Testing to the extreme22 July 2004
Current IAEA tests for radioactive materials transport packages appear to be rigorous enough to cover all realistic accident situations. By William Wilkinson
The design and performance standards for packages used for the transport of radioactive materials, including nuclear fuel cycle materials, are defined in the International Atomic Energy Agency’s (IAEA’s) Regulations for the Safe Transport of Radioactive Material (TS-R-1) to ensure safety under both normal and accident conditions of transport. The underlying philosophy is that safety is vested principally in the package. The design and performance criteria are related to the potential hazard; the more hazardous the material the tougher the package.
Appropriate tests are specified to ensure the integrity of the package during transport and there is a large body of evidence demonstrating that the IAEA tests are severe, and cover all the accident situations that could realistically be envisaged in the transport of nuclear fuel cycle materials.
The radiological hazard from unirradiated front end fuel cycle materials – uranium ore concentrate (UOC), uranium hexafluoride (Hex), uranium oxide powder (UO2) and fresh fuel – is low, except in the event of a criticality excursion. Appropriate tests for these packages are defined in the regulations.
Industrial and Type A packages
Industrial and Type A packages for non-fissile low activity materials such as UOC and low-level wastes are required to maintain their integrity during normal transport conditions and are designed to withstand a series of tests simulating these conditions, for example, a water spray, a free drop, a stacking test and a puncture test.
Packages for fissile materials
Industrial and Type A packages may be designed for fissile materials, notably enriched uranium oxide powder and fresh fuel, and they are then designated Type IF and Type AF. Special criticality safety assessments are required for these packages, both in isolation and in arrays. Tests are specified appropriate to the duty and design of the package.
Packages for Hex
Hex is a volatile solid that can give off toxic products on reaction with moist air. The steel cylinders used as packages for natural and depleted Hex are internationally standardised and are subjected to a pressure test, which they must withstand without leakage and unacceptable stress. In addition, they have to be evaluated against a thermal requirement.
Enriched front end materials (enriched Hex, uranium dioxide powder and new fuel assemblies) are fissile. The potential hazard associated with these materials is an unwanted criticality excursion. For this reason, the packages are subjected to tests to guarantee that criticality could not occur under accident conditions of transport including crashes, fires and submergence.
HIGHLY RADIOACTIVE MATERIALS
Type B packages, which are used for the more highly radioactive materials – spent fuel, high-level vitrified wastes, and also plutonium and MOX fuel – require a demonstration of the successful performance of the package under severe tests, including impact tests relevant to crashes, thermal tests which simulate fires and water immersion tests.
The IAEA impact tests include a requirement for Type B packages to survive a 9m drop test onto a flat unyielding surface (a completely rigid surface) without giving rise to a significant release of radioactivity. This drop test is very severe because in an impact with an unyielding surface all the kinetic energy of the falling package is absorbed by the package in the deformation and damage it sustains.
Impacts in real-life situations
An unyielding surface is a hypothetical concept. The surfaces that a package could impact in real-life situations, such as concrete roads, bridge abutments and piers, would yield to some extent and therefore a proportion of the energy of the moving package would be absorbed by the surface. If the surface were much weaker than the package, then the surface would absorb most of the energy.
The 9m drop onto an unyielding surface is therefore relevant to impacts onto a real-life surface as a result of a high speed crash. The partitioning of the impact energy between a spent fuel cask and a real surface can be used to obtain impact speeds onto real surfaces that would cause the same damage as impacts onto rigid targets. This comparison is the key to relating the IAEA drop test to real-life accident scenarios. A number of independent analyses, both theoretical and experimental, have been successfully carried out.
Impact studies on spent fuel transport casks
Spent fuel transport is becoming increasingly international and on occasion has given rise to public concern in some quarters. The same is true for high-level vitrified waste transport, and the equipment used for these two operations is similar.
In his early work, D J Ammerman showed that for a cask with an impact limiter the speed onto a hard soil to give the same forces on the cask would have to be twice that of the 9m drop test onto an unyielding surface – 26.8m/s (60mph) rather than 13.4m/s (30mph). Without an impact limiter the speed of 26.8 m/s onto hard soil would be equivalent to only 1.74m/s onto an unyielding surface. This work also shows the value of impact limiters.
A study by Ammerman published in 2001 showed that the real-life surface equivalent velocities for a monolithic steel spent fuel rail cask without an impact limiter would be at least three times the velocity for impact on an unyielding surface. For example, at 30mph (13.4m/s), which is the speed corresponding to a 9m drop, the equivalent speed for an end-on collision onto hard soil or a concrete slab would be more than 150mph.
Other similar studies on Type B packages for the transport of spent fuel confirm that casks will maintain their integrity, and realistic accidents would be less severe than the IAEA 9m regulatory drop test.
Impact studies for plutonium dioxide transport
R E Vallee also studied the impact behaviour of containers for the transport of plutonium dioxide powder, under the same conditions as for the study on spent fuel casks, for a range of real surfaces and configurations. Again, the results showed that the container maintained its integrity and real transport accidents would be less severe than the regulatory drop test.
Impact studies for uranium hexafluoride transport
K Shirai and coworkers carried out experimental drop tests on a cylinder used for the transport of Hex onto an unyielding surface and also onto hard soils. This showed that a 9m drop onto an unyielding surface was much more severe than a 14m drop onto hard soil.
FIRE TESTS FOR TYPE B PACKAGES
Fire also is a consideration in the transport of nuclear fuel cycle materials since it increases the potential for release of radioactive material to the environment and, for this reason, the IAEA regulations specify that the packages for the more radioactive nuclear fuel cycle materials must be able to withstand fires. The IAEA thermal test specifies that Type B packages have to withstand a fully engulfing fire of 800ºC for 30 minutes without a significant release of activity.
Spent fuel transport
Several studies have been carried out to investigate the ability of nuclear fuel cycle packages to withstand long duration, fully engulfing fires that could be caused by the rupture of an oil or gas pipeline, or fires resulting from a train crash involving highly inflammable cargoes such as gasoline.
For example, in the work of C Ito, a spent fuel cask was subjected to a regulatory fire test at 800ºC for 30 minutes and an analysis was carried out to determine the response to a realistically severe fire accident resulting from a collision with a tanker truck. The results indicated that the cask remained sound and the conditions generated in the regulatory test were more severe than in the realistic accident.
From a fire test on a cargo ship combined with modelling work, it was concluded that even if a ship fire reaches a hold where spent fuel or high-level vitrified waste packages are stowed, the cask would not fail and would not release significant quantities of radioactivity. For such a release, a hot long duration fire, well in excess of the regulatory thermal test, would be needed with the massive casks used to transport these materials.
WATER IMMERSION TESTS
The immersion test specified in the IAEA regulations is designed mainly to ensure safety in the event of accidents at sea. Type B packages for the more radioactive materials have to undergo an immersion test equivalent to a water depth of 15m for 8 hours without loss of shielding or significant release of radioactivity. In addition, packages for spent fuel (and high-level vitrified waste) are subjected to immersion for one hour at 200m and the containment system must not rupture.
The IAEA carried out a research project in 2001 to determine whether the current IAEA regulations were adequate to cover accidents at sea, taking account of the probabilities of accidents and their consequences. The sea transport of spent fuel and high-level vitrified waste were the main focus of the study.
For collisions, extensive analytical work on the structural behaviour of ships and spent fuel and high-level vitrified waste packages was carried out. It was concluded that ship collisions are unlikely to damage the casks because the collision forces would be relieved by the collapse of the ship structures and not by the casks. The forces on the cask would be less than the forces imposed by the 9m drop test.
In the highly improbable event of the cask sinking, the rate of release of radioactive material into the sea would be very slow since the containment of the cask is unlikely to have been lost. In this hypothetical scenario the radiation doses received by people who consume marine foods affected by the accident would be negligible compared with doses from the natural background, due to the refractory nature of the material and the vast dilution that would occur. The same would apply to other nuclear fuel cycle materials, the activity of which is much less.
COMPARISON OF ACCIDENTS
The US Department of Energy recently commissioned a detailed study of severe transport accidents that have occurred in the USA over the past 20 years involving hazardous cargoes. The accidents did not involve radioactive materials.
Accident reports for twelve very severe road and rail accidents, involving high impacts, fires, explosions or water immersion were studied to determine how the conditions generated in these accidents compare with the regulatory tests and how such conditions would have affected spent fuel transport casks. Some of these accidents involved impacts such as high speed train derailments and the collapse of bridges and viaducts, which resulted in road vehicles falling onto concrete roads or plunging into rivers, while others involved fires and explosions.
The study concluded that even under these extreme accident conditions the casks would not have been significantly damaged and would have retained their integrity.
MORE SEVERE TESTS?
There is a large body of evidence to show that the IAEA tests are severe tests that cover all the situations that could realistically be envisaged in the transport of spent fuel, high-level vitrified waste, and other fuel cycle materials. Proposals for more severe tests, which have little technical justification, should therefore be treated with caution. These proposals could result in a loss of public confidence in the current regulations and the ratcheting up of design requirements, which would not be warranted on quantitative safety grounds.
William L Wilkinson, World Nuclear Transport Institute, 7 Old Park Lane, London W1K 1QR, UK
|Nuclear fuel cycle materials|
Nuclear fuel cycle materials come in a variety of chemical and physical forms and the potential hazards they present differ widely. The main features are as follows:
|Packages for fuel cycle materials|
TS-R-1 provides for five different primary packages, designated as Excepted, Industrial, Type A, Type B and Type C. Criteria are set for the design based on the nature of the radioactive material they are to contain. The regulations prescribe additional criteria for packages containing fissile material. The regulations also prescribe the appropriate test procedures. This graded approach to packaging is important for safe and efficient commercial nuclear fuel cycle transport operations. Road, rail and sea are all commonly used for transporting nuclear fuel cycle materials. Air transport has been used to a limited extent.
The detailed requirements for all these packages are set out in the IAEA regulations and appropriate tests are specified.