Effect of fuel burnup history on neutronic characteristics of WWER-1000 Core

Abstract

In preparation of few-group cross-section libraries to be used in WWER macro-calculations, change in the nuclide composition during fuel burnup is commonly defined under invariable characteristics, averaged over the entire core (power, fuel and moderator temperature, moderator density etc.). In reality, conditions of fuel burnup are changing and this factor affects the fuel nuclide composition (the so-called spectral history effect). To account for real burnup history, it is necessary to take into consideration the dependence of cross-sections not only on burnup but also on history of neutron spectrum in the fuel burnup process. This paper analyzes fuel burnup history effect on neutronic characteristics of WWER-1000 core with use of the DYN3D code. The DYN3D code employs the local Pu-239 concentration as an indicator of burnup spectral history. The calculations have been performed for the first four fuel loadings of Khmelnitsky NPP unit 2 and stationary fuel loading with TVSA. The effect of fuel burnup history is shown both on macro-characteristics of the reactor core (boric acid concentration, fuel cycle duration, reactivity coefficients) and on local values of burnup and power.