“Alternative Requirements ”
8.9.2. Design considerations and approvals
The design considerations which apply to lightweight/low density Type B(U) or Type B(M) packages are the same as those that apply to any other Type B(U) or Type B(M)
packages. Nevertheless, because of the crush test requirement, consideration has to be given to the external forces on the packaging. This might lead to the design having some sort of rigid framework. These packages are submitted to the same requirements as for other Type B packages.
8.9.3. Examples
Packages containing large amounts of alpha emitters are generally lightweight/low density packages because of the requirement for less shielding. This includes for example, plutonium oxide powders, and plutonium nitrate solutions, which are radioactive materials with high potential hazards.
8.10. Type B packages containing high quantities of radioactivity 8.10.1. Requirements
In addition to the requirements discussed earlier, a Type B package for radioactive contents with activity greater than 105 A2 has to be designed so that if it were subjected to the enhanced water immersion test there would be no rupture of the containment system (paragraph 730 of TS-R-1). Originally, this requirement only applied to irradiated fuel with an activity greater than 37 PBq, but in TS-R-1 it was expanded to include all radioisotopes in large quantities.
For the enhanced water immersion test, the specimen must be immersed under a head of water of at least 200 m for a period of not less than one hour. For demonstration purposes, an external gauge pressure of a least 2 MPa is considered to meet these conditions. Again, this test is more fully discussed later in this chapter.
8.10.2. Design considerations and approvals
Clearly, to meet the additional water pressure more attention has to be paid to seals, closures, and valves to provide assurance of leak tightness. Except for the additional deep submersion test, these packages are designed to the same requirements as for other Type B packages.
8.10.3. Example 8.10.3.1. Wet/dry fuel flasks
Fuel flasks normally carry intact fuel pins and fuel elements. A typical fuel flask design is shown in Figure 8.12. Because of the activity associated with irradiated fuel, a large amount of gamma and neutron shielding is required to bring the radiation dose rate down to Regulatory levels. The type of shielding chosen is normally steel, although thick steel and lead, or cast iron may also be used.
FIG. 8.12. Typical wet/dry irradiated fuel package design.
The loading of fuel into these flasks is normally carried out in a pond attached to irradiated nuclear fuel storage pools at reactor sites. Depending on the fuel type, the number of fuel pins carried, and the heat output from the pins, these flasks may be transported either full or drained of water. The presence of water provides some neutron shielding and enhances heat transfer from the fuel pins to the flask’s walls, and hence to the environment and the fuel pins are kept cool. However, the presence of water in the container does impose extra loads on the flask body and lid under impact conditions.
On the other hand, although fuel flasks transported dry may be subjected to less onerous loading under impact conditions, they have to be equipped with a basket which provides efficient heat transfer. Otherwise, overheating of the fuel could result in cladding rupture, release of fission product gases, and possible oxidation of the fuel itself. An additional improvement over heat transfer is possible using helium as a coolant.
Helium has the added advantage that it counteracts the problem of radiolysis.
Radiolysis is the breakdown of water molecules under the influence of strong gamma radiation into a potentially flammable or explosive mixture of hydrogen and oxygen. Even in containers where the water has been drained, some water is inevitably still present in the form of vapour, water droplets, or trapped free water. Vacuum drying can be employed to remove any water present thus negating the potential problem.
Because fuel flasks contain potentially releasable mixtures of fission product gases, volatile solid radionuclides and particular matter, an efficient and effective sealing system must be provided. This may take the form of a double O-ring seal. Such a seal also permits verification of the leak tightness of the container before shipment, by pressure testing the space between the seals.
One popular method of fabrication of fuel flasks is by forging steel into a monolithic flask body. An alternative cheaper method of fabrication is casting steel or spheroidal graphite cast iron into a monolithic flask body. For this second type of flask construction, great care is
needed in controlling the production process and a very comprehensive Quality Assurance programme must be enforced. Yet another fabrication method includes the formation of large, concentric steel shells with interstitial layers of either lead, or depleted uranium, or both.
In many cases, it is also necessary to provide a neutron shield, usually on the outer portion of the package design. Frequently, impact-protecting devices are provided as part of the package design.
8.11. Type C packages 8.11.1. Requirements
Type C packages must generally meet the same requirements as Type B(U) packages up to, and including, the normal conditions of transport tests. However, some of the accident-simulating tests are more severe (paragraph 667 of TS-R-1).
Type C package designs are required to undergo the cumulative test sequences 1 and 2 outlined below and detailed later in this chapter. After the tests, they must be shown to retain their radioactive contents and shielding properties within defined limits. Like Type B(U) and Type B(M) packages, they must restrict loss of radioactive contents to no more than A2 in a week (10 A2 in a week for 85Kr), and restrict the radiation level at 1 m from the package surface to not more than 10 mSv/h (paragraph 669(b) of TS-R-1).
Additionally, Type C package designs must be shown to meet the same containment and shielding criteria as those above when subjected to burial (paragraph 668 of TS-R-1) and must be shown not to rupture when immersed in water 200 meters deep (paragraph 670 of TS-R-1). The requirement to demonstrate the ability to withstand burial is done by calculation assuming the package to be undamaged and buried in dry soil at 38°C in a steady state condition. The burial test was introduced because packages involved in high-speed crashes may be covered by debris or buried in soil. If a package whose contents generate heat becomes buried, an increase in package temperature and internal pressure may result.
8.11.1.1. Sequence 1
This test sequence must be carried out in the given order on the same specimen:
- The specimen is subjected to the accident conditions of transport drop I (paragraphs 734(a) and 727(a) of TS-R-1), being dropped from 9 metres onto an effectively unyielding target so as to suffer maximum damage (the impact speed is 13.3 meters per second);
- The specimen is subjected to the accident conditions of transport dynamic crush test (drop III), by placing it on the same unyielding target, in the worst orientation, and dropping a 500 kg plate onto it from a height of 9 meters (paragraphs 734(a) and 727(b) of TS-R-1);
- The specimen is subjected to a puncture/tearing test being dropped from 3 metres onto a conical probe (paragraphs 734(a) and 735 of TS-R-1);
- The specimen is subjected to an engulfing 800°C thermal test for a period of one hour (paragraphs 734(a) and 736 of TS-R-1).
8.11.1.2. Sequence 2
This test may be carried out on a separate specimen from the first sequence. It requires that the specimen be subjected to an impact at a speed of 90 m/s onto an essentially unyielding target so as to suffer maximum damage (paragraphs 734(b) and 737 of TS-R-1).
A separate specimen may be used for this test. It is considered that exposure to long duration fires in the aftermath of a high-speed aircraft impact is highly unlikely. Because the fuel on board an aircraft is typically dispersed in a crash, it will not form pools to supply long lasting fires in a single location.
8.11.2. Design considerations
The tests for Type C packages are considerably more stringent than those applying to Type B(U) and Type B(M) packages and, for a given radioactive content, the package may be expected to be considerably more robust and heavier. The 90 m/s impact test is especially demanding.
8.11.3. Approvals
Each Type C package design requires unilateral approval, except that a package design for fissile material requires multilateral approval (paragraph 806 of TS-R-1).
The Competent Authority has to issue an approval certificate stating that the approved design meets the applicable requirements for Type C packages (paragraph 827 of TS-R-1) and assigns an identification mark to that design.
The contents of an application for approval and of an approval certificate of the design of a package issued by a Competent Authority must include the same information as for a Type B(U) or Type B(M) approval (see 8.8.5.2 and 8.8.5.3 of this chapter).
The provisions for Type C packages first came into effect for international air shipment when Member States and the International Civil Aviation Organization implemented them on 1 January 2001. As no transitional arrangements were foreseen, the new provisions had immediate effect. This means that since that date, package design approvals for Type B(U) and Type B(M) packages have to be amended to reflect the content limit for transport by air.
8.11.4. Examples
At the time of the publication of this document there are no known Type C approved package designs.
In the USA in the 1970’s, packages were designed to carry plutonium dioxide (PuO2) powder under the requirements of US Nuclear Regulatory Commission Guide NUREG 0360 [4]. This text requires, amongst other things, that the package withstand a 129 m/s impact and a one-hour fire (as compared with the 90 m/s impact required in TS-R-1). Obviously, these so-called PAT-1 and PAT-2 packages would be candidates for Type C approval.
An interesting and innovative design is the FS 81 packaging, which is under development in France. The packaging has been designed to accommodate one of the three following contents:
- One PWR mixed oxide (MOX) fuel assembly, - Two BWR MOX assemblies, or
- Fuel rods in a canister.
To remain compatible with different operational and transport constraints and in view of the need to withstand the new regulatory tests, innovative principles have been introduced in this packaging design. One of the basic principles is to consider that a substantial part of the kinetic energy at the time of impact is absorbed by the containment components of the packaging and by the content itself. This leads to the concept of planned, controlled local plastic deformation of these different elements.
Therefore, a new containment concept excluding any mechanical assembly was used.
A leak-tight welded capsule without any orifices provides the containment vessel. This cylindrical capsule is closed at each end by a hollow hemisphere. This containment vessel has no opening or closure arrangement. Welding onto one hemispherical end after the contents and the internals have been inserted makes closure. Mechanical cutting at the same end carries out opening.
The packaging body is designed to protect the containment vessel during the different mechanical and thermal regulatory tests. It contains a layer of efficient neutron absorbing resin that contributes primarily to shielding but also acts as fire protection. Special devices designed to absorb a large part of the kinetic energy during impact protect the two ends of the body. These devices absorb more than 50 % of the kinetic energy during an axial impact at more than 130 m/s. They also remain efficient during angled impacts. Their efficiency is still approximately 35 % at an impact angled at 19°.
The body consists of two almost identical parts that can be separated to provide access to the containment vessel during loading and unloading of the fuel assemblies. Two aluminium caps have been designed to protect both ends of the containment vessel, and
“operate” at precise moments during impact.
Since the desire to transport different types of contents with the same packaging was one of the initial goals, only the internal arrangement is specific to the content transported.
These internal arrangements are designed to prevent damage to the internal wall of the containment vessel from shocks imposed under normal and accident conditions of transport.
The package body is connected to an aluminium frame by means of an anti-vibration system to ensure fuel integrity with respect of its future use in the power reactor.
8.12. Packages containing uranium hexafluoride
Earlier versions of the Regulations did not specifically address uranium hexafluoride (UF6), but dealt with its fissile and radiological considerations in the same manner as any other material. In 1984, the ship “Mont-Louis”, transporting 48Y cylinders from France to Russia, sank in the North Sea off the coast of Belgium. All the packages were recovered with no release of contents, and therefore with no chemical or radiological contamination.
However, this led the international transport community to question the adequacy of the regulations relating to uranium hexafluoride. This accident highlighted the chemical hazard potential, especially when linked to the reaction of uranium hexafluoride with water or humid
The decision to write separate regulations in the 1996 Edition of the Transport Regulations for a specific material was not taken lightly. It represented a significant change in the philosophy in the Regulations, which had always been formulated to apply generally to all radioactive material. The impetus for the change was driven more by a desire to ensure physical and chemical toxicity safety in the event of an accident involving a fire, rather than concerns about radiological or criticality safety. This change reflects the importance of UF6
within the fuel cycle, the very large quantities being shipped, and the peculiar physical and chemical properties of the material.
The new provisions (paragraphs 629–632 of TS-R-1) for packages containing UF6 are restricted to package performance standards and administrative requirements. Otherwise, the Regulations draw on the International Organization for Standardization document ISO 7195:1993 (E) “Packaging of Uranium Hexafluoride (UF6) for Transport” [5] for more detailed technical specifications.
Uranium hexafluoride is generally loaded under a pressure of 0.4 MPa and at a temperature of about 100°C, and under these conditions it is a liquid. The packages, therefore, also must comply with the requirements for pressurized equipment. Consequently, they are designed to resist a service pressure of 2 MPa (14 bar). The combination of these two sets of Regulations imposes stringent requirements. In particular, the packagings must be tested every five years to verify their resistance to a hydrostatic pressure of twice the service pressure.
The Revision Process dealt with a number of difficult items concerning UF6. These included:
- Cylinder pressure test requirements (paragraphs 630(a) and 718 of TS-R-1), - Drop test requirements (paragraphs 630(b) and 722 of TS-R-1),
- Thermal test requirements (paragraphs 630(c) and 728 of TS-R-1), - Criticality safety (paragraph 677(b) of TS-R-1), and
- Need for Competent Authority design approval (paragraph 805 of TS-R-1).
Adopting the needed regulatory changes was simplified by the publication of the International Standards Organization’s ISO 7195. This document provides the necessary detailed container design specifications and is directly incorporated by reference into TS-R-1.
8.12.1. Requirements
Uranium hexafluoride must be packaged and transported in accordance with ISO 7195 [5] (see paragraph 629 of TS-R-1). Packages designed to contain 0.1 kg or more of uranium hexafluoride must meet the regulatory requirements pertaining to the radioactive and fissile properties of the material.
In addition, packages containing 0.1 kg, or more of UF6, must be designed to meet the following requirements (paragraphs 630 and 632 of TS-R-1):
- Withstand, without leakage and without unacceptable stress, an internal pressure of at least 1.4 MPa. However, cylinders withstanding less than 2.8 MPa require multilateral approval.
- Withstand, without loss or dispersal of the uranium hexafluoride, the drop test for normal conditions of transport, with graduated heights from 0.3 to 1.2 m depending on the package mass.
- Withstand, without rupture of the containment system, the thermal test of 800°C for 30 minutes if designed to contain less than 9000 kg of UF6.
- Either meet the thermal test requirement or have multilateral approval if designed to contain 9000 kg or more of UF6.
- If containing fissile UF6, meet the test conditions applicable to fissile packages (i.e.
the accident conditions of transport impact and thermal tests). These must be met with no contact between the valve and other normally non-contacting parts of the packaging, and no leakage from the valve. Other operational requirements must also be met before the designer can assume no in-leakage of water for the criticality safety analysis (paragraphs 671 and 673–682 of TS-R-1).
8.12.2. Design considerations
Designing UF6 packages is complicated because they have to meet such a large variety of standards. These include those related to the radioactive nature of the contents, the possible fissile nature of the contents, as well as the uranium hexafluoride’s unusual triple point at 64 C and material properties. The need for valves for filling and emptying the containers complicate the design since these are the weak points of any package design.
8.12.3. Approvals
With certain exceptions, the Regulations require that packages designed to contain 0.1 kg of UF6 or more, must have at least unilateral Competent Authority design approval after 31 December 2003. If a UF6 package design cannot withstand a test pressure of 2.8 MPa or cannot meet the thermal test criteria, it is required to have multilateral approval after 31 December 2000.
These provisions are represented in Table 8.6, which shows the approval criteria for uranium hexafluoridepackages. The complexity of these requirements comes from a desire to address the areas of safety concern, while pragmatically recognizing the real world situation with respect to existing containers and older package designs.
8.12.4. Examples
There are many types of packages used for the transport of UF6 throughout the world.
Examples of these are described in the following sub-sections. Some typical UF6 packages are shown in Figure 8.13.
8.12.4.1. 48 Y cylinders
UF6 coming from natural uranium is currently transported in a number of different types of packages (i.e. cylinders) and the so-called 48 Y cylinder is one of these. The 48Y cylinders are made with a cylindrical shell, which is 15.9 mm (5/8 inch) thick with ellipsoidal ends. At both ends, metal skirts complete the shell. The front end is fitted with a valve, to allow filling and emptying of the cylinder. The rear end is fitted with a plug.
These cylinders allow 12 500 kg of UF6 to be shipped, with a total weight of the filled cylinder in the range of 15 000 kg. It is more or less obvious that those cylinders meet the
TABLE 8.6. DECISION CHART FOR EVALUATING AND CERTIFYING URANIUM HEXAFLUORIDE PACKAGE DESIGNS ACCORDING TO TS-R-1
Requirement Satisfaction of Requirements for Certification or Competent Authority Approval
H(U) H(M) H(M) H(M)
Special Arrangement Mass of UF6 < 0.1 kg ⇐⇐⇐⇐ None Required ⇒⇒⇒⇒⇒⇒
0.1 kg < Mass of UF6 < 9,000 kg x x x x
9,000 kg < Mass of UF6 x
Provisions of ISO 7195 x x As far as practicable
x Any other combination Para. 718 of TS-R-1
(without leakage and unacceptable stress) Test Pressure < 1.4 MPa
1.4 MPa < Test Pressure < 2.8 MPa x
2.8 MPa < Test Pressure x x
Para. 722 of TS-R-1
(without loss or dispersal of the UF6) Normal Condition of Transport Drop Test
x x x
Para. 728 of TS-R-1 (without rupture of the containment system)
Accident Condition of Transport Thermal Test
x x
Para. 631 of TS-R-1
Not provided with pressure relief devices x x x Controlling TS-R-1 paragraphs 629 629,
632(b)
632(a) 632(c)
FIG. 8.13. Typical UF6 shipping cylinders.
A significant amount of research has been undertaken to study the problem of the fire test. Included in this is an IAEA Co-ordinated Research Programme (CRP) on the behaviour of UF6 cylinders in thermal environments. At the time of writing the final conclusions have not been drawn.
Should the multi-national efforts show that the 48Y cylinders are not able to withstand the fire test, various solutions are available to enhance the thermal resistance of the cylinders.
One would be to review the cylinder design with the purpose of making it more resistant to fire. This could lead to selecting upgraded steel for the shell, increasing its thickness, adding reinforcement rings, reviewing the valve design, and providing similar upgrades. All or part of these improvements would certainly render the new cylinder able to survive the regulatory fire test.
Another complex solution would be to fit the cylinder with an outer packaging system
Another complex solution would be to fit the cylinder with an outer packaging system