PRESSURE TUBE-CALANDRIA TUBE THERMAL CONTACT CONDUCTANCE

Abstract

<p>CANDU reactors are of the heavy water moderated, pressure tube type. The core consists of several hundred horizontal fuel channels surrounded by a heavy water moderator. Fuel channels consist of a Zircaloy-2.5%Nb pressure tube enclosed within a Zircaloy-2 calandria tube. There is an annulus gas gap between the pressure tube and the calandria tube. Under extreme accident conditions such as a critical break Loss Of Coolant Accident (LOCA), the pressure tube may deform. If the fuel channel remains pressurized, the hot pressure tube can balloon into contact with the calandria tube.</p> <p>Thermal contact conductance between the pressure tube and calandria tube must be understood as it is a key factor in studying fuel channel integrity. Once PT-CT contact occurs, heat is transferred from the hot pressure tube to the relatively cool calandria tube. The heat flux to the calandria tube is a function of the temperatures of the two tubes as well as the thermal contact conductance between them. For high heat flux levels the calandria tube temperature can increase enough for film boiling to occur on the outer surface. Film boiling will severely limit heat transfer to the moderator and cause overheating of the calandria tube which could lead to fuel channel failure. It is therefore important to understand the mechanisms involyed in thermal contact conductance and to study the transient behaviour of contact conductance during a PT-CT contact event.</p> <p>This paper presents a new approach to calculating the contact conductance transient during the initial contact and post-contact phases of a postulated critical break loss of coolant accident .The contact pressure at the interface between the tubes is a critical parameter in determining the thermal contact conductance. An iterative method is used to solve for creep strain in the pressure tube and calandria tube which determines the interfacial pressure. A modern correlation for contact conductance is then applied. The results show high contact conductance at first contact in the initial contact phase. This is followed by a rapid decrease in conductance across the interface. These results are due to the interfacial pressure being high at initial contact. In the post contact phase, as the pressure tube transfers heat to the calandria tube and cools down, thermal expansion of calandria tube and thermal contraction of the pressure tube cause the conductance to rapidly decrease.</p>