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ISSN : 1738-1894(Print)
ISSN : 2288-5471(Online)
Journal of Nuclear Fuel Cycle and Waste Technology Vol.19 No.2 pp.243-253
DOI : https://doi.org/10.7733/jnfcwt.2021.19.2.243

# A Study on Estimation of Radiation Exposure Dose During Dismantling of RCS Piping in Decommissioning Nuclear Power Plant

Taewoong Lee1*, Seongmin Jo1, Sunkyu Park1, Nakjeom Kim1, Kichul Kim2, Seongjun Park2, Changyeon Yoon3
1Global Institute of Technology, KEPCO KPS, 211, Munhwa-ro, Naju-si, Jeollanam-do 58326, Republic of Korea
2Nuclear Maintenance Engineering Center, KEPCO KPS, Munsan 2Sandan-1ro, Oedong-eup, Gyungju-Si, Gyungsangbuk-do 38206, Republic of Korea
3Central Research Institute, Korea Hydro & Nuclear Power, 70, Yuseong-daero 1312beon-gil, Yuseong-gu, Daejeon 34101, Republic of Korea
* Corresponding Author. Taewoong Lee, KEPCO KPS, E-mail: superdapo@kps.co.kr, Tel: +82-61-345-0554

March 29, 2021 ; April 14, 2021 ; June 8, 2021

## Abstract

In the dismantling process of a reactor coolant system (RCS) piping, a radiation protection plan should be established to minimize the radiation exposure doses of dismantling workers. Hence, it is necessary to estimate the individual effective dose in the RCS piping dismantling process when decommissioning a nuclear power plant. In this study, the radiation exposure doses of the dismantling workers at different positions was estimated using the MicroShield dose assessment program based on the NUREG/CR-1595 report. The individual effective dose, which is the sum of the effective dose to each tissue considering the working time, was used to estimate the radiation exposure dose. The estimations of the simulation results for all RCS piping dismantling tasks satisfied the dose limits prescribed by the ICRP-60 report. In dismantling the RCS piping of the Kori-1 or Wolsong-1 units in South Korea, the estimation and reduction method for the radiation exposure dose, and the simulated results of this study can be used to implement the radiation safety for optimal dismantling by providing information on the radiation exposure doses of the dismantling workers.

## 1. Introduction

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].

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

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 [18].

$D o s e t o t a l = ∫ E ∫ V { C ( E ) S 0 ( E ) 4 π V } ⋅ B ( E , μ t ) e − b R 2 d V d E$
(1)

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:

$b = ∑ i = 1 N μ i t i$
(2)

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.

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 [26]. Therefore, the radiation source terms using DF = 30 were used to estimate the radiation exposure dose in all dismantling tasks.

## 3. Results

### 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 [27]. 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.

## Figures

Dismantling process of RCS piping in the decommissioning of a nuclear power plant [14, 15].

Configuration of the RCS system: (a) RCS system and (b) schematic diagram of the simulation setup.

Comparison of individual effective dose rates estimated by MicroShield program according to distance and DF values: (a) hot-leg and (b) cold-leg.

Individual effective doses with DF = 30 for each dismantling task (Table 1): (a) preparation tasks and (b) removal tasks.

Effective dose rates to each tissue in dismantling task B (DF = 30): (a) hot-leg and (b) cold-leg.

Comparisons of individual effective dose rates of estimated dose points for dismantling tasks (hot-leg, DF = 30): (a) 10 cm and (b) 100 cm.

Comparisons of individual effective dose rates of estimated dose points for dismantling tasks (cold-leg, DF = 30): (a) 10 cm and (b) 100 cm.

Comparison of individual effective dose rates with and without lead shielding according to the distance of dismantling tasks: (a) hot-leg and (b) cold-leg.

Comparison of reduction method of radiation exposure dose for dismantling tasks (DF = 30): (a) hot-leg and (b) cold-leg.

## Tables

Tasks in the dismantling process [14, 15]

Specifications and input parameters of RCS piping

Results of individual effective doses with DF = 30 for various dismantling tasks (Table 1)

Individual effective dose rates using radiation exposure dose reduction methods

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