I.54.
The dosimetric models described in the previous section apply to the vast majority of radionuclides of interest and may be used to determine their Q values and associated A1 and A2 values. However, in a limited number of cases, the models are inappropriate or require modification. The special considerations applying in such circumstances are discussed in this section.
Consideration of parent and progeny radionuclides
I.55.
The earlier Q system assumed a maximum transport time of 50 d, and thus radioactive decay products with half-lives of less than 10 d were assumed to be in equilibrium with their longer lived parents. In such cases, the Q values were calculated for the parent and its progeny, and the limiting value was used in determining the A1 and A2 values of the parent. In cases where a daughter radionuclide has a half-life either greater than 10 d or greater than that of the parent nuclide, such progeny, with the parent, were considered to be a mixture.
I.56.
The 10 d half-life criterion is retained. Progeny radionuclide products with half-lives of less than 10 d are assumed to be in secular equilibrium with the longer lived parent; however, the daughter’s contribution to each Q value is summed with that of the parent. This provides a means of accounting for progeny with branching fractions of less than unity; for example, Ba-137m is produced in 0.946 of the decays of its parent Cs-137. If the parent’s half-life is less than 10 d and the daughter’s half-life is greater than 10 d, then the mixture rule is to be used by the consignor. For example, a package containing Ca-47 (4.53 d) has been evaluated with its Sc-47 (3.351 d) daughter in transient equilibrium with the parent. A package containing Ge-77 (11.3 h) will be evaluated by the consignor as a mixture of Ge-77 and its daughter, As-77 (38.8 h).
I.57.
In some cases, a long lived daughter is produced by the decay of a short lived parent. In these cases, the potential contribution of the daughter to the exposure cannot be assessed without knowledge of the transport time and the buildup of progeny nuclides. It is necessary to determine the transport time and the buildup of progeny nuclides for the package and establish the A1/A2 values using the mixture rule. As an example, Te-131m (30 h) can be considered, which decays to Te-131 (25 min), which, in turn, decays to I-131 (8.04 d). The mixture rule should be applied by the consignor to this package with the I-131 activity derived on the basis of the transport time and the buildup of progeny nuclides. It should be noted that the above treatment of the decay chains, in some cases, differs from the 1996 BSS Table I of Schedule I. This table assumes that secular equilibrium exists for all chains.
Alpha emitters
I.58.
For alpha emitters, it is not, in general, appropriate to calculate QA or QB values for special form radioactive material, owing to their relatively weak gamma and beta emissions. In the 1973 Edition of the Transport Regulations, an arbitrary upper limit for special form alpha sources of 103 A2 was introduced. There is no dosimetric justification for this procedure and in recognition of this, coupled with the good record in the transport of special form radioactive material and the reduction in many QC values for alpha emitters resulting from the use of the ICRP recommendations [I.8], a tenfold increase in the arbitrary factor of 103 above was used. Thus, an additional Q value, QF = 104 × QC, is defined for special form alpha emitters and is listed in the column headed QA, where appropriate, in the tabulation of Q values.
I.59.
A radionuclide is defined as an alpha emitter if, in greater than 10–3 of its decays, it emits alpha particles or it decays to an alpha emitter. For example, Np-235, which decays by alpha emission in 1.4 × 10–5 of its decays, is not an alpha emitter for the purpose of special form consideration. In the same way, Pb-212 is an alpha emitter, since its daughter, Bi-212, undergoes alpha decay. Overall, the special form limits for alpha emitters have increased with increases in QC.
I.60.
Finally, with respect to the ingestion of alpha emitters, arguments analogous to those used for beta emitters in the discussion on QD apply and the inhalation, rather than the ingestion pathway, is always more restrictive; hence, the latter is not explicitly considered.
Neutron emitters
I.61.
In the case of neutron emitters, it was originally suggested under the Q system that there were no known situations with (α,n) or (γ,n) sources or the spontaneous neutron emitter Cf-252 for which neutron dose would contribute significantly to the external or internal radiation pathways considered earlier [I.4]. However, neutron dose cannot be neglected in the case of Cf-252 sources. Data given in ICRP Publication 21 [I.29] for neutron and gamma emissions indicate a dose rate of 25.4 Sv/h at 1 m from a 1 g Cf-252 source. Combined with the dose rate limit of 0.1 Sv/h at this distance cited earlier, this led to a QA value for Cf-252 of 0.095 TBq. The increase by a factor of about 2 in the radiation weighting factor for neutrons recommended by the ICRP [I.8] gives a value of 4.7 × 10–2 TBq for QA, which was used to determine the value of A1 in the 1996 Edition of the Transport Regulations. This is more restrictive than the QF value of 28 TBq obtained on the basis of the revised expression for special form alpha emitters. The neutron component dominates the external dose due to a Cf-252 source and similar considerations apply to the two other potential spontaneous fission sources, Cm-248 and Cf-254. The QA values for these radionuclides were evaluated by assuming the same dose rate conversion factor per unit activity as that for the Cf-252 source quoted above, with allowance made for their respective neutron emission rates relative to that of this source. The A1 value for Cf-252 was updated as described by Eckerman et al. [I.30] in accordance with later recommendations of the ICRP [I.31], and this revised value was used in the 1996 (As Amended) Edition and subsequent editions of the Transport Regulations.
Bremsstrahlung
I.62.
The A1 and A2 values tabulated in the 1973 Edition of the Transport Regulations were subject to an upper cut-off limit of 1000 Ci, in order to protect against the possible effects of bremsstrahlung. Within the Q system, this cut-off was retained at 40 TBq. It was recognized as an arbitrary cut-off and is not specifically associated with bremsstrahlung radiation or any other dosimetric consideration. It remains unchanged.
I.63.
A preliminary evaluation of bremsstrahlung, in a manner consistent with the assumptions of QA and QB, indicates that the 40 TBq figure is a reasonable value. However, explicit inclusion of bremsstrahlung within the Q system might limit A1 and A2 for some nuclides to about 20 TBq, a factor of two lower. This analysis supports the use of an arbitrary cut-off.
Tritium and its compounds
I.64.
During the development of the Q system, it was considered that liquids containing tritium should be considered separately. The model used considered spillage of a large quantity of tritiated water in a confined area followed by a fire. Resulting from these assumptions, the A2 value for tritiated liquids was set in the 1985 Edition of the Transport Regulations at 40 TBq, with an additional condition that the concentration be less than 1 TBq/L. For the 1996 Edition of the Transport Regulations, no change was considered necessary.
Radon and its progeny
I.65.
As noted earlier, the derivation of QE applies to noble gases which are not incorporated into the body and whose progeny are either a stable nuclide or another noble gas. In a few cases, this condition is not fulfilled and dosimetric routes other than external exposure due to submersion in a radioactive cloud must be considered [I.32]. Within the context of the Transport Regulations, the only case of practical importance is that of Rn-222, where the lung dose associated with the inhalation of the short lived radon progeny has received special consideration by the ICRP [I.33].
I.66.
The corresponding QC value in the 1985 Edition of the Transport Regulations was calculated to be 3.6 TBq. However, allowing for a 100% release of radon, rather than the 10–3–10–2 aerosol release fraction incorporated in the QC model, this reduces to a QC value in the range 3.6 × 10–3 to 3.6 × 10–2 TBq. Further, treating Rn-222 and its progeny as a noble gas resulted in a QE value of 4.2 × 10–3 TBq; towards the lower end of the range of QC values, and this is still the Type A package non-special form limit cited for Rn-222 in the tabulation of Q values. Radon dosimetry is ongoing and these values may be revised in the future.
I.67.
The above excludes consideration of chemical toxicity, for which a daily intake limit of 2.5 mg is recommended by the ICRP [I.34].