Hironari Obayashi

Conceptual Design Study of Beam Window for Accelerator-Driven System

The conceptual design study of the beam window of the spallation target is one of the critical issues in the R&D of the accelerator-driven system (ADS). In this study, the investigation to create a feasible concept of the beam window for the ADS was performed by changing the proton beam profile from the Gaussian distribution to the parabolic and the flat distributions. Detailed analyses were performed by considering the particle transport of protons and neutrons in the spallation target region, the thermal hydraulics of lead bismuth eutectic (LBE) around the beam window, and the structural strength of the beam window. The calculation results showed that the difference in the temperature between the inner and outer surfaces at the top of the beam window was reduced by changing the beam profile from the Gaussian to the parabolic and the flat distributions. By this reduction, in the parabolic case, the thermal stress at the top position was also reduced and the buckling pressure increased by about 20%. On the other hand, in the flat case, the thermal stress at the peripheral region instead of at the central position increased and the buckling pressure slightly deteriorated. The buckling mode was also changed in the flat case. From these calculation results and discussions, it was confirmed that all three cases, the Gaussian, the parabolic, and the flat, were feasible and it was concluded that the concept with the parabolic distribution would be the most feasible under the current ADS design condition.

Study on the evaluation of erosion damage by using laser ultrasonic integrated with a wavelet analysis technique

Spallation targets are the key components of accelerator driven systems (ADSs) that are being developed in the world. Erosion damages on the target vessels are anticipated. To prevent accidents occurrence due to erosion of spallation target vessel, the damage evaluation technique is desirable. The excited vibration of LBE target vessel will be monitored remotely to establish the technique. In this study, the basic researches were carried out through experiments and numerical simulations to investigate the interaction between ultrasonic waves and damage to understand the correlation between structural vibration and damage degree. Specimens with distributed erosion damage was irradiated by laser shots, and the vibration was detected by a laser vibrometer subsequently. A technique, Wavelet Differential Analysis (WDA), was developed to quantitatively and clearly indicate the differences caused by damage in the vibration signals. The results illustrated that the developed technique is sensitive to erosion damage with small size and is capable of quantitatively evaluating erosion damage. It is expected that the developed techniques can be applied to monitor the real spallation targets in the future.

Effects of Grain Size on Ultrasonic Attenuation in Type 316L Stainless Steel

A lead bismuth eutectic (LBE) spallation target will be installed in the Target Test Facility (TEF-T) in the Japan Proton Accelerator Research Complex (J-PARC). The spallation target vessel filled with LBE is made of type 316L stainless steel. However, various damages, such as erosion/corrosion damage and liquid metal embrittlement caused by contact with flowing LBE at high temperature, and irradiation hardening caused by protons and neutrons, may be inflicted on the target vessel, which will deteriorate the steel and might break the vessel. To monitor the target vessel for prevention of an accident, an ultrasonic technique has been proposed to establish off-line evaluation for estimating vessel material status during the target maintenance period. Basic R&D must be carried out to clarify the dependency of ultrasonic wave propagation behavior on material microstructures and obtain fundamental knowledge. As a first step, ultrasonic waves scattered by the grains of type 316L stainless steel are investigated using new experimental and numerical approaches in the present study. The results show that the grain size can be evaluated exactly and quantitatively by calculating the attenuation coefficient of the ultrasonic waves scattered by the grains. The results also show that the scattering regimes of ultrasonic waves depend heavily on the ratio of wavelength to average grain size, and are dominated by grains of extraordinarily large size along the wave propagation path.

TEF Beam Window Design and Evaluation of Structural Integrity

To promote R&D for accelerator-driven systems (ADS), the ADS Target Test Facility (TEF-T) is planned under the framework of the J-PARC project. Design of a proton beam window (BW) of a lead-bismuth eutectic (LBE) target is one of key issues for TEF-T construction. The objective of this study is to evaluate of structural integrity of the BW. That is, thermal-fluid behavior of LBE around the BW, structural integrity against thermal stress and internal pressure, and effects of cavitation bubbles generated by pressure waves due to pulsed proton beam injection were evaluated. As a result, the integrity of the BW for the thermal stress and the internal pressure was confirmed, and the candidate of the initial operation condition for the target system was suggested. Furthermore, it was confirmed that the cavitation bubbles would not cause meaningful damages on the target vessel.

Thermal-Hydraulic Analysis of the LBE Spallation Target Head in JAEA

Abstract To perform basic research and development to realize future accelerator-driven systems, a lead-bismuth eutectic (LBE) alloy spallation target will be installed within the framework of the Japan Proton Accelerator Research Complex (J-PARC) project, Japan Atomic Energy Agency. The target will be bombarded by high-power pulsed proton beams (250 kW, 400 MeV, 25 Hz, and 0.5 ms in pulse duration). The beam window (BW) of the spallation target is critical because it should survive under severe conditions that occur, i.e., high temperature, high irradiation, intense stress, and various kinds of damage. Therefore, the target vessel should be carefully designed to obtain an adequate safety margin. Our previous research indicates that there is a stagnant flow region in the LBE at the BW tip due to the symmetric configuration of the target, which causes high temperature and concentration of stress on the BW. On the basis of our previous work, three types of upgraded target head designs are studied in the current research to reduce/move the stagnant flow region from the BW tip and to increase the target safety margin. Thermal-hydraulic analyses and structural analyses for the target head designs are carried out numerically under a steady-state condition. Results illustrate that the designs can almost eliminate the stagnant flow region in the LBE. As a consequence, the concentration of thermal stress on the BW is released and greatly decreased. The safety margin of the target is improved through this study.