The evaluation of skyshine distribution, release of airborne radioactive nuclides, and soil activation and groundwater migration were required for radiological assessment of the impact on the environment surrounding In-Flight (IF)-system facility of the RAON (Rare isotope Accelerator complex for ON-line experiment) accelerator complex.
Monte Carlo simulation by MCNPX code was used for evaluation of skyshine and activation analysis for air and soil. The concentration model was applied in the estimation of the groundwater migration of radionuclides in soil.
The skyshine dose rates at 1 km from the facility were evaluated as 1.62×10−3 μSv·hr−1. The annual releases of 3H and 14C were calculated as 9.62×10−5 mg and 1.19×10−1 mg, respectively. The concentrations of 3H and 22Na in drinking water were estimated as 1.22×10−1 Bq·cm−3 and 8.25×10−3 Bq·cm−3, respectively.
Radiological assessment of environmental impact on the IF-facility of RAON was performed through evaluation of skyshine dose distribution, evaluation of annual emission of long-lived radionuclides in the air and estimation of soil activation and groundwater migration of radionuclides. As a result, much lower exposure than the limit value for the public, 1 mSv·yr−1, is expected during operation of the IF-facility.
A heavy-ion accelerator complex, named Rare isotope Accelerator complex for ON-line experiment (RAON), is under a construction in Korea for use in basic science research and various applications [
Radiation facilities are generally designed with sufficiently thick shield to attenuate the dose rate to less than the regulation limit. Because high energy particles are produced from the accelerator, very thick shield walls are installed in the accelerator facility. For instance, an IF-system using a uranium beam at 400 MeV/u requires an approximately 10 m-thick concrete wall to obtain a dose rate less than the public exposure limit, 1 mSv·yr −1, outside of the facility [
During operations, the air inside of an accelerator facility is activated by irradiation of energetic particles, mainly neutrons, which are then discharged to outside the facility. The concentration of radioactive nuclides in the air is controlled by cooling, delaying, and filtering, and must be kept to less than the release limit as set by domestic regulations. The IF-system uses ventilation to reduce the release of airborne radioactive nuclides to less than that limit [
The soil of the facility site is also activated. Energetic particles, mainly neutrons, penetrate the basement concrete floor and are distributed in the soil region. Then, radioactive nuclides are produced by the neutrons distributed in the soil. Most radioactive nuclides produced in soil remain in place where it was produced, but some mobile nuclides, such as 3H and 22Na, move into groundwater [
In this study, evaluation of skyshine distribution, releases of airborne radioactive nuclides, soil activation and groundwater migration were performed as radiological assessment of the impact on the environment surrounding the IF-system facility of the RAON complex.
The facility model and source terms from the previous study [
MCNPX version 2.7 code [
The facility model and source terms from the previous study [
Although total annual operation time of the IF-system is 2,000 hours, 200 hours of maximum continuous operation time is applied in the activation calculation for air. The IF-system facility of RAON uses a ventilation system to circulate air inside and decrease the concentration of induced activities in air. However it was assumed in the activation calculation, conservatively, that the ventilation system had not been installed.
MCNPX version 2.7 code and the ENDF/B-VI.8 nuclear data library were used in the neutron transport calculation, the FISPACT-2010 code [
The IF-system facility is built on a rock bed of granite and no groundwater flows through the facility site. However, it was conservatively assumed that groundwater flowed under the facility. The ground region, filled with soil, was described in the calculation model as shown in
MCNPX version 2.7, ENDF/B-VI.8, FISPACT-2010 patched with spallation reaction function, and EAF-2010 were used in the activation calculation for soil under the IF-system facility. For the evaluation of the groundwater migration of radioactive nuclides, the concentration model was applied [
The boundary of the soil activation calculation region was determined using the “99% volume” method [
Naturally percolating groundwater becomes contaminated by leaching out of radioactivity, as it migrates through the soil to the underlying aquifer. 3H, 7Be, 22Na, 45Ca, and 54Mn are known as dominant radioactive nuclides produced in soil. Of these only 3H and 22Na may significantly impact groundwater resources, because of their long half-lives and leachabilities [
where,
Granite and weathered soil including 30% water in volumetric fraction were applied and the leaching factors were estimated at 0.9 and 0.7 for 3H and 22Na, respectively. The leaching factor from Malensek study [
where,
As can be seen in
Results of estimation of radionuclides migration in ground water were summarized in
With the weight fraction of water (30%) and leaching factors, initial concentrations of 3H and 22Na, in aquifer of soil were estimated as 3.06×10−3 Bq·cm−3 and 1.37×10−3 Bq·cm−3. The reduction factors during flow through groundwater were 0.4 and 0.006 for 3H and 22Na, respectively. The final concentrations of 3H and 22Na, in drinking water were obtained as 1.22×10−1 Bq·cm−3 and 8.25×10−3 Bq·cm−3. The calculated final concentrations were compared to the recommended the World Health Organization (WHO) guidance levels [
Radiological assessment of environmental impact of IF-facility of RAON was performed through evaluation of skyshine dose distribution, evaluation of annual emission of long-lived radionuclides in the air and estimation of soil activation and groundwater migration of radionuclides. As a result, much lower exposure than the regulation limit for the public, 1 mSv·yr −1, is expected during operation of the IF-facility. Skyshine dose was evaluated as 1.62×10−3 μSv·hr −1 at 1 km. Annual releases of 3H and 14C were estimated as 9.62× 10−5 mg and 1.19×10−1 mg, respectively. The concentrations of 3H and 22Na in drinking water were conservatively estimated as 1.22×10−1 Bq·cm−3 and 8.25×10−3 Bq·cm−3, these values are lower than WHO recommendations by a factor of 10.
No potential conflict of interest relevant to this article was reported.
Formal analysis: Lee CW. Methodology: Han MH, Whang WT. Writing-original draft, Lee CW. Software: Lee CW. Writing-review & editing: Kim EH, Lee S, Jeong S, Jeong HS.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (Ministry of Science and ICT) (No. NRF-2017M2A8A4015251).
(A) Horizontal section view and (B) vertical section view of the skyshine calculation model for the IF-system facility of the RAON (IF-target is at the center of the concentric circles). IF, in-flight; RAON, Rare isotope Accelerator complex for ON-line experiment.
Internal configuration of the IF-system of the RAON and calculation regions for the air activation. IF, in-flight; MCNP, Monte Carlo N-Particle code; RAON, Rare isotope Accelerator complex for ON-line experiment.
(A) Depth directional view and (B) radial directional view of the soil activation calculation model for the IF-system facility of the RAON (Rare isotope Accelerator complex for ON-line experiment). IF, in-flight.
Reaction ratio to total reaction rate in (A) depth direction and (B) radial direction for soil region under the IF-system facility of the RAON. IF, in-flight; RAON, Rare isotope Accelerator complex for ON-line experiment.
Distribution of the dose rate inside the IF-system facility. IF, in-flight.
Distribution of the skyshine dose rate against to the distance from the IF-target. IF, in-flight.
Activities of radionuclide produced in air in the pre-separator room of IF-target facility. IF, in-flight.
Chemical Composition of Soil at RAON and Nuclear Reactions Producing Dominant Radionuclides
Element | Weight |
Reaction | Daughter nuclide |
---|---|---|---|
O | 62.5 | (n, t) | 3H |
(n, 2α+2n) | 7Be | ||
| |||
Si | 27.2 | (n, x) | 3H, 7Be |
(n, α+2n+p) | 22Na | ||
| |||
Al | 3.9 | (n, α+2n) | 22Na |
| |||
K | 1.6 | - | - |
| |||
Fe | 0.8 | - | - |
| |||
Mg | 1.75 | - | - |
| |||
Na | 1.4 | (n, 2n) | 22Na |
(n, γ) | 22Na | ||
| |||
Ca | 0.7 | (n, γ) | 22Na |
RAON, Rare isotope Accelerator complex for ON-line experiment.
From Lee et al. [
Concentration and Release of the Long Half-Lived Radioactive Nuclides, 3H and 14C, in the IF-System Facility of the RAON
Area | Concentration of induced activity (Bq·cm−3) | Activity (Bq) | ||
---|---|---|---|---|
|
| |||
3H | 14C | 3H | 14C | |
High energy beam transport room | 1.92×10−4 | 4.57×10−5 | 1.44×106 | 3.43×105 |
| ||||
Pre-separator room | 1.63×10−4 | 1.31×10−4 | 2.00×106 | 1.61×106 |
| ||||
Main separator room | 7.59×10−8 | 1.72×10−7 | 4.44×103 | 1.01×104 |
| ||||
Total release during an year | ||||
Activity (Bq) | - | - | 3.44×107 | 1.96×107 |
Mass (mg) | - | - | 9.62×10−5 | 1.19×10−1 |
IF, in-flight; RAON, Rare isotope Accelerator complex for ON-line experiments.
Summary of Radionuclide’s Concentration in Soil and Groundwater by the Concentration Model
Radionuclide | Activity in soil (Bq·cm−3) | Weight fraction of water in soil | Leaching factor | Initial concentration, |
Reduction factor | Final concentration, |
---|---|---|---|---|---|---|
3H | 1.02×10−1 | 0.3 | 0.9 | 3.06×10−1 | 0.4 | 1.22×10−1 |
7Be | 3.04×10−2 | 0.3 | 0 | - | - | - |
22Na | 5.89×10−2 | 0.3 | 0.7 | 1.37×10−1 | 0.06 | 8.25×10−3 |
24Na | 2.03×102 | 0.3 | 0 | - | - | - |
45Ca | 5.08×10−1 | 0.3 | 0 | - | - | - |
54Mn | 9.50×10−2 | 0.3 | 0 | - | - | - |
55Fe | 5.74×1 | 0.3 | 0 | - | - | - |