Conference Proceedings Paper
Mechanistic Modeling of Bearing Pad to Pressure Tube Contact Under Localized High Temperature Conditions in a CANDU Fuel
NURETH-14 - 2011 September 25-30
Farshad Talebi (McMaster University)
John Luxat (McMaster University)
During a postulated critical break LOCA (loss of coolant accident) in a CANDU reactor the coolant flow rates in the fuel channels of the flow pass of the reactor core downstream of the pipe break can rapidly reduce to very low values and remain very low for a period of tens of seconds following the break. Under the sustained low flow conditions, the fuel sheath (cladding) temperature in the affected channels rapidly increases and the coolant in the channels becomes significantly voided. The pressure tubes in the affected pass heat up under a combination of convection heat transfer from the low flows of superheated seam and thermal radiation heat transfer from the hot fuel. Additionally, hot spots may develop on the inner surface of pressure tubes at locations where the fuel bearing pads are in direct contact with the pressure tube. Localized thermal creep strain deformation at the hot spots is a potential pressure tube failure mechanism which could challenge fuel channel integrity. This paper evaluates the local thermal- mechanical deformation of a pressure tube in a CANDU reactor under critical break LOCA conditions tube using a coupled thermal-mechanical finite element COMSOL multi-physics model and investigates the conditions resulting in fuel channel failure due to localized contact between bearing pad and pressure. The mechanistic models are validated against data obtained from COG funded experiments performed at WRL (Whiteshell Research Laboratory). Multi- physics calculations are performed in which the heat transfer, thermal-mechanical and creep strain equations are solved, simultaneously. Heat conduction from bearing pads to the inner surface of the pressure tube is modeled with appropriate convective and radiation heat transfer boundary conditions. Thermal creep strain deformation of the Zr-2.5%Nb pressure tube is modeled using correlations derived from separate uniaxial tests that are reported in the literature. Contact conductance models based on experimental correlation are employed to calculate heat conduction through contacting boundaries of the bearing pads and pressure tube. The results of the analysis establish that localized pressure tube thermal strain causing failure of the pressure tube requires both sufficiently high local temperatures and pressures, and also depends on the extent of steam cooling of the pressure tube away from the contact point between the bearing pads and pressure tube in the fuel channel. Parametric analysis is presented that evaluates the sensitivity of pressure tube local strain to modeling the contact conductance, contact width and zircaloy surface emissivity.
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