The lifetime of a mercury target vessel installed in the Materials and Life Science Experimental Facility at J-PARC (Japan Proton Accelerator Complex) is affected by radiation damage caused by protons and neutrons. Quantitatively evaluating the changes in the mechanical properties of vessel materials is essential. We devised a method to quantitatively evaluate mechanical properties similar to tensile test results by conducting inverse analysis using the finite-element method on the microindentation test results of ductile materials.

Furthermore, the disposal of highly activated equipment, such as used mercury target vessels, remains challenging. Conducting laser processing from outside the radiation-shielding glass window is on way to reduce waste volume. In such cases, understanding the changes in the mechanical properties of glass due to laser irradiation is important.

In this study, using microindentation tests, we applied the devised evaluation method to radiation-shielding glass, which is a brittle material. The evaluation objects were three glass types with different lead contents and an area of lead glass irradiated with a high-power Nd:YAG laser. The flow stress (the true stress at the bend in the line) decreased due to the increase in lead content and irradiation (Fig. 1a). We focused on the microplastic flow representing liquid-like behavior. According, the difference in the plastic zone at the crack tip upon occurrence of a crack was confirmed. Subsequently, the possibility of quantitatively evaluating radiation damage (change in brittle properties; Fig. 1b) was demonstrated. As part of future research, this evaluation method will be applied to used target vessels that have become embrittled through irradiation to evaluate their lifetime based on radiation damage.

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