Status and validation of the fast reactor lattice code KAPER-2 for slab and pin cells

The cell programm KAPER for the calculation of the neutronics parameters of plate cells has been used frequently for the analysis of experiments performed in fast critical assemblies, especially at the facility SNEAK at Karlsruhe. KAPER determines cell-averaged group cross sections and diffusion coefficients. So far, it was restricted to slab geometry, because heterogeneity effects are usually much larger in plate-type critical assemblies than in fast power reactors. However, to allow an adequate prediction of the physics parameters of a sodium-cooled fast power reactor, especially in voided configurations, where streaming is important, a pin-cell code is required. Therefore, the version KAPER-2 was developed recently, which has options both for slab and for pin cells. In this paper, the method and the approximations used in KAPER-2 will be discussed. Probably the most serious limitation of the present version is the use of the simple Benoist formula to calculate anisotropic diffusion coefficients. Parallel with the code development, an effort is going on to check the validity of the different approximations by a Monte Carlo technique. The status of this validation effort, especially concerning the range of validity of the Benoist equation for slab lattices, will also be described. A more accurate method to calculate parallel streaming, similar to the method by Koehler and Ligou, is suggested, which may be interpreted as including parallel leakage in the collision probabilities. For pin lattices, the Benoist equation is compared with the most elaborate method available at Karlsruhe, the one developed by Eisemann. (author)