Stress-testing the ALFRED design - Part II: Quantification of uncertainties on the fuel assembly temperature field

Abstract

The advancement of the design of the Advanced Lead-cooled Fast Reactor European Demonstrator (ALFRED) beyond the conceptual phase, passes through the analysis of the impact of uncertainties, notably to what concerns safety-related conditions. Compliancy of plant safety to Design Extension Conditions is, according to IAEA and in line with the meaning itself of these beyond-design conditions, usually investigated by best estimates only. Due however to the demonstration nature of ALFRED, it was decided to assess the actual safety performances of this system even in ultimate conditions. To this regard, the emphasis was put on unprotected events like the UTOP (unprotected transient of over-power) and ULOOP (unprotected loss of offsite power, resulting from the combination of a loss of flow and loss of heat sink under unprotected conditions), pinpointed as the most challenging situations sought for the plant. The purpose of the present work, which has been divided in three parts, was then to assess the ultimate ALFRED safety margins against failure of the key core components and systems (Part III). To target this objective, the evaluation of uncertainties coming, on one hand, from nuclear data was performed at first, to retrieve their impact on the reactivity coefficients, thereby on the transient behavior driven by the latter (Part I); then, uncertainties from material properties, fabrication procedures, operation and computational tools were propagated to assess their influence on the thermal-hydraulics of the system (Part II). In this work the focus is on the latter uncertainties. The adopted methodology is presented at first, namely the semi-statistical vertical approach – characterized by an optimal degree of conservatism among the classical approaches – targeting a 3σ confidence interval. Then, the identification and propagation of each effect are shown, by means of the heat equations, so to retrieve the actual uncertainties on the parameters of interest (the temperatures themselves). Finally, a hot spot analysis to quantify the uncertainty-distorted temperature field is elaborated and presented. The performed analysis has revealed the great impact of fabrication tolerances for the coolant, film and clad temperature rises, particularly affecting safety margins during an ULOOP, while models and material properties uncertainties seem to dominate for the gap and fuel rises, which concur notably in challenging the respect of the fuel melting limit in an UTOP.