Basis of the Q system

Under the Q system a series of exposure routes are considered, each of which might lead to radiation exposure, either external or internal, to persons in the vicinity of a Type A package involved in a severe transport accident. The dosimetric routes are illustrated schematically in Eig. I.l and lead to five contents limit values Q, Qg, Q, Qp and Qg, for external photon dose, external beta dose, inhalation dose, skin and ingestion dose due to contamination transfer, and submersion dose, respectively. Contents limits for special form alpha and neutron emitters and tritium are considered separately. 

Type A package contents limits are determined for individual radionuclides, as in the 1985 edition of the Regulations. The Aj value for special form materials is the lesser of the two values and Qg, while the Aj value for non-special form radioactive materials is the least of the Aj and the remaining Q values. Specific assumptions concerning the exposure pathways used in the derivation of individual Q values are discussed below, but all are based upon the following radiological criteria  

Schematic representation of exposure pathways employed in the Q system. 

In this section the dosimetric models and assumptions underlying the derivation of five principal Q values are described in detail. The specific radiation pathways considered are outlined, and the considerations affecting the methods of derivation used are discussed. 

In the earlier Q system, was calculated by using the mean photon energy per disintegration taken from ICRP Publication 38 [1.11]. Furthermore, the conversion to effective dose per unit exposure free-in-air was approximated as 6.7 m Sv/R from photon energies between 50 keV and 5 MeV. 

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Following the 1985 edition of the Regulations, the application of the Q system as described here treats the derivation of each Q value, and hence each potential exposure pathway, separately. In general this will result in compliance with the dosimetric criteria defined earlier, provided that the doses incurred by persons exposed near a damaged package are dominated by one pathway. However, if two or more Q values closely approach each other this will not necessarily be the case. For example, in the case of a radionuclide transported as a special form radioactive material for which = Qg, the effective dose and skin dose to an exposed person could approach 50 mSv and 0.5 Sv, respectively, on the basis of the Q system models. Examination of Table 1.2 shows that this consideration applies only to a relatively small number of radionuclides, and for this reason the independent treatment of exposure pathways is retained within the Q system. 

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