Heat transfer experiment in a partially (internally) blocked 19-rod bundle with wire spacers cooled by LBE

Highlights: • The studied blockage scenarios represent worst-case conditions in a conservative approach. • Temperature is measured at blockage mid-high and in the wake region. • The thermal resistance decreases asymptotically with increasing flow rate. • A non-dimensional model, the Karlsruhe Blockage Equation is developed. • Pressure drop increase is negligible and thus blockages cannot be detected. - Abstract: Internal blockages in fuel rod bundles can be rather small and may have a negligible global effect on the flow rate in a fuel assembly, and yet they can still produce significant local hot spots potentially leading to clad failure. Within the European project MAXSIMA the thermal-hydraulic effects of internal flow blockages in a heated rod bundle cooled by lead-bismuth eutectic (LBE) are investigated in prototypical experiments. This work builds up on previous experiments in an unblocked 19-rod bundle with wire spacers within the project SEARCH. In all cases, operating conditions representative of the scenarios expected in the MYRRHA reactor, developed by SCK-CEN in Belgium, are imposed. Similarly the characteristics of the blockage scenarios are selected according to the worst-case postulated events, in a conservative approach. Only solid blockages are installed, while a certain porosity can be expected if they are formed by accumulation of particles. Moreover, they are constructed using a filling material of low thermal conductivity, instead of being metallic. All blockage elements have the same length (54.6 mm, i.e. one sixth of the wire pitch) but different position and size. A first scenario includes two small blockages covering one central and edge sub-channel each. In the second scenario, a large blockage covering six sub-channels surrounding the central rod is investigated. The experimental results are presented following two approaches. First, selected reference cases are described in detail, allowing for a direct comparison with numerical simulations. Second, the results for all cases are correlated in dimensionless terms. An empirical model named Karlsruhe Blockage Equation (KBE) is proposed for representing the heat transfer resistance at the hottest spots in the blocked region for each element. Extrapolating these results to full-power conditions, the temperature increase produced by solid blockages with low thermal conductivity can be acceptable if they are small and excessive if they are larger. Further results include the wall and fluid temperature in the wake region and differential pressure in blocked and unblocked sections of the bundle.