe our recent efforts to capture and tabulate the effects of different neighbors on the important parameters of a given assembly. Our effort is part of a larger collaborative project that is developing a new reactor-analysis methodology that uses quasi- diffusion equations for core-level analysis [3-5].