Evaluation of Worker Radiation Exposure during the Kori Unit 1 Steam Generator Dismantling Process

Kori Unit 1 was permanently shut down on June 18, 2017. Since then, Korea is actively preparing for the decommissioning of the nuclear power plant. Because decommissioning work is performed in a radioactive environment, worker radiation exposure is a signifcant consideration. In this study, worker radiation exposure is evaluated during the steam generator, one of the heavy components of nuclear power plant, dismantling process. A radiation evaluation for the dismantling process is performed using the code RESRAD-BUILD. A steam generator dismantling scenario and optimal cutting method are designed to evaluate worker radiation exposure, considering pipe dimensions, cutting tool speed, and experience in steam generator replacement. Te evaluation results are derived for each work type and year. As a result of the evaluation, worker radiation exposure is 7.5man-mSv at the year of planned decommissioning.


Introduction
In 2023, Korea operates 27 nuclear power plants (NPPs), among which Kori Unit 1 holds historical importance as the nation's frst commercial NPP.Kori Unit 1, a pressurized water reactor, successfully generated electricity for a period of 30 years, commencing from April 19, 1978.Following its approval for continued operation in 2008, Kori Unit 1 continued to operate for an additional 9 years, resulting in a total operational period of 39 years.On June 18, 2017, Kori Unit 1 was permanently shut down.After permanent shut down, Korea is actively preparing for the decommissioning of the nuclear power plant.Te adopted decommissioning strategy in Korea is known as DECON, which involves preparatory period (about 10 years) preceding the actual decommissioning process [1][2][3].
NPPs are comprised of two main systems: the primary system and the secondary system.Te primary system contains components such as the reactor, pressurizer, steam generator (SG), reactor coolant pumps (RCP), reactor coolant system (RCS) pipes, residual heat removal system (RHRS), and chemical and volume control system (CVCS).Because these components operated in a radioactive environment, worker radiation exposure is considered for worker safety when decommissioning an NPP.To predict worker radiation exposure, a radiation evaluation is needed [4].
Tis study evaluates worker radiation exposure during the Kori Unit 1 SG dismantling process, which is difcult in terms of time and cost, as well as a complex process.To evaluate worker radiation exposure, the RESRAD-BUILD code was used.

SG Dismantling
2.1.Kori Unit 1 SG.Kori Unit 1 is a 2-loop system, as shown in Figure 1.Te SG of Kori Unit 1 was replaced in 1998.It is Westinghouse Delta 60 model, which has 4,934 tubes, with a weight of 326 tons and length of 20.654 meters [5,6].

SG Dismantling Scenario.
Te SG dismantling scenario includes the following processes: (i) Te installation process for a temporary crane for moving tools and thermal insulation, as well as a lifting device for removing the steam generator.
(ii) Te installation process for a rail and transportation cart to move the SG and thermal insulation to the exit.(iii) Te installation process for a protection plate for blocking the reactor core.(iv) Because the SG is surrounded by thermal insulation, the process of removal and transportation for the insulation is included.(v) Cutting connections between the steam generator and its associated components and the process required to release the steam generator to the outside.
Te scenario for the SG dismantling is based on the experience obtained from the previous SG replacement in 1998.Te SG dismantling scenario processes are shown in Table 1 [7].

Cutting Method.
To dismantle the steam generator, it is necessary to cut pipes connected between the steam generator and the surrounding components.Te pipes' material is stainless steel or carbon steel.Tese pipes have varying inner diameters, ranging from 0.6 cm to 74 cm, and thickness ranging from 0.15 cm to 7.1 cm.It is classifed as low and intermediate level waste (LILW) or very low level waste (VLLW).Te material, dimension, and contamination of pipe connected to SG are shown in Table 2.
Because the characteristics of the pipes are diferent, each pipe optimal cutting methods are determined diferently considering material, dimension, contamination level, cutting cost, cutting time, and the management of secondary waste.
Te RCS pipe of the SG consists of the hot leg pipe and cold leg pipe.Te hot leg pipe is connected to the reactor core and SG.Te cold leg pipe is connected to the SG and the RCP.Te inner diameter of the hot leg pipe and cold leg pipe are about 70 cm, and they have thickness of about 7 cm.Due to their direct contact with the reactor and RCP, the contamination level of the hot leg and cold leg pipes is expected to be LILW.Terefore, remote control should be employed to minimize worker radiation exposure.Additionally, because the thermal cutting method generates aerosols and dust, it is not available to cut hot leg and colt leg pipes.Based on the above reason, an orbital cutter, one of the mechanical cutting methods, is selected as the cutting tool.Te orbital cutter can cut through stainless steel while minimizing the generation of aerosols and dust.It is available with remote control.Moreover, it facilitates collection of small metal fragments generated during cutting, simplifying secondary waste management [8,9].
Te main steam pipe has an inner diameter of about 66 cm and a thickness of about 3.5 cm.Main feedwater pipe has inner diameter of about 36 cm and thickness of about 3 cm.Considering that these pipes do not have direct contact with the primary system, the contamination level of these pipes is expected to be VLLW.Tus, workers can work directly.As the pipes are made of carbon steel and expected as VLLW, both mechanical and thermal cutting methods are feasible options.Considering the cost and working time, the oxygen cutting method, which is a type of thermal cutting, was selected to cut these pipes.Additionally, the main steam pipe will be cut into two parts because interference may occur when lifting the steam generator [10,11].
For the drain pipe, the intake and outlet pipes, wet well vent pipe, level/fow measurement pipe, and sample collection pipes, the inner diameter ranges from about 0.5 cm to 4 cm, with a thickness of about 0.1 cm-0.5 cm.Tese pipes are relatively small.Considering that these pipes do not have direct contact with primary system, contamination level of these pipes is expected to be VLLW.Tus, workers can work directly.Considering pipe size, cost, and work preparation time, a circular saw cutting method, which is a type of mechanical cutting, was selected to cut these pipes [10,12].

Working Time.
Te working time is determined based on the experience from replacement of the steam generator in 1998 and the pipe cutting time considering cutting tool speed, pipe dimensions, and work difculty factor.

Cutting Speed.
Each cutting speed is based the applicable equipment manual and technical documents.Cutting speeds are shown in Table 3 [12].In the case of the orbital cutter, which cuts the pipe while rotating, the cutting time is calculated by dividing the pipe thickness by the tool speed.Additionally, for the use of the orbital cutter, the worker requires time for tool installation and removal.Installation and removal time assumed to be 1 hour.When pipe is cut using the orbital cutter, chips are generated and must be periodically removed.Chip removal time is assumed to be 10 minutes per 1 hour of cutting.
Te cutting time for oxygen fuel cutting is calculated by dividing the circumference of the pipe by the tool speed.Work preparation time for oxygen fuel cutting is established at 30 minutes.
Te cutting time for the circular saw is calculated by dividing the outer diameter of the pipe by the tool speed.Te cutting preparation time for circular saw cutting is set at 10 minutes.  5. Tere are four work difculty factors.Te frst is an as low as reasonably achievable (ALARA) factor.Because the cutting process is performed in radioactive area, an ALARA factor is considered.Te ALARA factor can have a value of from 10% to 15%.Te second is an accessibility factor.In case of cutting process for the main steam pipe, main feedwater pipe, drain pipe, intake and outlet pipe, wet well vent pipe, level/fow measurement pipe, and sample collection pipes, work is performed on scafolding and ladders.Te limited degree of motion possible under these working conditions reduces worker productivity.Terefore, an accessibility factor is considered.Te accessibility factor can have a value of from 10% to 20%.Te third is a protective clothing factor.Because the cutting process is performed in radioactive area, workers must wear protective clothing.Te protective clothing factor can have a value of from 10% to 30%.Te fourth is a workbreak factor.Because worker needs rest periods, a work break factor is considered.Te work break factor can have

SG Dismantling Working Time.
Te working time is determined based on the experience obtained during the SG replacement in 1998 and the previously calculated pipe cutting time.Among the various work involved, the installation of temporary SG lifting devices takes the most time, followed by cutting of the hot leg and cold leg pipe.SG dismantling working times are shown in Table 6.Te exposure pathways for RESRAD-BUILD are shown in Figure 2. Te exposure pathways are as follows: Te frst pathway is external exposure to penetrating radiation emitted directly from the source.Te second pathway is internal exposure through the inhalation of aerosol indoor radon decay products and tritiated water vapor.Te third pathway is internal exposure through inhalation of airborne radioactive particulates.Te fourth pathway is external exposure to penetrating radiation due to submersion in airborne radioactive particulates.Te ffth pathway is external exposure to penetrating radiation emitted from radioactive particulates deposited on the foors of the    Te individual efective dose is calculated as follows: For an external or internal exposure, the absorbed doses of diferent organs and tissues are estimated.Because the same absorbed dose from diferent types of radiation of diferent energy have diferent biological efects, the absorbed dose is multiplied by the quality factor to obtain the dose equivalent for each organ and tissue.Te dose equivalent of each organ and tissue is multiplied by the organ and tissue weighting factor, and the weighted dose equivalents of diferent organs and tissue are added to obtain the efective dose equivalent for the whole body.For an internal exposure, the dose equivalent or efective dose equivalent is integrated over a period of time after the exposure to account for the retention of radionuclides in the body and the radiation continuously emitted by the radionuclides.Te integrated dose equivalent and the efective dose equivalent are called the committed dose equivalent and the committed efective dose equivalent, respectively.Finally, the efective dose equivalent associated with external exposure and the committed efective dose equivalent associated with internal exposure are added to obtain an estimate of the total efective dose a receptor would incur.

Radiation Exposure Evaluation
Building parameters, time parameters, receptor parameters shielding parameters, and source parameters are required to evaluate the worker radiation exposure using RESRAD-BUILD [13,14].

Room Parameter.
To evaluate the worker radiation exposure, the room is designed where the steam generator and hot leg pipe and cold leg pipe are located.Te height of the room is determined based on the diference in elevation between the bottom support structure of the steam generator and the main steam pipe.A room size of 63 m 2 is assumed, considering the dimensions of the steam generator.Te height of the room is set at 27 m, considering the height of the main steam pipe and lower support structure.Te ventilation rate of the room is assumed to be the same as the condition of the Kori Unit 1 SG replacement.During replacement the SG, a 500 cfm temporary ventilation facility was installed [7].

Time Parameter.
Te time parameter is set from 2017, the year of permanent shutdown, to 2037 with intervals of 5 years.

Receptor Parameter.
Te receptor parameter is set in consideration of the worker position, breathing rate and ingestion rate.Te position of the worker is set for each work.Te breathing rate is set 1.2 m 3 /h for 8 hours working day, the standard breathing rate specifed in ICRP 60.Te ingestion rate is set to 0.0001 m 2 /h which is the RESRAD-BUILD default value [7,13,15].

Source Term Parameter.
Since there are no source term data for the SG of Kori unit 1 after replacement, the source term data for the SG of Kori Unit 1 before replacement are used for the evaluation.Te steam generator before replacement operated for about 20 years, and the steam generator after replacement operated for about 19 years.Tus, operating period is only one year diferent.Te source term data for the Kori Unit 1 SG before replacement are shown in Table 7 [7].
Te source term data for the RCS pipes are measured by the smear test method at the time of replacement of the steam generator of Kori Unit 1. From a conservative point of view, the high radioactive value source term data between SG A and B are applied.Te RCS pipe source term data for the Kori unit 1 SG before replacement are shown in Table 8 [16].

Results
Te results of worker radiation exposure for each work type are shown in Table 9. Te resulting worker radiation exposure during the SG dismantling process in 2027 is 7.53 man-mSv.Among each work type, the installation of temporary the lifting device for the SG, which required the longest work time, resulted in the highest worker radiation exposure.Additionally, the hot leg and cold leg pipe are at a higher contamination level than another pipe.So, despite assuming remote cutting for the hot leg and cold leg pipe, a higher worker radiation exposure is found compared to other work types.
Te results of the annual total worker radiation exposure evaluation are shown in Table 10.Worker radiation exposure is 25.7 man-mSv in 2017, the year of shutdown.After fve years of permanent shutdown in 2022, a reduced radiation exposure of 14.6 man-mSv.Compared to 2017, there is a diference of 11.1 man-mSv.In 2027, the year of planned decommissioning, a reduction of 8.34-times is found.Compared to 2022, there is a diference of 7.1 man-mSv.Over time, the worker radiation exposure decreases.
Te segregated radiation exposure doses in 2027 are shown in Table 11.Most radiation exposure is external exposure, especially external exposure directly from source.It is evaluated that internal exposure rarely occurred.

Conclusion
Decommissioning of NPP takes place in a radioactive area.Terefore, it is necessary to predict the worker radiation exposure in advance and lower worker radiation exposure if necessary for the worker safety.Terefore, worker radiation exposure over time and process is estimated using RESRAD-BUILD code in this study.
Tere was a limitation in obtaining the source term data of SG and RCS pipe in performing this study.Because the source term data are private.So, source term data, measured when replacement of the steam generator, are used.Tese source term data are diferent from the actual data in terms of model and 1 year operation period diferences.However, through the experience data of the steam generator, it was evaluated based on specifc data such as the steam generator dismantling process, manpower, and time.
To evaluate worker radiation exposure, a scenario for SG dismantling, cutting methods, and relevant factors is considered.Te results indicate that the total worker radiation exposure for the entire process is about 7.53 man-mSv in 2027, 10 years after permanent shutdown.During the cutting process of the hot leg and cold leg pipes, it is evaluated that a signifcant amount of worker radiation exposure occurs compared to other process.Terefore, it is necessary to take this into consideration for worker safety during the cutting process of the hot leg and cold leg pipes.In the planned decommissioning year of 2027, it is evaluated that the worker radiation exposure is about 3.42 times lower than the exposure at the point of permanent shutdown, resulting to 7.5 man-mSv.Over time, amount of worker radiation exposure reduction declines.Terefore, considering economic aspects such as the maintenance cost of a permanently shut down nuclear power plant, it is deemed appropriate to determine a certain point in time when worker radiation exposure reaches manageable level and proceed with decommissioning.Trough this study, it was possible to predict worker radiation exposure during dismantling of the SG.
Furthermore, studies may be performed for the evaluation of worker radiation exposure during another large component to expect worker radiation exposure during the total process of NPP decommissioning.For the improvement of the reliability, a study will be performed that evaluates and compares using diferent codes.

2. 4 . 3 .
Work Difculty Factor.During actual work, various difculties can occur.Tus, work difculty factor is considered in the calculation of the working time.Work diffculty factors are shown in Table
SG: steam generator.RCS: reactor coolant system. 2 Science and Technology of Nuclear Installations 2.4.2.Cutting Time.Te cutting time for each pipe is calculated by considering the dimensions of the pipe and the speed of the cutting tool.Te cutting time of each pipe is shown in Table 4.

Table 2 :
Te material, dimension, and contamination of the pipe connected to the SG.: steam generator.RCS: reactor coolant system.LILW: low and intermediate level waste.VLLW: very low level waste. SG

Table 4 :
Pipe connected to SG cutting time.
ALARA: as low as reasonably achievable.

Table 6 :
SG dismantling working time.

Table 7 :
SG source term data before replacement.

Table 8 :
Hot leg and cold leg pipe of SG source term data before replacement.

Table 9 :
Worker radiation exposure during SG dismantling in 2027.

Table 10 :
Annual total worker radiation exposure.