Help:Transport & Packaging
Level: Introductory, Intermediate
Each year more than 10 million packages of radioactive materials are transported worldwide. Radionuclides are used for a variety of purposes e.g. in nuclear medicine, materials testing, oil exploration etc. For these purposes radioactive materials must be packaged and transported to the location of interest. Before these materials can be shipped, care must be taken that the shipping regulations have been strictly followed. The purpose of these regulations, of course, is to ensure safety by containing the radioactivity to make sure that there is no negative effect on the environment, to control the radiation emitted from the package, make sure that nuclear fission criticality conditions cannot be met, and to dissipate any heat generated within the package.
In Germany more than 500,000 packages with radioactive substances are transported annually. Over 90 per cent of these originate not from nuclear power stations, but from hospitals, research establishments, universities and industrial applications. These substances are weakly radioactive and are transported by truck. Waste packages from public or industrial organisations are transported to national collection points in the Federal states. The remaining 10 per cent are highly radioactive, usually spent fuel elements, are transported from nuclear power stations into temporary storage facilities in the direct proximity. In the year 2010, 234 transportations of nuclear fuels were subject to approval. Permission is granted by the responsible Federal Office for Radiation Protection (BfS).
For the purpose of transportation, radioactive materials were previously defined as those materials which spontaneously emit ionising radiation and have a specific activity in excess of 0.002 microcuries per gram (0.002μCi/g or 74 Bq/g) of material. In 2001, new regulations on the transport of radioactive materials were introduced with lower limits on the specific activity of individual nuclides .
The choice of packaging depends on the radionuclides involved, the amounts of radioactivity to be shipped and the form of the radionuclides. Restrictions on the amounts of material are determined by the so-called “A1” and “A2” values. “A1” is the maximum amount of activity for a special form radionuclide that is allowed in Type A packaging, whereas “A2” refers to the maximum amount of activity in a Type A package for normal form materials. Usually the A1 or A2 values can not exceed 37 terabecquerels (37 × 1012 Bq) or 1000 curies (Ci). For some materials, however, the limits have been set to 40 TBq or more (in the case of 238U).
It is important to note that the A2 values refer to normal form radioactive materials and to both external and internal exposure. In contrast to the A1 value, the A2 value assumes that dispersal and contamination of the package content is probable. On this basis, the A2 values are always lower than the A1 values.
A1 and A2 Values
The values of the quantity A1 arise through worst-case assumptions with regard to external gamma radiation from a known source at a certain distance. More exactly, the A1 value for a particular radionuclide is that quantity of radionuclide which will give rise to a dose rate of 0.1 Sv/h at a distance of 1m from the package. Since only external radiation is considered, it is assumed that the radioactive material inside the package will not be dispersed if the package is damaged.
The A2 value also relates to the worst-case assumptions, but five different exposure pathways are considered rather than just the single pathway associated with the A1 value. The five pathways are:
• external gamma radiation
• external beta radiation to the skin
• external gamma radiation from immersion in a gaseous cloud of radioactive material released from a damaged package
As an example, consider the radionuclides 137Cs and 60Co. The A1 and A2 values are shown in Table 9.8 where it can be seen that the values for 137Cs are quite different and for 60Co are the same.
Table 9.8. Maximum activities for special (A1) and normal form (A2) materials
|Nuclide||A1 (special form)||A2 (normal form)|
|137Cs||2 TBq||0.6 TBq|
|60Co||0.4 TBq||0.4 TBq|
|238U||No limit||No limit|
In the case of 60Co, this means that even if five different exposure pathways are considered, there is no greater risk than if only the external gamma radiation pathway were considered. This is not the case with 137Cs which does indeed depend on the exposure pathway.
The special form referred to above refers to the fact that if the material were released from the package, the only hazard would be from external gamma radiation. An example of such a special form is that of a sealed (encapsulated) source of radiation. Here the durable metal capsule with high physical integrity ensures that the radioactivity will not disperse. In addition, only solid materials are classified as “special form”. Special form encapsulation is designed such that the capsule cannot be opened unless it is destroyed.
In contrast to special form materials, normal form materials may be solid, liquid, or gaseous. Examples here are waste materials in a plastic bag, a liquid-containing bottle housed with a metal contained, powder in a glass or plastic bottle, contaminated soil in a drum, or gas in a cylinder.
Type A packaging is required for shipping radioactive materials when the radioactivity inside the package does not exceed the A1 or A2 values. If the radioactivity is higher, typeA packaging, which is foreseen for normal transportation conditions and minor accidents only, cannot be used. The basic purpose of type A packaging is to prevent loss or dispersal of the package contents while maintaining proper radiation shielding under normal transportation conditions. TypeA packaging must withstand water spray, drop, puncture and crash tests.
When the level of radioactivity exceeds the A1 and A2 values, type B or type C packaging is required. Type B and C packaging must meet all the conditions of type A packaging and in addition have the ability to withstand serious accidents. Examples of type B packaging are spent nuclear fuel casks.
TheTransport Index “TI” is the dose rate in units of millisieverts per hour (mSv/h) at a distance of one meter from the external surface of a package containing radioactivity, multiplied by a factor 100. The Transport Index is printed on the label of a package so that interested persons can assess the relative radiation hazard and the control to be exercised upon handling. In special cases (tanks, big containers), an additional multiplication factor must be used.
The Transport and Packaging Module
The transport & packaging application is used to create nuclear transport reports.
Before a nuclear transport report can be created a least one nuclear source has to be defined.
A nuclear source consists of a list of nuclides, a name and a form. The desired form can be selected from the 'Form' combobox and the name can be entered in the 'Name of the source' input field. 'Sealed source' has to be checked for sealed sources. A new nuclide is added to the nuclide list by selecting the desired nuclide in the 'Element' and 'Isotope' comboboxes and clicking the 'Add' button. A nuclide can be removed from the nuclide list by selecting the nuclide in the nuclide list and clicking the 'Remove' button on top of the nuclide list. The nuclide list is cleared by clicking the 'Remove All' button.
After entering the data of the nuclear source a new nuclear source is created by clicking the 'Update' button. After this the new source is added to the 'Other Sources' combobox which lists all previously definded nuclear sources.
To change an existing nuclear source it has to be selected from the 'Other Sources' combobox. This will display the data of the selected nuclear source. After changing the source data clicking the 'Update' button will overwrite the source displayed in the 'Name of the source' input field. If the name in the 'Name of the source' input field doesn't exist a new nuclear source will be created. An existing nuclear source can be deleted by selecting the source in the 'Other Sources' combobox and clicking the 'Remove' button right beneath the 'Update' button.
When all nuclear sources have been defined the nuclear transport report is created and printed by clicking the 'Create' button. Clicking the 'Show' button will only show the nuclear transport report.