Neutron activation or contamination by activation and corrosion products (Chalk River unidentified deposit: CRUD) cause high levels of activity in large components that are part of the primary circuit of a nuclear power plant (NPP) [1-3]. Hence, during the decommissioning of NPPs, the radiation exposure dose of dismantling workers should be estimated along with the establishment of a radiation protection plan to ensure radiation safety [4-6]. In previous research, the radiation exposure dose of dismantling workers was estimated using computer codes, with a focus on dismantling the main components because of the high radiation exposure dose. Thus, estimations of the radiation exposure dose during the dismantling of various contaminated components have been reported and analyzed [7-13].
Park et al. optimized the dismantling process for a research reactor and nuclear facility . To estimate the radiation exposure dose of dismantling workers, a virtual dismantling environment was modeled, and the radiation exposure dose was calculated using the Monte Carlo N-particle radiation transport code version 4C (MCNP-4C) based on the inventory of radioactive materials. In 2010, Bonavigo et al. proposed cutting scenarios for main component segmentation by considering the radiological characterization of the primary system at the Enrico Fermi NPP in Italy . Radioactive surface contamination of the primary circuit components and piping should be less than 1×104 Bq·cm−2. In 2014, Jeong et al. analyzed the radiological characteristics to estimate the radiation exposure dose during the decommissioning of a reactor pressure vessel (RPV). The radioactivity of the RPV surface and concrete shield was calculated using the MCNP code . The effective doses and their uncertainties during the dismantling of large-diameter pipes from the emergency core cooling system of the Ignalina NPP in Lithuania were evaluated in 2015 . Four alternatives for dismantling the large-diameter pipes were considered. As a result, the optimal dismantling process, according to the cost analysis and the individual effective doses, was determined. In 2016, Hornác ̂k et al. presented dismantling scenarios for a steam generator and evaluated the radiological impact on the public and environment using external and internal radiation exposure doses . In 2018, Lee presented an assessment of the radiation exposure dose resulting from decommissioning under the guidelines in Korea. The results were also compared and analyzed based on various overseas guidelines to evaluate the radiological environmental impact of NPPs . To derive the optimal working time, the internal radiation exposure dose was compared and analyzed using various computer codes in 2018. The analyzed data, which were “as low as reasonably achievable” enabled the determination of the optimal working time .
In this study, the radiation exposure dose of dismantling workers at different positions was estimated using the MicroShield dose assessment program. The individual effective dose, i.e., the sum of the effective dose for each tissue considering the working time, was used to estimate the radiation exposure dose. The detailed dismantling process of a reactor coolant system (RCS) piping was analyzed based on the NUREG/CR-1595 report. The radioactivity due to the inner surface contamination of RCS piping was calculated by considering the decontamination factor (DF) and half-life of the 60Co radionuclide. Finally, the individual effective dose in the dismantling process was estimated by considering the radiation exposure reduction methods.
2. Materials and methods
2.1 Detailed dismantling process of RCS piping
Based on the NUREG/CR-1595 report, the dismantling process of RCS piping is shown in Fig. 1 [14, 15]. Dismantling activities for chemical decontamination, cutting equipment installation, and nozzle and valve removal are often performed remotely. Most cutting operations are carried out using a bandsaw. Because dismantling workers perform both preparation and removal tasks, the distance between the radiation source and work areas varies depending on the requirements of the dismantling task. Table 1 summarizes the dismantling process of RCS piping, including the estimated working time and the distance between the radiation source and the work area.
2.2 Simulation input parameters of the MicroShield program
In the dismantling process, the radiological characteristics must be considered, such as the radioactivity of the inner surface contamination of components or the work area. In the primary coolant system of a NPP, the radioactive corrosion products (CRUD), which are activated by neutrons, results in an increase in the radiation exposure dose. Because the CRUD is saturated or deposited in the cold leg according to the concentration of hydrogen (pH), the radioactivity associated with the inner surface contamination of the cold leg is higher than that of the hot leg  (cf. Table 2). The most significant γ-ray-emitting radionuclides are 51Cr, 54Mn, 59Fe, 58Co, 60Co, and 95Zr . When compared to other radionuclides, 60Co radionuclide has a relatively long half-life and high energies (1,173 and 1,332 keV) and was therefore utilized to estimate the radiation exposure dose for dismantling workers. The radiological characterization data for RCS piping was obtained from actual measurements, which was then used for decommissioning at the Trino NPP in Italy . The decommissioning of the Trino NPP, operated from 1965 to 1987, is planned to end by 2031. The primary system includes four primary loops from A to D, and each system consists of two valves and a reactor coolant pump (RCP). To conservatively estimate the radiation exposure dose, the maximum radioactivity of the inner surface contamination was selected from loop C of the primary system (Table 2).
The MicroShield program provides various geometric models for the evaluation of radiation shielding and estimation of the radiation exposure dose [18-20]. Moreover, the trends of the MicroShield results and the estimation of the Monte Carlo code were similar to each other in previous studies [21-22]. The total radiation dose (Dosetotal) at the dose point was calculated using Eq. 1 based on the point-kernel method, which divides the volume source into a large number of small point sources, regards each as a point source, and obtains the sum of the respective contributions .
where C(E) is the flux-to-dose conversion factor (Sv·h−1 /#/cm2·s) and S0(E) is the photon production rate (#/s). B(E,μt) is the buildup factor and R is the distance from the source volume to the dose point (cm). b can be obtained as follows:
where μi and ti are the linear attenuation coefficient of the ith shield (cm−1) and the thickness of the ith shield (cm), respectively. The Dosetotal (mSv·h−1) at the dose point is integrated for the total photon energy and total source volume.
Table 2 lists the simulation input parameters, such as the geometry of the RCS piping and the radioactivity of the inner surface contamination [8, 14, 23]. In the MicroShield program, the calculated photon fluence rates at the dose points were converted into the effective dose. The weighting factors of radiation (WR) and tissues (WT) in the ICRP-60 report are the same as those in the ICRP-74 report [24, 25]. Because the weighting factors of the ICRP-60 dose model are used in Korea, the effective dose was calculated by applying the effective dose conversion factor of the ICRP-74 in the MicroShield program. The individual effective dose, which is defined as the sum of the effective dose to each tissue considering the working time, was used to estimate the radiation exposure dose. The detailed RCS piping dismantling process was analyzed based on the NUREG/CR-1595 report. In actual dismantling tasks, the radiation exposure dose will be affected by not only the radiation source of RCS piping but also by various radiation sources, which were attributed to the various dismantling tasks. However, in this study, the radiation exposure dose for each dismantling task was estimated to provide information on the radiation exposure dose for the optimized RCS piping dismantling process design. The radioactivity of the inner surface contamination of RCS piping was calculated by considering the DF. The individual effective dose for the dismantling process was also estimated by considering the radiation exposure dose reduction methods, such as the decay with half-life of the 60Co radionuclide and lead shielding.
The positions for the radiation exposure dose estimation and the simulation setup are illustrated in Fig. 2. Considering the structure of RCS piping, the geometry of the simulation was modeled by applying a cylinder surface-external dose point in the MicroShield program. The effective dose to each tissue and the individual effective dose were estimated and analyzed for the different positions of dismantling workers. Because the results of the radiation exposure dose estimation can be utilized for decommissioning the Kori-1 unit at the Kori NPP in South Korea, the DF was assumed to be 30, which is the target of DF for the unit . Therefore, the radiation source terms using DF = 30 were used to estimate the radiation exposure dose in all dismantling tasks.
3.1 Estimation of radiation exposure dose in dismantling tasks
After the detailed dismantling process for the RCS piping was chosen (Table 1), the radiation exposure dose of the workers dismantling the RCS piping was estimated by the MicroShield program. The comparisons of the estimated individual effective dose rates according to the distance from the RCS piping are shown in Fig. 3. Because the radioactivity of the inner surface contamination of the cold leg was higher than that of the hot leg, the individual effective dose rates for dismantling tasks at the cold leg were higher than those at the hot leg. The individual effective dose rates decreased with increasing distance from the center of the RCS piping. As expected, the individual effective dose rates decreased by approximately one-third when radiation sources with DF = 30 were applied.
Fig. 4 shows the individual effective dose for each dismantling task. The individual effective dose of task B was the highest because it has the shortest distance from the radiation source and the longest working time. The individual effective doses of dismantling preparation tasks were higher than those of removal tasks because the latter were often performed remotely. Table 3 summarizes the individual effective doses of dismantling tasks according to the defined distances between the radiation source and work area.
Fig. 5 shows the results of the highest effective dose rates to each tissue in dismantling task B. The effective dose rates to tissues with high radiosensitivity were high in all dismantling tasks. Based on the distance between the radiation source and the work area, the comparisons of the individual effective dose rates for dismantling tasks at various dose estimation points are shown in Figs. 6 and 7. The individual effective dose rates decreased with increasing distance between the center of the RCS piping and the estimated dose points. Therefore, to reduce the radiation exposure dose of dismantling workers, it is recommended that the cutting equipment be remotely controlled from the side of the RCS piping.
3.2 Comparisons of reduction methods for radiation exposure dose
The radiation exposure dose of dismantling workers must remain stable. To reduce the radiation exposure dose of dismantling workers, we considered two radiation exposure dose reduction methods. First, the individual effective dose rates were calculated using the lead shielding typically used in NPPs. The thickness of the lead shielding was set to 3 mm . As shown in Fig. 8, the individual effective dose rates with lead shielding are lower than those without lead shielding. The use of lead shielding resulted in dose reductions of approximately 15% to 30%.
Second, the decay of radioactive nuclides was used to reduce the radiation exposure dose. The major radioactive nuclide causing radiation exposure to dismantling workers is 60Co, whose half-life is approximately 5.27 years. Thus, the dismantling of major components may begin approximately 10 years after permanent shutdown. Based on the results in Fig. 9 and Table 4, the individual effective dose rates for dismantling workers significantly decreased upon considering the 10 years of decay and lead shielding. The effectiveness of the radiation exposure dose reduction using the radionuclide decay decreased by approximately 70% after 10 years.
Based on the characteristics of RCS piping dismantling tasks, the radiation exposure dose was estimated for dismantling workers at different positions using the MicroShield program. The individual effective dose, which is defined as the sum of the effective dose to each tissue considering the working time, was used to estimate the radiation exposure dose. The simulation results were compared in terms of the working distance, estimated working time, and methods for reducing the radiation exposure dose. Because the radioactivity of the inner surface contamination of the cold leg was higher than that of the hot leg, the individual effective dose rates for dismantling tasks at the cold leg were higher. However, the individual effective doses for preparation and removal tasks were below the dose limit of ICRP-60. The results of chemical decontamination, radioactive nuclide decay, and shielding planning demonstrate the reduction of the radiation exposure dose in the various RCS piping dismantling tasks. This study offers guidelines for the radiation safety of dismantling workers engaged in dismantling RCS piping.