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Shiota, Yuki; Ariyoshi, Hideo; Shiohama, Yasutaka; Isobe, Yuta; Takeuchi, Ryotaro; Kudo, Junki; Hanaki, Shotaro; Hamano, Tomoharu; Takagi, Tsuyohiko
JAEA-Technology 2022-019, 95 Pages, 2022/09
JAEA-Technology-2022-019.pdf:7.59MB
In the first stage of "Monju" decommissioning project, "Fuel Unloading Operations" have been carrying out. The operations consists of two processes. The first process is "Fuel Treatment and Storage" is that the fuel assemblies unloaded from the Ex-Vessel fuel Storage Tank (EVST) are canned after sodium cleaning, and then transferred to the storage pool. The second process is "Fuel Unloading" that the fuel assemblies in the reactor core are replaced with dummy fuel assemblies and stored in the EVST. "Fuel Treatment and Storage" and "Fuel Unloading" are performed alternately until 370 fuel assemblies in the core and 160 fuel assemblies in the EVST are all transferred to the storage pool. This is a summary of "Fuel Unloading" in the third quarter of "Fuel Unloading Operation". In fiscal 2020, as "Fuel Unloading", 72 fuel assemblies and 74 blanket fuel assemblies were unloaded from the core, and stored in the EVST. From the EVST, 145 dummy fuel assemblies and 1 fixed absorber were loaded in the core instead. During these operations, a total of 36 cases alarming or equipment malfunctions classified into 4 types occurred. However, these events were estimated in advance, there were no significant events that menaces to safety of fuel assemblies and equipment. Therefore, there were no serious problem like fall of fuel assemblies and events that may affect schedule of the project like stick of gripper of ex-vessel fuel transfer machine. When equipment's work or performance fail, the operation continued with safety by elimination of causes of problem. Fuel handling system of Monju has function that is endemic to sodium cooling fast breeding reactor. Because continuous operations of fuel handling system with actual fuel assemblies start recently, we don't have as much experience as PWR and BWR. With estimation of various troubles, reduction of frequency of trouble occurrence and minimization of impacts on schedule performed.
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.