Abstract
A whole-body counter (WBC) is a device employed in nuclear power plants (NPPs) to identify radionuclides and measure the content of radioactivity in humans. In this study, several experiments were conducted to suggest an optimal and practical method to improve the accuracy of in vivo measurements using WBCs at NPPs. First, countings from the front and back using a phantom were carried out to set up a discrimination programme between internal and external radioactive contamination in NPPs. Second, experiments were performed to select the optimal geometry of the WBC and to locate the contaminated area of radionuclides.
INTRODUCTION
A whole-body counter (WBC) is used in nuclear power plants (NPPs) to identify and measure radioactivity in humans(1–3). In most NPPs, several WBCs are used to monitor the internal radioactive contamination of workers. However, it has been found that external contamination is occasionally estimated as internal radioactive contamination in whole-body counting. The amount of radioactivity detected can be much higher than that of the actual radioactivity owing to this misclassification, as radioisotopes attached to the skin come in close proximity with the detectors of the WBC. Finally, this leads to not only misjudgement of external contamination as internal contamination, but also an excessively conservative estimate of radioactive contamination(4).
A WBC provides four geometries that compensate for the detector's efficiency depending on the location where radionuclides are deposited in the body in order to measure the exact amount of radioactivity inside a worker's body(5). In most NPPs, efficiency calibrations of the WBC for all geometries, i.e. the whole-body, thyroid, lung and gastrointestine (GI), are conducted when a major change of measurement is found after the initial commissioning of the WBC. Examples include cases in which important parts are replaced, including photomultiplier tubes, or in cases where periodical checks show that limits of quality assurance and control have been exceeded. These calibration results are then reported to check the performance and mechanical conditions of the WBC. However, only whole-body geometry is used to detect internal radioactivity during whole-body counting and the most conservative radioactivity is finally recorded. This is attributed to the absence of information and standard processes to choose and use the geometry of the WBC appropriately.
In this study, several experiments were carried out to set up a discrimination programme between internal and external radioactive contamination using a humanoid phantom and a WBC. In addition, experiments to select the optimal WBC geometry and to locate areas contaminated with radionuclides were conducted so that the radioactivity levels detected could be analysed depending on the geometry. Finally, an appropriate procedure for selecting the geometry of a WBC is proposed.
MATERIAL
Experiments were conducted using a WBC, phantoms and radiation sources. The WBC utilised for the experiments was the vertical linear Fastscan (Model 2250) manufactured by Canberra Inc., which is used for in vivo measurements of radionuclides with energies from 300keV to 1.8 MeV at most NPPs(6) (Figure 1). This WBC system consists of two large sodium iodide detectors (7.6 × 12.7 × 40.6 cm), which typically provide an a priori lower limit of detection of about 150 Bq for 60Co with a count time of 1 min for a normal person containing 40K(7,8). Prior to the experiments, energy and efficiency calibration of the WBC was performed to obtain reliable experimental results. After the calibration, the results of the performance test met the requirements of quality control; the relative bias was within −0.25 to +0.50 and the relative precision was less than 0.4(9). In addition, whole-body counting was conducted under stable temperature (20 ± 3°C) and humidity (50–55%) conditions.
Two types of phantoms were used. The first was a humanoid phantom of a typical Korean male developed by the Radiation Health Research Institute for radiation protection purposes and the second was the Canberra Transfer phantom developed for the calibration of a WBC. The humanoid phantom satisfied the reference Korean physical model (height: 170.9 cm, weight: 68.1 kg, etc.) and is sliced into 2 cm sections to facilitate dose mapping(10). Figure 2 shows the front, back and flank of the humanoid phantom with tags numbered 1–14 attached to the front and 15–28 to the back. Tags numbered F0–F3 are attached onto the flank to mark the position of the radioactive source. The Canberra Transfer phantom was designed for efficiency calibrations with the Canberra linear geometry of a WBC(11). This phantom can simulate four geometries: the whole-body, thyroid, lung and GI, and thus can provide more accurate and reliable measurement results using compensation for the detector's efficiency depending on the area where radionuclides are deposited in the body (Figure 1). Although these two phantoms do not match a person's physical characteristics exactly, they are designed to be accepted as a standard test phantom for the total body (or whole-body), thyroid and lung measurements of ANSI N13.30, which describes the performance criteria for radiobioassay(9).
In the experiments, a mixed gamma source of 137Cs and 60Co manufactured by North American Scientific, Inc. and used in the WBC daily quality assurance assessment was attached to the humanoid phantom surface. The activity of this mixed gamma source was 8.8 ± 2.7 kBq for 137Cs and 9.3 ± 2.8 kBq for 60Co (reference date: 1 October 1996). In addition, respective point sources of 137Cs (167.7 ± 0.9 kBq) and 60Co (110.9 ± 1.3 kBq), manufactured by the Korea Research Institute of Standards and Science, were inserted into the humanoid phantom. The date of certification for 137Cs and 60Co was 1 October 1993 and 8 February 1995, respectively. These two radionuclide sources were used for the experiments because they are the most important and common internal dose contributors for pressurised water reactors (PWRs). For the Canberra Transfer phantom, a liquid mixed gamma source was inserted into four efficiency calibration holes of the phantom. The liquid mixed gamma source was made by Analytics, Inc. (USA) and demonstrated traceability to the National Institute of Standards and Technology (NIST). The activity of this liquid mixed gamma source was 3.7 ± 0.1 kBq for 137Cs and 6.1 ± 0.2 kBq for 60Co (reference date: 1 April 2006).
METHOD
In this study, two types of experiments were carried out: an experiment to distinguish internal from external radioactive contamination and an experiment to optimise the WBC geometry. First, using a WBC counting was done from the front and back of the humanoid phantom to differentiate external contamination from internal contamination. It was assumed that the difference in the detected radioactivity levels between the front and back counts was higher than that of the internal contamination when the radionuclides were attached to the surface of the skin(4,12). As the body of the phantom shields the radiation from the source to the detectors, and because the distance from the source to the detectors changes when the phantom is turned around in the WBC, the measurement ratios of counting from the front and back would be high in the case of external contamination. In the case of internal contamination, there is no significant change in the distance between the body of the phantom and the detectors. Furthermore, radiation shielding is not a consequential factor, as it is located inside the phantom; thus, there is no discernable difference between the front and back counts.
In the WBC geometry experiment, two detailed experiments were performed(4,13). First, an experiment to select an optimal WBC geometry was carried out to compare the radioactivity levels detected depending on the WBC geometries. In this experiment, the radionuclide source was placed on each of four efficiency calibration locations of the phantom, assuming that the radionuclide was deposited on the whole-body, thyroid, lung and GI area. The radioactivity was then measured using four geometries for each deposition area. This makes it possible to compare and analyse radioactivity levels using a suitable WBC geometry for deposition area and, conversely, using an inappropriate WBC geometry for the deposition area. Second, an experiment was conducted to locate an area contaminated with radionuclides. Here, the locations of deposited nuclides were determined using the count rates of the upper and lower detectors of the WBC. In this experiment, the radionuclide sources were placed between slices in the centre, from top to bottom in sequence, of the humanoid phantom and counting was then done for each location. As the count rates of the upper and lower detectors of the WBC would change according to the locations of deposited nuclides, it was possible to locate the contaminated area of radionuclides approximately using the detected count rates.
where S is the net peak area, V the sample volume (or mass), ε′ the attenuation corrected efficiency, Y the branching ratio of the peak energy, T1 the live time of the collect in seconds, Uf the conversion factor required to have the activity in μCi, KC the correction factor for the nuclide decay during counting and KW the correction factor for the nuclide decay from the time the sample is created.
where σR is the user-defined random uncertainty (%), σS the uncertainty of the net peak area S, σV the uncertainty of the sample quantity V, σε′ the uncertainty of the effective efficiency ε′, σY the uncertainty of the branching ratio Y and σK the uncertainty of the composite decay, with correction factor K. In the experiments, the uncertainties of the weighted mean activity for detected nuclides were calculated as 5.7 ± 0.3% for 137Cs and 4.2 ± 0.3% for 60Co.
EXPERIMENTS AND RESULTS
In this study, three detailed experiments were conducted to set up a discrimination programme for internal and external radioactive contamination. In addition, two detailed experiments were conducted with the aim of selecting the optimal WBC geometry. All experiments were carried out using four geometries: the whole-body, thyroid, lung and GI. However, because the obtained results have similar trends, the result in which the whole-body geometry was used is shown in this section. The counting time was 3 min for all experiments.
Experiments to distinguish between internal and external contamination
Internal contamination
For a case of internal radioactive contamination, an experiment in which radiation sources were located inside the phantom was conducted. Two point sources of 137Cs and 60Co were placed in the centre of the phantom slice and counting from the front and the back was subsequently carried out while the phantom was turned around in the WBC. The finding showed that the average ratio of the front and back counts was 1.5 ± 0.7 for 137Cs and 1.4 ± 0.6 for 60Co; thus, there was no significant difference between the front and backside counts (Figure 3). To check whether the modelling assumptions were valid, the experimental data were plotted as a bias value (observed ratio–mean ratio)/mean ratio, against the location variable x. In Figure 4, the bias value of 137Cs is shown, as a result of 60Co which followed a trend similar to that of 137Cs. As all data points were close to the fitted line and there was no outlier whose bias value was larger than 3. The ratios obtained were considered appropriate. The Wilcoxon signed-rank test was also performed to compare the differences between the measurements of two related samples, the front and back counts. This result found W = 22.0 with the p-value = 0.25 for 137Cs and W = 23.0 with the p-value = 0.35 for 60Co. Hence, there was no difference between the front and back counts for internal contamination (p > 0.05)(15).
Bias values for the measurement ratios of the front and back counts for internal contamination (137Cs).
To simulate the internal contamination of the volume source assuming a whole-body distribution, an additional experiment was performed using the Bottle Manikin Absorber (BOMAB) phantom manufactured by Eckert & Ziegler Analytics (USA)(16). This phantom complied with traceability to the NIST and the activity of the source was 19.1 ± 0.5 kBq for 137Cs and 30.1 ± 0.8 kBq for 60Co (reference date: 1 April 2008). The results showed that the ratios of the front and backside counts were 1.1 ± 0.1 for 137Cs and 1.6 ± 0.5 for 60Co, which was similar to the previous test involving two point sources. Thus, it was found that for internal contamination, there was no significant difference between the front and back counts using either point sources or volume sources.
External contamination
Experiments that tested front and back external radioactive contamination were conducted. The experimental results indicated that the average ratios of the front and back counts were 13.4 ± 5.1 for 137Cs and 8.3 ± 3.1 for 60Co with the attachment of a mixed source to the front side of the phantom. For a source positioned on the back of the phantom, similar results were obtained. The result of a Wilcoxon signed-rank test found W = 105.0 with a p-value = 0.00 for both 137Cs and 60Co; thus, there was a difference between the front and back counts (p < 0.05). Figures 5 and 6 depict the ratio of the front and back counts and the bias values of 137Cs for the front external contamination assessment, respectively.
Bias values for the measurement ratios of the front and back counts for external contamination (137Cs).
The second experiment evaluated flank contamination, where it was assumed that external radioactive contamination occurs on both flanks of the worker. Two point sources of 60Co and 137Cs were attached to the humanoid phantom flank (F0–F3): 137Cs for the left flank and 60Co for the right flank. Counting from the front and back was then carried out as the phantom was turned around in the WBC. The result showed no difference between the front and back counts.
The third experiment evaluated simultaneous contamination of the worker's front and back. Four point sources of 60Co and 137Cs were attached to both the front and the back of the humanoid phantom surface. In the first case, two point sources of 60Co and 137Cs were fixed to the back of the phantom (position No. 21), while two other point sources of 60Co and 137Cs were positioned on the front side of the phantom from locations No. 1 to No. 14 in sequence. For the second case, two point sources of 60Co and 137Cs were fixed on the front side of the phantom (position No. 7), while two other point sources of 60Co and 137Cs were positioned on the back of the phantom from locations No. 15 to No. 28 in sequence. According to the experimental results, there were no differences between the front and back counts for both cases.
Simultaneous internal and external contamination
To evaluate the simultaneous contamination of both the internal and external body, two sets of experiments were carried out: the two sets assessed front and back contamination, respectively. Four point sources of 137Cs and 60Co were used, with two sources of 137Cs and 60Co placed in the centre inside the phantom, and the other two sources were attached to either the front or the back of the phantom surface. Counting from the front and back was then done while the phantom was turned around in the WBC. The experimental results indicated that the average ratio of the front and back counts was 2.1 ± 0.6 for 137Cs and 2.8 ± 0.7 for 60Co; thus, there was no significant difference between the front and back counts for simultaneous contamination of both the internal and external body. The results of a Wilcoxon signed-rank test showed W = 116.0 with a p-value = 0.00 for 137Cs and W = 105.0 with a p-value = 0.00 for 60Co (p < 0.05).
The second experiment entails simultaneous contamination of both the internal body and the flank region. Similar to the previous experiments, two point sources of 137Cs and 60Co were located inside the phantom and two sources were attached to the flank of the phantom. The results showed no discernable differences between the front and back counts.
Experiments on WBC geometry
Selecting the optimal WBC geometry
To select the optimal WBC geometry, two experiments were carried out to compare the detected activities depending on the geometries of the WBC. In the first experiment, point sources of 60Co and 137Cs were respectively placed at four locations, for efficiency calibration, inside the humanoid phantom, where it was assumed that radionuclides would be deposited on the whole-body, thyroid, lung and GI area. Counting was then done under all geometries for each location. To compare the results depending on the phantom, the experiment used Canberra's Transfer phantom and a liquid mixed gamma source instead of the humanoid phantom and point sources were also assessed. These two experiments produced similar results regardless of the type of phantom and radiation source. Both sets of results indicated that the activity detected under the whole-body geometry was higher compared to those of any other geometry (Table 1). A chi-square test was used to calculate the p-value. These results were χ2 = 8.449 with a p-value = 0.04 for 137Cs and χ2 = 9.088 with a p-value = 0.03 for 60Co using the humanoid phantom. Additionally, they were χ2 = 7.555 with a p-value = 0.05 for 137Cs and χ2 = 8.222 with a p-value = 0.04 for 60Co using the Canberra phantom(15). In Figure 7, the bias values of detected activities depending on the WBC geometry for 137Cs using the humanoid phantom are shown. The result of 60Co followed a trend similar to that of 137Cs.
Bias values of detected activities depending on the WBC geometry using the humanoid phantom (137Cs).
Comparison of detected activities depending on the WBC geometry.
| Source position | Geometry | Humanoid phantom | Canberra's phantom | ||
|---|---|---|---|---|---|
| Activity (kBq) | Activity (kBq) | ||||
| 60Co | 137Cs | 60Co | 137Cs | ||
| Whole body | Whole body | 24.8 | 84.5 | 5.8 | 4.0 |
| Thyroid | 11.4 | 32.7 | 2.6 | 1.6 | |
| Lung | 19.3 | 61.6 | 4.1 | 2.9 | |
| GI | 18.8 | 66.2 | 4.4 | 3.0 | |
| Thyroid | Whole body | 43.3 | 173.1 | 12.5 | 10.0 |
| Thyroid | 19.7 | 66.3 | 5.6 | 3.9 | |
| Lung | 33.3 | 126.0 | 9.6 | 7.5 | |
| GI | 32.9 | 131.4 | 9.5 | 8.2 | |
| Lung | Whole body | 34.4 | 124.2 | 7.3 | 5.5 |
| Thyroid | 15.7 | 48.4 | 3.3 | 2.1 | |
| Lung | 26.3 | 91.2 | 5.7 | 3.7 | |
| GI | 25.5 | 96.9 | 5.8 | 4.1 | |
| GI | Whole body | 29.3 | 100.2 | 7.2 | 5.3 |
| Thyroid | 13.2 | 38.4 | 3.3 | 1.9 | |
| Lung | 22.5 | 73.1 | 5.8 | 3.7 | |
| GI | 22.3 | 77.9 | 5.5 | 3.8 | |
| True activity | 27.7 ± 0.1 | 84.8 ± 1.0 | 5.6 ± 0.1 | 3.7 ± 0.1 | |
| Source position | Geometry | Humanoid phantom | Canberra's phantom | ||
|---|---|---|---|---|---|
| Activity (kBq) | Activity (kBq) | ||||
| 60Co | 137Cs | 60Co | 137Cs | ||
| Whole body | Whole body | 24.8 | 84.5 | 5.8 | 4.0 |
| Thyroid | 11.4 | 32.7 | 2.6 | 1.6 | |
| Lung | 19.3 | 61.6 | 4.1 | 2.9 | |
| GI | 18.8 | 66.2 | 4.4 | 3.0 | |
| Thyroid | Whole body | 43.3 | 173.1 | 12.5 | 10.0 |
| Thyroid | 19.7 | 66.3 | 5.6 | 3.9 | |
| Lung | 33.3 | 126.0 | 9.6 | 7.5 | |
| GI | 32.9 | 131.4 | 9.5 | 8.2 | |
| Lung | Whole body | 34.4 | 124.2 | 7.3 | 5.5 |
| Thyroid | 15.7 | 48.4 | 3.3 | 2.1 | |
| Lung | 26.3 | 91.2 | 5.7 | 3.7 | |
| GI | 25.5 | 96.9 | 5.8 | 4.1 | |
| GI | Whole body | 29.3 | 100.2 | 7.2 | 5.3 |
| Thyroid | 13.2 | 38.4 | 3.3 | 1.9 | |
| Lung | 22.5 | 73.1 | 5.8 | 3.7 | |
| GI | 22.3 | 77.9 | 5.5 | 3.8 | |
| True activity | 27.7 ± 0.1 | 84.8 ± 1.0 | 5.6 ± 0.1 | 3.7 ± 0.1 | |
Bold type represents the highest detected activities in the four geometries.
Comparison of detected activities depending on the WBC geometry.
| Source position | Geometry | Humanoid phantom | Canberra's phantom | ||
|---|---|---|---|---|---|
| Activity (kBq) | Activity (kBq) | ||||
| 60Co | 137Cs | 60Co | 137Cs | ||
| Whole body | Whole body | 24.8 | 84.5 | 5.8 | 4.0 |
| Thyroid | 11.4 | 32.7 | 2.6 | 1.6 | |
| Lung | 19.3 | 61.6 | 4.1 | 2.9 | |
| GI | 18.8 | 66.2 | 4.4 | 3.0 | |
| Thyroid | Whole body | 43.3 | 173.1 | 12.5 | 10.0 |
| Thyroid | 19.7 | 66.3 | 5.6 | 3.9 | |
| Lung | 33.3 | 126.0 | 9.6 | 7.5 | |
| GI | 32.9 | 131.4 | 9.5 | 8.2 | |
| Lung | Whole body | 34.4 | 124.2 | 7.3 | 5.5 |
| Thyroid | 15.7 | 48.4 | 3.3 | 2.1 | |
| Lung | 26.3 | 91.2 | 5.7 | 3.7 | |
| GI | 25.5 | 96.9 | 5.8 | 4.1 | |
| GI | Whole body | 29.3 | 100.2 | 7.2 | 5.3 |
| Thyroid | 13.2 | 38.4 | 3.3 | 1.9 | |
| Lung | 22.5 | 73.1 | 5.8 | 3.7 | |
| GI | 22.3 | 77.9 | 5.5 | 3.8 | |
| True activity | 27.7 ± 0.1 | 84.8 ± 1.0 | 5.6 ± 0.1 | 3.7 ± 0.1 | |
| Source position | Geometry | Humanoid phantom | Canberra's phantom | ||
|---|---|---|---|---|---|
| Activity (kBq) | Activity (kBq) | ||||
| 60Co | 137Cs | 60Co | 137Cs | ||
| Whole body | Whole body | 24.8 | 84.5 | 5.8 | 4.0 |
| Thyroid | 11.4 | 32.7 | 2.6 | 1.6 | |
| Lung | 19.3 | 61.6 | 4.1 | 2.9 | |
| GI | 18.8 | 66.2 | 4.4 | 3.0 | |
| Thyroid | Whole body | 43.3 | 173.1 | 12.5 | 10.0 |
| Thyroid | 19.7 | 66.3 | 5.6 | 3.9 | |
| Lung | 33.3 | 126.0 | 9.6 | 7.5 | |
| GI | 32.9 | 131.4 | 9.5 | 8.2 | |
| Lung | Whole body | 34.4 | 124.2 | 7.3 | 5.5 |
| Thyroid | 15.7 | 48.4 | 3.3 | 2.1 | |
| Lung | 26.3 | 91.2 | 5.7 | 3.7 | |
| GI | 25.5 | 96.9 | 5.8 | 4.1 | |
| GI | Whole body | 29.3 | 100.2 | 7.2 | 5.3 |
| Thyroid | 13.2 | 38.4 | 3.3 | 1.9 | |
| Lung | 22.5 | 73.1 | 5.8 | 3.7 | |
| GI | 22.3 | 77.9 | 5.5 | 3.8 | |
| True activity | 27.7 ± 0.1 | 84.8 ± 1.0 | 5.6 ± 0.1 | 3.7 ± 0.1 | |
Bold type represents the highest detected activities in the four geometries.
In addition, Table 2 indicates that when the related WBC geometry is used for the contaminated area, the detected activity is consistent for all geometries (at a confidence level of 2 sigma). Thus, if only the whole-body geometry is used for whole-body counting without regard to the locations where radionuclides are deposited, the results will always be more conservative than the actual internal radioactivity. In particular, when a whole-body geometry was used for the contaminated area of the thyroid, the detected activity was greatly overestimated by approximately 104–171% for 137Cs and 56–124% for 60Co.
Consistency of detected activities when the related WBC geometry is used for the contaminated areas.
| Source position | Geometry | Humanoid phantom | Canberra's phantom | ||
|---|---|---|---|---|---|
| Activity (kBq) | Activity (kBq) | ||||
| 60Co | 137Cs | 60Co | 137Cs | ||
| Whole body | Whole body | 24.8 | 84.5 | 5.8 | 4.0 |
| Thyroid | Thyroid | 19.7 | 66.3 | 5.6 | 3.9 |
| Lung | Lung | 26.3 | 91.2 | 5.7 | 3.7 |
| GI | GI | 22.3 | 77.9 | 5.5 | 3.8 |
| Experimental mean | 23.3 ± 2.9 | 80.0 ± 10.6 | 5.6 ± 0.1 | 3.9 ± 0.1 | |
| True activity | 27.7 ± 0.1 | 84.8 ± 1.0 | 5.6 ± 0.1 | 3.7 ± 0.1 | |
| Source position | Geometry | Humanoid phantom | Canberra's phantom | ||
|---|---|---|---|---|---|
| Activity (kBq) | Activity (kBq) | ||||
| 60Co | 137Cs | 60Co | 137Cs | ||
| Whole body | Whole body | 24.8 | 84.5 | 5.8 | 4.0 |
| Thyroid | Thyroid | 19.7 | 66.3 | 5.6 | 3.9 |
| Lung | Lung | 26.3 | 91.2 | 5.7 | 3.7 |
| GI | GI | 22.3 | 77.9 | 5.5 | 3.8 |
| Experimental mean | 23.3 ± 2.9 | 80.0 ± 10.6 | 5.6 ± 0.1 | 3.9 ± 0.1 | |
| True activity | 27.7 ± 0.1 | 84.8 ± 1.0 | 5.6 ± 0.1 | 3.7 ± 0.1 | |
Consistency of detected activities when the related WBC geometry is used for the contaminated areas.
| Source position | Geometry | Humanoid phantom | Canberra's phantom | ||
|---|---|---|---|---|---|
| Activity (kBq) | Activity (kBq) | ||||
| 60Co | 137Cs | 60Co | 137Cs | ||
| Whole body | Whole body | 24.8 | 84.5 | 5.8 | 4.0 |
| Thyroid | Thyroid | 19.7 | 66.3 | 5.6 | 3.9 |
| Lung | Lung | 26.3 | 91.2 | 5.7 | 3.7 |
| GI | GI | 22.3 | 77.9 | 5.5 | 3.8 |
| Experimental mean | 23.3 ± 2.9 | 80.0 ± 10.6 | 5.6 ± 0.1 | 3.9 ± 0.1 | |
| True activity | 27.7 ± 0.1 | 84.8 ± 1.0 | 5.6 ± 0.1 | 3.7 ± 0.1 | |
| Source position | Geometry | Humanoid phantom | Canberra's phantom | ||
|---|---|---|---|---|---|
| Activity (kBq) | Activity (kBq) | ||||
| 60Co | 137Cs | 60Co | 137Cs | ||
| Whole body | Whole body | 24.8 | 84.5 | 5.8 | 4.0 |
| Thyroid | Thyroid | 19.7 | 66.3 | 5.6 | 3.9 |
| Lung | Lung | 26.3 | 91.2 | 5.7 | 3.7 |
| GI | GI | 22.3 | 77.9 | 5.5 | 3.8 |
| Experimental mean | 23.3 ± 2.9 | 80.0 ± 10.6 | 5.6 ± 0.1 | 3.9 ± 0.1 | |
| True activity | 27.7 ± 0.1 | 84.8 ± 1.0 | 5.6 ± 0.1 | 3.7 ± 0.1 | |
Locating the radionuclide contaminated area
An experiment was conducted to locate the area contaminated by radionuclides and to find the locations of deposited nuclides using the count rates of the upper and lower detectors of the WBC. Point sources of 60Co and 137Cs were placed between slices of the humanoid phantom from the top to the bottom in sequence. Counting was then performed for each location. The experimental results demonstrated that the count rate of the upper and lower detectors of the WBC varied according to the location of the radiation sources. The count rate of the upper and lower detectors was approximately 90:10 at the top slice of the phantom (head). The count rate of the upper detector decreased and the count rate of the lower detector increased as the locations of the radiation sources were moved downward. The approximate count rates of the upper and lower detectors were 85:15 for the thyroid, 75:25 for the lung, 60:40 for the whole-body and 35:65 for the GI (Figure 8). It is, therefore, possible to determine the location of an area contaminated by radionuclides using the count rate of the upper and lower detectors if radioactivity is detected during whole-body counting.
DISCUSSION AND CONCLUSION
In general, it is not easy to exactly determine the internal radioactive contamination while simultaneously measuring the exact internal radioactivity of workers at NPPs in a single measurement. Due to the absence of a complete procedure for whole-body counting, both misjudgements of external contamination as internal contamination and excessively conservative estimates of radioactive contamination can occur at NPPs. This deteriorates the accuracy of internal activity measurements and consequently diminishes the reliability of radiation safety management at NPPs. Thus, it is essential to have an appropriate whole-body counting procedure at NPPs.
In addition, most radiation workers in NPPs consider that internal contamination is more dangerous than external contamination or external exposure regardless that the exposure dose is identical. As the radionuclides remain and accumulate inside the body for a specific time period, the workers carry a burden of internal contamination psychologically and eventually become reluctant to do radiation work that can cause internal contamination. Thus, it is important to determine internal contamination accurately and to estimate the exact amount of internal radioactivity to assure the workers of the level of safety at NPPs.
In this study, several experiments involving counting from the front and back using a WBC and a phantom were conducted to set up a programme that discriminates between internal and external forms of radioactive contamination. The results found that front and back counts can be used to distinguish between internal and external radioactive contamination. The approximate ratio of detected radioactivity levels between the front and back counts was greater by more than a factor of two for external contamination. It is, however, necessary to carry out further tests to discriminate external contamination from internal contamination for ratios that are less than a factor of two. In this case, several recounts at regular intervals should be performed within a fixed period of time. Finally, a method to determine whether internal contamination has occurred was employed. This method was based on a comparison between recounting results and the intake retention function graphs of corresponding nuclides. It is recommended that this counting method be used only when radioactivity is detected in normal counting.
In all of the experiments involving front and back counts, the point source and the humanoid phantom were used to simulate the internal and external radioactive contamination of workers. Thus, the conditions of actual contamination scenarios, such as complicated cases in which the radionuclides are distributed through different organs and tissues or are present on both the internal and external areas of the body, can differ from the experimental conditions in this study. Although simultaneous contamination is not likely to occur in normal situations at NPPs, the results of simultaneous internal and external contamination experiments showed that the average ratio of the front and back counts was greater than a factor of two. In such a case, it is difficult to apply the ratio factor of two for the determination of either internal or external contamination. It is therefore necessary to develop a more detailed process to cover potentially complicated cases of internal or external contamination.
In general, 58Co, 60Co, 131I, 137Cs and 54Mn are the dominant contributors to internal radioactive contamination for radiation workers at PWRs. All radionuclides deposit in specific critical organs and WBC provides four geometries that consider such locations in order to measure the internal radioactivity more accurately. For example, although 131I is likely to be deposited in the thyroid inside the body, if whole-body geometry is used instead of thyroid geometry for the measurement, the activity detected will likely be more conservative than the actual internal radioactivity. This can be a considerable burden for both radiation workers and health physicists at NPPs due to the increase in the level of apprehension and the likelihood of exceeded dose limits caused by overestimations. Thus, it is essential to develop a method that provides detected activity at more accurate levels.
In this study, several experiments were carried out to analyse the detected radioactivity levels depending on the geometry of the WBC. The locations of the areas of radionuclide contamination were also assessed. From the results of the experiments, it was found that the detected activity under the whole-body geometry is always more conservative than that of any other geometry. Moreover, the count rates of the upper and lower detectors of the WBC varied according to the locations of the deposited radiation sources. For this reason, it was possible to determine the optimal geometry to measure the exact amount of internal radioactivity considering the physical and biological characteristics of radionuclides, including the deposited locations using the count rates of the upper and lower detectors of the WBC.
In all experiments involving the WBC geometry, it was assumed that inhaled radionuclides were deposited on a single specific organ. A point source and liquid mixed gamma source were used to simulate this type of internal contamination. There is a likelihood of multiple contaminations of several different organs, although this is unlikely to occur under the normal operating conditions of NPPs. In such cases, it is difficult to determine the locations of radionuclide contamination using the count rates of the upper and lower detectors of the WBC. Furthermore, in complicated contamination conditions such as contamination from several large-volume sources, the conditions can differ from those of the experiments in this study. Thus, it is necessary to consider the number of nuclides, the organs affected, and the type of distribution (such as point or volume) prior to applying a method of locating an area contaminated by radionuclides.
In conclusion, several experiments involving front and back counts and the WBC geometry were carried out to determine a practical and optimal method of improving the accuracy of in vivo measurements using WBCs at NPPs. The results showed that front and back counts can be applied to determine the degree of internal contamination of radiation workers more accurately. The count rates of the upper and lower detectors of the WBC can also be used to assess the areas contaminated by radionuclides and then the related WBC geometry can be used to measure the internal radioactivity more exactly.
FUNDING
This research was carried out with financial support from the Korea Hydro & Nuclear Power Corporation.
