A1, A2 Values for the assessment of radioactive material for transport in a Type A package.
A1 is the activity limit of special form (see Note 1) radioactive material in a Type A package (see Note 2). The values of the quantity A1 arise through transport accident conditions with release of the sealed source but no dispersal of its radioactive content. The A1 value for a particular radionuclide is that quantity of radionuclide which will rise to a gamma dose rate of 0.1 Sv/h or a beta dose rate of 1 Sv/h at a distance of 1 m from the sealed source.
For alpha emitters it was decided to set A1 = 10000 x A2.
The A2 value also relates to transport accident conditions, but five different exposure pathways are considered rather than just the two pathways associated with the A1 value. The five pathways are:
• external gamma radiation
• external beta radiation to the skin
• immersion: external gamma radiation from immersion in a gaseous cloud of radioactive material released from a damaged package
Note 1: Special form radioactive material shall mean either an indispersible solid radioactive material or a sealed capsule containing radioactive material (from IAEA, TS-R-1)
Note 2: 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, type A 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. Type A 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.
As an example, consider the radionuclides Cs-137 and Co-60. The A1 and A2 values are shown in the Table where it can be seen that the values for Cs-137 are quite different and for Co-60 are the same.
Table 1. Maximum activities for special (A1) and normal form (A2) materials
|Nuclide||A1 (special form)||A2 (normal form)|
|Cs-137||2 TBq||0.6 TBq|
|Co-60||0.4 TBq||0.4 TBq|
|U-238||No limit||No limit|
|Pm-147||40 TBq||2 TBq|
|Am-241||10 TBq||10-3 TBq|
|Pu-239||10 TBq||10-3 TBq|
In the case of Co-60, this means that even if the 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 Cs-137 which does indeed depend on the exposure pathway.
In the case of Pm-147, 40 TBq is an upper numerical cut-off limit applied also to many other radionuclides.
Am-241 is an example for A1 = 103 A2