Measurement of large negative reactivity in WWER-440. Sources of uncertainty

The sources of uncertainty in measurement of large negative reactivity in WWER-440 by the inverse point kinetics method, are evaluated quantitatively on the example of measurement of the reactivity worth of the shutdown control rod system of WWER-440 at zero power. Considering the specific features of the control rod system of WWER-440, it is demonstrated that using an appropriate formulation of the inverse solution of the equations of point kinetics, the uncertainty of measured reactivity ρ/β introduced by the assumption of constancy of the parameters of kinetics can be reduced to <3–5% for the case of the discussed rod-drop test at zero power. Based on an analysis of both numerically simulated and actual rod-drop transients, it is shown that the uncertainty of measured reactivity ρ/β can be quite considerable due to the underlying delayed neutron data set—the values of ρ/β obtained using different data sets can differ by 15%. Inexact accounting of the share of ²³⁹Pu in the fission neutron source is estimated to contribute to the total uncertainty of measured ρ/β of 1%, whereas possible spatial effects are expected to result in a relative error in ρ/β of 5%.