Practical preprocessed nuclear data files for fast reactor calculations

Basic nuclear data libraries, such as ENDF/B-III, are used today for the generation of multigroup cross-section sets. Since the multigroup sets are determined by calculating the weighted averages of the cross-section curves, knowledge of the weighting function within each group is required. In principle, this function is not only dependent on the specific reactor, but is also a function of position in the reactor. With existing codes and methods the weighting function in the Bondarenko-type library is not iterated, since each time the weighting function is changed, a code that handles the basic data library and consumes much computer time must be run.A new method is presented that enables the user to obtain from a preprocessed library, in a few computer seconds, effective self-shielded multigroup cross-section sets averaged over any desired weighting function without reference to the basic data library. Specifications of the preprocessed library (only 2 to 3 times as large as a Bondarenko-type library) are given. The preprocessed library may be generated using existing codes.A family of functions with terms into which any weighting function can be expanded is used. Group averages of each cross section with respect to terms of the family and a set of shielding factors form the preprocessed library. Two families of functions, fn(E), are considered, and two methods are given for the expansion of W(E) into each series of fn(E). Nuclear parameters obtained by these methods are compared with the same parameters obtained using the exact weighting function (SNEAK flux) to check the methods. Very good agreement is found.A preprocessed compact library of ENDF/B-III materials is available, upon request, from the authors.