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Radiation-enhanced diffusion (RED) of copper (Cu) in iron (Fe) is essential for understanding solute/impurity diffusion in nuclear materials, especially reactor pressure vessel steel, but has been rarely reported experimentally. In this study, we performed a high-precision investigation of RED using well-controlled electron irradiation and three-dimensional atom probe (3D-AP). Cu-Fe diffusion pairs were created using high-purity Fe and Cu as base materials, and irradiated by 2 MeV electron at a temperature of 773 – 893 K controlled to within ±3 K. Cu diffusion into the Fe matrix was observed at the atomic level using 3D-AP, and the diffusion coefficient was obtained directly using Fick's law. RED was clearly observed, and the ratio of diffusion under irradiation to thermal diffusion was increased as the irradiation temperature decreased. RED was quantitatively evaluated using the reaction kinetics model, and the model which consider only vacancies gave a good agreement. This gave experimental clarification that RED was dominated by irradiation-induced vacancies. In addition, the direct experimental results on the effect of irradiation on the solubility limits of Cu in Fe was obtained; solubility limits under irradiation were found to be lower than those under thermal aging.

Charged particle beams from accelerators driven by radiofrequency or with beam scanners possess time structures (i.e., pulse beam operations). However, it is not clear whether the irradiation effects of these pulse beam operations are the same as those of continuous beams, although the average damage rate is the same in both cases. In this study, the difference between continuous beams and pulse beams, and the effect of the repetition frequency and pulse duration on the defect accumulation are evaluated by the rate equation analysis for Al and Fe. The growth rate of interstitial type dislocation loops in Al is simulated with fixed duty ratios (constant pulse duration over the irradiation period) at 453 K. The effect of pulse duration on a constant period of 1 s is simulated in Fe, changing the pulse duration from 1 to 10⁻⁶ s, while the damage (dpa) caused by one pulse remains the same. The results are analyzed and the effects of pulse duration and repetition rate on the damage structure evolution are demonstrated in relation to materials parameters.

In reactor pressure vessel surveillance data, it was found that the concentration of matrix defects was very low even after nearly 40 years of operation, though a large number of precipitates existed. In this paper, defect structures obtained from surveillance data of A533B (high Cu concentration) were simulated using reaction kinetic analysis with 11 rate equations. The coefficients used in the equations were quite different from those obtained by fitting a Fe-0.6 wt%Cu alloy irradiated by the Kyoto University Reactor. The difference was mainly caused by alloying elements in A533B, and the effect of alloying elements was extracted. The same code was applied to low-Cu A533B irradiated with high irradiation damage rate, and the formation of voids was correctly simulated.

For developing an accelerator-driven system (ADS), many studies, such as those on accelerator development, subcritical system neutronics, materials irradiation effects, and thermal hydraulics, are required. In the Research Reactor Institute at Kyoto University (KURRI), ADS is planned as a future neutron source. Recent activities of relevant ADS studies at KURRI are introduced herein.

Positron lifetime measurements were used to study the effects of alloying elements on the defect structure during the incubation period for void swelling in several fcc model alloys. Pure Ni, four model alloys (Fe–Cr–Ni, Fe–Cr–Ni–Mo–Mn, Fe–Cr–Ni–Mo–Mn–Si and Fe–Cr–Ni–Mo–Mn–Si–Ti), and four commercial alloys (SUS316LSS, SUS316SS, SUS304SS and Ti added modified SUS316SS) were irradiated with electrons and neutrons. Even at 363 and 573 K to a dose of 0.2 dpa, an effect of alloying elements was observed. At 363 K irradiation, voids were formed only in Ni and Fe–Cr–Ni. At 573 K irradiation, voids were formed in Ni and all model alloys, though the concentration depended on the alloying elements. In commercial alloys, precipitates were formed instead of vacancy clusters, which prevented void growth.

He and H can be produced by nuclear reaction. In addition, energetic particles of He and T and D, which are isotopes of H, in the plasma of fusion reactor induce the damage in the surface of materials, such as erosion, sputtering and blistering. To investigate the interaction of He and D, amorphous and crystalline FeBSi alloys were irradiated by He or D[2] or He + D[2] ions with 5 keV. The results of thermal desorption indicated that more He atoms were trapped in both the amorphous and crystalline alloy irradiated by He ions than D atoms in those alloys irradiated by D[2] ions. Although He and D atoms were trapped in FeBSi alloy irradiated by He + D[2] ions, desorption peaks of D[2] and He were separated. Absorption of D in an amorphous alloy was influenced by the presence of He; however, absorption of He was independent of D[2] irradiation in both alloys.

Microstructral evolution of electron-irradiated F82H and Fe-8%Cr at 77 K was studied using positron annihilation lifetime measurements. Irradiation-induced vacancies started to migrate at 300 K and 180 K in F82H and Fe-8%Cr, respectively. Solute Cr atoms did not suppress vacancy migration, but they made di-vacancies more stable. Microvoids were not formed by annealing. In F82H, solute atoms acted as trapping site of irradiation-induced defects and annihilation of vacancies and interstitials was facilitated. Pre-existing dislocations and precipitates were also their sinks. These lead to the suppression of microvoids formation. In Fe-8%Cr, small vacancy-type dislocation loops were formed by isochronal annealing test, "16th International Conference on Positron Annihilation (ICPA-16)" 19–24 August 2012, H H Wills Physics Laboratory, University of Bristol, UK

It is interesting to investigate the formation of He bubbles in amorphous alloys because point defects do not exist in amorphous materials. In the present study, the microstructural evolution of amorphous Fe79B16Si5 alloy, either irradiated with 5 keV He^{+} ions or implanted with 150 eV He^{+} ions without causing displacement damage, and then annealed at a high temperature, was investigated using transmission electron microscopy (TEM). Vacancy-type defects were formed in the amorphous alloy after irradiation with 5 keV He^{+} ions, and He bubbles formed during annealing the irradiated samples at high temperature. On the other hand, for samples implanted with 150 eV He^{+} ions, although He atoms are also trapped in the free volume, no He bubbles were observed during annealing the samples even up to 873 K. In conclusion, the formation of He bubbles is related to the formation and migration of vacancy-type defects even in amorphous alloys.

It is well known that the minor elements Si and Sn can suppress the formation of voids in Ni alloys. In the present study, to investigate the effects of Si and Sn on the retention of helium in Ni alloys, Ni, Ni–Si, and Ni–Sn alloys were irradiated by 5 keV He ions at 723 K. Thermal desorption spectroscopy (TDS) was performed at up to 1520 K, and microstructural observations were carried out to identify the helium trapping sites during the TDS analysis. Two peaks, at 1350 and 1457 K, appeared in the TDS spectrum of Ni. On the basis of the microstructural observations, the former peak was attributed to the release of trapped helium from small cavities and the latter to its release from large cavities. Small-cavity helium trapping sites were also found in the Ni–Si and Ni–Sn alloys, but no large cavities were observed in these alloys. In addition, it was found that the oversized element Sn could trap He atoms in the Ni–Sn alloy.

The growth and shrinkage of interstitial clusters on {113} planes were investigated in electron irradiated Czochralski grown silicon (Cz-Si), floating-zone silicon (Fz-Si), and impurity-doped Fz-Si (HT-Fz-Si) using a high voltage electron microscope. In Fz-Si, {113} interstitial clusters were formed only near the beam incident surface after a long incubation period, and shrank on subsequent irradiation from the backside of the specimen. In Cz-Si and HT-Fz-Si, {113} interstitial clusters nucleated uniformly throughout the specimen without incubation, and began to shrink under prolonged irradiation at higher electron beam intensity. At lower beam intensity, however, the {113} interstitial cluster grew stably. These results demonstrate that the {113} interstitial cluster cannot grow without a continuous supply of impurities during electron irradiation. Detailed kinetics of {113} interstitial cluster growth and shrinkage in silicon, including the effects of impurities, are proposed. Then, experimental results are analyzed using rate equations based on these kinetics.

Structure cahnges of vacuum-deposited amorphous Au-Si and liquid-quenched amorphous Fe-B and Co-Fe-Si-B alloys by irradiation with high energy electrons have been observed by high resolution and high energy electron microscopy. The irradiation is found to induced the transformation to metastable phase and also to stable phase at much lower temperatures than those of the similar reactions by simple thermal treatment. In Au-Si, no visible structure change takes place at a temperataure lower than -120℃ even by a strong irradiation. The speed of the transformation to metastable phase by irradiation gradually increases with temperature, and precipitation of pure silicon takes place at above 0℃. The precipitation grows very fast until the matrix is covered with metastable phase. The existence of a very rapid diffusion of gold through amorphous phase and a strong tendency to maintain a homogeneous mixture within the amorphous is concluded. Detailed structure of the metastable phase has been observed as lattice images. Irradiation with no observable change at low temperature is found in all the observed materials to enhance the nucleation of crystalline phase which appears by heating after the irradiation. From the dependence on the irradiation dose and intensity, not the formation of the crystalline nucleus but the homogeneous change of the amorphous sturcture is concluded to be induced by irradiation. The enhancement of the thermal nucleation is more pronounced by the irradiation at higher temperatures. The variation of the phenomena with temperature of each reaction serves us to propose 'variable vacancy mechanism' of diffusion and crystallization in amorphous solids. The mechanism characterizes the diffusion in amorphous materials by the existence of multi-component diffusion carriers. The existence of the thermal equilibrium change among pre-existing latent vacancies, which accompanies a large change in their mobility, is suggested.