Abstract
The paper presents our first results of the exercise III-I-2c from the OECD-NEA UAM-LWR benchmark intended to an elaboration of the methodology of uncertainty propagation. The considered case studied a full PWR core behavior in fast (~0.1 sec) rod ejection transient. According to the benchmark, the core represented a Hot Zero Power state. Authors used brute-force sampling propagating nuclear data and thermo-fluid uncertainties using 3D computational IRSN chain HEMERA. It couples the reactor physics code CRONOS and thermal-hydraulic core code FLICA4. The nuclear data uncertainties were represented in a form of cross sections standard deviations (in percentage of the mean cross sections values) supplied by the UAM team. In addition to the original benchmark, the study includes a case with an increased power peak by supplementary rod ejection, i.e. with higher reactivity. Both the results are similar to what we obtained in the mini-core rod ejection: the power standard deviation follows, in percentage of the mean power, the mean power curve. We split the variance with a direct calculation: once the cross sections are modified and the thermal-hydraulics inputs are kept constant, another time the contrary. The results show that uncertainties dues to nuclear data dominate over ones due to the thermal-flow area. Furthermore, the major contributors in peak-of-power variance lie in a fast group of cross sections.
Highlights
The stringent and confident characterization of the accuracy of modern best-estimate simulations tools is, for a long time, of a particular interest expressed by the international, scientific and engineering communities
We show the results of uncertainty propagation on a rod ejection accident, on a whole core; exercise III-I-2c [4]
The cross sections and the kinetic parameters were modified according to a normal Probability Distribution Function (PDF) with a standard deviation supplied by the UAM team [9]
Summary
The stringent and confident characterization of the accuracy of modern best-estimate simulations tools is, for a long time, of a particular interest expressed by the international, scientific and engineering communities This interest concerns the design and safety requirements imposed on existing and advanced nuclear reactors.[1]. For being able to provide effective assistance, it is necessary to always be at state-of-art of scientific knowledge and to anticipate the potential needs of tomorrow's expertise For this reason, aiming to increase competence and to estimate the robustness of one or other methodologies applied in the uncertainty propagation domain, the IRSN took part in UAM-LWR activities with contributions in the benchmark. Applied in the studies brute-force sampling methodology, i.e. numerous transient computations where XS and thermal-hydraulic parameters were varied within given bounds of uncertainties, allowed estimate standard deviations of outputs including peak-of-power, deposit energy and time of peak.
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