Simulation of subcooled flow instability for high flux research reactors using the extended code ATHLET

Covering the wide range of reactor safety analysis of power reactors, consi sting of leak and transients, the thermohydraulic code ATHLET is being deve loped by the Gesellschaft for Anlagen- und Reaktorsicherheit (GRS-Society f or Plant and Reactor Safety) (Lerchel, G., Austregesilo, H., 1998. ATHLET,E T Mode 1.2 Cycle A, User's Manual, GRS-p-1/Vol. 1, Rev. 1, GRS). In order t o extend the code's range of application to the safety analysis of research reactors, a model was developed and implemented permitting a description o f the steam formation in the subcooled boiling regime (Hainoun, A., 1994. M odellierung des unterkuhlten Siedens in ATHLET und Anwendung in wassergekuh lten Forschungsreaktoren, D 294 Diss. Univ. Bochum, Jul-2961). Considering the specific features of high flux research reactors given by both high hea t flux and high flow velocity, the model of void condensation in subcooled flow has been extended and a new correlation of critical heat Aux (CHF) is implemented. To validate the extended program, the Thermal-Hydraulic Test L oop (THTL) of Oak Ridge National Laboratory (ORNL) was modeled with ATHLET and an extensive series of experiments concerning the onset of thermohydrau lic flow instability (OFI) in subcooled boiling regime were calculated. The comparison between experiments and ATHLET-postcalculation shows that the e xtended code can accurately simulate the thermohydraulic conditions of Row instability in a wide range of heat flux up to 15 MW m(-2) and inlet flow v elocity up to 20 m s(-1) The thermohydrauhc design limit characterized by t he mass flux, at which the flow just becomes unstable (OFI), has been predi cted in very good agreement with the experiment. However the calculated pre ssure drop at OFI is overestimated by a maximum deviation of about 25%. The calculated exit bulk temperature of subcooled coolant and the maximum wall temperature at OFI show a maximum deviation from experiment of 12 and 7%,, respectively. (C) 2001 Elsevier Science B.V. All rights reserved.