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Unformatted text preview: ENU 4134 – Gap Conductance D. Schubring November 10, 2009 (Driving Towards) SingleChannel Analysis I Conduction in fuel – 1+ I Gap conductance – 1 I Conduction in cladding – << 1 I Convection to coolant – 23 Conduction in cladding is such a simple topic that it is grouped with these slides. Gap Conductance (1) In a typical LWR, the second biggest Δ T is across the gas gap. At BOL, this is a narrow (order of 100 μ m) space between the UO 2 fuel and Zircalloy cladding. It is filled with an inert gas, usually helium. The gap gets smaller as the reactor first comes to temperature; both the width and composition of the gap vary with burnup. Width: fuel swelling and cracking leads to shrinking gap and fuelclad contact. Composition: fission gases displace some of the helium and reduce conductance through the gap. Gap Conductance (2) The term gap conductance can be confusing. It is possible to formulate this Δ T using a parameter with dimensions of W K 1 , W m 1 K 1 , or W m 2 K 1 . Strictly, thermal conductance should have a dimension of W K 1 , but modeling as h g (with dimensions of W m 2 K 1 – like a heat transfer coefficient) is conventional. q 00 g = h g ( T fo T ci ) (1) Since the thickness of the gap δ g << D , the gap is usually modeled without regard to curvature effects. Three sorts of the heat transfer must be considered – conduction through the gas, radiation between the surfaces, and enhanced conduction due to gap closure effects. (Why not convection?) Conduction through Gas...
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This note was uploaded on 07/14/2011 for the course ENU 4133 taught by Professor Schubring during the Spring '11 term at University of Florida.
 Spring '11
 Schubring

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