Transfer functions for Q_A /Q_B international regulatory limits for the safe transport of radioactive materials

Abstract

This paper presents a proposed revision of the International Atomic Energy Agency (IAEA) transport regulations, related to the A1 and A2 limit values used to determine the radioactive transport classification. Based on the “Q system”, a novel methodology was introduced to derive QA and QB values related to scenarios involving external exposure from a distant source. These values are key parameters that respectively represent the total effective dose and total equivalent dose to the skin, from all primary and secondary particles contributing to radiation exposure. The International Working Group (WG A1/A2) is established and associated with the TRANSSC Technical Expert Group on Radiation Protection (TTEG-RP). A review of the A1 and A2 values is performed in response to identified limitations within the existing Q system. The followed approach is based on Monte Carlo simulations that enabled the development of transfer functions aimed at reducing computational time and increasing the flexibility of dose evaluations for any radionuclide with known particle emission spectra. This method allows updating the QA and QB values to account for future data evolutions (decay data, fluence-to-dose conversion coefficients) and standardizing the calculation of regulation limits across all referenced radionuclides and scenarios related to external exposure. The transfer functions are established using three Monte Carlo simulation codes — FLUKA, Geant4, and MCNP — and address the previous limitations of the “Q system”, reflecting the latest ICRP recommendations and improvements in calculation techniques. The results of the WG show consistent agreement across the codes, with minor discrepancies observed at low primary energies due to statistical uncertainties and different handling of stopping power for electrons/positrons in the codes. This revised approach aligns with current standards and recommendations, ensuring that the radiological consequences of transport accidents are acceptable for the new A1 and A2 limits from a radiological protection perspective.