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atw Vol. 63 (2018) | Issue 1 ı January OPERATION AND NEW BUILD 30 on the ingested area, but it is assumed that, per ingestion event, indirect ingestion occurs only from a limited fraction of the surface of the hands, i.e. f ing A hands , with f ing =0.01. Activity on the hands is assumed not to be affected by removal through indirect ingestion or transfer to the face ( conservative approach). Moreover, it is assumed that a certain fraction of activity is re-suspended from the object surface and becomes airborne, therefore producing an effective dose contribution through inhalation. The dose conversion factors for the inhalation and ingestion pathways are those indicated in ICRP Publication 72 [3] for an adult member of the public. More specifically regarding the inhalation dose, when dose conversion coefficients for different lung absorption types are available, the most conservative value is chosen. To study the role of the characteristics (type and energy) of the emitted radiation, the dose calculation for this scenario was performed for several radionuclides (βγ- or pure β-emitters) and for a radioisotopic composition typical of a nuclear power plant, as indicated in the first row of Table 1. The latter is labelled “NPP” in Table 1 and Figure 1, and corresponds to the nuclide vector of the whole site of the nuclear power plant in Doel, Belgium, in the year 2015-2016. Radioactive progeny is here considered to contribute to the dose only if equilibrium can be reached within the time-integration period of one year and for sufficiently large branching ratios. For the list of considered nuclides, this is the case for Cs-137 (including Ba-137m) and Sr-90 (including Y-90). 2.2 Alternative scenarios Starting from the reference scenario described in Sect. 2.1, several alternative scenarios were developed, by varying one parameter at a time. This was done to analyse the effect of separate parameter variations on the effective dose and therefore to identify the most relevant parameters, to which the results are most sensitive. This study then serves as basis for a more detailed sensitivity analysis. In this study, five alternative scenarios were developed from the reference scenario. In scenario 2, the distance of the worker to the contaminated bookcase is increased from 3 m to 4.5 m. In scenario 3, wipe-off events are assumed to occur with an increased frequency of once per hour (during use), instead of once every three hours (the ingestion frequency, instead, remains unvaried with respect to the reference scenario). In scenarios 4 and 5, the transfer efficiency f oth is decreased from 0.2 to 0.1 and 0.05, respectively. In scenario 6, the worker benefits from six weeks of holiday, thus is only exposed during 46 weeks per year. As a result, the duty factor decreases from 0.24 to 0.22. 3 Preliminary results The obtained results are summarised in Table 1, reporting the total annual effective dose in the six scenarios for all considered nuclides and for the NPP nuclide vector. It can be noticed from Table 1 that the dose values for the considered nuclides range from about 10 -1 µSv/y for isotopes as Ni-63 and Co-57, to values as high as 10² µSv/y for Pu-241. The dose values resulting from exposure to the NPP isotopic vector are similar to those of its most abundant radionuclide, i.e. Co-60. The (large) differences among the considered nuclides are related to the characteristics of the emitted radiation (type and energy of emitted particles), the half-life of the nuclides and the metabolic behaviour of these elements when ingested or inhaled. Note that, in general, results of a dose evaluation will also strongly depend on the type of object (geometry, surface area, distance) and how exactly it is used or handled. Effective doses presented in Table 1 for the bookcase may thus differ significantly from those for other objects released from a nuclear facility, because the relevant exposure pathways may contribute differently to the effective dose, in absolute and relative sense. Variations between nuclides may then also be different from those observed in Table 1, depending on their dominant exposure pathways. The comparison of results for several objects is currently under investigation. Furthermore, Figure 1 illustrates, for each nuclide, the relative dose, defined as the ratio of the dose in a specific scenario and the dose in the reference scenario. Elimination of the absolute differences by such normalisation enables a way to compare the relative impact of parameter changes for the considered nuclides, thus a comparison of parameter sensitivity between nuclides. It can be observed that, in most of the alternative scenarios, the variation of the dose with respect to the reference scenario is rather heterogeneous for the considered radionuclides. For example, considering scenario 2, in which the distance to the object is increased, the total dose for βγ-emitters decreases as a consequence of the reduction of the external-gamma-radiation dose, which is here the only contribution affected by (a change in) distance. The relative decrease, however, is not the same for all nuclides, as it depends on the relative contribution of the external-gamma-radiation pathway to the total dose, which differs per nuclide. Accordingly, reducing the distance with respect to the object would lead to an increase of the total dose, which is more pronounced when the external-gammaradiation exposure term is more dominant: it can be shown that, for the βγ-emitters considered here, the total dose increases by a factor between two and four when the distance is reduced to 1 m. For purebeta emitters, in which the externalgamma-radiation component is absent, the dose is not affected by a variation of the distance. A certain dose contribution could result from external-beta radiation, but is not considered here. In scenario 3, in Scen. Na-22 Mn-54 Co-56 Co-57 Co-58 Co-60 Zn-65 Cs-134 Cs-137 Eu-152 Ni-63 Sr-90 Pu-241 NPP 1 3.86 0.97 1.87 0.21 0.56 5.18 1.38 8.05 6.91 3.65 0.10 17.04 87.64 4.20 2 2.63 0.58 1.11 0.14 0.34 3.77 1.14 7.15 6.53 2.91 0.10 17.04 87.64 3.30 3 2.07 0.55 1.31 0.12 0.40 2.73 0.81 4.35 3.59 1.90 0.05 8.85 45.66 2.21 4 3.71 1.03 1.87 0.21 0.54 5.38 1.10 6.21 5.48 3.98 0.09 13.94 104.59 4.11 5 3.58 1.06 1.87 0.21 0.53 5.46 0.92 4.96 4.49 4.16 0.08 11.76 114.91 4.01 6 3.58 0.90 1.70 0.19 0.50 4.82 1.27 7.47 6.43 3.40 0.09 15.86 81.55 3.90 | | Tab. 1. Total annual effective dose [µSv/y] for all the considered nuclides in the six scenarios (see Sect. 2.1 and 2.2) for the contaminated bookcase. Operation and New Build Clearance of Surface-contaminated Objects from the Controlled Area of a Nuclear Facility: Application of the SUDOQU Methodology ı F. Russo, C. Mommaert and T. van Dillen
atw Vol. 63 (2018) | Issue 1 ı January | | Fig. 1. Variation of the total dose values in the six analysed scenarios for the bookcase with respect to the reference scenario (i.e. scenario 1). which the wipe-off frequency is increased, a decrease of the total dose is observed for all nuclides. This can be attributed to a more rapid removal from the surface, which leads to a reduction of the time-integrated surface- and air-contamination levels, and thus to a decrease of all dose contributions. This decrease is of course larger when wipe-off is a more dominant mechanism for removal of surface activity. As a result, in the case of long-lived radionuclides, for which radioactive decay does not constitute a competing removal mechanism, the wipe-off process will have a larger relative contribution, and the final result will be more sensitive to a variation in this mechanism: such radionuclides will, therefore, show a larger decrease than shorter-lived nuclides as Co-56 and Co-58. In scenarios 4 and 5, a decrease in the transfer efficiency has two opposite effects. On the one hand, activity residing on the object surface will be removed at a slower rate, leading to an increase of the time-integrated surface-contamination level (TISC). As a result, more activity is available for resuspension, thus the time-integrated air-contamination level (TIAC) also increases. Since the external-gamma-radiation dose is proportional to TISC and the committed effective dose from inhalation is proportional to TIAC, both dose contributions increase with respect to the reference scenario. On the other hand, the effective-dose contributions from indirect ingestion and skin contamination are both proportional to the product f oth TISC (f oth decreases, TISC increases). For the assumptions made here, the product f oth TISC decreases, thus the latter dose contributions decrease. Altogether, the total annual effective dose is the result of the balance between the opposite trends of these considered dose contributions. For some nuclides (e.g. Co-60, Mn-54, Pu-241, and Eu-152) the total dose increases as a result of the increase of the external-radiation exposure or inhalation contribution (or a combination of both). For other nuclides (e.g. Cs-137, Cs-134, Zn-65, Sr-90) the total dose follows the decreasing trend of its leading contribution, i.e. ingestion. In other cases (Co-56 and Co-57), the total dose marginally changes, due to the fact that the opposite effects approximately cancel each other out. Finally, in scenario 6, a decrease of the exposure duration leads to an (approximately) identical decrease in the total dose for all nuclides (the relative values in this scenario range between 0.90 and 0.95). 3.1 Benchmarking study The results obtained with SUDOQU were compared to the results obtained with the model described in RP101 [2]. A graphical illustration of this comparison is provided in Figure 3.2. The RP101-model was chosen for the benchmarking study because one of the scenarios studied in RP101 considers a surface-contaminated tool cabinet, which is comparable to the bookcase considered in this paper. Moreover, like SUDOQU, the RP101- model assumes a non-constant surface activity. However, a fundamental difference between the two models is that the RP101-model only considers radioactive decay as a removal mechanism, whereas the SUDOQU model considers other processes affecting the evolution of the contamination level (Sect. 1). Another important difference concerns the removability of surface contamination: in SUDOQU, 100 % of the surface activity is assumed to be remov able, with a transfer efficiency of 20 %; in RP101, only 10 % of the total surface activity is removable, and the transfer efficiency is equal to 10 %. These differences lead to dissimilar (relative) contributions of the exposure pathways in the two models. In this study, parameter values defining the exposure geometry and duration in SUDOQU were harmonised with those in RP101. In this way, differences in dose results between the two models are only related to differences in model construction and the (remaining) underlying assumptions. As a first step of the benchmarking study, values of the remaining parameters were left unvaried in SUDOQU (i.e. values from Sect. 2.1), with the aim of comparing the two models based on their main, default assumptions and to investigate their impact on the results. The assumption in RP101 that only 10 % of the total surface activity is removable enhances the dose contribution from externalgamma-radiation exposure, as the remaining 90 % of the surface activity contributes exclusively to this pathway, while only being modified by radioactive decay. On the other hand, the contribution of the other exposure pathways, related to activity removal from the surface (resuspension and wipe-off), will be reduced in RP101 with respect to those in SUDOQU, for which 100 % of the surface activity is removable and may thus contribute to these pathways (inhalation, ingestion and skin contamination). Again, the net outcome depends on the balance OPERATION AND NEW BUILD 31 Operation and New Build Clearance of Surface-contaminated Objects from the Controlled Area of a Nuclear Facility: Application of the SUDOQU Methodology ı F. Russo, C. Mommaert and T. van Dillen
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