Uncertainty Analyses with Nuclear Covariance Ddata in Reactor Core Ccalculations

Abstract

Evaluated nuclear data are continuously improved. During the last years, the European library was updated from JEF-2.2 to JEFF-3.1 [1], the American library from ENDF/B-VI to ENDF/B VII [2], and the Japanese library from JENDL-3.2 to JENDL3.3/AC-2008 [3], with the main aim to achieve better agreement between calculated and measured results for integral systems. Along with this, growing attention is paid to uncertainty and sensitivity studies concerning the nuclear data evaluations, accompanied by improvements in the covariance data files describing the uncertainties of nuclear cross section data, and the calculation methods using these covariance data. For the validation of the nuclear data libraries, a large number of integral experiments are used. Descriptions of such experiments are found in the “International Handbook of Evaluated Criticality Safety Benchmark Experiments” [4]. Most of these validation calculations refer to multiplication factors, although other measured quantities like reaction rates and reactivity coefficients are increasingly considered; such experiments are described in the “International Handbook of Evaluated Reactor Physics Benchmark Experiments” [5]. Most systems considered are compact assemblies, mainly at room temperature. Likewise, uncertainty and sensitivity investigations based on covariance data, as performed, e.g., with the TSUNAMI code package [6], primarily consider the multiplication factors of critical assemblies. Such compact critical systems at low temperatures are not necessarily representative for power reactors at operating conditions.