Experimental investigation on single-phase pressure losses in nuclear debris beds: Identification of flow regimes and effective diameter

Abstract

During a severe nuclear power plant accident, the degradation of the reactor core can lead to the formation of debris beds. The main accident management procedure consists in injecting water inside the reactor vessel. Nevertheless, large uncertainties remain regarding the coolability of such debris beds. Motivated by the reduction of these uncertainties, experiments have been conducted on the CALIDE facility in order to investigate single-phase pressure losses in representative debris beds. In this paper, these results are presented and analyzed in order to identify a simple single-phase flow pressure loss correlation for debris-bed-like particle beds in reflooding conditions, which cover Darcean to Weakly Turbulent flow regimes.The first part of this work is dedicated to study macro-scale pressure losses generated by debris-bed-like particle beds, i.e., high sphericity (>80%) particle beds with relatively small size dispersion (from 1mm to 10mm). A Darcy–Forchheimer law, involving the sum of a linear term and a quadratic deviation, with respect to filtration velocity, has been found to be relevant to describe this behavior in Darcy, Strong Inertial and Weak Turbulent regimes. It has also been observed that, in a restricted domain (Re=15 to Re=30) between Darcy and Weak Inertial regimes, deviation is better described by a cubic term, which corresponds to the so-called Weak Inertial regime.The second part of this work aims at identifying expressions for coefficients of linear and quadratic terms in Darcy–Forchheimer law, in order to obtain a predictive correlation. In the case of monodisperse beds, and according to the Ergun equation, they depend on the porosity of the medium, empirical constants and the diameter of the particles. Applicability of the Ergun equation for debris-bed-like particle beds has been investigated by assessing the possibility to evaluate equivalent diameters, i.e., characteristic length allowing correct predictions of linear and quadratic terms by the Ergun equation. It has been observed that the Sauter diameter of particles allows a very precise prediction of the linear term, by less than 10% in most cases, while the quadratic term can be predicted using the product of the Sauter diameter and a sphericity coefficient as an equivalent diameter, by about 15%.