NUCLEAR GAMBLE

BY JIM HUTTON, PE, CSP

Why is the Department of Energy (DOE) gambling with the safety of emergency responders and the public? Nuclear materials are shipped on the public byways in various forms from medical isotopes to transuranic waste. The criteria used to protect nuclear materials from fire are outdated and flawed. Fuel fires during a transportation accident may result in sudden and violent rupture of nuclear containers before tactical response is possible. The potential exposure of emergency responders and the public to nuclear and chemical hazards during transportation continues to be ignored and denied.

FIRE TEST CRITERIA FOR PROTECTIVE OVERPACKS

Fissile uranium hexafluoride (UF6), uranium and plutonium isotopes, transuranic waste, and so on, are shipped in protective overpacks. The fire temperatures used by the DOE and the regulatory criteria for protective packaging under hypothetical accident conditions have a flawed technical basis and insufficient safety margins to protect the public.

The regulatory fire exposure requirements for protective overpacks adopted by the Atomic Energy Commission (AEC) in the 1960s and, subsequently, the Department of Transportation (DOT) preceded the experimental research and computer modeling knowledge available today. The regulatory fire temperature and duration of 1,475°F for 30 minutes was chosen by representatives of the nuclear industry based on fire criteria for building fires and is not representative of a fuel fire typically associated with a vehicle accident.

In a literature search of regulatory fire temperature origins, it was stated, “ellipse the test proposed of 1,475°F for half an hour is felt to be as searching as the British standard fire for one hour. No proof was given.”1 Standard building fire test criteria are published by England and the United States and are used to evaluate building construction and assemblies. Building fires are fueled by solid combustibles, and oxygen supplied to the fire is controlled by door or window openings. These constraints produce a slower-developing fire when compared with a fire involving hydrocarbon fuels typical of a transportation accident.

The American Society of Testing Materials (ASTM), Underwriters Laboratories (UL), and the American Petroleum Institute, along with several other organizations, including the DOE’s own National Laboratory, have developed other criteria that characterize hydrocarbon fuel fires. The original time-temperature curve, ASTM E-119, was published in 1917 to simulate interior structure building fires. While still useful for structure fires involving solid Class A combustible materials, ASTM E-119 does not correlate well with the actual time-temperature and heat flux experienced during a hydrocarbon spill fire.

A fire involving hydrocarbons, typical of a transportation accident, is more accurately represented by the fire exposure criteria in ANSI/UL 1709.2 The fire tests in these standards require an average temperature of 2,000 ± 200° within five minutes from the start of the tests and provide reasonable safety margins for protection of nuclear materials.

The DOE has funded research that supports the fact that average fire temperatures involving hydrocarbon fuels typically found at a transportation accident exceed the regulatory criteria. However, the DOE is unwilling to adopt its own research and more stringent accident conditions for its protective packaging criteria.

The DOE frequently uses computer modeling and bench scale oven testing rather than real fire testing to analyze failures. It is important to use the correct assumptions and input codes in computer modeling. The DOE appears unwilling to accept higher verifiable fire temperature data and perpetuates a pattern of flawed decisions with regard to these safety concerns and protection of the public.

You must appreciate the relationship between temperature and heat to understand the significance of higher fire temperatures for hypothetical accidents than those adopted by the DOE. The laws of heat transfer and physics state that the radiant heat emission per unit area from a black surface is proportional to the fourth power of its absolute temperature. Thus, you can calculate that increasing regulatory criteria temperature to the “real fire” conditions increases radiant heat flux by approximately 50 percent-this is the dramatic effect of the fourth power. Comparing the level of radiant heat flux referenced by the DOE with actual experimental data from the literature, it is clear that the DOE underestimates the real radiant heat flux when evaluating transportation risks.

URANIUM HEXAFLUORIDE HAZARDS

Emergency responders and the public should be aware of new information on the fire hazards associated with uranium hexafluoride cylinders owned and transported by the DOE and others. Recent mathematical modeling has determined that cylinders may rupture in six to 15 minutes from postulated fire exposures typically associated with a transportation accident.

In 1986, the DOE censored my recommendation to perform research and fire testing on the fire hazards of large UF6 cylinders. The DOE remains in denial about the public transportation risks and research and analysis conducted by its own National Laboratory and contractors. The quantity of fuel necessary to transfer enough heat to a cylinder and cause rupture depends on the fire temperature, pool size, and spill rate. Internal safety analysis reports have determined that as little as 30 to 80 gallons of diesel fuel would be sufficient to rupture a 14-ton cylinder.3

Uranium hexafluoride is a radioactive and corrosive material that may have varying degrees of uranium 235 enrichment. When the uranium is enriched to more than one percent, the UF6 is classified as a “fissile” material for transportation and is protected by overpacks. Cylinders with up to 14 tons of material containing less than one percent enrichment are transported by common carriers without protective overpacks. Regulations do not require the use of pressure-relief devices for cylinders.

UF6 is a solid at room temperature; with continued heating, the material changes to the liquid state until, at approximately 290°F, the cylinder becomes liquid full; any additional heat will cause a hydraulic failure. UF6 escaping from a ruptured cylinder in a fire can react explosively with hydrocarbons to create a large fireball. The material also will react rapidly with the moisture in the air to form large quantities of uranium oxyfluoride (UO2F2) and hydrogen fluoride (HF). Uranium oxyfluoride presents the hazard of heavy metal poisoning, which affects mainly the kidneys. Hydrogen fluoride is a very corrosive material that produces severe burns to the skin and to lung tissue if inhaled. Contamination may affect a large area; however, no fire testing of large cylinders has been conducted to validate evacuation distances.

There is much confusion concerning public evacuation distances. The DOT’s Emergency Response Guidebook (Guide 166 for UF6) recommends an evacuation distance of 1,000 feet (300 meters) for a major fire situation. It recommends a one-mile (1,600-meter) distance for hydrogen fluoride (the major constitutent of a UF6 release). The French Commissariat a l’Energie Atomique (CEA) postulates the lethal effects of a release will be observed at distances of 500 to 1,000 meters.

The DOE must stop ignoring and denying the fire risks of transporting nuclear materials and must establish protective criteria and controls that ensure public safety. Protective overpacks should be designed to withstand higher fuel temperatures. All UF6 cylinder shipments should be provided with protective overpacks. Failure to do so is gambling with emergency responder and public safety.

Endnotes

1. Marvin Resnikoff, et al., The Next Nuclear Gamble, Council on Economic Priorities Publication, 1983. 196.

2. ANSI/UL 1709, Fire Tests Protection Materials for Structural Steel, Underwriters Laboratories Inc., 1989.

3. D.A. Lombardi; W.R. Williams; W.R. Brock; J.C. Anderson; and J.H. Clinton, Development of the FSAR Fire Accident Analysis, June 6, 1997.

Jim Hutton, PE, CSP, is a senior fire protection engineer with the U.S. Department of Energy. The views expressed in this article are his own and do not represent official DOE policies.

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