SPORTS-M Prediction of the Transition from Forced to Natural Circulation in a MAPLE Reactor

Abstract

MAPLE (Multipurpose Applied Physics Lattice Experimental) is a new research reactor-developed by-Atomic Energy of Canada Limited. It is a light-water-cooled, pool-type research reactor using fuel bundles in flow tubes. The reactor is designed to generate a maximum thermal power output ranging from 1 to 30 MU. SPORTS-M is a one-dimensional computer code developed to analyse the system behavior, thermal margin and natural circulation capability of a piping network during normal and upset conditions. It is particularly suited for the analysis of slow transients and natural circulation flows in the presence of a large volume of fluid. The code solves the conservation equations for a transient flow of a two-phase mixture using a fully implicit, forward- marching, iterative, finite-difference scheme. A heat transfer package and a radial heat conduction module are coupled to the hydraulic modules. The package contains correlations that address all heat transfer regimes of a boiling curve and a void fraction model especially suited to subcooled boiling conditions of low pressure and high subcooling. The subcooled boiling heat transfer regime is the dominant cooling mode in MAPLE for most upset conditions studied. In this paper, the integral behavior of the MAPLE primary cooling system (PCS) is studied using the SPORTS-M code. The study is based on the long-term scenario of a loss of primary flow to the reactor core. The primary objective of this study is to determine whether the transition from forced to natural circulation takes place smoothly without causing adverse thermal stratification in the PCS. The results indicate that the transition is influenced mainly by the time lag between the generation and removal of decay heat and the pump run-down time. In some cases, the PCS experiences a long period of oscillatory flow that eventually converges to the natural circulation flow. The predicted thermal margins from the onset of nucleate boiling (ONB) and onset of significant void (OSV) during the transient conditions are also presented.