Center for Reactor Information
Safe Transportation of Spent Nuclear Fuel
Part 1 of 2
Eugene E. Voiland 1
Recently Congress approved the Yucca Mountain site in Nevada for the disposal of High Level Nuclear Waste (HLW), which includes spent nuclear fuel. 2 The U.S. Department of Energy is now authorized to seek licensing of the repository.
Because spent nuclear fuel is highly radioactive and therefore dangerous, it is a common misconception that its transportation from the nuclear reactor where it originates to the disposal site in Nevada will pose a great hazard and grave risk to the general public.
However, many materials are dangerous under some circumstances, but by simple control measures, the risks can be eliminated or reduced to acceptable levels.
Ammonium nitrate, for example, is used safely by the ton as a fertilizer. On the other hand, it can be a terrible explosive.
Just so with spent nuclear fuel. If no provision is made to control the radiation, it is truly "deadly" and exposure at close range for relatively short periods of time can be lethal.
Fortunately, control of the radiation is done rather simply, by storing or manipulating the spent fuel under water or behind thick layers of iron, lead, or concrete. These "shielding" materials absorb the radiation and eliminate the risk inherent in the unshielded spent fuel.
Thick-walled containers to absorb radiation during transport of spent fuel have been used from the first such shipment in 1946 to the present time, during which several thousand shipments have been made. No container in normal use or involved in an accident has released any of its contents, nor has any increase in emitted radiation above levels allowed by the design ever been noted.
Shipping containers (casks) for the transport of spent nuclear fuel are designed, fabricated, and operated under regulations prescribed by Title 10, Part 71, Code of Federal Regulations, "Packaging and Transportation of Radioactive Material." Such casks are strong, massive metallic cylinders that are designed to retain their contents under the most unlikely accident conditions. These casks are inherently safe containers for transporting large quantities of highly radioactive materials including spent nuclear fuel.
Exposure of the public to radiation while the cask is in-transit is inconsequential.
The transportation section of the U.S. Department of Energy's "Environmental Impact Statement (EIS) for Yucca Mountain" considers the frequency that accidents can occur, their severity and their consequences, 3 both radiological and nonradiological. It concludes that in more than 99.99 percent of rail and truck accidents no cask contents would be released. Hypothetical accidents that could cause damage to a cask are very serious, very improbable, and expected to occur extremely infrequently. One such postulated accident could be expected to occur three times in each trillion truck accidents, a second such accident, three times in 100 trillion accidents.
Confidence that casks will perform as designed arises from validated engineering analyses and from many tests using scale models and actual casks.
That this confidence is not misplaced is borne out by the performance of casks in actual use.
The record is perfect!
"The safety record for spent fuel shipments in the U.S. and other industrialized nations is enviable. Of the thousands of shipments completed over the last 30 years, none has resulted in an identifiable injury through release of radioactive material."4
* * * *
A reliable transport system for the movement of spent nuclear fuel was recognized very early as an essential part of the process of making electricity from nuclear energy. Originally, it was expected that the spent fuel would be moved to a reprocessing plant for recovery of recyclable nuclear fuel materials and packaging of the radioactive waste materials for disposal. For a variety of reasons, large-scale commercial reprocessing of the spent fuel did not materialize in the United States. Instead a decision was made to treat the spent fuel as a waste for disposal. The Nuclear Waste Policy Act of 1982 (as amended) specifies that the federal government will take ownership of the spent fuel and assume responsibility for its disposal.
Users of nuclear-produced electricity pay for disposing of spent fuel by a surcharge of a tenth of a cent per kilowatt-hour, which is included in their electric bills.
In July 2002 the Congress approved the site at Yucca Mountain in Nevada as the U.S. repository for disposal of high-level nuclear waste (which includes spent nuclear fuel). The U.S. Department of Energy is applying for site license from the U.S. Nuclear Regulatory Commission.
One concern is that adopting a central location in the western part of the country for spent fuel disposal will require shipping highly radioactive material across the country. According to some, the possible release of spent fuel in a transportation accident is an unacceptable risk.
This report provides information about the safety of the spent-fuel transportation system that has been in use for the past 40 years and under which several thousand spent-fuel shipments have been made with no release of any of the transported materials.
Design Considerations. The chief factor that influences the design of the transport system is the need to protect the general public from exposure to the radiation emitted by the radioactive materials contained in the spent fuel. Thus, the shipping containers for spent fuel have to:
Ensure that the spent fuel remains contained even under severe accident conditions.
Ensure that radiation levels at the surface of the container are well below allowed limits during normal transport and under accident conditions.
Ensure that the transported spent fuel cannot accidentally undergo a nuclear fission reaction.
From the beginning, it was believed that it was well within technical capabilities to design and fabricate a transport system that meets these performance requirements. It was also believed necessary to provide a regulatory framework to assure that the resulting system would consistently meet performance requirements.
Regulations Affecting Transportation Systems. The basic criteria for packages for shipping high-level nuclear materials and spent fuel originated in 1946 and were based on recommendations of the National Academy of Sciences. These recommendations served as guidance for manufacture of the early shipping casks for spent fuel and have been adopted by the International Atomic Energy Agency and by 53 nations. 5
Regulations were formalized based on the above criteria and in 1974 the United States Nuclear Regulatory Commission issued Title 10, Part 71, of the Code of Federal Regulations (10CFR71). This directive on the Packaging and Transportation of Radioactive Material is a detailed and comprehensive listing of requirements that must be met for safely shipping radioactive materials.
A few of the 75 subjects addressed in 10CFR71 that pertain to spent fuel transport include (i) application procedure for package approval, (ii) the approval standards that a shipping package must meet for irradiated nuclear fuel transport, (iii) the tests that the package must meet, (iv) quality control procedures that apply, and (v) operational procedures to be followed in use of the shipping containers.
In conformance with this regulatory document, a typical spent-fuel container -- generally referred to as a cask -- is a 20- to100- ton cylinder consisting of concentric layers of steel alloy (for strength), a dense metal such as lead or uranium between the steel layers (for absorption of gamma radiation), a neutron shield wrapped around the cask, and a grid-work within the cask to hold the spent fuel. One end of the cask is fitted with a closure and seals capable of maintaining cask integrity under severe impact.
For example, casks for shipping spent fuel to Yucca Mountain are about 20 feet long and about six feet in diameter, depending on design.
Performance Under Accident Conditions. A cask built in conformance with 10CFR71 has proven to be invulnerable to damage from the most serious accidents experienced on the nation's highways or railroads.
In December 1977 the Nuclear Regulatory Commission released its environmental statement on the transportation of radioactive material, NUREG-0170, which concluded that the risks associated with shipment in casks licensed to the standards of 10CFR71 are small.
In February 1987 another comprehensive study was completed. 6 This study relied on historical highway and railroad accident information to define realistic accident scenarios. By applying engineering analyses to casks hypothetically involved in such accidents, realistic assessments of damage were made. The conclusions were that radiological risks are "...less than risks previously estimated in the NUREG-0170 document."
In the mid-1970s Sandia National Laboratory conducted impact tests of shipping casks.7 These tests simulated actual accidents. A highway-transported cask was mounted on its trailer and the trailer was attached to its tractor in the usual fashion. Similarly, a rail-transported cask was mounted on its special rail car and the car attached to a locomotive. These casks containing unirradiated nuclear fuel were subjected to a variety of tests including (i) being impelled by rocket motors at speeds of more than 60 miles per hour into a massive 688-ton concrete barrier backed by 1700 tons of dirt, and (ii) being struck broadside (truck transported cask) by a locomotive. These tests showed that the casks could be expected to retain their radioactive contents "... in extremely severe transportation accidents".
Such accidents are very improbable. The 60-mile per hour highway impact was judged to occur with a probability of once every 70 years assuming seven million transport miles per year for spent fuel. Accidents corresponding to the other test scenarios are considered much less probable with average number of years between accidents ranging from 1,000 to 18,000 years.
Even more important than the demonstration that these cask systems could perform as designed was the confirmation that scale models could be reliably used to predict full-scale performance and that the engineering analytical methods used were valid.
The impregnability of spent-fuel shipping casks is well accepted. For example in a Smithsonian article, the author states "Gasoline...is far and away the most dangerous cargo on the nation's highways. It would be possible to build gasoline trucks that are as well protected as those that haul nuclear fuel, but doing so, the carriers say, would mean paying much more for gas at the pump."8
Response to a Fire. Full-scale fire tests 9, 10 under extreme conditions disclosed that a cask could be exposed to twice the heat and three times the duration specified by the current regulations before cask degradation occurred. The response of the cask was well predicted analytically. In one test, the lead melted completely and some lead escaped, but no fuel was released. The probability of a railroad cask fire of this magnitude is estimated at once every 700 years.
Response to a Sabotage Attack. A test to determine the amount of spent fuel that would be pulverized and become breathable if a cask were subjected to explosive penetration was undertaken in late 1981. 11 The results of this experiment in which a 26-ton cask containing a single unirradiated fuel assembly was penetrated by a shaped charge explosive device, disclosed that very little of the fuel was converted to particles of a size breathable by humans.
Using the information garnered in this experiment, an analysis was made of the radiological consequences of a hypothetical accident in which a truck mounted cask containing spent fuel is assumed to undergo such an explosive attack in Manhattan, New York City. The results from nuclear causes were no early deaths (within weeks after exposure), no early fatality (deaths within a year after exposure), and possibly one cancer fatality later. (The study did not speculate on the number of deaths that might result from the explosion.)
Certainly the nuclear effects were inconsequential as compared to homicides or vehicular accidents occurring in the same period in New York City.
Conclusion. A combination of good regulatory standards, good engineering design, quality controlled fabrication and inspection processes, validated performance of scale-model and full scale casks under a variety of test conditions provide a high degree of confidence that containers used for the shipment of spent nuclear fuel will do their job safely, both in normal use and in the event of very serious accidents.
Hypothetical Accident Conditions from 10CFR71.73
A cask for shipment of spent fuel must be able to survive the following tests:
A drop through 30 feet onto an unyielding surface. ("Unyielding" is the main point here, as this ensures that the kinetic energy of the drop is all applied to the cask. We all know that a light bulb will usually survive a fall onto a carpet, but will not live through impact with a concrete floor.) In practice, in addition to the collapsible structural components of the cask system, engineered collapsible "impact limiters" are attached to the casks to provide a "yielding" surface in the event of accident.
Impact by an 1,100-pound mass falling through 30 feet.
Fall of the cask from a height of 40 inches onto a steel post 6 inches in diameter and 8 inches long.
Exposure to an engulfing fire at 1,475 degrees Fahrenheit for 30 minutes.
Immersion under three feet of water. (An undamaged specimen of the cask must not leak when immersed to a depth of 50 feet -- or a pressure equivalent of 21.7 pounds per square inch.)
Orientation of the cask in each test is to be such as to assure maximum damage. The impact tests are to be applied successively, followed by the fire and water tests. Thus the cask must maintain its integrity through all of these tests.
Spent Nuclear Fuel
Spent nuclear fuel looks just like it did when it was loaded into the reactor. Within the fuel rods there is a change in the atomic species due to fission of uranium and plutonium in the reactor and radioactive decay after removal from the reactor. There is a minuscule loss of weight because in the fission process some mass is converted into energy (heat).
Typically, nuclear fuel12 consists of 1/2 -inch diameter by 1/2 -inch long ceramic uranium dioxide pellets stacked in 13-foot long tubes of a zirconium alloy. From 63 to 264 of these fuel rods, depending on the particular fuel design, are mounted in metal fixtures. These fuel-rod aggregations are called fuel assemblies.
A large number of fuel assemblies -- containing a total of 120 to 140 tons of uranium13 -- are grouped in the "core" of the reactor. When the reactor is operating, fission and radioactive decay produce heat mostly within the fuel rods. This heat is transferred to water pumped through the fuel assemblies and the water is directly or indirectly converted to steam. The steam powers turbine-generators that produce electricity.
This is the same way that electricity is usually generated except that the heat is provided by a nuclear reaction rather than by a chemical combustion process such as the burning of coal, gas, oil, or other fuel. A significant difference is that in the nuclear powered system there are almost no gases -- greenhouse or otherwise -- emitted.
The fuel rods remain in the reactor typically for three or four years, in 600-plus degree Fahrenheit water and at a pressure exceeding 1,000 pounds per square inch. In some instances, reactors have run continuously for over a year at essentially full power -- a quite remarkable technical feat. The fuel rods survive this harsh environment very well.
About every 18 to 24 months, one-fourth to one-third of the fuel rods are no longer suitable for continued use in the reactor. They become "used" or "spent" fuel and are removed and replaced by fresh fuel.
Upon removal from the reactor, the spent fuel is stored under water in a basin to allow the short-lived radionuclides to decay to more-stable isotopes and to reduce the heat emitted by radioactive decay.
When a nuclear power plant is shut down, the rate of heat production immediately drops by a factor of 16 to about 6% of what it was when the fission process was ongoing. An hour later, it is down by a factor of 100 to about 1%. After a month, the power reduction factor is about 1,000 (0.1%). After a year it is about 5,000 (0.02%) and after 5 years about 30,000 (0.003%).
Freshly discharged spent fuel is stored in deep pools at the reactor covered by at least nine feet of water. The water provides both shielding from the radiation and removal of heat from the radioactive decay.
After about five years of water storage the heat output of the spent fuel is reduced to the extent that it can be stored in massive concrete containers that are air cooled by convection.
According to federal regulation, the spent fuel is considered to be High Level Nuclear Waste.
Go to part 2: Safe Transportation of Spent Nuclear Fuel
Footnotes to part 1
1. Member of the Center for Reactor Information (CFRI). The author has had personal managerial experience with the safe transportation of spent nuclear fuel in over 500 truck shipments and over 100 railcar-shipments of spent fuel with no accidents and no consequences to the public. He believes that the existing procedure for shipping spent fuel is a well established and safe procedure that has proven more than adequate over the test of time. (Return to article)
2. Nuclear fuel is the uranium material that undergoes a nuclear reaction in a nuclear reactor to produce heat and steam for the production of electricity. Spent nuclear fuel is, for a variety of reasons, no longer useful to sustain the nuclear heat-generating reaction. The spent fuel can be thought of as the ashes of the nuclear reaction. Spent fuel is exceedingly radioactive, is inherently dangerous, and must be handled with great caution. (Return to article)
3. " Final Environmental Impact Statement for a Geologic Repository for the Disposal of Spent Nuclear Fuel... DOE/EIS-0250", U.S. Department of Energy, Office of Civilian Radioactive Waste Management. February 2002. (Return to article)
4. As cited in the Congressional Research Service Report for Congress, Transportation of Spent Nuclear Fuel. Updated May 29, 1998. (Return to article)
5. OTA Report " Transportation of Hazardous Materials ", Office of Technology Assessment, U.S. Congress. July 1986. (Return to article)
6. Fischer, L. E. et al, Shipping Container Response to Severe Highway and Railway Accident Conditions, NUREG/CR-4829. Lawrence Livermore National Laboratory. February 1987. (Return to article)
7. Yoshimura, H. Richard, Full Scale Simulations of Accidents on Spent-Nuclear-Shipping Systems. Sandia National Laboratory. Undated. (Return to article)
8. MacFadyen, J. Tevere, Routine Shipments of Essential Goods Are Freighted with Special Risks. Smithsonian. April 1984. (Return to article)
9. Vigil, Manuel, et al. Experimental Program Assessing Thermal Response of a Spent Fuel Transport Cask, SAND82-0110C. June 1982. (Sandia National Laboratories.) (Return to article)
10. Eggers, P. E. et al. Thermal Analysis of HNPF Spent Fuel Shipping Container in Torch Environments. July 1980 (Ridalgh, Eggers, and Associates, and Sandia National Laboratories). (Return to article)
11. Sandoval, R. P. (Sandia National Laboratories) and Newton, George J. (Lovelace Foundation) A Safety Assessment of Spent Fuel Transportation through Urban Regions. March 1982. (Return to article)
12. Table A-18, p. A-25. Final Environmental Impact Statement for a Geologic Repository for the Disposal of Spent Nuclear Fuel DOE/EIS-0250. February 2002. (Return to article)
13. Typical Boiling Water Reactor. Personal communication Exelon Corporation. (Return to article)