Tabulation of Q values

In general, the new values lie within a factor of about 3 of the earher values there are a few radionuclides where the new Aj and values are outside this rauge. A few tens of radionuclides have new A values higher than previous values by factors ranging between 10 and 100. This is mainly due to the improved modelhng for beta emitters. There are no new A or A values lower than the previous figures by more than a factor of 10. A few radionuclides previously listed are now excluded but additional isomers are included, namely both isomers of Eu-150 and Np-236. 

Values and limits for special form (Aj) and non-special form (A2) materials 

A further factor considered by the Special Working Group meeting was the need to apply additional limits for materials whose physical properties might render invalid the assumptions used in deriving the Q values discussed above. Such considerations are relevant to materials that may become volatile at the elevated temperatures which could occur in a fire, or which may be transported as very finely divided powders, and especially for the model used to evaluate the values. However, on balance it was considered that only in the most extreme circumstances would the assumed intake factor of 10 be exceeded and that special modification of the Qf. model was unnecessary for these materials. 

As in the case of the 1985 edition of the Regulations, no consideration was given to the chemical form or chemical properties of radionuclides. However, in the determination of values the most restrictive of the dose coefficients recommended by the ICRP [1.8] were used. 

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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Type A package non-special fonn limit cited for Rn-222 in the tabulation of Q values. Radon dosimetry is ongoing and these values may be revised in the future. 

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