Testing to the extreme

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.

UNIRRADIATED 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.

Author Info:

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: Uranium ore concentrate Uranium ore concentrate (UOC) is a low specific activity material and the radiological hazard is very low. It is normally transported in sealed 200 litre drums in standard ISO transport containers by road, rail or sea. There would be a minor risk due to the toxicity of the powder if it was released and ingested. In this respect UOC is no different from most heavy metal compounds. Uranium hexafluoride Uranium hexafluoride (‘Hex’) also is a low specific activity material and the radiological risk from natural and depleted material is low. However there would be a chemical hazard in the unlikely event of a release because it produces toxic byproducts on reaction with moist air. This is the case with many industrial chemicals. Enriched Hex is fissile – it can, under certain circumstances, go critical and sustain a nuclear chain reaction. The package requirements in the transport regulations (which specify the quantity of material in a consignment and its configuration) guard against a criticality excursion. Uranium dioxide powder Uranium dioxide (UO2), of less than 5% enrichment, used in the manufacture of new uranium fuel elements also is classified as a low specific activity material. Whereas there would be a minor toxic risk if the powder were to be ingested, the primary hazard would be radiological in the event of a criticality incident. This again is prevented by the design of the package, prevention of moderation, and the configuration of the packages in transport. Fabricated uranium fuel New fuel assemblies typically consist of sintered ceramic UO2 pellets formed into assemblies and transported in specially designed packages. The fuel is refractory and stable. The chemical hazard is negligible and the radiological hazard is low. The design and configuration of the packages during transport guarantee that criticality excursions could not occur. Spent fuel and high-level vitrified waste Spent fuel and high-level vitrified waste are intensely radioactive and need to be heavily shielded. However, they are inherently stable and refractory and very difficult to disperse. The chemical and toxic risks are negligible when compared to the radiological risk. Mixed oxide fuel Mixed plutonium/uranium oxide (MOX) fuel elements, in which the enriched uranium isotope is replaced by plutonium, are very similar to uranium fuel elements. The chemical hazard again is negligible and the radiological hazard is low except in the event of a criticality excursion. As is the case for enriched uranium fuel, this is controlled by the design of the package and the configuration of the packages during transport. Plutonium Plutonium is a special case. This material is hazardous since it is very toxic and in its powder form can be easily dispersed. The primary risk is due to toxicity except in the event of criticality, which is controlled by the package design. To keep the toxic risk to a minimum, it is preferable to transport it as MOX fuel, which is a stable, refractory ceramic not easily dispersed.

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. Industrial packages are used for low specific activity materials, typically uranium ore concentrate or low-level waste, which can be transported in sealed 200 litre drums packed into a standard transport container. Type A packages are typically used for fresh fuel. Uranium hexafluoride is a special case. Type B packages are high duty packages which are used to transport some of the more radioactive nuclear fuel cycle materials, notably spent nuclear fuel, high-level vitrified wastes, and MOX fuel. Type C packages for air transport have only recently been specified in the regulations and are intended for potentially hazardous materials. They are likely to be used only for air transport of plutonium and MOX fuel. The main potential hazard is chemical rather than radiological. The detailed requirements for all these packages are set out in the IAEA regulations and appropriate tests are specified.