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ISSN : 1738-1894(Print)
ISSN : 2288-5471(Online)

Journal of Nuclear Fuel Cycle and Waste Technology Vol.22 No.4 pp.465-476
DOI : https://doi.org/10.7733/jnfcwt.2024.036

Sensitivity Analysis for Factor of the On-link Public Assessment Model in Radioactive Waste Overland Transportation

Ga Eun Oh

, Min Woo Kwak

, Dong Gyu Lee

, Kwang Pyo Kim*

Kyung Hee University, 1732, Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Republic of Korea

^* Corresponding Author.
Kwang Pyo Kim, Kyung Hee University, E-mail: kpkim@khu.ac.kr, Tel: +82-31-201-2560

Received August 8, 2024 ; Revised September 5, 2024 ; Approved September 23, 2024

Abstract

The increase in radioactive waste increased the demand for transportation to the disposal facility. Prior to transporting radioactive waste, confirming that the potential exposure is insignificant is crucial. Overland transportation risk assessment models were developed tailored to domestic characteristics. Dose assessment using this model requires selecting appropriate factors. However, users may struggle to derive appropriate values, leading to inaccuracies. Additionally, if assessment results show outliers, prioritizing factors for review can be challenging. Therefore, sensitivity analysis is necessary to prioritize factors for accurate assessment. In this study, sensitivity analysis was conducted on the on-link public risk assessment model factors for radioactive waste overland transportation. Initially, assessment models were analyzed by each detailed exposure scenario. Subsequently, uncertainty propagation-based sensitivity analysis methodology was applied. The default values for the assessment model factors were set, and sensitivity analysis was conducted based on road type for maximum individual and collective dose assessment models. For the maximum individual dose model, the distance to the samedirection vehicle was the most sensitive, whereas for the collective dose model, vehicle velocity was the most sensitive. The results of this study can be used as the basic data on radioactive waste transportation risk assessment in Korea in the future.

Key Words : Radioactive waste , Overland transportation , Sensitivity analysis , Assessment model factor , On-link public

초록

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1. Introduction

The growing volume of low and intermediate radioactive waste, spent fuel, and decommissioned waste is increasing the demand for transporting to the disposal facility [1-4]. When transporting such radioactive waste, it is necessary to confirm in advance that the possible exposure is not significant as it may occur depending on the transport link. For this purpose, Korea currently uses the overseas assessment code within the country. However, due to difficulties in reflecting the domestic characteristics, an overland transportation risk assessment model was developed reflecting these characteristics in preliminary research. When calculating the radiation dose using the developed assessment model, the result has variations with the uncertainty of the factor.

The International Atomic Energy Agency and Sandia National Laboratories performed the sensitivity analysis of the assessment model factors based on the uncertainty propagation equation for the international transport risk assessment codes INTERTRAN and RADTRAN, respectively [5-7]. Kamboj et al. performed a sensitivity analysis using the normalized dose difference method on the factors in RESRAD, a code used for assessing the radiation dose for the public from site contamination [8, 9]. Mill et al. and Reardon et al. conducted a factor sensitivity analysis of the RADTRAN’s transport accident condition assessment model for the factor samples extracted using the Latin hypercube sampling method and the Monte Carlo sampling method, respectively [10, 11].

In the case of overland transportation risk assessment models developed in previous studies, there are various factors that affect the dose assessment results because many scenarios were considered. As a result, users may have difficulty in deriving all appropriate factor values for a given transport situation and may assign factor values that are not appropriate for the transport situation. Furthermore, if the dose assessment results show outliers, prioritizing factors that should be reviewed first can be challenging. Therefore, for the user to derive a dose assessment value with a certain level of accuracy by substituting factors and to efficiently review the dose assessment results, it is necessary to perform a sensitivity analysis to derive a priority for factors.

The objective of this study is to perform a sensitivity analysis on the risk assessment model factors of the radioactive waste overland transportation incident-free condition on-link public to derive a priority for the factors. For this purpose, this study analyzed the on-link public risk assessment models of overland transportation in incident-free conditions developed in previous studies. Furthermore, an analysis methodology was established for the model factor sensitivity analysis and factor defaults were set based on assumed transport situations. Based on the finalized methodology, a sensitivity analysis was performed to derive the priority of the assessment model factor.

2. Materials and Methods

In this study, an uncertainty propagation equation–based sensitivity analysis methodology was used to conduct a sensitivity analysis of the overland transportation incident-free condition on-link public dose assessment models. To this end, the incident-free condition on-link public exposure scenario was analyzed by categorizing the assessment model into three detailed exposure scenarios: 1) same-direction vehicle occupants, 2) opposite-direction vehicle occupants, and 3) overtaking vehicle occupants. Further, the factor sensitivity analysis was performed based on the uncertainty propagation equation. To perform this analysis, we set default values for the factors by assessment model.

2.1 Analysis of the Radiation Dose Assessment Model for Each Detailed Exposure Scenario

The on-link public indicates the public inside a vehicle that shares the link traveled by a radioactive waste transportation vehicle. The overland transportation risk assessment model for the on-link public analyzed in this study was developed to more realistically reflect the domestic transportation situation than the existing overseas assessment models. For example, the exposure scenarios were improved by considering the exposure to the occupants of the overtaking vehicle separately, whereas the existing models do not. Fig. 1 shows a detailed exposure scenario for the on-link public. Exposure scenarios for the on-link public can be categorized into same-direction vehicle occupants, opposite-direction vehicle occupants, and overtaking vehicle occupants. Additionally, the individual and collective dose assessment models were analyzed for each detailed exposure scenario.

Fig. 1

Schematic diagram of exposure scenarios for on-link public.

2.1.1 Analysis of the radiation dose assessment mode for same-direction vehicle occupants

Same-direction vehicle occupants refer to those traveling in vehicles in the same lane as the transportation vehicle. The assessment model was developed under the assumption that the velocity of the same-direction vehicle is identical to that of the transportation vehicle. The maximum individual dose assessment model for the same-direction vehicle occupants sharing a transportation vehicle link is as follows:

\begin{matrix} d_{o n, S} = 2 \cdot D R_{v} \cdot k_{0} \cdot C S F \cdot \frac{D I S T}{V} \\ \times (F G \cdot \frac{e^{- μ G r} B_{G} (r)}{x_{s a m e}^{2}} + F N \cdot \frac{e^{- μ N r} B_{N} (r)}{x_{s a m e}^{2}}) \end{matrix}

(1)

- d_on,S: maximum individual dose of the same direction vehicle occupant (mSv)
- DR_v: 1 m dose rate from transportation vehicle (mSv-hr⁻¹)
- k₀: point source shape factor (m²)
- CSF: vehicle shielding factor (−)
- DIST: transportation link distance (km)
- V: transportation vehicle velocity (km-hr⁻¹)
- FS: fraction of dose rate from radiation S (−) (FG: gamma radiation fraction, FN: neutron radiation fraction)
- e^−μSr: degree of attenuation during the radiation S moves by distance r within the medium (−)
- B_S(r): buildup factor during the radiation S moves by distance r within the medium (−)
- x_same: minimum distance from the same direction adjacent vehicle (m)

Shape factor k₀ is derived from the critical dimension which is the factor determined by the longest value of the packaging or vehicle.

When assessing the collective dose for same-direction vehicle occupants, consideration should be given to the number of overtaking vehicles in the transport link and number of occupants in those vehicles. Therefore, the collective dose assessment model for the same-direction vehicle occupants is as follows:

\begin{array}{l} D_{o n, S} = 2 \cdot Q \cdot D R_{v} \cdot k_{0} \cdot C S F \cdot (1 - N T) \cdot \frac{N \cdot P P V \cdot D I S T}{V^{2}} \\ \times (F G \cdot \int_{x_{s a m e}}^{\infty} \frac{e^{- μ G r} B_{G} (r)}{r \sqrt{r^{2} - x^{2}}} d r + F N \cdot \int_{x_{s a m e}}^{\infty} \frac{e^{- μ N r} B_{N} (r)}{r \sqrt{r^{2} - x^{2}}} d r) \end{array}

(2)

- D_on,S: the collective dose of the same direction vehicle occupants (person-Sv)
- Q: unit conversion factor (10⁻⁶ Sv-km-mSv⁻¹ m⁻¹)
- DR_v: 1 m dose rate from transportation vehicle (mSv-hr⁻¹)
- k₀: point source shape factor (m²)
- CSF: vehicle shielding factor (−)
- NT: overtaking vehicle ratio (−)
- N: vehicle traffic (vehicle∙hr⁻¹)
- PPV: number of adjacent vehicle occupants (person-vehicle⁻¹)
- DIST: transportation link distance (km)
- V: transportation vehicle velocity (km-hr⁻¹)
- FS: fraction of dose rate from radiation S (−) (FG: gamma radiation fraction, FN: neutron radiation fraction)
- e^−μSr: degree of attenuation during the radiation S moves by distance r within the medium (−)
- B_S(r): buildup factor during the radiation S moves by distance r within the medium (−)
- x_same: minimum distance from the same direction adjacent vehicle (m)

2.1.2 Analysis of the radiation dose assessment model for opposite-direction vehicle occupants

The opposite-direction vehicle occupants are those traveling in vehicles in the direction opposite to the transportation vehicle in the transport link. It was assumed that the velocity of the opposite-direction vehicle is the same as that of the transportation vehicle and that the transportation vehicle and opposite-direction vehicle, which has started from a very far away location, pass through the shortest distance point and then move away from each other again. As a result, the maximum individual dose assessment model for the opposite-direction vehicle occupants is as follows:

\begin{matrix} d_{o n, O} = Q \cdot \frac{D R_{v} \cdot k_{0} \cdot C S F}{V} \times (F G \cdot \int_{x_{a p p}}^{\infty} \frac{e^{- μ G r} B_{G} (r)}{r \sqrt{r^{2} - x_{o p p}^{2}}} d r \\ + F N \cdot \int_{x_{a p p}}^{\infty} \frac{e^{- μ N r} B_{N} (r)}{r \sqrt{r^{2} - x_{o p p}^{2}}} d r) \end{matrix}

(3)

- d_on,O: maximum individual dose of opposite direction vehicle occupant (mSv)
- Q: unit conversion factor (10⁻³ km-m⁻¹)
- DR_v: 1 m dose rate from transportation vehicle (mSv-hr⁻¹)
- k₀: point source shape factor (m²)
- CSF: vehicle shielding factor (−)
- V: transportation vehicle velocity (km-hr⁻¹)
- FS: fraction of dose rate from radiation S (−) (FG: gamma radiation fraction, FN: neutron radiation fraction)
- e^−μSr: degree of attenuation during the radiation S moves by distance r within the medium (−)
- B_S(r): buildup factor during the radiation S moves by distance r within the medium (−)
- x_opp: minimum distance from opposite direction adjacent vehicle (m)

The assessment of the collective dose for opposite-direction vehicle occupants requires consideration of traffic in the transport link and the number of occupants in those vehicles. Therefore, the collective dose assessment model for the opposite-direction vehicle occupants is as follows:

\begin{matrix} D_{o n, O} = 2 \cdot Q \cdot D R_{v} \cdot k_{0} \cdot C S F \\ \cdot \frac{N \cdot P P V \cdot D I S T}{V^{2}} \times (F G \cdot \int_{x_{a p p}}^{\infty} \frac{e^{- μ G r} B_{G} (r)}{r \sqrt{r^{2} - x_{o p p}^{2}}} d r \\ + F N \cdot \int_{x_{a p p}}^{\infty} \frac{e^{- μ N r} B_{N} (r)}{r \sqrt{r^{2} - x_{o p p}^{2}}} d r) \end{matrix}

(4)

- D_on,O: the collective dose of opposite direction vehicle occupants (person-Sv)
- Q: unit conversion factor (10⁻⁶ Sv-km-mSv⁻¹ m⁻¹)
- DR_v: 1 m dose rate from transportation vehicle (mSv-hr⁻¹)
- k₀: point source shape factor (m²)
- CSF: vehicle shielding factor (−)
- N: vehicle traffic (vehicle∙hr⁻¹)
- PPV: number of adjacent vehicle occupants (person-vehicle⁻¹)
- DIST: transportation link distance (km)
- V: transportation vehicle velocity (km-hr⁻¹)
- FS: fraction of dose rate from radiation S (−) (FG: gamma radiation fraction, FN: neutron radiation fraction)
- e^−μSr: degree of attenuation during the radiation S moves by distance r within the medium (−)
- B_S(r): buildup factor during the radiation S moves by distance r within the medium (−)
- x_opp: minimum distance from opposite direction adjacent vehicle (m)

2.1.3 Analysis of the radiation dose assessment model for overtaking vehicle occupants

Overtaking vehicle occupants are those traveling in vehicles in the transport link in the same direction as the transportation vehicle but at a higher velocity. Therefore, the relative velocity of the overtaking vehicle to the transportation vehicle was considered when developing the dose assessment model for occupants in an overtaking vehicle. The maximum individual dose assessment model for the overtaking vehicle occupants sharing the transport link is as follows:

\begin{matrix} d_{o n, O v} = 2 \cdot Q \cdot \frac{D R_{v} \cdot k_{0} \cdot C S F}{V^{'}} \times (F G \cdot \int_{x_{o v e r}}^{\infty} \frac{e^{- μ G r} B_{G} (r)}{r \sqrt{r^{2} - x_{o v e r}^{2}}} d r \\ + F N \cdot \int_{x_{o v e r}}^{\infty} \frac{e^{- μ N r} B_{N} (r)}{r \sqrt{r^{2} - x_{o v e r}^{2}}} d r) \end{matrix}

(5)

- d_on,Ov: maximum individual dose of overtaking vehicle occupant (mSv)
- Q: unit conversion factor (10⁻³ km-m⁻¹)
- DR_v: 1 m dose rate from transportation vehicle (mSv-hr⁻¹)
- k₀: point source shape factor (m²)
- CSF: vehicle shielding factor (−)
- V': relative velocity of overtaking vehicle (km-hr⁻¹)
- FS: fraction of dose rate from radiation S (−) (FG: gamma radiation fraction, FN: neutron radiation fraction)
- e^−μSr: degree of attenuation during the radiation S moves by distance r within the medium (−)
- B_S(r): buildup factor during the radiation S moves by distance r within the medium (−)
- x_over: minimum distance from overtaking vehicle (m)

The assessment of the collective dose for overtaking vehicle occupants should consider the number of overtaking vehicles in the transport link and the number of occupants in those vehicles. Therefore, the formula for calculating the collective dose for the overtaking vehicle occupants is as follows:

\begin{matrix} D_{o n, O v} = 2 \cdot Q \cdot D R_{v} \cdot k_{0} \cdot C S F \cdot N T \\ \cdot \frac{N \cdot P P V \cdot D I S T}{V \cdot V^{'}} \times (F G \cdot \int_{x_{o v e r}}^{\infty} \frac{e^{- μ G r} B_{G} (r)}{r \sqrt{r^{2} - x_{o v e r}^{2}}} d r \\ + F N \cdot \int_{x_{o v e r}}^{\infty} \frac{e^{- μ N r} B_{N} (r)}{r \sqrt{r^{2} - x_{o v e r}^{2}}} d r) \end{matrix}

(6)

- D_on,Ov: the collective dose of overtaking vehicle occupants (person-Sv)
- Q: unit conversion factor (10⁻³ Sv-mSv⁻¹)
- DR_v: 1 m dose rate from transportation vehicle (mSv-hr⁻¹)
- k₀: point source shape factor (m²)
- CSF: vehicle shielding factor (−)
- NT: overtaking vehicle ratio (−)
- N: vehicle traffic (vehicle∙hr⁻¹)
- PPV: number of adjacent vehicle occupants (person-vehicle⁻¹)
- DIST: transportation link distance (km)
- V: transportation vehicle velocity (km-hr⁻¹)
- V': relative velocity of overtaking vehicle (km-hr⁻¹)
- FS: fraction of dose rate from radiation S (−) (FG: gamma radiation fraction, FN: neutron radiation fraction)
- e^−μSr: degree of attenuation during the radiation S moves by distance r within the medium (−)
- B_S(r): buildup factor during the radiation S moves by distance r within the medium (−)
- x_over: minimum distance from overtaking vehicle (m)

2.2 Uncertainty Propagation–Based Sensitivity Analysis Methodology

Uncertainty propagation means that when a physical quantity is obtained indirectly by calculating it from a measurement, the uncertainty in the measurement affects the physical quantity to be obtained [12]. The following equation shows the relation between the uncertainty in the calculated value and the measured value:

\in_{y}^{2} = \sum_{i = 1}^{n} {(\frac{\partial f}{\partial x_{i}})}^{2} \cdot \in_{x_{i}}^{2}

(7)

- ϵ_y: uncertainty of physical quantity y calculated by factor x
- $\frac{\partial f}{\partial x_{i}}$ : variation of function f for factor x
- $\in_{x_{i}}$ : uncertainty of factor x_i

Defining importance as the extent to which the radiation dose changes owing to a change in the value of a particular factor, the importance of a factor x for a detailed scenario-specific radiation dose assessment model d based on Equation 7 is as follows [5]:

I_{c} = \in_{x} \cdot x_{c} \cdot \sum \frac{\partial d_{x c}}{\partial x_{c}}

(8)

- I_c: Importance of factor x
- ϵ_x: uncertainty of factor x
- x_c: value of factor x
- $\sum \frac{\partial d_{c}}{\partial x_{c}}$ : sum of the variation for factor x_c of detailed exposure dose asses model d

In this study, we used Equation 8 to calculate the importance of the factors of the radiation dose assessment model for 1% uncertainty in the factors.

2.3 Setting Default Values of Factors for Sensitivity Analysis

Default values of factors must be set for the factor sensitivity analysis of the assessment model. For this purpose, we set the default values of factors by assuming the transport situation of transporting low and intermediate level radioactive waste in IP-2 type containers for 10 km with two containers per vehicle. The link distance default value was referred to as the standard interval of interchanges at the major industrial areas around the urban area and was set for the conservative assessment [13]. Among the factors of the assessment model, the vehicle velocity and distance from the transportation vehicle may vary depending on the road type due to Korean laws and regulations; therefore, sensitivity analysis was performed by setting defaults for each road type. The factors and default values used in the radiation dose assessment model for the on-link public are shown in Table 1. The 1 m dose rate was set as the default radiation dose rate for 1 m of the surface of the transport container specified in the regulations on technical standards for radiation safety control, etc. [14]. Fig. 2 shows the specifications of transportation vehicle and IP-2 type container. We set the critical dimension of the vehicle calculated from the Fig. 2 as the default value [15, 16]. The distance from the transportation vehicle for each detailed exposure scenario was set by considering the width of the lane and median strip, as specified in the criteria for structure and facility of the road [17]. The velocities of the transport and overtaking vehicles were set to the minimum and maximum design velocity provided by the applicable laws and regulations for each road type [17, 18]. However, in the case of a highway, the velocity of the transportation vehicle was set to the velocity limit of the cargo vehicle as its velocity limit is lower than the design velocity of the road. In terms of the traffic, for conservative assessment, we set the 95th percentile of the traffic by the road type provided by the Ministry of Land, Infrastructure and Transport as the default value [19].

Table 1

The default value of input factor for on-link public by road type

Input factor	Highway	General road	Regional road

Same dir. vehicle dist.^a,b	80	35	25
Opp. dir. vehicle dist.^c,d	5.5	3.75	3.5
Overtake vehicle dist.^e,f	3.5	3.25	3
Overtake vehicle relative velocity^e,f	30	30	20
Vehicle velocity^a,b,c,d,f	80	50	40
Traffic^b,d,f	6,964.83	1,700.38	1,244.42
1 m dose rate		0.1
Gamma-ray friction		1
Neutron friction		0
Critical dimension		7.38
Link distance^a,b,d,f		10
Adjacent vehicle occupants^b,d,f		4
Vehicle shielding factor		1
Overtake vehicle ratio^f		0.5

a: For the same direction vehicle occupant individual exposure scenarios
b: For the same direction vehicle occupant collective exposure scenarios
c: For opposite direction vehicle occupant individual exposure scenarios
d: For opposite direction vehicle occupant collective exposure scenarios
e: For overtaking vehicle occupant individual exposure scenarios
f: For overtaking vehicle occupant collective exposure scenarios

Fig. 2

Specification of transportation vehicle and packaging in supposed situation.

3. Results and Discussion

In this study, an uncertainty propagation–based sensitivity analysis methodology was used to conduct the factor sensitivity analysis for factor of the on-link public radiation dose assessment models in overland transportation developed in previous studies. For this purpose, the assessment model factors were selected based on the road type. Accordingly, the maximum individual dose assessment model and collective dose assessment model factor sensitivity analyses were performed for each road type.

3.1 Sensitivity Analysis Results of the Maximum Individual Dose Assessment Model

The results of sensitivity analysis for the on-link public maximum individual dose assessment model by the road type are shown in Fig. 3. With varying degrees of importance, the sensitivity analysis shows that the distance to the same-direction vehicles is the most sensitive factor among those considered in the same-direction vehicle occupant scenario on all roads. The same-direction vehicle occupant is exposed from the transportation vehicle throughout the transport path. Therefore, in the assessment model, the exposure time was considered based on the link distance and transportation vehicle velocity, unlike the assessment model for other scenarios. Accordingly, the same-direction vehicle occupant scenario has the greatest impact on the on-link public scenario. Among the factors considered, the distance to the same-direction vehicle is the most dominant factor in the assessment model. It is therefore the most sensitive in the factor sensitivity analysis for the overall onlink public.

Fig. 3

Result of input factor sensitivity for on-link public (Maximum individual dose).

In the sensitivity analysis, the least sensitive factor is found to be the distance to the opposite-direction vehicle, which is considered in the opposite-direction vehicle occupant scenario. For occupants in the opposite direction and overtaking vehicles, the exposure time was not considered as the exposure is only for an instant during which they pass the transportation vehicle. Furthermore, the oppositedirection occupant has a longer exposure distance than the overtaking vehicle occupant. As a result, the sensitivity analysis for the transport on-link public has the smallest impact on the factor sensitivity analysis for the opposite-direction vehicle occupant scenario, indicating that the factor considered only in the opposite-direction vehicle occupant scenario is the least sensitive.

The importance values for the factors and variations in their importance with the road type change are shown in Table 2. As the road type changes from highway to general roads and regional roads, the factor importance of the overtaking vehicle’s relative velocity, overtaking vehicle’s distance, and distance to the opposite-direction vehicle reduces. Conversely, the factor importance of the distance to the same-direction vehicle, transportation vehicle velocity, and link distance increases dramatically on general roads and regional roads compared with highways. The difference between the decreasing and increasing importance of the factors is whether or not they are considered in the same-vehicle occupant scenario. As the road type changes, the distance to the transportation vehicle decreases under all scenarios, increasing in the radiation dose. However, among the factors, the distance to the same-direction vehicle factor decreases the most when the road type changes, leading to an increase in the radiation dose for that scenario. As a result, the impact of the same-direction vehicle occupant exposure scenario becomes dominant in the sensitivity analysis, this reduces the sensitivity of the factors considered only in other scenarios.

Table 2

Importance value and amount of change of importance for on-link public individual by road type

Input factor	Highway	General road	Regional road

Same dir. vehicle dist.	1.08×10⁻²	1.78×10⁻² (+7.00×10⁻⁴)	1.85×10⁻² (+7.00×10⁻⁴)
Vehicle velocity	5.88×10⁻³	9.12×10⁻³ (+3.24×10⁻³)	9.39×10⁻³ (+2.70×10⁻⁴)
Link distance	5.38×10⁻³	8.89×10⁻³ (+3.24×10⁻³)	9.26×10⁻³ (+3.70×10⁻⁴)
Overtake vehicle relative velocity	4.12×10⁻³	8.78×10⁻⁴ (−3.24×10⁻³)	6.06×10⁻⁴ (−2.72×10⁻⁴)
Overtake vehicle dist.	4.12×10⁻³	8.78×10⁻⁴ (−3.24×10⁻³)	6.06×10⁻⁴ (−2.72×10⁻⁴)
Opp. dir. vehicle dist.	4.92×10⁻⁴	2.28×10⁻⁴ (−2.46×10⁻⁴)	1.30×10⁻⁴ (−9.80×10⁻⁵)

3.2 Sensitivity Analysis Results of the Collective Dose Assessment Model

Fig. 4 shows the results of the collective dose assessment model factor sensitivity analysis for the on-link public by road type. The sensitivity analysis indicates that the vehicle velocity factor, which is common to all scenarios on all roads, is the most sensitive, even though its importance value varies. Unlike the maximum individual dose assessment model, the collective dose assessment model considers the velocity of the transportation vehicle under all exposure scenarios. In particular, it is the dominant factor in the same- and opposite-vehicle occupant scenarios, which is likely why it appears to be the most sensitive in the sensitivity analysis.

Fig. 4

Result of input factor sensitivity for on-link public (Collective dose).

The least sensitive factor is the distance to the same-direction vehicle considered in the same-direction vehicle occupant scenario. In the maximum individual dose assessment, the occupant in the same-direction vehicle has a longer exposure time, resulting in a larger dose assessment result, which greatly influences the sensitivity analysis. However, in the collective dose assessment, the radiation dose is calculated for the entire transport link, therefore, the exposure time remains the same for the three exposure scenarios. As a result, the scenario’s impact becomes the smallest in the sensitivity analysis due to the farthest exposure distance for the same-direction vehicle occupants. Finally, the distance to the same-direction vehicle factor, which is only considered in the same-direction vehicle occupant scenario, has the smallest effect on the overall dose. Therefore, this factor appears to be the least sensitive in the sensitivity analysis of the collective dose assessment model factors for the on-link public.

The importance values of the factors and variations in their importance with the road type change are shown in Table 3. As the road type changes from highway to general roads, the factor importance of the transportation vehicle velocity and distance to the opposite-direction vehicle increases temporarily. Conversely, the factor importance of the overtaking vehicle’s relative velocity, distance to the overtaking vehicle, and overtaking vehicle’s ratio decreases. This is due to the significant reduction in the transportation vehicle velocity compared with other factors when comparing the factor default values for highways and general roads. Because the transportation vehicle velocity is a more dominant factor in the other two scenarios compared with the overtaking vehicle occupant scenario, changes in this factor result in larger increase in the dose assessment results for the two scenarios. As a result, it seems that the impact of the assessment models of two scenarios in the sensitivity analysis is higher than that of the highway, which temporarily increases the sensitivity values of the factors considered in the scenarios.

Table 3

Importance value and amount of change of importance for on-link public group by road type

Input factor	Highway	General road	Regional road

Vehicle velocity	1.33×10⁻²	1.52×10⁻² (+1.90×10⁻³)	1.48×10⁻² (−4.00×10⁻⁴)
Opp. dir. vehicle dist.	3.20×10⁻³	4.96×10⁻³ (+1.76×10⁻³)	4.47×10⁻³ (−4.90×10⁻⁴)
Same dir. vehicle dist.	1.10×10⁻⁴	2.66×10⁻⁴ (+1.56×10⁻⁴)	3.13×10⁻⁴ (+4.70×10⁻⁵)
Overtake vehicle relative velocity	6.69×10⁻³	4.77×10⁻³ (−1.92×10⁻³)	5.22×10⁻³ (+4.50×10⁻⁴)
Overtake vehicle dist.	6.69×10⁻³	4.77×10⁻³ (−1.92×10⁻³)	5.22×10⁻³ (+4.50×10⁻⁴)
Overtaking vehicle ratio	6.59×10⁻³	4.51×10⁻³ (−2.08×10⁻³)	4.90×10⁻³ (+3.90×10⁻⁴)

4. Conclusion

The objective of this study is to perform a sensitivity analysis on the risk assessment model factors of the radioactive waste overland transportation incident-free condition on-link public to derive a priority for the factor importance. For this purpose, this study first analyzed the incident-free condition on-link public risk assessment model among the overland transportation assessment models developed in previous studies. We established a methodology for the sensitivity analysis of the assessment model factors and set default values of factors. Subsequently, the sensitivity analysis of the risk assessment model factors for the on-link public under the incident-free conditions of the overland transportation of radioactive waste was performed for the maximum individual dose assessment model and collective dose assessment model for each road type.

The result of the sensitivity analysis for the maximum individual dose assessment model factors showed that the distance to the same-direction vehicle was the most sensitive factor, while the distance to the opposite-direction vehicle was the least sensitive factor. This is likely due to differences in the dose assessment result values for the detailed exposure scenarios and the impact of various factors in those scenarios. As the road type changed, the sensitivity of the factors considered in other scenarios, such as the relative velocity of the overtaking vehicle, decreased as the impact of the same-direction vehicle occupant exposure scenario became dominant due to the change in default values.

The collective dose assessment model factor sensitivity analysis showed that vehicle velocity was the most sensitive factor in all cases. Unlike the maximum individual dose model, the distance to the same-direction vehicle was the least sensitive factor in this case. This is because the collective dose calculation assesses the collective dose for the same exposure time, resulting in the smallest dose for the same-direction vehicle occupants, which are at the farthest distance. As the road type changed from highway to general roads, the impact of the same- and opposite-direction vehicle occupant exposure scenarios increased with the default value of factor change. As such, the sensitivity of the factors considered in both scenarios increased.

The results of this study can be utilized as input and in the review of factor data for dose assessments to the onlink public under the incident-free conditions of the overland transportation of radioactive waste. In addition, the results can be used as a reference for evaluating the risk of the overland transportation of radioactive waste in Korea in the future. While this study can be applied in the aforementioned aspects, it is limited to the evaluation model for overland transportation. In Korea, maritime transportation for low and intermediate level radioactive waste is also conducted. Accordingly, risk assessment models in maritime transportation have also been developed in our preliminary study. Sensitivity study of input factors for maritime transportation assessment models is planned for the next study.

Acknowledgements

This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (RS-2024-00349003, Support for on-site practical training linked to public enterprises).

This work was supported by the Nuclear Safety Research Program through the Korea Foundation of Nuclear Safety (KoFONS) using the financial resource granted by the Nuclear Safety and Security Commission (NSSC) of the Republic of Korea (RS-2021-KN059910).

Conflict of Interest

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

Figures

Fig. 1.

Schematic diagram of exposure scenarios for on-link public.

Fig. 2.