Detection of the Departure from Nucleate Boiling in Nuclear Fuel Rod Simulators

In the thermal hydraulic experiments to determin parameters of heat transfer where fuel rod simulators are heated by electric current, the preservation of the simulators is essential when the heat flux goes to the critical point. One of the most important limits in the design of cooling water reactors is the condition in which the heat transfer coefficient by boiling in the core deteriorates itself. The heat flux just before deterioration is denominated critical heat flux (CHF). At this time, the small increase in heat flux or in the refrigerant inlet temperature at the core, or the small decrease in the inlet flux of cooling, results in changes in the heat transfer mechanism. This causes increases in the surface temperature of the fuel elements causing failures at the fuel (burnout). This paper describes the experiments conducted to detect critical heat flux in nuclear fuel element simulators carried out in the thermal-hydraulic laboratory of Nuclear Technology Development Centre (CDTN). It is concluded that the use of displacement transducer is the most efficient technique for detecting critical heat flux in nuclear simulators heated by electric current in open pool. 1. Introduction One of the most important limits in the design of the water-cooled nuclear reactors is the condition in which the heat transfer coefficient for boiling in the core deteriorates. The critical heat flux (CHF) is the heat flux at which a boiling crisis occurs that causes abrupt rise of the fuel rod surface temperature and, subsequently, a failure of the cladding material [1]. Design of a water-cooled reactor requires a sufficient safety margin with regard to the critical heat flux. The importance of CHF in nuclear engineering has led to intensive investigations worldwide over several years. Experiments show that near the CHF time of occurrence, a small increase of the heat flux or in the inlet temperature of the core coolant or a small decrease in the inlet flux of the coolant results in a change in the heat mechanism transfer, causing an abrupt increase of the temperature in the surface of the fuel rods, causing the cladding burnout. The boiling crisis that causes the critical flux is usually classified into two types as described later. The drydout that happens when the thickness of the layer of water drops to zero, usually happens in areas of high steam quality and low heat flux. The departure from nucleate boiling (DNB) happens in the area of low steam quality when there is nucleus formation of bubbles. This results in a departure from nucleate boiling (DNB) in which