Research Papers: Design and Analysis

Development, Use, and Accuracy of a Homogenized Fuel Region Model for Thermal Analysis of a Truck Package Under Normal and Fire Accident Conditions

[+] Author and Article Information
Krishna Kumar Kamichetty, Venkata Venigalla

Graduate Research Assistant
Department of Mechanical Engineering,
University of Nevada,
Reno, NV 89557

Miles Greiner

Fellow ASME
Professor of Mechanical Engineering
Department of Mechanical Engineering,
University of Nevada,
Reno, NV 89557
e-mail: greiner@unr.edu

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received September 25, 2012; final manuscript received November 14, 2013; published online February 12, 2014. Assoc. Editor: Jong Chull Jo.

J. Pressure Vessel Technol 136(2), 021208 (Feb 12, 2014) (12 pages) Paper No: PVT-12-1156; doi: 10.1115/1.4026065 History: Received September 25, 2012; Revised November 14, 2013

In the current work, a geometrically-accurate two-dimensional model is developed of an isolated fuel assembly within isothermal compartment walls. Finite difference thermal simulations are performed to determine the cladding temperature for a range of compartment wall temperatures and assembly heat generation rates. The results for zero-heat-generation-rate are used to determine a temperature-dependent effective thermal conductivity of the fuel region. The effective volumetric specific heat of the region is determined from a lumped capacity model. These effective properties are then applied to a two-dimensional model of a legal weight truck cask with homogenized (smeared) fuel regions. Steady-state normal conditions of transport simulations are performed for a range of fuel heat generation rates. The generation rate that brings the zircaloy cladding to its radial-hydride formation temperature, predicted by the homogenized model, is greater than that determined by simulations that employ an accurate-geometry fuel region model. Transient regulator fire accident simulations are then performed for a range of fire durations. The critical fire duration is defined as the minimum that brings the fuel cladding to its burst-rupture temperature. That duration is found to decrease as the fuel heat generation rate increases. The critical durations predicted by the homogenized fuel-region model are shorter than those predicted by the accurate-geometry model.

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Fig. 1

Isolated fuel region computational mesh for a 15 × 15 PWR. (a) Structure used to construct full mesh. Detail of (b) course Mesh with 21,600 nodes, (c) nominal Mesh with 86,400 nodes, and (d) fine Mesh with 316,800 nodes.

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Fig. 2

Temperature-dependent thermal conductivities. Solid lines show conductivity of each material. Dashed lines show effective conductivities of homogenized fuel region.

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Fig. 3

Temperature contours within the upper right-hand quarter of the isolated fuel region model for N2 with TB = 25 °C and Q ′ = 166 W/m (a) AG simulation (b) HG simulation with a uniform effective conductivity kU.

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Fig. 4

Temperature along w-axis shown in Fig. 3 minus boundary temperature versus distance from the fuel region center for N2 gas, TB = 25 °C and Q ′ = 166 W/m. Results are shown for an AG simulation, and for HG simulations using uniform conductivity kU, and temperature-dependent conductivity kT.

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Fig. 5

Ratio of center-to-boundary temperature difference within AG domains for different numbers of mesh points NG, to that for NG = 316,800. Results are given for Q ′ = 166 W/m, He and N2 gases, and TB = 25 and 1000 °C.

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Fig. 6

Center-to-boundary temperature difference versus axial heat generation rate for different boundary temperatures for an isolated fuel region. Diamonds show AG simulation results while lines show HG simulation results (a) N2 gas and (b) He gas

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Fig. 7

Uniform effective thermal conductivity versus axial heat generation rate for different boundary temperatures (a) N2 gas and (b) He gas

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Fig. 8

Full package cross-section AG model, with NG = 86,400.

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Fig. 9

NCT Temperature contours for Q ′ = 750 W/m-assembly and N2 gas from the upper half of the full package simulations (a) AG, (b) HG with temperature-dependent fuel region effective thermal conductivity kT.

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Fig. 10

NCT basket surface temperature profiles along s-coordinate (Fig. 8) for Q ′ = 750 W/m for He and N2 from AG (solid lines) and HG (dashed lines) simulations

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Fig. 11

Peak clad temperature versus axial heat generation rate from AG (solid lines) and HG (dashed lines) package simulations for He and N2 cover gases

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Fig. 12

Peak cladding temperatures versus time during and after a D = 30 min fire, from HG (dashed lines) and AG (solid lines) simulations, with He and N2 cover gases. Dots show the maximum peak clad temperatures.

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Fig. 13

Fire duration of concern for cladding versus axial heat generation rate for He and N2 cover gases from HG (dashed lines) and AG (solid lines) simulations




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