EXERCISE 5.2. Graded Approach to Package Integrity
6. ACTIVITY LIMITS AND MATERIAL RESTRICTIONS
6.1.3. Dosimetric models and assumptions
In this section, the dosimetric models and assumptions underlying the derivation of five principal Q values are briefly summarized. Further details can be obtained by referring to Appendix I of TS-G-1.1 [6].
6.1.3.1. QA — External dose due to photons
The QA value for a radionuclide is determined by consideration of the external radiation dose due to gamma or X-rays to the whole body of a person exposed near a damaged Type A package following an accident. The shielding of the package is assumed to be completely lost in the accident and the consequent dose rate at a distance of 1 m from the edge (or surface) of the unshielded radioactive material is limited to 100 mSv/h. It is further assumed that the damaged package may be treated effectively as a point source.
In the earlier Q-system, QA was calculated using the mean photon energy per disintegration taken from ICRP Publication 38 [7]. Furthermore, the conversion to effective dose per unit exposure free-in-air was approximated as 6.7 mSv/R from photon energies between 50 keV and 5 MeV.
In the revised Q-system, the QA values have been calculated using the complete X-ray and gamma emission spectrum for the radionuclides as given in ICRP Publication 38 [7]. The energy dependent relationship between effective dose and exposure free-in-air is that given in ICRP Publication 51 [8] for an isotropic radiation geometry.
6.1.3.2. QB — External dose due to beta emitters
completely lost in the accident. However, a residual shielding factor for beta emitters, which is associated with materials such as package debris was used. This concept assumed a very conservative shielding factor of 3 for beta emitters of maximum energy 2 MeV, and within the Q-system this practice is extended to include a range of shielding factors dependent on beta energy based on an absorber of approximately 150 mg/cm2 thickness.
In the revised Q-system, QB is calculated using the complete beta spectra for the radionuclides of ICRP Publication 38 [7]. The spectral data for the nuclide of interest is used along with data from Cross [10, 11] on the skin dose rate per unit activity of a mono-energetic electron emitter. The self-shielding of the package is taken to be a smooth function of the maximum energy of the beta spectrum, as depicted in Figure 6.2.
FIG. 6.2. Shielding factor as a function of beta energy.
Three additional factors were considered in the revised Q-system. Firstly, although the dose limit for the lens of the eye is lower than that for the skin, consideration of the depth doses in tissues for beta emitters indicates that the dose to the skin is always limiting for
maximum beta energies up to approximately 4 MeV [12, 13, 14]. Thus, specific consideration of the dose to the lens of the eye was judged to be unnecessary.
Secondly, in accounting for conversion electrons in the determination of Q values, they were treated as mono-energetic beta particles, and weighted according to their yields.
Thirdly, relative to the treatment of positron annihilation radiation, this was not included in the evaluation of the beta dose to the skin since it contributes only an additional few per cent to the local dose to the basal layer. However, the 0.51 MeV gamma rays are included in the photon energy per disintegration used in the derivation of QA.
6.1.3.3. QC — Internal dose via inhalation
The QC value relates to a radionuclide transported in a non-special form, where it has the potential to become airborne and inhaled in the event of an accident. It is determined by consideration of the inhalation dose to a person exposed to the activity released from a damaged Type A package following an accident. Compliance with the limiting doses cited earlier was ensured by restricting the intake of activity under accident conditions to the annual limit on intake (ALI) recommended by the ICRP [15].
Under the Q-system, a range of accident scenarios is considered, including that originally proposed for the derivation of QC. The accidents encompassed those occurring both indoors and out of doors and included the possible effects of fires. In the 1973 Edition of the Regulations, it was assumed that 10–3 of the package contents might escape as a result of a median accident and that 10–3 of this material might be taken into the body of a person involved in the accident. This resulted in a net intake factor of 10–6 of the package contents.
Although this value has been retained within the Q-system, it is now recognized as representing a range of possible release fractions and a range of uptake factors. The model considers intake factors in terms of these two parameters independently.
The range of release fractions now recognized under the Q-system is 10–3–10–2 of the package contents. This covers the range represented by the earlier assumption in the 1973 Edition of the Regulations, and also the range used in the original Q-system. Underlying this is the tacit assumption that the likelihood of a major accident, which could cause the escape of a large part of the contents of a Type A package, is small. This approach is borne out by the behaviour of Type A packages in severe accident environments [16, 17, 18].
For inhalation, it is the respirable aerosols released from the package which need to be taken into consideration. Data on the respirable aerosol fractions produced under accident conditions are generally sparse and are only available for a limited range of materials. For example, for uranium and plutonium specimens under enhanced oxidation rate conditions in air and carbon dioxide, respirable aerosol fractions up to approximately 1% have been reported [19]. However, below this level the aerosol fractions showed wide variations dependent on the temperatures and local atmospheric flow conditions. In the case of liquids, higher fractional releases are obviously possible. However, the multiple barriers provided by typical Type A packaging materials, potentially including absorbent material, a two-component containment system, and a lead shielding pot, remain an effective containment system even after severe impact or crushing accidents [18]. Indeed, in an example cited of an
131I source, which was completely crushed in a highway accident, less than 2% of the package contents remained on the road after removal of the package debris [20].
Potentially the most severe accident environment for many Type A packages is the combination of severe mechanical damage accompanied by exposure to a fire. However, even in this situation the role of debris may be significant in retaining released activity. This is indicated by data from the 1979 Athens DC8 aircraft accident [17, 18]. Frequently, fires produce relatively large particles of material, which would tend to minimize any intake via inhalation, while at the same time providing a significant surface area for the absorption of volatile species and vaporized liquids. A further mollifying factor is the enhanced local dispersion associated with the convective air currents due to the fire, which would also tend to reduce intake via inhalation.
On the basis of considerations of the type outlined above, the release fraction in the range of 10–3–10–2 was established, and was assumed to be appropriate for the determination of Type A package contents limits within the Regulations.
The range of uptake factors (i.e. the fraction of material released that is inhaled) now recognized under the Q-system is 10–4–10–3. This is based upon consideration of a range of possible accident situations, both indoors and out of doors. The original Q-system considered exposure within a storeroom or cargo-handling bay of 300 m3 volume with four room air changes per hour. Assuming an adult breathing rate of 3.3 × 10–4 m3/s, this resulted in an uptake factor of approximately 10-3 for a 30-minute exposure period. An alternative accident scenario might involve exposure in a transport vehicle of 50 m3 volume with ten air changes per hour, as originally employed in the determination of the Type B package normal transport leakage limit in the 1985 Edition of the Regulations. Using the same breathing rate and exposure period as above, this led to an uptake factor of 2.4 × 10–3, which is of the same order as the value obtained above.
For accidents occurring out of doors, the most conservative assumption for the atmospheric dispersion of released material is that of a ground level point source. Tabulated dilution factors for this situation at a downwind distance of 100 m range from 7 × 10–4 to 1.7 × 10–2 s/m3 [21], corresponding to uptake factors in the range 2.3 × 10–7 to 5.6 × 10–6 for the adult breathing rate cited above. These values apply to short term releases and cover the range from highly unstable to highly stable weather conditions; the corresponding value for average conditions is 3.3 × 10-7, which is towards the lower end of the range quoted above.
Extrapolation of the models used to evaluate the atmospheric dilution factors to shorter downwind distances is unreliable, but reducing the exposure distance by an order of magnitude to 10 m would increase the above uptake factors by about a factor of 30. This indicates that as the downwind distance approaches a few metres the uptake factors would approach the 10–4–10–3 range used within the Q-system. However, under these circumstances other factors, which would tend to reduce the activity uptake, come into effect and may even become dominant. The additional turbulence to be expected in the presence of a fire has been mentioned earlier. Similar reductions in airborne concentrations can be anticipated as a result of turbulence originating from the flow of air around any vehicle involved in an accident or from the effects of nearby buildings. Thus, on balance, it can be seen that uptake factors in the range of 10–4–10–3 appear reasonable for the determination of Type A package contents limits.
When this range in uptake factors is taken in conjunction with the range in release fractions discussed earlier, the overall intake factor for a Type A package becomes 10–6. This is the same as that used in the original Q-system. However, it is emphasized that this value represents a combination of releases typically in the range up to 10–3–10–2 of the package contents as a respirable aerosol, combined with an uptake factor of up to 10–4–10–3 of the released material.
The ranges of release and uptake noted above are partially determined by the chemical form of the material and particle size of the aerosol. The chemical form consideration has a major influence on the dose per unit intake. The intake fraction derived above is consistent with the value used in the earlier Q-system. In calculating QC the most restrictive chemical form has been assumed and the effective dose coefficients, for an aerosol characterized by an aerosol median aerodynamic diameter (AMAD) of 1 micron, where applicable, are assumed [5, 7]. The 1 μm AMAD value used in the earlier Q-system is retained even though other AMAD values can give more conservative dose coefficients for some radionuclides.
For uranium, the QC values are presented in terms of the lung absorption types (formerly referred to as lung clearance classes) assigned for the major chemical forms of uranium. This more detailed evaluation of QC was undertaken because of sensitivity of the dose per unit intake to the absorption type and the fact that the chemical form of uranium in transport is generally known.
6.1.3.4. QD – Skin contamination and ingestion doses
The QD value for beta emitters is determined by consideration of the beta dose to the skin of a person contaminated with non-special form radioactive material as a consequence of handling a damaged Type A package.
The model used within the Q-system assumes that:
- 1% of the package contents are spread uniformly over an area of 1 m2,
- handling of the debris is assumed to result in contamination of the hands to 10% of this level [22], and
- the exposed person is not wearing gloves but would recognize the possibility of contamination or wash his hands within a period of 5 hours.
Taken individually, these assumptions are somewhat arbitrary, but as a whole, they represent a reasonable basis for estimating the level of skin contamination that might arise under accident conditions. Values for QD were calculated using the beta spectra and discrete electron emissions for the radionuclides as tabulated by ICRP [7, 8]. The emission data for the nuclide of interest was used with data from Cross [23] on the skin dose rate for mono-energetic electrons emitted on the surface of the skin.
The models used in deriving the QD values for skin contamination were also employed to estimate the possible uptake of activity via ingestion. Assuming that a person may ingest all the contamination from 10–3 m2 (10 cm2) of skin over a 24 hour period [22], the resultant intake is 10–6 × QD, compared with that via inhalation of 10–6 × QC derived earlier. Since the dose per unit intake via inhalation is generally of the same order or greater than that via ingestion [4], the inhalation pathway will normally be limiting for internal contamination due to beta emitters. Where this does not apply, almost without exception, QD is much smaller than QC, and explicit consideration of the ingestion pathway is unnecessary.
6.1.3.5. QE – Submersion dose due to gaseous isotopes
The QE value for gaseous isotopes, which do not become incorporated into the body, is determined by consideration of the submersion dose following their release in an accident. A
QE/300 m–3, which falls exponentially with a decay constant of 4 h–1 as a result of ventilation over the subsequent 30 minute exposure period to give a mean concentration level of 1.44 × 10–3 × QE m–3. Over the same period the concentration leading to the dose limits cited earlier is 4000 × DAC (Bq/m3), where the DAC was the derived air concentration recommended by the ICRP for 40 hours per week and 50 weeks per year occupational exposure in a 500 m3 room. However, the use of the radiation protection quantity, DAC, is no longer deemed appropriate. Therefore the modified Q-system calculations use an effective dose coefficient for submersion in a semi-infinite cloud, from U.S.E.P.A. Federal Guidance Report No. 12 [24], as shown in Table 6.2.
TABLE 6.2. DOSE COEFFICIENTS hSUB FOR SUBMERSION (Sv·Bq–1·s–1·m3)
Nuclide hsub Nuclide hsub
Ar-37 0 Xe-122 2.19E-15
Ar-39 1.15E-16 Xe-123 2.82E-14
Ar-41 6.14E-14 Xe-127 1.12E-14
Ar-42 no value Xe-131m 3.49E-16
Kr-81 2.44E-16 Xe-133 1.33E-15
Kr-85 2.40E-16 Xe-135 1.10E-14
Kr-85m 6.87E-15 Rn-218 3.40E-17
Kr-87 3.97E-14 Rn-219 2.46E-15
Rn-220 1.72E-17
Rn-222 1.77E-17
6.1.3.6. Special considerations
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 the associated A1
and A2 values. However, in a limited number of cases the models are inappropriate or require modification. The special considerations that apply in such cases are discussed in this section.
The original Q-system assumed a maximum transport time of 50 days and thus radioactive decay products with half-lives less than 10 days 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 A1 and A2 of the parent.
In cases where a daughter radionuclide has a half-life either greater than 10 days or greater than that of the parent nuclide then such progeny, with the parent, were considered to be a mixture. The ten-day half-life criterion is retained in TS-R-1. Progeny radionuclide products with half-lives less than 10 days 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 less than
one. For example, 137mBa is produced in 0.946 of the decays of its parent 137Cs. In addition, if the parent's half-life is less than 10 days and the daughter's half-life is greater than 10 days, then the mixture rule is to be used by the consignor. For example, a package containing 47Ca (4.53 d) is evaluated with its 47Sc (3.351 d) daughter in transient equilibrium with the parent.
A package containing 77Ge (11.3 h) is evaluated by the consignor as a mixture of 77Ge and its daughter 77As (38.8 h).
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 build up of progeny nuclides. It is, however, necessary to determine the transport time and the build up of progeny nuclides for the package in establishing the A1/A2 values using the mixture rule. As an example, consider 131mTe (30 h) which decays to 131Te (25 min) and which in turn decays to 131I (8.04 d). The mixture rule should be applied by the consignor to this package with the 131I activity derived based on the transport time and the build up of progeny nuclides. It should be noted that the above treatment of the decay chains, in some cases, differs from the BSS Table I of Schedule I. That table assumes that secular equilibrium exists for all chains. The decay chains for which the daughter's contribution is included in determining the Q value for the parent nuclide are listed in section AI.9 of Appendix I of TS-G-1.1 [6].