Assessment of Radiation Dose during the Installation and Removal of Steam Generator Primary Nozzle Dam in Overhaul Period

Ju Young Kim; Ji Woo Kim; Dae Ho Lee; Kwang Pyo Kim

doi:10.14407/jrpr.2024.00213

J. Radiat. Prot. Res > Volume 49(4); 2024 > Article

Kim, Kim, Lee, and Kim: Assessment of Radiation Dose during the Installation and Removal of Steam Generator Primary Nozzle Dam in Overhaul Period

Original Research

Journal of Radiation Protection and Research 2024; 49(4): 187-195.

Published online: December 31, 2024

DOI: https://doi.org/10.14407/jrpr.2024.00213

Assessment of Radiation Dose during the Installation and Removal of Steam Generator Primary Nozzle Dam in Overhaul Period

Ju Young Kim

, Ji Woo Kim

, Dae Ho Lee

, Kwang Pyo Kim

Department of Nuclear Engineering, Kyung Hee University, Yongin, Republic of Korea

Corresponding author: Kwang Pyo Kim, Department of Nuclear Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin 17104, Republic of Korea, E-mail: kpkim@khu.ac.kr, https://orcid.org/0000-0003-0544-2978

Received August 9, 2024 Revised September 6, 2024 Accepted October 22, 2024

This is an open-access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by-nc/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Background

The installation and removal of nozzle dam work is one of the representative maintenance works in the steam generator primary side and can result in high levels of exposure due to the Chalk River unidentified deposit deposited inside the steam generator. Therefore, it is necessary to assess the radiation dose to workers installing and removing the nozzle dam to determine the exposure level for each task. This study assessed the radiation dose to workers installing and removing the nozzle dam during the overhaul period.

Materials and Methods

The work scenarios for installation and removal of the nozzle dam were analyzed based on Advanced Power Reactor 1400 (APR1400) mock-up operation video and worker questionnaire. Then, based on the steam generator design data presented in the APR1400 design criteria document (DCD), the steam generator was simulated using Monte Carlo N-Particle (MCNP) code. Based on analyzed work scenarios and simulated steam generator, the radiation doses to workers were assessed.

Results and Discussion

The work scenarios were analyzed for the four sub-works and the steam generator was simulated using MCNP6. The simulated steam generator was used to derive dose rates for each work location. For each sub-work, the assessment results of radiation doses were in the range of 2.13–4.51 man·mSv. For each worker, the radiation doses were in the range of 1.01–2.70 mSv. The highest radiation dose to workers was about 5.4% of the annual dose limit for workers (based on a maximum of 50 mSv per year). In this study, the source term was set using the specific activity of each radionuclide provided as conservative values in the APR1400 DCD. Consequently, the results of the dose assessment are likely to be somewhat conservative.

Conclusion

The results of this study can be used for optimizing radiation exposure to workers installing and removing nozzle dam during the overhaul period.

Keywords: Dose Assessment, Steam Generator, Radiation Worker, Nozzle Dam, Overhaul

Introduction

Workers in nuclear power plants are exposed to radiation under various situations. Approximately 80% of the total radiation dose occurs during the overhaul period [1, 2]. During the overhaul period, workers perform maintenance works in the radiation area inside the containment vessel and the workers are primarily exposed to radiation from Chalk River unidentified deposit (CRUD) inside the primary system [3]. CRUD, a neutron activation corrosion product generated on the surface of the nuclear fuel assemblies, separates and resides in the primary coolant, containing high-energy gamma-ray emitting radionuclides [4]. Generally, CRUD deposits on the internal surfaces of the primary system; in the case of a steam generator, CRUD deposits on the surface of the water chamber and U-tubes [5]. The water chamber and U-tubes are parts of the primary system of the steam generator, and maintenance work on the steam generator includes eddy current testing, installation and removal of the nozzle dam, and the opening and closing primary manway [6, 7]. Among them, installation and removal of the nozzle dam work is performed before and after the steam generator maintenance to install and remove the nozzle dam from the nozzles at the hot and cold legs inside the water chamber to block the inflow of primary coolant [8, 9]. This work is performed inside the water chamber, close to the water chamber surface and the U-tubes, where CRUD is deposited; there is a possibility of relatively high exposure to radiation due to the high-dose-rate radiation field.

Bace et al. [10] evaluated gamma dose rates around the steam generator for shielding analysis using QAD-CGGP and Monte Carlo N-Particle (MCNP) transport computer code. Zohar and Snoj [11] evaluated the gamma dose rate around the steam generator considering the contribution from the activated primary coolant using the MCNP computer code. Maeng et al. [12] evaluated the lens dose of radiation workers using a lens phantom and the MCNP computer code to simulate the steam generator of OPR1000 model (Korea Electric Power Corporation E&C). Shin et al. [13] evaluated the inventory of radionuclides in the CRUD accumulated inside the steam generator of Kori Unit 1 Nuclear Power Plant and evaluated gamma dose rates around the steam generator using the QAD-CG computer code. Lee and Lee [14] simulated the radiation field in the water chamber of the steam generator using the MCNP computer code and evaluated the effective dose to workers using the computational anthropometric phantom.

Various studies have evaluated the gamma dose rates around the steam generator and the radiation dose to workers. However, the assessment of radiation fields and worker exposure to radiation in the steam generators of the latest nuclear power plant models remains insufficient. Furthermore, due to the high-dose-rate radiation field caused by the deposited CRUD in the primary side of the steam generator, the radiation dose to workers can vary depending on the work location, work time, shielding, and the number of workers. However, there is a lack of research on dose assessment for workers considering work location and work time for each task performed during the installation and removal of the nozzle dam work. Therefore, a task-specific dose assessment is required to determine the exposure level of workers installing and removing the nozzle dam. The purpose of this study is to assess the dose to workers installing and removing steam generator nozzle dam in the overhaul period. For this purpose, work scenarios of installation and removal of the steam generator nozzle dam were analyzed, and the steam generator was simulated using MCNP computer code. Based on the analyzed work scenarios and the simulated steam generator, the radiation dose to workers was assessed.

Materials and Methods

This study assessed the radiation dose to workers installing and removing nozzle dam. For this purpose, work scenarios for installation and removal of the nozzle dam were analyzed according to each sub-work. In addition, geometric structure and source term were investigated and analyzed for simulation of the steam generator using MCNP computer code. Furthermore, the radiation dose to workers was assessed from the results of the evaluated dose rate and work time at each work location.

1. Analysis of the Work Scenario

The work scenarios for installation and removal of the nozzle dam were analyzed based on (1) Advanced Power Reactor 1400 (APR1400) steam generator (Korea Electric Power Corporation E&C) mock-up operation video and (2) the worker questionnaire. The work scenarios were analyzed by each sub-work. Furthermore, scenarios were analyzed according to the work location and time. As there were limitations in identifying the actual work location, the work locations were generalized and classified into three points in total for inside and outside the water chamber. For the inside of the water chamber, the work location was generalized at the center point (A) of the water chamber. For the outside of the water chamber, the work locations were established according to the distance from the manway, generalized to 10 cm for the close point (B) and 300 cm for the standby point (C).

2. MCNP Simulation of a Steam Generator

To assess the radiation dose to workers installing and removing the nozzle dam, the steam generator was simulated. The simulation for the geometric structure and source term of the APR1400 steam generator was performed using MCNP version 6.2 computer code. Geometric structure was simulated based on the geometry of the steam generator’s interior and exterior structures. The source term was set for CRUD deposited inside the components of the steam generator. The geometric structure and CRUD information of the steam generator were obtained from the design specifications presented in the APR1400 design criteria document (DCD) and the safety analysis report (SAR) for construction and operating licenses for Korean APR1400 nuclear power plants.

1) Simulation of the geometric structure

Fig. 1 shows the simulated steam generator using MCNP code. In this study, the geometric structures for the primary and secondary components of the steam generator were simulated, focusing on the primary components where CRUD is mainly deposited. For the primary components, the U-tubes, tubesheet, stay cylinder, primary lower head, primary shell, and the primary divider plate were considered, and for the secondary components, the secondary shell and secondary upper head were considered [15]. For U-tubes, the APR1400 steam generator contains 13,102 U-tubes, and it can be somewhat difficult to simulate individually. Therefore, in this study, the area occupied by each of the U-tubes and tubesheet was homogenized and simulated.

2) Setting material properties

Material properties were set using the material specifications given in the APR1400 DCD for each primary and secondary component [15]. The elemental composition and density of each component were utilized in the simulation. For components with elemental composition ratios given as ranges, the median value of the range was set for simulation. As the U-tubes and tubesheet were simulated by homogenizing the occupied area, the elemental composition and density of each area were also homogenized.

3) Setting source term

The source term was set by considering only gamma rays emitted from CRUD. In the case of APR1400 nuclear power plant considered in this study, which is a pressurized water reactor, external exposure accounts for approximately 95% of the total radiation exposure for workers [16]. Although gamma-ray emitting nuclides in the CRUD can emit both gamma and beta rays, only gamma rays were considered in the source term due to the negligible impact on external exposure of beta rays, which has weak penetration power.

To set source term of the steam generator, an analysis of the CRUD deposited inside the steam generator was conducted. The analyzed source term was set for the CRUD deposition areas, namely (1) the water chamber surface and (2) the U-tubes and tubesheet. The CRUD includes various radionuclides, with APR1400 DCD and SAR presenting six representative radionuclides [17–20]. The radionuclides considered in this study were ⁵¹Cr, ⁵⁴Mn, ⁵⁹Fe, ⁵⁸Co, ⁶⁰Co, and ⁹⁵Zr. For the water chamber, gamma-ray emissions per unit area were derived for each gamma-ray energy by considering the specific activity of each radionuclide, the gamma-ray yield by energy, and the equilibrium thickness for deposited CRUD. Since the U-tubes and tubesheet were simulated homogeneously, the source term was set to gamma-ray emissions per unit mass, taking into account the gamma-ray emissions per unit surface area for U-tubes and tubesheet, CRUD deposited surface area of U-tubes and tubesheet, and mass of each homogenized area. Specific activity and equilibrium thicknesses were obtained from the APR1400 SAR. For the equilibrium thicknesses, the water chamber surface was set to 1.0×10⁻³ g·cm⁻² and the U-tubes and tubesheet were set to 1.0×10⁻⁴ g·cm⁻². Gamma-ray yield and energy were obtained from the radionuclide data presented in the International Commission on Radiological Protection (ICRP) Publication 107 [21].

3. Assessment of Radiation Dose

Fig. 2 shows the radiation dose rate assessment points for the steam generator. Radiation dose from the installation and removal of the nozzle dam work was assessed for each sub-work and worker. The radiation dose to workers was derived by multiplying the dose rates assessed for each work location A, B, and C with the corresponding work time based on the analyzed work scenarios. For radiation dose calculation for workers, deep dose equivalent, operation quantity for whole body absorbed dose of workers was calculated. For the dose conversion factor, H*(10) provided in ICRP Publication 74 was applied in this study [22].

Validation of the radiation dose calculated by MCNP6 was performed. For the validation, the Microshield code (Grove Software) was used to calculate radiation dose rates at each work location. The dose rates calculated by the Microshield code, which uses a deterministic method for the dose calculation, were then compared to the dose rate results calculated by the MCNP6 code.

Results and Discussion

This study assessed the radiation dose to workers installing and removing nozzle dam. For this purpose, the work scenarios of installation and removal of the nozzle dam work were analyzed. Then, a simulation of the steam generator was performed using the MCNP computer code, and the radiation dose to workers was assessed based on the analyzed work scenarios.

1. Results of Work Scenario Analysis

Tables 1 –4 show work scenarios of each sub-work for installation and removal of the nozzle dam work. The work scenarios were analyzed by following sub-works: (1) nozzle dam installation on the cold leg side, (2) nozzle dam installation on the hot leg side, (3) nozzle dam removal from the hot leg side, and (4) nozzle dam removal from the cold leg side. For the nozzle dam installation work, the work scenarios were analyzed for a total of four workers on both the cold leg side and hot leg side. For the nozzle dam removal work, the work scenarios were analyzed for a total of six workers on both the cold leg side and hot leg side.

2. Results of Simulation for the Steam Generator

1) Setting material properties

Table 5 shows the materials by components of the steam generator. The simulation of the geometric structure of the steam generator was performed, focusing on the primary side components. Since the U-tubes and tubesheet areas were simulated by homogenizing, the material properties were also homogenized. For the U-tubes area, the SB-163 NiCrFe alloy of U-tubes, secondary coolant, and air were considered and homogenized evaluating density and weight percent by elements. For the tubesheet area, SA-508 grade 3 class 1 steel for tubesheet, and air were considered and homogenized.

2) Setting source term

Tables 6 and 7 show the source term for the U-tubes and tubesheet area, and water chamber surface and divider plate. The source term was set as the gamma-ray emission rate by energy for the U-tubes and tubesheet area, and water chamber surface and divider plate, respectively. The source term for the U-tubes and tubesheet area was set to gamma-ray emission rate per unit mass, assuming a homogeneous volumetric radiation source. The source term for the water chamber surface and divider plate was set to gamma-ray emission rate per unit area, assuming a homogeneous surface radiation source. The total radioactivity of the U-tubes and tubesheet area, and water chamber surface and divider plate surface were derived to be 1.02×10¹⁴ Bq and 2.49×10¹² Bq, respectively.

The specific activity by radionuclides utilized to set the source term was based on the APR1400 DCD and SAR. Equation (1) shows the specific activity assessment formula presented in the APR1400 DCD and SAR.

(1)

Ai=∑i∅(1-e-λitres)AcAT

From the above equation, A_i denotes the specific activity of radionuclide i in the CRUD (Bq·g⁻¹), while the activation rate is represented by the sum of ø for all radionuclides (reaction·g⁻¹·s⁻¹). λ_i denotes the decay constant of radionuclide i (s⁻¹). The core residence time is indicated by t_res (s). Additionally, A_c represents the core surface area (cm²), and A_T denotes the total primary system area (cm²).

The core residence time (t_res) of the CRUD in the core is derived by considering the average and maximum activity of the CRUD measured in various reactors and the design parameters of each reactor system. This value was found to be generally two to six times more conservative than the actual residence time of CRUD in the core [19, 20].

The source term used in this study was derived based on the specific activity of each radionuclide in CRUD as presented in the APR1400 DCD and SAR. The specific activity values provided in these documents are design values that do not account for any radioactivity reduction operations. In the case of actual maintenance work in nuclear power plant, radioactivity reduction operations such as decontamination work are performed on the system before maintenance. Therefore, the source term for dose calculation in this study may be conservative, thus resulting in conservative dose estimation.

3. Results of Dose Assessment

The radiation doses for installation and removal of the nozzle dam were assessed for each work scenario. To assess radiation dose to workers, radiation dose rates inside and outside the water chamber were assessed. For the outside water chamber, the dose rates at each work location were assessed when the manway was open. The dose rate assessment results of each work location showed that the radiation dose rates were 8.95×10¹, 1.14×10⁰, and 3.78×10⁻³ mSv·hr⁻¹ for points A, B, and C, respectively.

Fig. 3 shows radiation doses at each work location calculated by MCNP6 and Microshield codes. The dose results from MCNP6 and Microshield codes were generally similar. The dose differences were 12.4%, 5.3%, and 3.5% at location A, B, and C, respectively.

Fig. 4 shows the results of the dose assessment for the installation and removal of the nozzle dam work. The dose assessment of each sub-work showed that the radiation doses were in the range of 2.13–4.51 man·mSv. The highest radiation dose was observed during the nozzle dam removal from the cold leg side among the sub-works. The dose assessment for each worker showed that the radiation doses were in the range of 1.01–2.70 mSv. The average radiation dose to workers was 2.03 mSv. The Nuclear Safety Act sets effective dose limits for radiation workers to 100 mSv in 5 years, with the further provision that the effective dose should not exceed 50 mSv in any single year. On the basis of a maximum of 50 mSv per year, the highest radiation dose to workers was about 5.4% of the annual dose limit for workers. In this study, source terms were set using conservative values of the specific activity to assess radiation dose to workers. Therefore, the radiation dose for the installation and removal of the nozzle dam workers is likely to be conservatively assessed.

Among the sub-works of installation and removal of the nozzle dam, removal and installation work on the cold leg side accounted for 37.1% and 25.7% of the total radiation dose to workers, respectively. The APR1400 steam generator has two cold leg nozzles and one hot leg nozzle. Therefore, the radiation dose was found to be higher during the work on the cold leg side than that on the hot leg side because of the longer work time required for the nozzle dam installation and removal work on the cold leg.

Conclusion

This study assessed the radiation dose to workers installing and removing the steam generator nozzle dam during the overhaul period. First, work scenarios were analyzed for each sub-work. Second, the steam generator was simulated using the MCNP computer code. Third, the radiation dose assessment for the workers was conducted using a simulated steam generator and the analyzed work scenarios.

In the first step, work scenarios for the installation and removal of the nozzle dam work were analyzed. In this study, work scenarios for nozzle dam installation on cold leg and hot leg, nozzle dam removal from cold leg and hot leg were analyzed by work locations and time.

In the second step, the steam generator was simulated using the MCNP computer code. The APR1400 DCD and SAR of Korean APR1400 nuclear power plants were utilized in the simulation. The geometric structure of the steam generator was simulated focusing on the primary side. The source term was set by simulating the CRUD deposited on the U-tubes and tubesheet, and the water chamber surface and divider plate inside the steam generator.

In the third step, the radiation doses to workers were assessed for each work scenario. The assessment results for each sub-work showed that the radiation doses were in the range of 2.13–4.51 man·mSv. The highest radiation dose was observed during the nozzle dam removal from the cold leg side. The assessment results for each worker showed that the radiation doses were in the range of 1.01–2.70 mSv. Among the sub-works, nozzle dam installation work, and removal work on the cold leg side showed relatively higher radiation dose compared to other sub-works. It is judged that the work time of installing and removing the nozzle dam is relatively longer for the cold leg side compared to the hot leg side. The water chamber and the manway of the steam generator, where the nozzle dam installation and removal workers primarily perform their work, showed high radiation dose rates. To minimize radiation dose to the nozzle dam installation and removal workers during the overhaul period, an optimized work plan should be established through the dose assessments. The results of this study can be used for optimizing radiation exposure to workers installing and removing nozzle dam during the overhaul period.

Notes

Conflict of Interest

No potential conflict of interest relevant to this article was reported.

Ethical Statement

This article does not contain any studies with human participants or animals performed by any of the authors.

Author Contribution

Conceptualization: Kim KP. Methodology: Kim KP. Data curation: Kim JY, Kim JW, Lee DH. Formal analysis: Kim KP. Funding acquisition: all authors. Project administration: Kim JY. Investigation: Kim JY, Kim KP. Visualization: Kim JW, Lee DH. Software: Kim JY, Kim KP. Validation: Kim KP. Writing - original draft: Kim JY, Kim KP. Writing - review & editing: Kim KP. Approval of final manuscript: all authors.

Acknowledgements

This work was supported through the National Research Foundation of Korea (NRF) using the financial resource granted by the Ministry of Science and ICT (MSIT) (No. RS-2022-00143994).

References

1. Kim HG. Development of the most appropriate algorithm for estimating personal dose of radiation workers under high exposure rate and non-homogeneous radiation field in nuclear power plants [dissertation]. Chosun University. 2015;(Korean).

2. Kim HG, Kong TY, Jeong WT, Kim ST. An effects of radiation dose assessment for radiation workers and the member of public from main radionuclides at nuclear power plants. J Radiat Prot. 2010;35(1):12-20. (Korean).

3. Kim HG, Kong TY. An analysis of radiation field characteristics for estimating the extremity dose in nuclear power plants. J Radiat Prot. 2009;34(4):176-183. (Korean).

4. Song JS, Yoon TB, Lee SH. A study on corrosion product behavior prediction for domestic PWR primary system by using CRUDTRAN. J Nucl Fuel Cycle Waste Technol. 2015;13(4):253-262. (Korean).

5. Kong TY, Kim SY, Cho M, Jung Y, Son JK, Jang H, et al. A preliminary evaluation of the implementation of a radiation protection program for the lens of the eye in Korean nuclear power plants. Nucl Eng Technol. 2021;53(9):3035-3043.

6. Song CJ, Kong TY, Kim SJ, Son J, Kim H, Kim JW, et al. Classification of radiation work in Korean nuclear power plants. J Radiat Ind. 2023;17(3):239-256.

7. Song CJ, Kong TY, Kim S, Son J, Kim H, Kim J, et al. High-radiation-exposure work in Korean pressurized water reactors. Nucl Eng Technol. 2024;56(5):1874-1879.

8. Korea Institute of Nuclear Safety. Identification of radiation facilities of significant risk and characterization of associated risk parameters. KINS/HR-1000. KINS. 2010;68-79. (Korean).

9. Korea Institute of Nuclear Safety. Analysis of technologies & experiences for reducing occupational radiation dose and study for applying to regulations. KINS/HR-490. KINS. 2003;36-38. (Korean).

10. Bace M, Jecmenica R, Smuc T. Radiological shielding analysis of special building for the old NPP Krsko steam generators. Proceedings of the International conference: Nuclear Option in Countries with Small and Medium Electricity Grids. 2000 Jul 19–22. Duvrovnik, Croatia. 431-438.

11. Zohar A, Snoj L. Gamma dose field due to activated cooling water in a typical PWR. Proceedings of the 26th International Conference Nuclear Energy for New Europe. 2017 Sep 11–14. Bled, Slovenia. 802.

12. Maeng SJ, Kim J, Cho G. Evaluation of eye lens dose to workers in the steam generator at the Korean Optimized Power Reactor 1000. Radiat Prot Dosimetry. 2018;181(4):374-381.

13. Shin SW, Son JK, Cho CH, Song MJ. Analysis of dose rates from steam generators to be replaced from Kori unit 1. J Radiat Prot. 1998;23(3):175-184.

14. Lee CS, Lee JK. Characterization of radiation field in the steam generator water chambers and effective doses to the workers. J Radiat Prot. 1999;24(4):215-223. (Korean).

15. Korea Electric Power Corporation; Korea Hydro & Nuclear Power Co. Ltd. APR1400 design control document tier 2. Chapter 5. Reactor coolant system and connecting systems (APR1400-K-X-FS-14002-NP). KEPCO, KHNP. 2018.

16. Lim YK. Radiation exposure on radiation workers of nuclear power plants in Korea: 2009–2013. J Radiat Prot Res. 2015;40(3):162-167. (Korean).

17. Korea Hydro & Nuclear Power Co. Ltd. Preliminary safety analysis report of Shin Kori Unit 5 & 6. Chapter 11. Radioactive waste management [Internet]. KHNP; 2013 [cited 2024 Dec 11]. Available from: https://npp.khnp.co.kr/board/view.khnp?boardId=BBS_0000026&menuCd=DOM_000000104001002000&orderBy=REGISTER_DATE%20DESC&startPage=3&dataSid=881 (Korean)

18. Korea Hydro & Nuclear Power Co. Ltd. Preliminary safety analysis report of Shin Kori Unit 5 & 6. Chapter 12. Radiation protection [Internet]. KHNP; 2017 [cited 2024 Dec 11]. Available from: https://npp.khnp.co.kr/board/view.khnp?boardId=BBS_0000026&menuCd=DOM_000000104001002000&orderBy=REGISTER_DATE%20DESC&startPage=3&dataSid=882 (Korean)

19. Korea Electric Power Corporation; Korea Hydro & Nuclear Power Co. Ltd. APR1400 design control document tier 2. Chapter 11. Radioactive waste management (APR1400-K-X-FS-14002-NP). KEPCO, KHNP. 2018.

20. Korea Electric Power Corporation, Korea Hydro & Nuclear Power Co. Ltd. APR1400 design control document tier 2. Chapter 12. Radiation protection (APR1400-K-X-FS-14002-NP). KEPCO, KHNP. 2018.

21. Eckerman K, Endo A. Nuclear decay data for dosimetric calculations. ICRP Publication 107. Ann ICRP. 2008;38(3):7-96.

22. International Commission on Radiological Protection. Conversion coefficients for use in radiological protection against external radiation. ICRP Publication 74. Ann ICRP. 1996;26(3–4):1-205.

Fig. 1

Simulated steam generator of APR1400.

Fig. 2

Radiation dose rate assessment points for steam generator.

Fig. 3

Radiation doses rate at each work location calculated by Monte Carlo N-Particle 6 (MCNP6) and Microshield (Grove Software).

Fig. 4

Results of dose assessment for installation and removal of nozzle dam work by (A) sub-works and (B) workers.

Table 1

Work Scenario for Nozzle Dam Installation on the Cold Leg Side

Procedure	Task	Work time (s)	Work location

			Worker 1	Worker 2	Worker 3	Worker 4
Installation of central segment of nozzle dam	Cover the central segment of nozzle dam and diaphragm seal with a canvas wrap	120	C	C	C	C
	Install lightning inside the water chamber	120	B	B	C	C
	Inspect for damage in the nozzle dam installation area	60	B	B	C	C
	Entry into the water chamber	5	B	B	B	C
	Convey the central segment of nozzle dam and diaphragm seal into the water chamber	15	B	B	A	C
	Install the central segment of nozzle dam and diaphragm seal and verify their integrity	45	C	C	A	C
	Exit the water chamber	5	C	C	B	C

Installation of side segments of nozzle dam	Entry of alternate worker into the water chamber	5	B	B	C	B
	Convey the side segments of nozzle dam into the water chamber	10	B	B	C	A
	Install the side segments of nozzle dam and verify their integrity	50	C	C	C	A
	Exit of alternate worker from the water chamber	5	B	B	C	B

Table 2

Work Scenario for Nozzle Dam Installation on the Hot Leg Side

Procedure	Task	Work time (s)	Work location

			Worker 1	Worker 2	Worker 3	Worker 4
Installation of central segments of nozzle dam	Cover the central segment of nozzle dam and diaphragm seal with a canvas wrap	120	C	C	C	C
	Install lightning inside the water chamber	120	C	C	B	B
	Inspect for damage in the nozzle dam installation area	60	C	C	B	B
	Entry into the water chamber	5	B	C	B	B
	Convey the central segment of nozzle dam and diaphragm seal into the water chamber	10	A	C	B	B
	Install the central segment of nozzle dam and diaphragm seal and verify their integrity	30	A	C	C	C
	Exit the water chamber	5	B	C	C	C

Installation of side segments of nozzle dam	Entry of alternate worker into the water chamber	5	C	B	B	B
	Convey the side segments of nozzle dam into the water chamber	5	C	A	B	B
	Install the side segments of nozzle dam and verify their integrity	35	C	A	C	C
	Exit of alternate worker from the water chamber	5	C	B	B	B

Table 3

Work Scenario for Nozzle Dam Removal from the Hot Leg Side

Procedure	Task	Work time (s)	Work location

			Worker 1	Worker 2	Worker 3	Worker 4	Worker 5	Worker 6
Removal of side segments of nozzle dam	Entry into the water chamber	5	B	C	B	B	C	C
	Disassemble bolts	15	A	C	C	C	C	C
	Removal of first side segment of nozzle dam	15	A	C	C	C	C	C
	Convey the removal segment	10	A	C	B	B	C	C
	Exit the water chamber	5	B	C	B	B	C	C

Removal of central segments of nozzle dam	Entry of alternate worker into the water chamber	5	C	B	B	B	C	C
	Removal of second side segment and central segment of nozzle dam	30	C	A	C	C	C	C
	Convey the removal segments	15	C	A	B	B	C	C
	Remove foreign materials and equipment from the water chamber	10	C	A	C	C	C	C
	Exit of alternate worker from the water chamber	5	C	B	B	B	C	C

Table 4

Work Scenario for Nozzle Dam Removal from the Cold Leg Side

Procedure	Task	Work time (s)	Work location

			Worker 1	Worker 2	Worker 3	Worker 4	Worker 5	Worker 6
Removal of nozzle dam on the first cold leg nozzle	Entry into the water chamber	5	C	C	B	C	B	B
	Disassemble bolts	15	C	C	A	C	C	C
	Removal of first side segment of nozzle dam on the first cold leg side	15	C	C	A	C	C	C
	Convey the removal segment	10	C	C	A	C	B	B
	Exit the water chamber	5	C	C	B	C	B	B
	Entry of alternate worker into the water chamber	5	C	C	C	B	B	B
	Removal of second side segment and central segment of nozzle dam on the first cold leg side	30	C	C	C	A	C	C
	Convey the removal segments	15	C	C	C	A	B	B
	Exit of alternate worker from the water chamber	5	C	C	C	B	B	B

Removal of nozzle dam on the second cold leg nozzle	Entry into the water chamber	5	B	B	C	C	B	C
	Disassemble bolts	15	C	C	C	C	A	C
	Removal of first side segment of nozzle dam on the second cold leg side	15	C	C	C	C	A	C
	Convey the removal segment	10	B	B	C	C	A	C
	Exit the water chamber	5	B	B	C	C	B	C
	Entry of alternate worker into the water chamber	5	B	B	C	C	C	B
	Removal of second side segment and central segment of nozzle dam on the second cold leg side	30	C	C	C	C	C	A
	Convey the removal segments	15	B	B	C	C	C	A
	Remove foreign materials and equipment from the water chamber	10	C	C	C	C	C	A
	Exit of alternate worker from the water chamber	5	B	B	C	C	C	B

Table 5

Materials by Components of Steam Generator

Components	Material (density)
Upper shell and head	SA-508 grade 3 class 1 (7.8 g·cm⁻³)
Lower shell and head
Stay cylinder
Economizer plate
Manway

U-tubes	Homogenized material based on NiCrFe Alloy 690 (SB-163) (1.29 g·cm⁻³)

Tubesheet	Homogenized material based on SA-508 grade 3 class 1 (4.48 g·cm⁻³)

Divider plate	SA-240 type 410s (7.732 g·cm⁻³)

Table 6

Source Term for U-Tubes and Tubesheet Area

Nuclides	Specific activity (Bq·g⁻¹)	Energy (MeV)	Gamma-ray yield	Gamma emissions per unit mass^a) (γ·s⁻¹·g⁻¹)
⁵¹Cr	7.31×10⁹	0.320	9.922×10⁻²	4.55×10⁴

⁵⁴Mn	1.99×10⁷	0.835	9.998×10⁻¹	1.25×10³

⁵⁹Fe	4.18×10⁷	0.143	1.020×10⁻²	2.67×10¹
		0.189	9.000×10⁻⁶	2.36×10⁻²
		0.192	3.080×10⁻²	8.07×10¹
		0.335	2.700×10⁻³	7.08×10⁰
		0.382	1.800×10⁻⁴	4.72×10⁻¹
		1.099	5.650×10⁻¹	1.48×10³
		1.292	4.320×10⁻¹	1.13×10³

⁵⁸Co	1.77×10⁹	0.811	9.945×10⁻¹	1.10×10⁵
		0.864	6.900×10⁻³	7.66×10²
		1.675	5.200×10⁻³	5.77×10²

⁶⁰Co	6.22×10⁷	0.347	7.500×10⁻⁵	2.93×10⁻¹
		0.826	7.600×10⁻⁵	2.96×10⁻¹
		1.173	9.985×10⁻¹	3.89×10³
		1.332	9.998×10⁻¹	3.90×10³
		2.159	1.200×10⁻⁵	4.68×10⁻²
		2.506	2.000×10⁻⁸	7.80×10⁻⁵

⁹⁵Zr	4.90×10⁷	0.724	4.427×10⁻¹	1.36×10³
		0.757	5.438×10⁻¹	1.67×10³

^a) Equilibrium thickness for 1.0×10⁻⁴ g·cm⁻².

Table 7

Source Term for Water Chamber Surface and Divider Plate Surface

Nuclides	Specific activity (Bq·g⁻¹)	Energy (MeV)	Gamma-ray yield	Gamma emissions per unit area^a) (γ·s⁻¹·cm⁻²)
⁵¹Cr	7.31×10⁹	0.320	9.922×10⁻²	7.25×10⁵

⁵⁴Mn	1.99×10⁷	0.835	9.998×10⁻¹	1.99×10⁴

⁵⁹Fe	4.18×10⁷	0.143	1.020×10⁻²	4.26×10²
		0.189	9.000×10⁻⁶	3.76×10⁻¹
		0.192	3.080×10⁻²	1.29×10³
		0.335	2.700×10⁻³	1.13×10²
		0.382	1.800×10⁻⁴	7.52×10⁰
		1.099	5.650×10⁻¹	2.36×10⁴
		1.292	4.320×10⁻¹	1.81×10⁴

⁵⁸Co	1.77×10⁹	0.811	9.945×10⁻¹	1.76×10⁶
		0.864	6.900×10⁻³	1.22×10⁴
		1.675	5.200×10⁻³	9.20×10³

⁶⁰Co	6.22×10⁷	0.347	7.500×10⁻⁵	4.67×10⁰
		0.826	7.600×10⁻⁵	4.73×10⁰
		1.173	9.985×10⁻¹	6.21×10⁴
		1.332	9.998×10⁻¹	6.22×10⁴
		2.159	1.200×10⁻⁵	7.46×10⁻¹
		2.506	2.000×10⁻⁸	1.24×10⁻³

⁹⁵Zr	4.90×10⁷	0.724	4.427×10⁻¹	2.17×10⁴
		0.757	5.438×10⁻¹	2.66×10⁴

^a) Equilibrium thickness for 1.0×10⁻³ g·cm⁻².