Evaluation of the rod worth uncertainty from S-curve for neutronics start-up tests at China Experimental Fast Reactor

In China Experimental Fast Reactor (CEFR) Start-up tests, the reactivity change was measured by the control rod worths; the change of critical rod positions was measured first, and it was converted to worth by rod worth curve, S-Curve. Since the uncertainties in control rod worth are propagated to all other reactivity measurements, it is essential to assess the impact of rod worth uncertainty on the reactivity measurements. In practice, the control rod worth at specific position is obtained from S-Curve, which is an interpolated result of several rod worth computations. Therefore, the uncertainty of control rod worth is highly related with the interpolation parameters and errors in S-Curve, and those were quantified in this paper. It turns out that the uncertainty varies according to the control rod positions, and large uncertainty can be found when the control rod positions are changed significantly. The maximum uncertainty of rod worth was estimated about three times greater than that of criticality, but it was less than two for most cases. With the evaluated rod worth uncertainty, one can provide appropriate calculation uncertainty of reactivity measurement in CEFR Start-up test. (authors)