This paper develops a simple mathematical model to examine the heat transfer phenomena in a single-phase counter-current subcooled water flow in a volumetrically heated horizontal channel connected to an unheated vertical pipe at each end as shown in Figure 1. In Figure 1, the heated horizontal channel and the vertical pipes connected to it are initially filled with subcooled water up to a certain height in the vertical pipes. The vertical pipes can have horizontal runs. The piping arrangement in the model with horizontal fuel (i.e., heated) channels and vertical feeder pipes is relevant to a reactor such as the Canadian Deuterium Uranium (CANDU) reactor. The single-phase water flow condition considered in the model is relevant to CANDU in a shutdown, maintenance state where the main heat-transport-circuit pumps are shutoff and the shutdown-cooling pumps are or become unavailable. Under such postulated loss-of shutdown-cooling pump scenario, it is desirable to know whether the fuel fission-product decay heat can be adequately removed by single-phase subcooled water natural-circulation flow before the water in the fuel channels begins to boil. Boiling and the resulting two-phase conditions, condensation and changes in the buoyancy forces induce intermittent flow in the channel causing intermittent limited fuel heatup Ref [1–3]. Unlike counter-current flow of gas and liquid, counter-current flow of liquids, particularly the same miscible unequal-temperature liquids and in the geometry considered in this paper has not been studied either theoretically or experimentally to the authors’ knowledge.

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