Doses

Annual and cumulative doses (up to five years) were predicted for each of two exposure scenarios, for the ‘no countermeasures’ scenario (without protective actions, including remedial actions). The exposure scenario in Region 1 (a business area) assumed an adult who was spending 40 hours per week indoors (at work) and 5 hours per week outside the building (lunch breaks). Specified inhalation rates were assumed to be 0.5 m³/h indoors (sitting) and 1 m³/h outdoors (standing or walking). The exposure scenario in Region 2 (a park area) assumed an adult exercising in the park for 3 hours per week (0.5 hours per day, 6 days per week). The specified inhalation rate was assumed to be 1.5 m³/h (moderate exercise).

Predicted annual and cumulative doses are provided in Appendix VII (Tables VII.15 to VII.24) for external doses and inhalation doses. The discussion in this section is restricted to the cumulative doses, which depended primarily (and in some cases, totally) on the dose received during the first year. In other words, the annual doses after the first year were much smaller than the annual dose during the first year following the deposition event.

The main focus of the scenario, and hence, of this discussion, is external gamma doses from

60Co and inhalation (internal) doses from ²³⁹Pu. All participants also calculated inhalation doses from ⁶⁰Co, and three participants (ERMIN, CPHR, RESRAD-RDD) also calculated external doses from ²³⁹Pu; these are discussed briefly in the following Sections 4.3.5.1 and 4.3.5.2.

4.3.5.1. External doses from Cobalt-60 and Plutonium-239

Predicted cumulative external doses from ⁶⁰Co after 1 year and 5 years post-deposition are shown in Fig. 4.27 for different initial weather conditions (dry, light rain (3 mm/d), or heavy rain (20 mm/d)), for a summer release. Tabulated results for ²³⁹Pu for three models, ERMIN, CPHR and RESRAD-RDD are given in Appendix VII, Tables VII.16 to VII.18, respectively).

For ⁶⁰Co, consistent with the predictions for contamination densities and external dose rates, predicted cumulative external doses were higher under conditions of wet deposition than for dry deposition (heavy rain > light rain >> dry). Differences in cumulative external doses between summer and winter releases predicted by METRO-K and ERMIN were small or negligible (Tables VII.15 and VII.16). Predicted cumulative external doses for Region 2 (the park area) were very similar amongst four models (METRO-K, ERMIN, RESRAD-RDD and CHERURB), reflecting similar predicted radionuclide behavior at Location 5 (the centre of the park). Cumulative external doses predicted for Region 1 (the business area) showed more variability amongst models, reflecting the contribution from time spent indoors, as well as outdoors. This is because, in general, predicted external dose rates at indoor locations (Figs 4.14 to 4.16) varied more widely amongst models than did those at outdoor locations (Figs 4.17 to 4.19), reflecting differences amongst models in their treatment of surfaces contributing to external dose rates at indoor locations. For RESRAD-RDD, CPHR, and CHERURB, predicted cumulative external doses were higher in Region 1 than in Region 2, while for METRO-K and ERMIN, predicted cumulative external doses were higher in Region 2 than in Region 1.

As for predicted deposition and internal dose, predicted external doses for ²³⁹Pu were higher for wet deposition than for dry deposition. Predictions made using ERMIN were higher in Region 2 than in Region 1, while those made using CPHR were higher in Region 1 and those made using RESRAD-RDD were similar for the two regions.

4.3.5.2. Internal doses from Cobalt-60 and Plutonium-239

Predicted cumulative inhalation doses from ⁶⁰Co (Fig. 4.28) and ²³⁹Pu (Fig. 4.29) were predicted by all five models. The predicted cumulative internal doses from ²³⁹Pu reflect either the inhalation dose from the initial plume (CPHR and CHERURB) or the inhalation dose from resuspension (METRO-K, ERMIN, and RESRAD-RDD). None of the models used in this exercise included both the initial plume and resuspension. This leads to very different predictions of inhalation doses across these two groups of models for this exercise.

For ⁶⁰Co, the predicted inhalation doses from resuspension were three to four orders of magnitude lower than the corresponding predicted external doses from ⁶⁰Co for METRO-K, ERMIN, and RESRAD-RDD. For CHERURB, predicted cumulative internal doses from ⁶⁰Co were approximately a factor of 2 to 4000 less than the corresponding predicted external doses.

For CPHR, the predicted cumulative internal doses from ⁶⁰Co were a factor of 2.5 to 15 greater than the corresponding predicted external doses in Region 1 and a factor of 8 to 24 greater in Region 2. The CHERURB and CPHR models predict much higher inhalation doses than the other three models due to inclusion of inhalation from the initial plume, these inhalation doses being much higher than those from resuspension (see Fig. 4.28). ERMIN and RESRAD-RDD use a similar resuspension factor and the predicted doses were in general similar, reflecting the similar predicted deposition, particularly for dry deposition and deposition during light rain.

The METRO-K model assumes a resuspension factor about a factor of 10 higher than ERMIN and RESRAD-RDD, which in general leads to higher inhalation doses being predicted compared to the other models. Consistent with the predicted deposition and predicted external doses, predicted inhalation doses from resuspension for ⁶⁰Co and ²³⁹Pu (METRO-K, ERMIN, RESRAD-RDD) were higher under conditions of wet deposition than for dry deposition (heavy rain > light rain > dry). The predicted inhalation dose from the initial plume (CHERURB) did not vary with initial weather conditions for both ⁶⁰Co and ²³⁹Pu.

Predictions of inhalation doses for ²³⁹Pu made using CPHR and CHERURB indicate that tall the inhalation dose was received early on, i.e. from the initial plume, and the cumulative doses after 1 year or 5 years were the same. For the other three models that considered inhalation doses from resuspension, much or most of the cumulative inhalation dose was received during the first year, with only small additional contributions to the cumulative dose after the first year.

For both CPHR and CHERURB, the predicted cumulative inhalation dose in Region 1 was higher than that in Region 2, substantially so for CPHR. Cumulative inhalation doses from resuspended materials that were predicted using ERMIN and RESRAD-RDD were higher in Region 1 than in Region 2 by about an order of magnitude. METRO-K predicted higher cumulative internal doses in Region 2 than in Region 1 by about an order of magnitude under dry initial conditions, but only slightly higher doses were predicted in Region 2 than in Region 1 under initial conditions of light or heavy rain.

Predicted cumulative inhalation doses from ²³⁹Pu (see Appendix VII, Tables VII.16 to VII.18) were several orders of magnitude higher than the predicted cumulative external doses from

239Pu, for any given model, consistent with the very low external dose coefficient for ²³⁹Pu compared to the inhalation dose coefficient.

4.3.5.3. General comments

In general, the relative predicted external doses for a given model for the different locations and exposure scenarios were consistent with the predicted contamination densities and external dose rates. For example, higher predicted external doses under conditions of wet deposition versus dry deposition corresponded to higher predicted contamination densities under conditions of wet deposition versus dry deposition. Predicted inhalation doses from resuspension (METRO-K, ERMIN, RESRAD-RDD) were also consistent with predicted contamination densities, as resuspension is usually predicted as a function of the contaminated material on the soil or other exposed surfaces. Thus, inhalation doses that were predicted using these models were higher under conditions of wet deposition versus dry deposition.

Inhalation doses that had been predicted from the initial plume, without consideration of resuspension, were dependent only on the plume, and not on conditions of deposition (wet versus dry). Thus, predictions of internal dose from the plume (CHERURB) did not vary with initial weather conditions. The predicted inhalation doses by CHERURB and CPHR from the initial plume were much higher than those from resuspension predicted by METRO-K, ERMIN and RESRAD-RDD.

Comparisons of predicted external doses between Region 1 (the business area) and Region 2 (the park area) are complex functions of predicted external dose rates (dependent, in turn, on predicted contamination densities) from various surfaces, the importance of each surface to the total external dose rate at a location, and assumptions about time spent at a given location. There are not necessarily clear reasons for predicted external doses being higher in Region 1 than Region 2 for some models, or for the reverse being predicted for other models. For models predicting inhalation doses from resuspension (METRO-K, ERMIN, RESRAD-RDD), the same considerations would apply, as the resuspension depends on the predicted contamination densities at each location, as well as predicted changes in contamination densities and resuspension factors over time. For the models predicting inhalation doses only from the initial plume (CPHR and CHERURB), differences in predicted internal doses in the two regions are probably related to the assumptions about time spent at a given location.

FIG. 4.27. Predicted cumulative external doses (mSv) from ⁶⁰Co during the first year and the first 5 years after the deposition event in Region 1 (left; business area) and Region 2 (right; park area).

Predictions are shown for initial conditions in summer of dry weather (top), light rain (centre), and

FIG. 4.28. Predicted cumulative inhalation doses (mSv) from ⁶⁰Co during the first year and the first 5 years after the deposition event in Region 1 (left; business area) and Region 2 (right; park area).

Predictions are shown for initial conditions in summer of dry weather (top), light rain (centre), and heavy rain (bottom).

FIG. 4.29. Predicted cumulative inhalation doses (mSv) from ²³⁹Pu during the first year and the first 5 years after the deposition event in Region 1 (left; business area) and Region 2 (right; park area).

Predictions are shown for initial conditions in summer of dry weather (top), light rain (centre), and