Geometrically-Accurate and Homogenized Fuel Region Models to Predict Cladding Temperatures within a Truck Cask under Normal and Fire Accident Conditions
The temperature of spent nuclear fuel cladding within transport casks must be determined for both normal conditions of transport and hypothetical fire accident conditions to assure that it does not exceed certain limit conditions. In the current work a two-dimensional finite-element thermal model of a legal-weight truck cask is constructed that accurately models the geometry of the fuel rods and cover gas. Computational fluid dynamics (CFD) simulations are performed that include buoyancy induced motion in, and radiation and natural convection heat transfer across the cover gas, as well as conduction in all solid components. Separate simulations are performed using helium or nitrogen cover gas. Stagnant-gas CFD (SCFD) simulations are preformed and compared to CFD simulations to determine the effect of gas motion. These results are compared to those from finite element models that employ Effective Thermal Conductivity properties in the fuel regions. For normal conditions of transport, the peak clad temperature is determined for a range of fuel heat generation rates to determine the thermal dissipation capacity based on peak cladding and surface temperature, QC and QS. These are respectively, the fuel heat generation rates that bring the peak cladding temperature to 400°C, or the peak surface temperature to 85°C (their allowed limits for normal transport). Transient fire/post fire simulations are then performed for a range of fire durations to determine the critical durations for cladding Creep Deformation or Burst Rupture, DCD or DBR. These are the fire durations that bring the cladding temperature to 570°C or 750°C, respectively. In the current work, a geometrically-accurate two-dimensional model is developed of a single fuel assembly within isothermal compartment walls. Finite element thermal simulations are performed to determine the cladding temperature for a range of compartment wall temperatures and assembly heat generation rates. These results 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 tubes to their radial hydride formation temperature is determined. Transient regulator fire accident simulations are performed for a range of fire durations. The minimum fire durations that bring the fuel cladding to its creep deformation or burst rupture temperatures are determined.
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