1. Introduction

On contrary to the operational waste, which tend to have a consistent generation rate and limited variety in the waste types, the radioactive wastes generated during the decommissioning of the NPP, such as facilities and structures within the NPP, demolition and decontamination equipment, work clothes, and miscellaneous goods used for a wide range of decommissioning works, have a wide variety of waste types, sizes, and radiation characteristics (clearance to intermediate level). Furthermore, these decommissioning wastes tend to be generated in large quantities within a short period of time [1].

Due to such differences between operational and decommissioning wastes, 200 L or 320 L drums, and IP-2 type transport containers may be inefficient or impossible to be used for packaging and transporting decommissioning wastes. Hence, through the study of “Development of Waste Package, Transport, and Disposal Containers for Decommissioning Wastes of Nuclear Power Plant”, decommissioning wastes are categorized according to waste type, size, weight, and radiation characteristics, then containers, which consider characteristics of each waste group, are under development.

These containers are designed to be of larger sizes and higher design weights compared to the 200 L, and IP-2 type transport containers. Therefore, current transported system (handling equipment, transport vehicles, and transport vessels), which are tailored to the transport of the operational waste, is required to be changed or redesigned considering the characteristics of the container under development. Likewise, transport strategy, such as transport schedule, frequency, and route, should be established according to the generation characteristics of decommissioning wastes.

Meanwhile, in the process of transporting the radioactive waste, the public and the workers are likely to be exposed to the radiation emitted from the radioactive materials. If radioactive materials are released out of the package due to accidents, such as collisions, rollovers, and fire, more people may be affected by radiation in the broader environment. Therefore, when considering the transport of radioactive materials, it is necessary to evaluate the transport safety of the radioactive materials in advance, considering of the type of radioactive material, the transport route, and the transport environment.

So far, the safety evaluation of radioactive waste transport has focused mainly on the transport of operational waste. However, for the decommissioning waste, the safety evaluation is only under the conditions when on-road transport and existing transport containers are considered, excluding the characteristics and the transport strategies of the decommissioning wastes. The decommissioning wastes have relatively high activity levels compared to operational wastes and are transported in a large amount at once using the developed containers. Furthermore, both on-road and sea routes may be considered according to the transport strategy for decommissioning wastes. In this study, such conditions, which were previously excluded, are reflected in the safety evaluation by considering various types of waste, new containers, transport routes of both land and sea, etc.

The transport route is from the Kori NPP to the Gyeongju disposal facility, and the evaluation targets are the transport workers, the handling (loading and unloading of radioactive material package) workers, and the public near the transport route. For the evaluation, the RADTRAN 6 code, which is the most widely used code for transport risk assessment, was used.

Then, the evaluation result expressed as exposure dose and risk on the workers and public during the transport of the radioactive material is compared with the domestic regulatory limit to confirm the transport safety of the decommissioning wastes. Additionally, the sensitivity analysis was performed to evaluate the change in exposure dose according to the change in the release rate of radioactive materials after the accident.

2. Assessment Input Data and Assumption

The input parameters required for the transport risk assessment consist of seven basic items and detailed items as shown in Tables 1 and 2 [2].

Table 1

Assessment input parameter (normal)

Table 2

Assessment input parameter (accident)

2.1 On-Road Transport

On-road transport route is through which radioactive waste is transported by vehicle to national roads and highways.

2.1.1 Characteristics of the Transport Container and Package

In this study, transport risk on 7 types of packages including different contents, shown in Table 3, were evaluated. Realistically, the surface dose rate of the package would be lower than the transport regulation radiation level limit of 2 mSv·h⁻¹ [3], depending on the activity concentration in the package. However, for conservative evaluation, the surface dose rate of all packages is assumed to be 2 mSv·h⁻¹. In addition, the radiation dose rate (0.658 mSv·h⁻¹) at a point 1 m away from the package was calculated using the microshield computer code. Since there was no nuclide that emits neutrons in the wastes to be transported, the neutron fraction was set to 0%.

Table 3

Package specification

2.1.2 Transport Route

The input parameters related to the transport route are distance, vehicle speed, population density, and accident rate of each link that is a route segment. The transport route was set as shown in Table 4, considering the transport time, population density, and accident possibility to minimize the exposure dose. The transport link was divided into 16 sections according to administrative districts based on eup, myeon, and dong. The population and area of each section were calculated using Statistical Geographic Information Service (SGIS) [4] and website information for each administrative district. The total transport distance is 77.2 km (14.2 km on national roads and 63 km on highways), and the speed of the vehicle was assumed to be 60 km·h⁻¹ on highways and 30 km·h⁻¹ on national roads. The accident rate was calculated by dividing the number of accidents by the vehicle traffic and distance according to the calculation formula [5]. The information on accidents and vehicle traffic was obtained from the Traffic Accident Analysis System (TAAS) [6] and the Korean Statistical Information Service (KOSIS), respectively [7, 8].

Table 4

Transport route (on-road transport)

2.1.3 Transport Vehicle

Each transport vehicle is loaded with only one package, therefore the dose at 1m from the vehicle and gamma/neutron fraction was set to be the same value as the package. The number of vehicle occupants was set to two (the driver and radiation safety manager). The vehicle shielding factor was applied with a value (0.03) corresponding to the dose limit of 0.02 mSv·h⁻¹ at the occupant position [9].

2.1.4 Stop

If a transport vehicle stops for any reasons (eg. Crew change, passenger transfer, crew meals, refueling, storage, and inspection, etc.), then persons at or near the stop point can be exposed to external radiation from the shipment [10]. In this evaluation, it was expected that there will be no stopping during transport due to the short transport distance, but for a conservative evaluation, only one stop during a transport is assumed. The stop time was set as 1 hour, and the stop location was set to Nam-gu, Mugeo-dong, which had the highest population density among the transport links. In addition, the range of the exposed target was assumed to be the public within a radius of 800 m (default) [2].

2.1.5 Handling

The handling workers are likely to be heavily affected by radiation because they handle a container with radioactive materials at adjacent distances. In this study, it is assumed that 6 workers are involved in loading/unloading per vehicle for 1 hour at 1 m from the package.

2.1.6 Accident Condition

The fraction of radionuclides in the package that could be released in an accident, was assumed to be 0.1% as suggested in NUREG/CR-4370 [11]. For a conservative assessment, it was assumed that all the radionuclides released into the atmosphere due to an accident are aerosolized and the respirable fraction value is 1.0. The deposition velocity of all nuclides except gaseous nuclides (³H, ¹⁴C) was set to 0.01 m·s⁻¹, the recommended value of code [12]. Weather conditions are classified from strong instability A grade to strong stability F grade according to the atmospheric stability grade. The Pasquill Category D section (ambient lapse rate is the same as the adiabatic lapse rate) with the fastest wind speed at 10 m ground was applied to the evaluation [13].

2.1.7 Radionuclide Inventory

Table 5 shows the radionuclide inventory date for each package used in the evaluation [14].

Table 5

Radionuclide inventory

2.2 Sea Transport

Sea transport is a route from the Kori NPP lighter’s wharf to the Wolsung NPP lighters wharf (hereinafter referred to as wharf) using an exclusive ship. On-road transports from the waste storage facility to the Kori wharf, and from the Wolsung wharf to the disposal facility were not evaluated as the transports are carried out within the boundaries of the exclusive area of Kori NPP, and the distance is short.

2.2.1 Characteristics of the Transport Container and Package

In this study, the number of packages loaded per shipment was assumed as shown in Table 6 and five types (A to E) of transport risk assessments were performed.

Table 6

Package loaded per shipment

2.2.2 Transport Route

As shown in Table 7, the sea transport distance from the Kori wharf to the Wolsung wharf is about 73.2 km and links were classified into three sections. The average speed of the shipment is about 22.2 km·h⁻¹, and the accident rate was referred to in the data presented in the 2020 Marine Accident Statistical Data published by the Ministry of Oceans and Fisheries [15]. The accident rate was calculated as 2.55×10⁻³ and applied equally regardless of the link section.

Table 7

Transport route (sea transport)

2.2.3 Transport Vehicle

The transport ship “Cheong Jeong Nuri” is used to transport radioactive waste in Korea, which is a 2,600-ton ship with a length of 78 m and a width of 15 m. The ship’s passengers are a total of 20 persons that consist of 17 crew members, 1 transport manager, and 2 radiation safety managers.

The radiation dose in the work area is limited to 0.0075 mSv·h⁻¹ according to the technical standards [16]. Therefore, the shielding factor of the transport ship was applied with a value (0.84) corresponding to reducing the surface dose of 2 mSv·h⁻¹ of the package to 0.0075 mSv·h⁻¹ at the work position.

2.2.4 Handling

In sea transport, the distance between the package and the worker was set to 10 m because a crane and a lifting device are used for loading and unloading works. It was also assumed that 10 workers and 10 hours of working time are required.

2.2.5 Accident Condition

If a marine accident caused by a collision with other ships or obstacles, severe weather, and ship failure occurs, the normal operation of the ship becomes impossible. In this evaluation, it was assumed that radioactive materials are released outside of the ship after the ship is stranded in the near port due to collision or inability to operate. Other accident-related input data is the same as on-road transport accident conditions.

3. Transport Scenario

Transport scenarios can be divided into a normal transport condition and an accident transport condition, depending on whether radioactive materials are released or not.

3.1 Normal Condition

Normal transport (incident-free) is defined as “transport during which no accident, packaging, or handling abnormality or malevolent attack occurs” and radioactive materials leakage does not occur during routine, incident-free transport and during accidents when there is no breach of containment. Therefore, in normal conditions, the contents of the radioactive materials package do not matter; only the external dose is important [10].

The evaluation considered only external exposure caused due to the radiation emitted by radioactive materials loaded inside the container. Evaluation targets were the public residing on the transport route, the persons in vehicles sharing the transport link, the transport workers (operators and safety managers), and the handling workers. The exposure range was set to a radius of 800 m from the transport vehicle.

3.2 Accident Condition

Accident transport condition is in which radioactive materials are leaked out of the container and spread around the accident locations. The evaluation targets were the public living within 10 km from the point of accident and the five exposure pathways (inhalation, ground shine, cloud shine, ingestion, and resuspension) were considered as sources of internal and external exposures to the public during an accident [17].

3.3 Assessment Criteria

The assessment criteria for results are the dose limits for the public, the radiation workers, and the persons engaging in transport. The dose limit means the upper limit of the amount of radiation exposed which is the sum of the external and internal doses, and each level is suggested in the enforcement decree of the nuclear safety act [18].

As shown in Table 8, the cumulative radiation level limits are 100 mSv for five years within the scope of not exceeding 50 mSv·y⁻¹ for the radiation worker, 6 mSv·y⁻¹ for the transport worker, and 1 mSv·y⁻¹ for the public. For a conservative evaluation, the assessment criterion of the radiation worker is set as 20 mSv·y⁻¹ which is the annual average for the five-year.

Table 8

Dose limit

4. Results

4.1 On-Road Transport

4.1.1 Normal Condition

Tables 9 and 10 show the evaluation results for the collective exposure dose and the individual exposure dose for the on-road transport under normal conditions, respectively. According to the evaluation results, the collective exposure dose of the transport worker, the handling worker, and the public is 0.25 mSv to 0.35 mSv, 11.7 mSv to 14.7 mSv, and 4.12×10⁻² mSv to 3.67 mSv, respectively. Also, the individual exposure dose is 0.123 mSv to 0.178 mSv, 1.95 mSv to 2.45 mSv, and 5.07×10⁻⁷ mSv to 1.91×10⁻⁴ mSv, respectively. The results are between 1.69×10⁻⁷% to 9.23×10⁻⁵% of the national standard, and all the results satisfies the requirement that the exposure dose evaluated should be below the legal dose limit.

Table 9

On-Road-Normal collective dose (mSv)

Table 10

On-Road-Normal individual dose (mSv)

4.1.2 Accident Condition

Tables 11 and 12 show the evaluation results for the collective dose risk and the individual dose risk of the onroad transport under accident conditions, respectively. According to the evaluation results, the collective dose risk was evaluated as 1.77×10⁻¹¹ mSv to 4.53×10⁻⁸ mSv, and the individual dose risk was evaluated as 9.58×10⁻¹⁷ mSv to 4.38×10⁻¹³ mSv. Among the transport links, the highest risk appeared in section 16, because the accident rate is higher than in other sections. The results, which all met the does limit, are evaluated to be between 9.58×10⁻¹⁵% to 4.38×10⁻¹¹% of the national standard.

Table 11

On-Road-Accident collective dose (mSv)

Table 12

On-Road-Accident individual dose (mSv)

4.2 Sea Transport

4.2.1 Normal Condition

Tables 13 and 14 show the evaluation results for the collective exposure dose and the individual exposure dose on the sea transport under normal conditions, respectively. According to the evaluation results, the collective exposure doses for the transport worker, the handling worker, and the public are 5.94×10⁻³ mSv, 5.86 mSv to 20.3 mSv, and 5.81×10⁻³ mSv, respectively. Also, the individual exposure doses are 2.97×10⁻³ mSv, 0.586 mSv to 2.03 mSv, and 9.75×10⁻⁶ mSv, respectively. All the evaluation results satisfied the legal dose limit.

Table 13

Sea-Normal collective dose (mSv)

Table 14

Sea-Normal individual dose (mSv)

The reasons why the dose values of the transport worker and the public were equal in all shipments (A to E) are as follows:

In addition, the exposure doses about the on-link and the stop were not evaluated because it was assumed there are no other ships moving adjacent to the transport route, and the transport ship does not stop during sea transport.

4.2.2 Accident Condition

Tables 15 and 16 show the evaluation results for the collective dose risk and the individual dose risk of the sea transport under accident conditions, respectively. According to the evaluation results, the collective dose risk was evaluated as 3.96×10⁻⁵ mSv to 4.77 mSv and the individual dose risk was evaluated as 7.64×10⁻¹⁰ mSv to 5.05×10⁻⁷ mSv. The results are between 7.64×10⁻⁸% to 5.05×10⁻⁵% of the national standard, thus satisfying the dose limit.

Table 15

Sea-Accident collective dose (mSv)

Table 16

Sea-Accident collective dose (mSv)

5. Sensitivity Analysis

Sensitivity analysis was performed to confirm the amount of change in exposure dose according to the increasing ratio of radioactive materials released out of the packages from 0.1% to 100% under accident conditions. The analysis for the on-road transport was performed in section 16 where the highest exposure dose was evaluated among the transport links. The analysis results are shown in Fig. 1 and the highest individual dose risk among the results is 4.38×10⁻¹⁰ mSv, which is the case when releasing 100% of the contents from the T2-P2B package.

Fig. 1

Sensitivity analysis results-On-Road-Transport in Link 16 (mSv).

In the case of sea transport, sensitivity analysis was performed for section 2, and the results are shown in Fig. 2. As a result of the evaluation, the individual dose risk was the highest at 5.05×10⁻⁴ mSv, which is the case when releasing 100% of the contents from the package of shipment of B.

Fig. 2

Sensitivity analysis results-Sea-Transport in Link 2 (mSv).

Through the sensitivity analysis, it was confirmed that the dose risk increased proportionally with the amount of release of radioactive materials, and even though the contents were released 100%, the evaluation result was still significantly lower than the legal limit.

6. Conclusion

In this study, the risk assessment of the transport of the decommissioning wastes of Kori Unit 1 to disposal facilities was evaluated. The transport scenarios were divided into a normal condition and an accident condition, and the transport route was divided into on-road transport and sea transport. Also, the evaluation result expressed as dose and risk value on the public and the workers was compared with the domestic regulatory limit to confirm the safety of the transport.

As a result of the on-road transport, the exposure dose of the transport workers, the public, and the handling workers in the normal transport condition was evaluated as 2.97%, 7.94×10⁻⁵%, and 12.25% of the legal dose limit, respectively. Also, the dose risk of the public in the accident transport condition was evaluated as 4.38×10⁻¹¹% of the legal dose limit.

As a result of the sea transport, the exposure dose of the transport workers, the public, and the handling workers in the normal transport condition was evaluated as 4.95×10⁻²%, 9.75×10⁻⁴%, and 10.15% of the legal dose limit, respectively, and the dose risk of the public in the accident transport condition was evaluated as 5.05×10⁻⁵% of the legal limit.

Additionally, sensitivity analysis was performed to confirm the amount of change in exposure dose according to the increasing ratio of radioactive materials released out from the packages from 0.1% to 100% under accident conditions. As a result of the evaluation, when radioactive materials were released 100% during the on-road transport and the sea transport, the exposure dose was evaluated at 4.38×10⁻⁸% and 5.05×10⁻²% of the dose limit, respectively.

In summary, all the evaluation results satisfied the requirement, which states that the dose of the public and the workers received during the transport of radioactive materials should not exceed the legal dose limit. However, unlike the other results, the exposure dose of the handling workers was evaluated to be relatively high, and it may exceed the dose limit depending on the working distance and the time. Therefore, it is necessary to prepare appropriate handling systems and a working environment, which can lower the exposure dose of the workers.