**1. Introduction**

An acceleration-driven system (ADS) is a powerful tool for effectively transmuting minor actinides in the double-strata fuel cycle strategy for separation/conversion technology [1,2]. As a neutron source and coolant, such systems use a liquid lead–bismuth eutectic (LBE) alloy as a nuclear spallation target [3]. The LBE alloy is designed to flow through piping at 2 m/s. It causes erosion and corrosion in the pipes [4,5], which are suppressed by adjusting the oxygen concentration by about 10−<sup>7</sup> wt.% in the LBE alloy [6]. Therefore, an oxygen sensor is required to control the oxygen concentration in the LBE alloy.

Oxygen sensors based on yttria-stabilized zirconia (YSZ) and a Pt/air electrode are used worldwide to monitor the oxygen concentration in LBE alloys [7,8]. Such oxygen sensors must have high fracture toughness because a high load is applied by the flowing LBE alloy. The fracture toughness of YSZ depends on the yttrium concentration [9]. YSZ with 3 mol% yttrium is called partially stabilized zirconia (3Y–PSZ).

Under high stress, 3Y–PSZ undergoes a phase transition from a tetragonal (t) phase to a monoclinic (m) phase [10–12]. Because the m phase contributes to arresting crack propagation, it has been shown to play an important role in endowing this material with high fracture toughness. Therefore, if the fracture toughness can be maintained in the ADS operating environment, the PSZ may become an important material for use in ADS oxygen sensors.

As the LBE alloy flows through the entire ADS cooling system, it becomes radioactive through the spallation reaction. The LBE alloy is predicted to expose the oxygen sensor to a high radiation field of over 1 kGy/h [6], which is expected to lead to radiation damage in the oxygen-sensor material.

**Citation:** Okuno, Y.; Okubo, N. Phase Transformation by 100 keV Electron Irradiation in Partially Stabilized Zirconia. *Quantum Beam Sci.* **2021**, *5*, 20. https://doi.org/ 10.3390/qubs5030020

Academic Editors: Akihiro Iwase and Rozaliya Barabash

Received: 5 April 2021 Accepted: 11 June 2021 Published: 25 June 2021

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In a previous study of pure zirconia irradiated with 340 keV Xe and 800 keV Bi ion beams [13–15], an m-to-t phase transition was observed. The phase transition between m and t is considered to be caused by the cumulative transfer of energy laid down in the displacement cascades. On the other hand, the t phase of 3Y-PSZ is a metastable phase and is related to its strong mechanical properties. However, the t-to-m transition is triggered by mechanical stress and thermal annealing, and may be affected by the deposition energy of the radiation because of the metastable state phase.

To use the PSZ device in a radiation environment, the behavior of the PSZ in the metastable t phase must be investigated. In this study, the effect of PSZ on a 100 keV electron beam was investigated, using a crystal structure and radiation simulation analysis.
