Surfaces contributing to external gamma dose rates

For the predictions of external dose rates described in Sections 4.3.2 and 4.3.3, predictions were also made for four models (METRO-K, ERMIN, CPHR, RESRAD-RDD) of the surfaces contributing the most to the predicted external dose rate at each location at three times following deposition. The percentage that each surface contributed to the predicted external dose rate at each location and time point for an initial deposition of ⁶⁰Co and ²³⁹Pu in dry conditions was estimated. The predictions for ⁶⁰Co are shown in Figs 4.20 and 4.21 and a comparison is given between the four models. Figures 4.22 to 4.26 provide comparisons by location and initial weather conditions for ⁶⁰Co for each model. The initial time point ‘Year 0’ represents less than 1 day since the release. The time points ‘Year 1’ and ‘Year 5’ are one year and five years, respectively, following the release of the radionuclide. Participants were asked to provide the three most important surfaces contributing to external dose rate at each location. In some cases, more than three surfaces contributed; thus, the percentages do not always sum to 100%. In other cases, only one or two surfaces contributed to external dose rate. Tabulated results for an initial deposition of ²³⁹Pu are given in Appendix VII, Tables VII.11 to VII.14; however, the model predictions for ²³⁹Pu are discussed here to enable a direct comparison with those for ⁶⁰Co.

Different models included different specifications of surfaces; surfaces are designated in the figures as they were reported by the participants (soil and grass are considered together in the figures). The surfaces considered in each model are listed in Table 4.4. Understanding the variety of surfaces included by different models is important to understanding the model predictions of external dose rates (Sections 4.3.2 and 4.3.3) and external doses (Section 4.3.5), especially in terms of how the predicted external dose rates vary over time under different release conditions, by location, by isotope, and amongst models.

RESRAD-RDD also included two generic surfaces ‘from outside’ (for external dose rates at indoor locations), and ‘infinite area’ (for external dose rates at outdoor locations) representing all outdoor contributors to external dose rate (e.g. paved surfaces, soil, grass). CPHR also included the contribution from ‘air’ (the initial plume) at the initial time point (immediately after the release), for all locations.

4.3.4.1. Indoor locations (Locations 1–3)

For indoor locations (Fig. 4.20), the contributing surfaces were dependent on the location within the building. For Location 1 (ground floor), most models included contributions from outdoor surfaces (e.g. trees and pavement for METRO-K, trees, pavement, grass and external walls for ERMIN, and exterior walls and ‘from outside’, i.e. from outdoor surfaces, for RESRAD-RDD).

Contributions from indoor surfaces were included as ‘interior surfaces’ by ERMIN and internal walls and floors by RESRAD-RDD, whereas CPHR and METRO-K do not include consideration of internal deposition. CPHR included the contribution from ‘air’ (the initial air contamination or plume) at the initial time point after the release, for all indoor locations; the predicted external dose rate from the plume was high enough that no other surfaces contributed at the initial time point (Fig. 4.20 and considered further in Section 4.3.5). For Location 2 (10^th floor), RESRAD-RDD showed a major contribution ‘from outside’ (66–78% of the predicted external dose rate from ⁶⁰Co and 27–29% of the predicted external dose rate from

239Pu at this indoor location), while the other three models predicted that the major contribution to external dose rate at Location 2 was from external walls or interior surfaces (Fig. 4.20). For Location 3 (24^th or top floor), a major contribution to external dose rate from roofs was evident for all models, ranging from 17% to 98% of the predicted external dose rate from ⁶⁰Co and from 2% to 67% of the predicted external dose rate from ²³⁹Pu, depending on the model and the time point. At this location, RESRAD-RDD still predicted a significant (14–48%) contribution to external dose rate ‘from outside.’

For ⁶⁰Co at the ground floor location (Location 1), both METRO-K and ERMIN predicted an important contribution to the external dose rate from trees, representing 78% and 33% of the total external dose rate, respectively, at the initial time point (Fig. 4.20). For METRO-K, trees were the dominant surface initially contributing to the external dose rate (Year 0) (78%), less so (43%) at the time point Year 1 (1 year since the release), and not contributing at all (0%) at Year 5, while walls and paved surfaces became increasingly important over time (from 2% at Year 0 to 40% at Year 5 for walls and from 20% at Year 0 to 60% at Year 5 for paved surfaces;

see Fig. 4.20). For ERMIN, trees were important contributors to external dose rate (33%) only at the initial time point (Year 0); however, grass became increasingly important over time, contributing 69% of the external dose rate at Year 5 (Fig. 4.20).

ERMIN also predicted a decrease over time in the importance of interior surfaces as contributors to external dose rate, with corresponding increases in the percent contribution to external dose rate by walls for all indoor locations; this was especially the case at Location 2, where interior surfaces contributed 93% of the external dose rate at Year 0, but only 2% at Year 5, while walls contributed 7% of the external dose rate at Year 0, but 98% at Year 5 (Fig. 4.20). Location 2 is on the 10^th floor of the building, which is too high for significant contributions to external dose rate from trees, grass, or paved surfaces (unlike Location 1, on the ground floor) (see Section 4.3.4.1), but also not having a significant contribution to external dose rate from the roof (unlike Location 3 on the top floor, for which the contribution from the roof was significant) (see Section 4.3.4.1). METRO-K predicted that walls were the only surface contributing to the external dose rate at Location 2 at all the time points.

For Location 3 (top floor, i.e. the 24^th floor), ERMIN and RESRAD-RDD both predicted higher contributions to external dose rates from roofs than from walls or interior surfaces (after the initial time point for ERMIN and at all time points for RESRAD-RDD), increasing from 31%

at Year 0 to 87% at Year 5 for ERMIN and from 34% at Year 0 to 76% at Year 5 for RESRAD-RDD (Fig. 4.20). METRO-K predicted that roofs contributed 98% of the external dose rate at Location 3 at all the time points (Fig. 4.20). In general, surfaces that can act as

‘sinks’ for radioactivity (grass, walls, roofs) tended to contribute a greater percentage of the external dose rate at later time points than those surfaces presumed to have net losses of radioactive contamination over time (e.g. trees, interior surfaces).

For the three models that were used to predict external dose rates for ²³⁹Pu (CPHR, ERMIN, RESRAD-RDD), the same surfaces were important contributors to external dose rate as for

60Co, but the percentage contributions were different. In general, the nearer surfaces (e.g.

‘interior’, floors) were more important for ²³⁹Pu than more distant surfaces (e.g. trees, grass, paved surfaces, ‘from outside’, roofs). For example, for ERMIN, ‘interior’ surfaces were predicted to contribute 41–46% of the external dose rate at Location 1 for ⁶⁰Co, whereas 87–88% of the external dose rate at Location 1 was predicted to be contributed by ²³⁹Pu. For RESRAD-RDD, the predicted contribution ‘from outside’ at Location 1 was 73–83% of the external dose rate for ⁶⁰Co, but only 29–31% of the external dose rate for ²³⁹Pu. For predictions generated using either ERMIN or RESRAD-RDD, contributions from trees, interior surfaces, and floors became less important over time, while contributions from walls and roofs became more important over time, depending on the location within the building.

4.3.4.2. Outdoor locations (Locations 4–6)

Results for Locations 4 and 6 tended to be similar for any given model (Fig. 4.21). These two locations were both outdoors with mostly paved surfaces; Location 4 was near buildings, while Location 6 was not, but the walls at Location 4 were a minor contributor to external dose rate (representing 2–19%, depending on the model, the time point, and the radionuclide). The results for three models (METRO-K, ERMIN, CPHR) included a significant contribution to the external dose rate from paved surfaces at both locations: 47–81% at Location 4 and 73–100%

at Location 6 for ⁶⁰Co for METRO-K; 9–35% at Location 4 and 11–56% at Location 6 for ⁶⁰Co for ERMIN; 94% at both locations for both radionuclides for CPHR. METRO-K and ERMIN both predicted a contribution from trees at the initial time point, which decreased over time. For METRO-K predictions, the contributions to external dose rate from walls (Location 4) and pavement (Locations 4 and 6) increased over time. For ERMIN, the results showed a decrease in contribution over time from pavement and an increase over time from grass.

For Location 5 (the centre of a park area), the main contributors to external dose rate predicted by three models (METRO-K, ERMIN, CPHR) were grass and trees (Fig. 4.21). CPHR included the contribution from ‘air’ at the initial time point (immediately after the release), for all outdoor locations. The fourth model (RESRAD-RDD) was used to calculate the external dose rate for all outdoor locations in terms of an ‘infinite area’ and did not distinguish amongst individual surfaces or between ⁶⁰Co and ²³⁹Pu. CPHR also predicted the same percentages for contributing surfaces for ⁶⁰Co and ²³⁹Pu.

The predictions from ERMIN had the same percentages for contributing surfaces for ⁶⁰Co and

239Pu at Locations 5 and 6 at the initial time point and predicted small differences (e.g. 0.96%

for ⁶⁰Co versus 1.16% for ²³⁹Pu at Location 5) at the later time points, due to the shorter half-life of ⁶⁰Co. At Location 4, where the building walls also contributed to external dose rates, there were small differences predicted between ⁶⁰Co and ²³⁹Pu in terms of the percent contributions (e.g. 34.6% for ⁶⁰Co and 33.5% for ²³⁹Pu for paved surfaces at the initial time point; see Fig. 4.21).

For METRO-K (Figs 4.22 and 4.23), trees were predicted to contribute a larger percentage of the external dose rate (e.g. 51% versus 5%) for a summer release than for a winter release, as

which trees were a major contributor to external dose rate (Locations 1, 4, 5, and 6), the relative contribution to external dose rate decreased over time, with corresponding increases in the percent contributions from walls (Locations 1 and 4), soil (Location 5), or paved surfaces (Locations 1, 4, and 6). At Locations 2 (10^th floor inside the building) and 3 (top floor), no significant seasonal or weather related differences in contributing surfaces were apparent. At Location 2, the entire contribution to external dose rate was predicted to be from walls, while at Location 3, 97–98% of the external dose rate was predicted to be from roofs.

Results from the ERMIN model (Figs 4.24 and 4.25) also predicted a greater percent contribution to external dose rate from trees (Locations 1, 4, 5, and 6) for a summer release (20–40% under dry conditions) than for a winter release (5–13% under dry conditions), as well as under dry versus wet initial conditions (5–28% for a summer release under wet conditions and 1–6% for a winter release under wet conditions). Effects of season or initial weather conditions at indoor locations were less apparent for other surfaces. Differences in the importance of walls and ‘interior’ surfaces were seen for ⁶⁰Co versus ²³⁹Pu at indoor locations;

roofs were less important contributors to external dose rate for ²³⁹Pu than for ⁶⁰Co. At the outdoor locations, the importance of grass surfaces increased over time, whereas the predicted contribution from paved surfaces decreased.

At indoor locations, RESRAD-RDD (Fig. 4.26) predicted a greater percent contribution to external dose rate from floors for ²³⁹Pu (67–76% of the external dose) than for ⁶⁰Co (13–18%

of the external dose rate), and a corresponding decrease in the fraction contributed ‘from outside’ or from roofs. Roofs became increasingly important contributors to external dose rate over time at Location 3 (the top floor of the building, i.e. the 24^th floor), and were slightly less important under wet release conditions than under dry conditions; for example, for a ⁶⁰Co release under dry conditions, a 34% contribution to external dose rate was initially predicted and 76% was predicted after five years, and 27.5% was predicted initially and 73% after five years for a release under wet conditions.

FIG. 4.20. Percent contributions to the predicted external dose rates from specified surfaces at indoor locations, shown separately for ⁶⁰Co, by model and time point, for a summer release in dry conditions.

FIG. 4.21. Percent contributions to the predicted external dose rates from specified surfaces at outdoor locations, shown separately for ⁶⁰Co, by model and time point, for a summer release in dry conditions.

FIG. 4.22. Percent contributions to the predicted external dose rates from ⁶⁰Co from specified surfaces

FIG. 4.23. Percent contributions to the predicted external dose rates from ⁶⁰Co from specified surfaces at outdoor locations, shown for METRO-K, for the indicated time points and release conditions.

FIG. 4.24. Percent contributions to the predicted external dose rates from ⁶⁰Co from specified surfaces at indoor locations, shown for ERMIN, for the indicated time points and release conditions.

FIG. 4.25. Percent contributions to the predicted external dose rates from ⁶⁰Co from specified surfaces at outdoor locations, shown for ERMIN, for the indicated time points and release conditions.

FIG. 4.26. Percent contributions to the predicted external dose rates from ⁶⁰Co from specified surfaces at indoor locations, shown for RESRAD-RDD, for the indicated time points and release conditions.