Thermal Analysis of Spent LWR Fuel
Casks Under Normal and Accident
Conditions
Miles Greiner
University of Nevada, Reno
Systems Analysis Technical Guidance from
Bill Halsey, LLNL
GNEP Annual Meeting, October 1, 2007
Litchfield Park, AZ
Spent LWR Fuel Casks
Rail Cask
Storage
Packages
Internal
Structure
• Spent fuel is placed in thick-wall, gas-filled
casks for storage and transport
• Individual assemblies are supported in a basket
• Designed to provide protection under accident
and normal conditions
Container Analysis Fire Environment
(CAFE) CFD code
• Under development at Sandia to
predict cask response in large fires for
risk studies
• Employs physics-based models that
allow it to accurately calculate heat
transfer from large fires to objects even
when coarse grids are employed
• Parameters of the models must be
determined from large-fire experiments
Ongoing Work
800
DT (K)
600
Experiment
CAFE
(Initial
Simulation)
400
200
0
0
500
1000
1500
Time (s)
Rail-Cask-Size Pipe
Calorimeter in a JP8 Fire
CAFE Fire
Simulation
Measured and Simulated
Average Temperature
• July-September, 2007, performed three fire tests with a rail-casksized calorimeter.
– Measured calorimeter temperature and wind conditions
• Currently performing CAFE simulations using the measured wind as
boundary conditions
• Determining model parameters that bring simulation results close to
data
2000
Normal Conditions
38ºC still air, full solar heat flux
• Fuel generates heat but its cladding must not exceed 400ºC
– Affects the amount of time the fuel must be aged, and/or
– Limits number of assemblies that can be loaded (cask capacity)
• In the past
– Computer resources were not available to accurately model the fuel rod
geometry
– In Full Cross section models, the fuel was replaced by solids with an
Effective Thermal Conductivity (ETC) that conservatively overestimate
clad temperatures
Current Work
• Whole Rail Cask Cross Section Simulations, including
– Accurate fuel rod geometry
– CFD simulations of gas motion (using Fluent®)
• Calculate Peak Clad Temperature vs Fuel Heat Generation Rate
• Current CFD simulations predict lower cladding temperatures than ETC
model, and higher cask thermal dissipation capacity
– Are three-dimensional effects important?
– Vertical configuration requires three-dimensional simulations
500
Cover Gas Streamlines
4000C
400
T [0C]
ETC
CFD
300
200
200
300
400
500
600
700
800
Q [W/Assembly]
¼ of Full Cross-section
Including Fuel
Peak Clad Temperature vs
Assembly Heat Generation
Rate
3D CFD Fuel Assembly Simulations
Model Region Between Consecutive Spacer Plates
PWR
Assembly
Vertical (Storage)
Orientation
Horizontal (Transport)
Orientation
• Compare two- and three-dimensional CFD
simulations results
• Are these results quantitatively accurate?
Mock Spent Fuel Heat Transfer
Experiment
Mock Fuel
(8x8 Heater Rod Array)
Gas Filled Aluminum Enclosure
• 8x8 array of electrical heater rods within an air-filled
aluminum enclosure (region inside BWR channel)
• Measure rod and enclosure temperatures for a range
of heat generation rates in horizontal and vertical
orientations. Compare to simulation results.
• Future: He cover gas, 15x15 heater array (PWR)
Status
• 15 months through a 24 month program
• Benchmarking simulation tools to predict the
performance of LWR casks under normal and fire
accident conditions
• Developing expertise that will be applied to AFCI
packages, once they have been designed
• We are looking for guidance on ways to apply our
thermal analysis capabilities to new areas that are
central to AFCI development
– We have congressional funding