Thermal Analysis of Spent LWR Fuel Casks Under Normal and Accident

Conditions

Miles GreinerUniversity of Nevada, Reno

Systems Analysis Technical Guidance fromBill Halsey, LLNL

GNEP Annual Meeting, October 1, 2007Litchfield Park, AZ

Spent LWR Fuel Casks

• 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

Storage Packages

Internal Structure

Rail Cask

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

• July-September, 2007, performed three fire tests with a rail-cask-sized 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

0

200

400

600

800

0 500 1000 1500 2000

Time (s)

DT

(K

)

Experiment

CAFE(Initial

Simulation)

Measured and Simulated Average TemperatureCAFE Fire

SimulationRail-Cask-Size Pipe Calorimeter in a JP8 Fire

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

200

300

400

500

200 300 400 500 600 700 800Q [W/Assembly]

T [0C] ETCCFD

4000C

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

¼ of Full Cross-sectionIncluding Fuel

Cover Gas Streamlines

Peak Clad Temperature vs Assembly Heat Generation

Rate

3D CFD Fuel Assembly Simulations Model Region Between Consecutive Spacer Plates

• Compare two- and three-dimensional CFD simulations results

• Are these results quantitatively accurate?

PWR Assembly

Vertical (Storage) Orientation

Horizontal (Transport) Orientation

Mock Spent Fuel Heat Transfer Experiment

• 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)

Mock Fuel(8x8 Heater Rod Array) Gas Filled Aluminum Enclosure

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