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Unformatted text preview: ENU 4134 – Gap Conductance D. Schubring Fall 2011 Learning Objectives I 3d Evaluate models for gap conductance (“evaluate” meaning both getting a result and considering if the model is physical) I 5a Develop familiarity with terminology and mathematical symbols common to nuclear TH, including those symbols which do not map onetoone to terminology I 5b Use correlations and/or models to analyze problems in nuclear thermal hydraulics 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 dimensions of W K 1 , but modeling with dimensions of W m 2 K 1 is conventional (consider as a heat transfer coefficient) q 00 g = htc 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.modeled without regard to curvature effects....
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 Fall '11
 Schubring
 Heat, Noble gas, Black body, argon, gap conductance, htccontact

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