Analytical model for CHF in narrows gaps on plates and in hemispherical geometries

Abstract

This study deals with CHF in narrow gaps and the purpose is to propose a predictive model for the CHF, taking into account the effect of geometry and pressure. The modelling is based on a limitation by flooding of the flow entering the gap (CCFL for counter-current flow limitation). A model has been derived for both vertical plate and hemispherical geometry. The formulation proposed by Kutateladze (Eqs. (12) and (17) with a = 1 and b ∼ 1.7) provides best-fit results for both vertical channels and hemispherical geometry. The comparison with the results obtained by Köhler et al. [Köhler, W., Schmidt, H., Herbst, O., Krätzer, W., 1998. Thermohydraulische Untersuchungen zur Debris/Wand-Wechselwirkung (DEBRIS), Abschlussbericht Project No. 150 1017, November; Köhler, W., Schmidt, H., Herbst, O., Krätzer, W., 1998. Experiments on heat removal in a gap between debris crust and RPV wall. In: OECD/CSNI Workshop on In-Vessel Core Debris Retention and Coolability, Garching, Germany, March 3–6, also in First European-Japanese Two-Phase Flow Group Meeting, 36th European Two-Phase Flow Group Meeting, Portoroz 1–5 June, and Seventh Conference on Nuclear Engineering Tokyo, Japan, April 19–23, 2000 ICONE 7012.] seems to indicate that the validity of models based on CCFL controlled CHF is limited to gaps of less than 3–5 mm. Beyond this gap size, mechanisms other than CCFL might control the CHF. However, the experimental results are too scarce and affected by too large uncertainties to validate a theoretical model. Experimental uncertainties are mainly linked to the positioning of the structure (evolution of the gap with the temperature) and to the criteria that are applied to detect the CHF. The conclusion of applications to reactor situations at reduced pressure is that the corium mass that might be coolable through a gap is certainly much nearer to the mass observed in TMI2 (∼10–20 tonnes) than to the whole mass contained in a core (100 tonnes). The main uncertainty for reactor applications still remains the knowledge of the distribution and configuration of the relocated corium.