Kenji Koizumi

Prediction of the drying behavior of debris in Fukushima Daiichi Nuclear Power Station for dry storage

ABSTRACT Although treatment policies for debris from Fukushima Daiichi Nuclear Power Station have not been decided yet, they may include medium-and long-term debris storage. Dry storage may be desirable in terms of cost and handling, but before implementation, it is necessary to assess hydrogen generation that occurs during storage due to the radiolysis of the water accompanying the debris. Herein, Al2O3, SiO2, ZrO2, UO2, and cement paste pellets were prepared as simulated debris with various porosities and pore size distributions. The weight changes of the wet samples were measured at various drying temperatures (100°C, 200°C, 300°C, and 1000°C) via thermogravimetry under helium gas flow (50 cc/min) or reduced pressure conditions (rate: 200 Pa in 30 min), and the resulting drying curves were evaluated. All ceramic pellets exhibited similar drying characteristics in this experiment, indicating that cold ceramics could be used for predicting the drying behavior of ceramic debris. In compariosn with ceramic pellets, cement paste pellets exhibited different behavior and a longer drying time even under 1000°C. In conclusion, it is necessary to decide a standard level for the dry state of molten core concrete interaction (MCCI) products that accompany concrete.

Mechanical properties of cubic (U,Zr)O2

Evaluation of fuel debris properties in the Fukushima Daiichi Nuclear Power Plant (1F) is required to develop fuel debris removal tools. In the removal of debris resulting from the TMI-2 accident, a core-boring system played an important role. Considering the working principle of core boring, hardness, elastic modulus, and fracture toughness were found to be important fuel debris properties that profoundly influenced the performance of the boring machine. It is speculated that uranium and zirconium oxide solid solution (U,Zr)O2 is one of the major materials in the fuel debris from 1F. In addition, the Zr content of the fuel debris from 1F is expected to be higher than that of the debris from TMI-2 because the 1F reactors were boiling-water reactor (BWR). In this research, the mechanical properties of cubic (U,Zr)O2 samples containing 10%¬-65% ZrO2 are evaluated. The hardness, elastic modulus, and fracture toughness are measured by the Vickers test, ultrasonic pulse echo method, and indentation fracture method, respectively. In case of the (U,Zr)O2 samples containing less than 50% ZrO2, Vickers hardness and fracture toughness increased, and the elastic modulus decreased slightly with increasing ZrO2 content. Moreover, all of those values of the (U,Zr)O2 samples containing 65% ZrO2 increased slightly compared to (U,Zr)O2 samples containing 55% ZrO2. ZrO2 content affects fracture toughness significantly in the case of samples containing less than 10% ZrO2. Higher Zr content (exceeding 50%) has little effect on the mechanical properties.

Applicability of Single Mode Fiber Laser for Wrapper Tube Cutting

Japan Atomic Energy Agency (JAEA) has been developing a fuel disassembly system with reliability for FBR fuel reprocessing. Laser technology has a high cutting performance and stable operation and was apply to disassembly system in our previous studies. However, it was hard to produce the impeccable disassembly system, because it was occurred the problems, such as pin damage and dross adhesion between a wrapper tube and fuel pins.After that, the advance of the laser cutting technology has recently attracted a great deal of attention from industry. In particular, single mode fiber laser (SMFL), which has a small beam size and high beam quality, has been reported as a new oscillator. Then, it was presumed that SMFL might provide a possible to prevent the original matters in the disassembly.The main purpose of this study is to reevaluate an applicability of laser for the wrapper tube cutting by the basic cutting tests. Concretely, the authors researched whether every cutting condition such as SMFL etc, has the effects on the original matters or not. This experimental results show that the kerf width of SMFL is still narrower than that of multi mode fiber laser (MMFL). It is an important phenomenon to decrease the amount of dross. Therefore, the authors confirmed that SMFL is suitable for prevention of the original matters and the new feasibility method of wrapper tube cutting.© 2012 ASME

Development of Short Stroke Shearing Technology for FBR Fuel Pins

The short stroke shearing tests with simulated fuel pin bundle were carried out in engineering scale. The shearing device was designed to handle the simulated Monju (FBR prototype reactor) type fuel pin bundle. Monju type and Commercial reactor type simulated fuel pins were used for the test. The length of sheared pin and the opening ratio of sheared section were measured under several shearing settings such as the pressure to hold pin bundle, the shearing speed and the filling-ratio of pins in the pin magazine. Both types of fuel pin were able to be sheared accurately at the length of about 10mm, and the opening ratio of sheared section was not significantly reduced. As the results, fundamental data of the short stroke shearing characteristics were obtained and that shearing method was confirmed to be promising with the reliable shearing device.Copyright

Design and Fabrication of the FBR Fuel Disassembly System

Japan Atomic Energy Agency (JAEA) has been developing a reliable disassembly system for FBR fuel reprocessing as a part of Fast Reactor Cycle Technology Development (FaCT). As FBR fuel pins are installed in a hexagonal shaped wrapper tube made of stainless steel, the fuel pins should be separated from the wrapper tube prior to the shearing process. JAEA has been developing the laser beam cutting method and the mechanical cutting method as the disassembly system. Although Fiber laser system showed a good cutting performance, it couldn’t completely avoid fuel pin damage and adhesion during the cutting operation. So we focused on the mechanical method to minimize such troubles. Two types of mechanical cutting modes have to be developed to realize the disassembly procedure, namely, the slit-cut for the wrapper tube and the crop-cut for the end plug region of the fuel pin bundle. To ensure disassembly technology of commercial reactor fuel assemblies, we designed and fabricated the testing machine of disassembly system having the cutting modes in engineering scale. We confirmed basic functions of this machine and improved its performance. We will soon demonstrate engineering operation by a series of disassembling and pin bundle handling procedure; separating fuel pins from wrapper tube, transferring them to the fuel magazine for shearing. Scattering of cutting dust cause machine troubles and transition of it to the dissolution process together with pins causes unknown problems. To resolve the problems, collection device of cutting dust will be tested and the cutting condition to make the disassembly easy to cut will be improved.Copyright

Design Study of Mechanical Disassembly System for FBR Fuel Reprocessing

Japan Atomic Energy Agency (JAEA) has been leading feasibility study on commercialized fast reactor cycle systems in Japan. In this study, we have proposed a new disassembly technology by mechanical disassembly system that consists of a mechanical cutting step and a wrapper tube pulling step. In the mechanical disassembly system, high durability mechanical tool grinds the wrapper tube (Slit-cut (S/C) operation in circle direction), and then the wrapper tube is pulled out and removed from the fuel assembly. Then the fuel pins are cut (Crop-cut (C/C) operation at entrance nozzle side) and the entrance nozzle is removed. The fuel pins are transported to the shearing device in next process. The Fundamental tests were carried out with simulated FBR fuel pins and wrapper tube, and cutting performance and wrapper tube pulling performance has been confirmed by engineering scale. As results, we established an efficient disassembly procedure and the fundamental design of mechanical disassembly system.Copyright