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Transportation : Testing / Safety
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The safety record of shipping radioactive materials is well established. Transportation incidents involving radioactive materials occur at a rate of about 24 per year, representing only 0.4 percent of the total of all transportation accidents which number about 6,000 per year. Few, if any, of these events involve any actual release of radioactive materials. No deaths or serious injuries have ever been attributed to the radioactive nature of any materials involved in a transportation accident.
Radioactive materials are subject to the same transportation hazards as any other freight. However, radioactive materials have special regulations that govern their shipment. These regulations and procedures are shaped by two considerations:
- the methods for shipping radioactive materials from one location to another should minimize the chance that an accident will occur;
- the radioactive materials should be packaged in such a way that no significant radiation will be released even if an accident should occur. Packages used for transport of radioactive materials are therefore designed to contain their radioactive contents in the event of an accident.
The shipping containers for spent fuel are the most rigorously designed, manufactured, tested and licensed for use. In one cask design, the fuel assemblies are sealed into a water-filled stainless steel cylinder with walls 1/2-inch thick, clad with 4 inches of heavy metal shielding, enclosed by a shell of steel plate 1-1/2 inches thick, surrounded by 5 inches of water, and encircled by a corrugated stainless steel outer jacket. The overall package measures 5 feet by 17 feet and weighs 70 tons.
The Nuclear Regulatory Commission must certify all container designs. Before approval, containers must meet rigorous engineering and safety criteria and be able to pass a series of hypothetical accident conditions that create forces greater than the containers would experience in actual accidents. These accidents conditions are:
- A 30-foot fall onto a flat hard surface (as if the cask were dropped from an overpass onto a concrete highway)
- A 40-inch drop onto a metal pin 6 inches in diameter (as if the cask hit a sharp corner of a bridge abutment)
- A 30-minute exposure to a temperature of 1,475 Fahrenheit (800 Celsius) (as if a tank of gasoline ruptured in an accident and a fire ensued)
- Complete immersion in 3 feet of water for 8 hours (as if the cask rolled off into a creek along the highway). Also, by a separate test, containers are submerged under 50 feet of water for eight hours.
The container must undergo these destructive forces in sequence with no breach of containment and with no significant reduction in shielding. A recent NRC study projects that a Type B cask will withstand 99.99% of the accidents it may be in, and that, in the absence of a fire, the impact speed would have to exceed 90 mph to result in a release. In the case of a fire, the fire would have to burn at about 1800°F for about two hours to result in a release. Radioactive material transportation accident data have been maintained in a data base since 1970. As of 1999, 90 Type B casks have been involved in some sort of traffic accident or incident. None of these accidents or incidents have resulted in any release of radioactive material, even an "allowed release."
In the 1970s and 1980s, engineers and scientists at Sandia National Laboratories in New Mexico subjected used nuclear fuel containers to actual accidents to see what would happen in real-world conditions. Among other accidents, (1) a flatbed tractor-trailer carrying a container was run into a 700-ton concrete wall banked with 1,700 tons of dirt at 80 miles per hour, (2) a container on a tractor-trailer was broadsided by a rocket-assisted 120-ton train locomotive traveling 80 miles per hour, and (3) a container was dropped 2,000 feet onto soil as hard as concrete, traveling 235 miles an hour at impact. In all these cases, the containers survived intact. Post-crash assessments demonstrated that the containers would not have released their contents.
The transportation of spent fuel assemblies and radioactive waste is governed by numerous regulations:
- The International Atomic Energy Agency (IAEA)
- The Nuclear Regulatory Commission - responsible for licensing and regulating all commercial users and handlers of radioactive materials, including waste shippers and carriers
- The Department of Transportation - general authority for regulating the transportation of hazardous materials, including radioactive materials
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