Kenta Murakami

Role of nickel and manganese in recovery of resistivity in iron-based alloys after low-temperature proton irradiation

This study investigates the recovery of electric resistivity in pure iron, Fe–0.6Ni and Fe–1.5Mn as related to isochronal annealing following 1 MeV proton irradiation at lower temperature than 70 K, focusing on the relationship between solute atoms and irradiation defects. Both nickel and manganese prevent stage ID recovery, which corresponds to correlated recombination. Stage II recovery is also changed by the addition of a solute, which corresponds to the migration of small interstitial clusters. In both pure iron and Fe–0.6Ni, no evident difference was observed in the stage III region, which corresponds to the migration of vacancies. In contrast, two substages appeared in the Fe–1.5Mn at a higher temperature than stage IIIB appeared in pure iron. These substages are considered to represent the release of irradiation-induced defects, which was trapped by manganese.

Effect of insulator environment on thermal ageing estimations for ethylene propylene rubber insulated nuclear power plant cables

ABSTRACT Thermal ageing estimations merely focus on the degradation of cable insulator materials without considering their arrangement within the cable structure, by surrounding jacket or colorant pigments. This study reconsidered thermal ageing estimations based on the insulator elongation at break (EAB) degradation progress for flame retardant ethylene propylene rubber (FR-EPR) insulators within the whole cable structure. FR-EPR insulators were aged using two methods. In the first method, the insulators were heated, including jacket material at 125 °C, 150 °C, and 160 °C for 2208, 336, and 264 h, respectively. In the second method, the insulators were heated without jacket at 125 °C for 3480 h, and at 160 °C for 264 h. It was expected that the EAB degradation progress would be more severe when the FR-EPR insulators were aged without jacket. However, the results contradicted previous expectations, and the special heat-resistant polyvinyl chloride jacket accelerated the EAB degradation. In addition, the black FR-EPR insulators showed values that deviated from the predictions for the activation energy (Ea) and equivalent times compared to the white and red insulators due to the use of carbon black as a pigment. The reduced equivalent times for thermally aged FR-EPR insulators with jacket in this study were evident compared with the current degradation database for FR-EPR insulators degraded without jacket.

Effects of solute elements on hardening and microstructural evolution in neutron-irradiated and thermally-aged reactor pressure vessel model alloys

ABSTRACT Nanometer-sized Cu-enriched solute clusters containing Mn, Ni, and Si atoms are considered as the primary embrittling feature in reactor pressure vessel steels. In order to understand the effects of solute atoms Mn, Ni, and Si on hardening and cluster formation, reactor pressure vessel model alloys FeCu, FeCuSi, FeCuNi, and FeCuNiMn were irradiated at 290 °C in a research reactor. Thermal ageing at 450 °C was also carried out to compare with the results in the neutron irradiation. The addition of Mn resulted in larger hardening and higher cluster number density in both thermal ageing and neutron irradiation. In FeCu0.8NiMn alloy, the size distribution of Cu-enriched clusters formed in 62-h thermal ageing (almost peak hardening) was very similar to that formed in the neutron irradiation, indicating they are on a similar growing stage. But the average Ni and Mn composition in clusters formed in neutron irradiation was higher. A good linear relationship between hardening and the square root of cluster volume fraction for both neutron irradiation and thermal ageing data was found.

A positron beam study on vacancy formation in iron by ion beam irradiation at low temperature

This study intends to investigate cascade damage structure produced by energetic ion irradiation. Cascade damage structure is preserved at low temperature below stage I where interstitial atoms begin to migrate. Then positron beam is implanted to the irradiated surface as a vacancy probe to evaluate vacancy concentration remained in the irradiated specimens. By this method, defect production efficiency was evaluated for iron irradiated with proton and carbon ions. The defect production efficiency values indicate enhanced recombination for carbon irradiation due to primary knock-on atoms (PKA) with higher energies.

Microstructure and critical current properties of Bi(Pb)2212/metal tapes and single crystals

Abstract Flux pinning mechanism of the heavily lead-doped Bi(Pb)2212 single crystals was studied by the magneto-optical method. Largely different behaviors of J c (∥ a ) and J c (∥ b ) suggested that temperature-induced planar pinning site contributes to the excellent J c performance at high temperatures in magnetic field. For the Bi(Pb)2212/metal tapes, microstructure of superconducting layer was improved by addition of MgO fine particles or applying the amorphous to crystallization method. However, their transport J c s were still low compared with the lead-free Bi2212/Ag tapes.

Heterogeneity of ion irradiation-induced hardening in A533B reactor pressure vessel model alloys

Reactor pressure vessels comprise bainitic steel structures, and are heterogeneous on the mesoscale. Nanoindentation techniques were used to evaluate the hardness of these structures on the micrometer scale, and to evaluate the heterogeneity in a specimen using the distribution of the hardness. Three A533B model alloys were irradiated by 2.8 MeV Fe2+ ions at 563 K, and the effects of ion fluence, ion flux, and chemical composition on the change in the hardness distribution were examined. Heterogeneity of the hardening is observed in high-copper specimens irradiated up to (2–10) × 1014 ions/cm2, where the average hardness increases the most. In these specimens, the hardness distribution broadens, and demonstrates that the hardening in certain positions (possibly where the initial hardness is high) is greater than in other positions. Variation in initial chemical composition (especially copper and carbon) or sink strength may cause a difference in the curing behavior.

In search of extendable conditions for cable environmental qualification in nuclear power plants

ABSTRACT The environmental qualification (EQ) for cable insulators in nuclear power plants (NPPs) has been developed on the basis of the design basis accident (DBA) to prevent reactor core damage. However, the latest safety principles require extending the design concept to prepare the utilized equipment for scenarios after core damage. Thus, we propose a modification to the EQ for cables connecting utilized equipment at design extension conditions. This paper surveys all electrical components for accident management in boiling water reactor-4 (BWR-4), and identifies their connecting cables’ functional category as low-voltage power, instrumentation, and control cables. The EQ temperature profile of these cables during the incident phase was addressed for extension. This required postulating maximum temperature environments according to accident scenarios, knowledge of cable integrity degradation, and their current evaluation by the EQ. To evaluate whether these environments are suitable stressors, heat testing was conducted on flame-retardant ethylene propylene rubber (FR-EPR)-insulated cables. On the basis of those results, we suggest a maximum primary peak temperature of the EQ temperature profile of 250 °C. We also suggest increasing the primary peak period of the EQ temperature profile to 48 h without experiment, on the basis of inherent excessive margin for mechanical integrity during the ageing phase.

Fabrication of heavily Pb-doped Bi22212/Ag composite tapes

Abstract Heavily Pb-doped Bi2212/Ag composite tapes were fabricated by two methods, and their microstructures and superconducting properties were investigated. The amorphous-to-crystallization method was found to be effective for elimination of impurity phases, which grew largely by the conventional melt-solidification technique.

The counter effects of the accident at Fukushima Dai-ichi nuclear power station

The counter effects of the accident at the Fukushima Dai-ichi Nuclear Power Station are discussed in this paper. Though decommission and remediation have been conducted in the facility and surrounding area, ninety thousand residences near the facility are still under the evacuation order. Four nuclear power units have already restarted under the new regulatory framework, but two of them in Fukui prefecture stop the operation due to the provisional disposition declared by Ohtsu district court in Shiga prefecture. Reinforcement of the latter layer of the defense in depth has been improved in many ways. The improvement of decision-making process is very important in latter layers of the defense in depth, in contrast the plant behaviors are automatically decided based on their design in the prior layers.

The Experimental Approach to Observe Atomistic Behaviors of Metallic Surfaces by a Linked Facility Between an Ion Accelerator and a STM

This study focuses on an experimental approach to quantifying irradiation-induced defects in structural materials used in nuclear power plants at an atomic scale. As a preliminary step, a procedure for observing metallic surfaces using a facility comprising an ion accelerator and a scanning tunneling microscope was developed.Using this experimental setup, atomic-resolution images of Au(111) surfaces were successfully obtained. A herringbone structure, which is characteristic of reconstructed Au(111) surfaces, was clearly observed, with individual atoms being distinguishable.Copyright