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Takai, Shizuka; Shimada, Taro; Takeda, Seiji; Koike, Katsuaki*
Journal of Contaminant Hydrology, 251, p.104097_1 - 104097_12, 2022/12
Times Cited Count：3 Percentile：46.08(Environmental Sciences)When assessing the risk from an underground environment that is contaminated by radioactive nuclides and hazardous chemicals and planning for remediation, the contaminant plume distribution and the associated uncertainty from measured data should be estimated accurately. While the release history of the contaminant plume may be unknown, the extent of the plume caused by a known source and the associated uncertainty can be calculated inversely from the concentration data using a geostatistical method that accounts for the temporal correlation of its release history and groundwater flow modeling. However, the preceding geostatistical approaches have three drawbacks: (1) no applications of the three-dimensional plume estimation in real situations, (2) no constraints for the estimation of the plume distribution, which can yield negative concentration and large uncertainties, and (3) few applications to actual cases with multiple contaminants. To address these problems, the non-negativity constraint using Gibbs sampling was incorporated into the geostatistical method with groundwater flow modeling for contaminant plume estimation. This method was then tested on groundwater contamination in the Gloucester landfill in Ontario, Canada. The method was applied to three water soluble organic contaminants: 1,4-dioxane, tetrahydrofuran, and diethyl ether. The effectiveness of the proposed method was verified by the general agreement of the calculated plume distributions of the three contaminants with concentration data from 66 points in 1982 (linear correlation coefficient of about 0.7). In particular, the reproduced large spill of organic contaminants of 1,4-dioxane in 1978 was more accurate than the result of preceding minimum relative entropy-based studies. The same peak also appeared in the tetrahydrofuran and diethyl ether distributions approximately within the range of the retardation factor derived from the fraction of organic carbon.
Hanaki, Shotaro; Mashimo, Ryutaro; Nanri, Tomohiro; Hayashi, Hirokazu
no journal, ,
The assessment of contamination distribution evaluates the type, radioactivity and distribution of radioactive materials remaining in the facility for the purpose of reducing the exposure of radiation workers and the surrounding public, formulating dismantling and removal methods and procedures, and evaluating the amount of radioactive waste generated during dismantling and removal work. In Monju, some structural materials around the reactor and primary sodium have been activated, and radioactive materials remain as secondary contamination on the inner surfaces of equipment and piping in the primary cooling system, although the total amount of radioactive materials is small. This presentation provides an overall overview of the assessment and the results of the assessment carried out in the first phase.
Hanaki, Shotaro; Kinoshita, Takuma*; Kishimoto, Yasufumi*; Hayashi, Hirokazu
no journal, ,
The decommissioning of the "Monju" facility, a 30-year process, is divided into 4 phases. In Phase 1, spent fuel is transferred to a storage pool, while Phase 2 involves dismantling uncontaminated areas. Phase 3 focuses on dismantling sodium equipment. Ongoing evaluations in Phase 1 and 2 aim to identify radioactive materials in the facility, reduce worker and public exposure, and establish demolition methods. The assessment is categorized into activation contamination from structural material activation and secondary contamination from corrosion products produced by leaching. This summary provides an overview of the contamination distribution assessment and the activation contamination assessment for fuel handling equipment specific to sodium-cooled fast reactors (SFRs).
Kinoshita, Takuma*; Kishimoto, Yasufumi*; Hanaki, Shotaro; Hayashi, Hirokazu
no journal, ,
Dismantling in radiation-controlled area is planned in phase 3 of the "Monju" decommissioning program, but an assessment of contamination of the facilities is essential before starting the dismantling. The under-floor transfer car for fuel handling unique to "Monju" transports, undergoes gas replacement and preheats fuel assemblies. Due to neutrons generated from fuel, the structural materials are activated. The radioactivity of the car was assessed using the same methodology as the reactor's evaluation. The neutron flux distribution and activation levels were evaluated using a 2D RZ system, and a 3D analysis was conducted for validation. Additionally, it was confirmed that the activity of activation products is at most at L3 levels, and in most areas, it is below the clearance levels.
Kishimoto, Yasufumi*; Kinoshita, Takuma*; Hanaki, Shotaro; Hayashi, Hirokazu
no journal, ,
Dismantling in the radiation-controlled area is planned in phase 3 of the "Monju" decommissioning program, but an assessment of contamination of the facilities is essential before starting the dismantling. The unique Ex-Vessel Storage Tank (EVST) unique to "Monju" stores fuel in liquid sodium at 200 
C. It requires an evaluation of radioactivity due to neutron-induced structural material activation. The neutron flux distribution and radioactivity of the EVST were assessed by calculation using a 2D RZ system, validating the results through comparison with a 3D system. The assessment confirms that the maximum concentration of activity of activation product in the EVST structural material is at L3 level and generally below clearance level in most areas.
