# Presentation: 546333

## An Analysis Of A Spent Fuel Transportation Cask Under Severe Accident Conditions

**Christopher S. Bajwa**
**Spent Fuel Project Office**
**Office of Nuclear Material Safety and Safeguards**
**U.S. Nuclear Regulatory Commission**
**Finite Element Modeling Continuous Improvement Workshop**
**May 23, 2002**
**Greenbelt, MD**

## Introduction

**Baltimore Tunnel Fire**
**Spent Fuel Transportation Cask**
**Preliminary Results **
**Conclusions and Future Work **

**2**

**Notes:**
What I would like to discuss today are some of the details surrounding the Baltimore Tunnel Fire event of July 2001.  

Some of the questions that came out of this event as they relate to the transportation of spent nuclear fuel.  

I will also discuss a scoping analysis that we performed to determine the possible effects of a tunnel fire environment on a spent fuel transport cask, as well as present the preliminary findings of that analysis.  

Finally I will share some conclusions and discuss future work to be completed in this area.

## Baltimore Tunnel fire

**July 18****th****, 2001**
**Howard Street Tunnel**
**CSX Freight Train**
**Derailment and Fire **
**Tripropylene**

**3**

**Notes:**
The Baltimore Tunnel accident and ensuing fire occurred on July 18th of this year.  It took place at the Howard Street rail tunnel near downtown Baltimore.  This tunnel is a single rail tunnel, constructed in 1895 of brick and concrete.  It is 1.66 Miles long, and has a 1/10% grade, going East to West.  Tunnel section dimensions vary, but are approximately 14 feet high by 32 feet wide.

The incident involved a CSX freight train which derailed, and a tanker car full of Tripropylene was punctured and caught fire.  Tripropylene is a liquid that has a NFPA Flammability Rating of 3, which means that it can ignite under all ambient conditions.  Gasoline has the same flammability rating.

According to reports the Tripropylene tank car burned out in about 12 Hours.  It was about 12 Hours after the fire started that fire fighting personnel determined they could safely enter the tunnel.  There was a great deal of speculation as to the temperatures present in the fire during the event.  Numbers of between 1,000 and 1,500 degrees F have been reported in the media.

The truth is, we do not know the maximum temperature that was present in the fire, or how long that maximum temperature was sustained.  This information should be provided by the NTSB in the future.

## Spent Fuel Transportation Cask

**10 CFR 71.73 Fire Accident**
**Cask Performance**
**Finite Element (ANSYS****®****) Model**

**4**

**Notes:**
Needless to say, this event has raised questions about how a spent fuel transportation cask would respond if exposed to a severe tunnel fire, similar to the one that occurred in Baltimore.

10 CFR 71.73 outlines a regulatory fire condition for evaluation of transportation cask performance.  This regulatory fire is specified as a fully engulfing fire at an average flame temperature of 1475°F, for 30 minutes.  A turbulent fire environment must also be accounted for.  Casks subjected to a fire with these conditions must maintain their integrity and continue to provide shielding and criticality control for spent fuel.  In other words, we don’t want the cask to leak and we want to make sure the fuel stays intact.

In order to evaluate the performance of a cask under rail tunnel conditions, we developed a finite element model of a currently approved transportation cask in ANSYS.  Details of the model are on the next slide.

## Spent Fuel Transportation Cask

## ANSYS® Model

**Notes:**
Model is a 2-D circular cross section of a cask.  The Outer Shell of the Cask is carbon steel.  
Neutron shield region is a composite of the neutron shield polymer material and stainless steel stiffeners.
Gamma shield is layers of carbon steel plates.
Overpack inner shell is stainless steel material

Fuel region is a homogenization of fuel assemblies, and helium fill gas, with an effective thermal conductivity.
This homogenization of the fuel region is conducted to get an effective thermal conductivity and an effective density for the region.  This greatly simplifies modeling of the fuel, but has the drawback that it is not as precise as more detailed fuel modeling.  The homogenization is conducted through the following steps:

First, Model the fuel assemblies and fuel basket accurately to account for all heat transfer mechanisms involved, including convection, conduction, and radiation.  This gives you an accurate fuel assembly and basket temperature profile to work with.

Then, calculate an effective conductivity from the individual conductivity values of all the materials of the fuel  basket, based on the volumes of those individual materials present.

Finally, the effective conductivity region must be modeled to assure that the temperature profile closely matches that of the original detailed fuel model.

The model shown here is 2-D planar, and has 7500 elements.  (Plane 55 thermal elements, and Surf 151 elements were used)

## ANSYS® Model Mesh

## Heat Transfer Mechanisms
Initial Conditions
Fire conditions

**Initial Conditions**
**Fire conditions**

**Boundary Conditions**

**8**

**Notes:**
The model utilizes radiation and convection as external heat transfer mechanisms, and conduction as the sole internal heat transfer mechanism.

The initial condition for our model is an ambient of 100°F, with solar insolation (heat flux) based on values in 10 CFR 71.71.  Conduction is placed on the outside surface of the cask, with a coefficient equivalent to free convection.  There is an internal 20kW heat load placed on the fuel region.  This is run to steady state.  This is the starting point for the fire condition.

For the fire condition, because the conditions of the Baltimore tunnel fire have not yet been determined, we chose to use a temperature of 1500°F, which is slightly above the regulatory requirement.  We removed the solar heat flux from the model, and increased the convection coefficient to a conservative value, to model the turbulent nature of a fire.  This condition was run for 8 Hours.  

The slides that follow are plots of the results.

## Normal Condition Temperature Profile

**Notes:**
This is the “normal condition” temperature profile.

As you can see, the fuel center of the fuel region is at 665°F, and the skin of the cask is at 226°F.

## 8 Hour Fire Temperature Profile

**Notes:**
At 8 hours of fire exposure, the fuel region is around 862°F, while the surface of the cask is at 1445°F.

After the 8-hour fire the cask is returned to the 100 degree ambient condition with natural convection.

## Cask Component Time vs.Temperature Plot 

**Notes:**
What we see that after the fire condition is ended there is a rapid drop off of the skin temperature, as the external conditions are returned to 100°F ambient, and free convection.  The center and outer fuel region however, continue to increase in temperature well after the fire is over.  The center of the fuel region (basket) reaches a maximum of 1054°F at approx. 20.5 hours into the transient.  

This is not a peak cladding temperature, since the fuel region is homogenized, the peak cladding temperature for a fuel assembly in this case would be higher by as much as 40 degrees.

The accepted maximum short term fuel clad temperature for Zircalloy fuel is 1058°F.  Therefore, for this model, run for 8 hours, it is likely that we would exceed peak cladding temperature limits.

## 8 Hour Fire with 20 Hour Cooldown

## Fuel Cladding Temperature Limits 
Conservative Assumptions

**Conservative Assumptions**

**Preliminary Results**

**13**

**Notes:**
From this analysis, we are able to get a rough idea of how a cask may perform when subjected to a long term fully engulfing fire.  We can estimate that fuel cladding temperature limits may not be exceeded for a fire duration of close to 8 hours.

Several assumptions made for this analysis would be considered conservative.

Our fire temperature was above the regulatory maximum of 1475°F (800°C)

We did not model the rail car carrying the cask, which would provide an additional heat sink during a fire, and prevent the cask from being fully engulfed.

In addition, we assumed natural convective cooling after the fire event.  In reality, there would most likely be external cooling provided such as a hose-stream or a cooling fan.  This would combat the temperature rise in the center of the cask.

Finally, the internal heat load for the cask was the maximum allowed for the cask design (20 kW).  It is unlikely that a cask would be carrying the maximum design thermal load.

## Conclusions and Future Work

**Cask Performance**
**Refined Cask Model**
**Model Tunnel and Railcar**
**NIST Tunnel Fire Model**
**Revise Boundary Conditions**

**14**

**Notes:**
One of the next steps we take will be to more accurately model the fuel in the cask.  A homogenized region cannot accurately predict fuel cladding temperatures.

When the data from NTSB becomes available, we plan to re-evaluate this analysis, and re-perform it if necessary.  We also plan to add a railcar model, and model the tunnel enclosure around the cask, which could possibly make the fire exposure worse.  

Ultimately, the most accurate model would be a 3-dimensional CFD model of the cask and tunnel together.

## Refined Cask Model

## Acknowledgements

**Mr. Thomas E. Michener**
**Ms. Judith M. Cuta**
**Mr. Harold E. Adkins, Jr.**
**Pacific Northwest National Labs**

**16**