The Effect of Shielding on A1and A2values

Over the last 5 years, an international group of experts has reviewed and update the methodology to calculate the A1 and A2 values included in IAEA Transport regulations. The methodology known as the Q system, is described in IAEA SSG-26. This update has involved a review of input values and methods of calculation. As part of this, the Q system pathway scenarios that involve external radiation have moved from equations using attenuation and build-up factors to using Monte Carlo method particle transport codes such as MCNP. The use of MCNP has proved particularly useful in the review of calculation methods for the values of QA (external doses from gamma radiation) and QB (skin equivalent doses from beta radiation) and to resolve inconsistencies between the two methods. In particular, along with new dose coefficient data, Monte Carlo method calculations allowed the change from calculating effective dose and skin dose for single particles to assessing the effect of all radiations. Another area where the use of MCNP proved to be very useful was in the calculation of the impact of shielding from the source material or the capsule around it. This paper describes how MCNP was used to investigate the shielding effect of different materials and thicknesses on both external doses and doses to the skin. In the current Q system, a shielding factor of 150 mg cm-2 is used only in the calculation of skin equivalent doses from beta emissions to account for auto-absorption. The paper also illustrates how the effect of shielding of gamma ray radiations was also assessed, allowing the effective and skin equivalent dose pathway scenarios to be harmonized and for bremsstrahlung to be directly accounted for. The effects of the changes of the methodology on the QA and QB values and also on the A1 values for a number of key radionuclides are also presented.