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Ito, Michitane*; Mukai, Tomohisa*; Suzuki, Hiroshi; Nanbu, Yoshihito*; Suzuki, Junichi*; Matsuzawa, Koichi*; Kinugasa, Hideyuki*
Nihon Kenchiku Gakkai Gijutsu Hokokushu, 27(65), p.99 - 103, 2021/02
no abstracts in English
Ebihara, Kenichi; Yamaguchi, Masatake; Nishiyama, Yutaka; Onizawa, Kunio; Matsuzawa, Hiroshi*
Journal of Nuclear Materials, 414(2), p.328 - 335, 2011/07
Times Cited Count：4 Percentile：32.59(Materials Science, Multidisciplinary)According to the experiment, the grain-boundary (GB) phosphorus (P) segregation in neutron-irradiated reactor pressure vessel (RPV) steels causes intergranular embrittlement. However the dependence of the segregation on variables such as dose and dose-rate is not clear due to the lack of experimental databases. In this paper, we incorporated the effect of carbon atoms into the rate theory model, and simulated the GB P coverage in the neutron-irradiated RPV steels. As a result, the simulation reproduced the experimental GB P coverage. It was observed that the GB P coverage does not depend on the dose rate regardless of the presence of carbon atoms. Furthermore, it was confirmed that vacancies scarcely transport P atoms to grain-boundaries as compared to the transport by self-interstitial atoms and it was found that carbon atoms influence the irradiation-induced P segregation mainly by suppressing the migration of vacancies.
Nishiyama, Yutaka; Yamaguchi, Masatake; Onizawa, Kunio; Iwase, Akihiro*; Matsuzawa, Hiroshi*
Journal of ASTM International (Internet), 6(7), 8 Pages, 2009/07
A study on grain-boundary segregation and embrittlement in terms of the Charpy ductile-to-brittle transition temperature (DBTT) has been performed for the neutron-irradiated A533B steels with a typical contents of impurities of Japanese reactor pressure vessel ones. The neutron irradiation was conducted at 563K to a fluence of 1.3 
10
n/m
(E
1MeV) using Materials Testing Reactors. The neutron irradiation induced the phosphorus (P) and nickel (Ni) segregation and the reduction of segregated carbon (C) in some cases at grain-boundaries. The increase in the P segregation at high fluences (5 10
n/m, E1MeV) was less than 0.15 in monolayer coverage for the steels with the bulk content of P not exceeding 0.02 wt.%. The hardening more strongly affected the DBTT shift than the P segregation for those steels. The reduction of segregated C that enhances the grain-boundary cohesion by neutron fluence is not large enough to cause the DBTT shift.
Onizawa, Kunio; Matsuzawa, Hiroshi*
Proceedings of 2009 ASME Pressure Vessels and Piping Division Conference (PVP 2009) (CD-ROM), 8 Pages, 2009/07
The structural integrity of RPV during PTS is assessed by considering important factors related to fracture mechanics analysis such as fracture toughness, PTS transient, and irradiation embrittlement prediction and so on. The effects of such factors have been evaluated with regard to the fracture probability of RPV using probabilistic fracture mechanics (PFM) analysis code PASCAL2. Results from sensitivity analyses on the factors showed that the effect of fracture toughness curve is significant when the curve shape is different at high temperature range. Revised embrittlement correlation method gave slightly lower probability of fracture. The heat transfer coefficient for PTS transient and welding residual stress due to overlay cladding showed some effects to increase the probability of fracture. Uncertainties related to the RTNDT shift with regard to the probabilistic variables such as chemical composition and neutron fluence are compared with the deterministic result with a margin.
Ebihara, Kenichi; Yamaguchi, Masatake; Nishiyama, Yutaka; Onizawa, Kunio; Matsuzawa, Hiroshi*
Proceedings of 2009 ASME Pressure Vessels and Piping Division Conference (PVP 2009) (CD-ROM), 8 Pages, 2009/07
We incorporate the parameters obtained from first-principles calculations into the rate theory model which was developed for bcc lattice, and apply it to the simulation of irradiation-induced phosphorous segregation in bcc iron. We evaluate the grain boundary phosphorous coverage and discuss its dependence on dose-rate and irradiation temperature by comparing our results with previously reported results and experimental data. As results, we find that dose-rate does not affect the grain boundary phosphorous coverage within the range of our simulation condition and that the dependence on irradiation temperature differs remarkably from the previous results.
Nishiyama, Yutaka; Onizawa, Kunio; Matsuzawa, Hiroshi*
Proceedings of 16th Pacific Basin Nuclear Conference (PBNC-16) (CD-ROM), 6 Pages, 2008/10
The effects of grain-boundary P segregation and hardening on the embrittlement in terms of Charpy ductile-to-brittle transition temperature (DBTT) in several neutron-irradiated reactor pressure vessel (RPV) steels with different bulk contents of P have been investigated using a scanning Auger microbe, a local electrode atom probe and positron annihilation spectroscopy. Increasing the neutron fluence promotes intergranular P segregation, particularly in steels with high levels of P. The content of P significantly also affects irradiation hardening due to distinct formation of P-rich precipitates arising from the stabilization of vacancies. It was found that neutron irradiation mitigates an embrittling effect of segregated P, therefore the hardening more strongly affects the DBTT shift than the P segregation. The likelihood of intergranular embrittlement of RPV steels is discussed by these results.
Onizawa, Kunio; Matsuzawa, Hiroshi*
Nihon Kikai Gakkai M&M 2008 Zairyo Rikigaku Kanfarensu Koen Rombunshu (CD-ROM), p.OS0305_1 - OS0305_2, 2008/09
Concerning the probabilistic structural integrity analysis method, the effects of crack existing probability and transients on the fracture probability of reactor pressure vessel (RPV) have been investigated based on the pilot plant analyses performed at the U.S.NRC. In addition to the comparisons of stress intensity factor and temperature during typical PTS events, we studied the correlation between analysis results by deterministic and probabilistic methods using probabilistic fracture mechanics (PFM) analysis code PASCAL2. From the PFM analyses by PASCAL2, the effects of crack existing probability and transient loadings on the fracture probabilities of a typical RPV were investigated. It was concluded that the crack existing probability had much influence on the fracture probabilities. The model on the crack existing probability for a PWR plant in USA resulted in less fracture probability than the current PASCAL2 model developed at PNNL previously. It was also shown that the transient loading had little effect on the correlation between deterministic safety margin and corresponding probabilistic analysis result, i.e. fracture probability.
Yamaguchi, Masatake; Nishiyama, Yutaka; Shiga, Motoyuki; Kaburaki, Hideo; Matsuzawa, Hiroshi*
Proceedings of 3rd International Conference on Multiscale Materials Modeling, p.733 - 735, 2006/09
First-principles calculations have been performed to simulate the grain boundary decohesion (embrittlement) in ferromagnetic bcc iron (Fe) 
3(111) symmetrical tilt grain boundary by adding various impurity or solute atoms (B, C, N, O, Si, P, S, Mn, Ni, Cu) to the boundary. We have successfully derived the segregation energy of the solute atom, tensile strength, and cohesive energy of the grain boundary, which are consistent with many empirical facts.
Yamaguchi, Masatake; Nishiyama, Yutaka; Shiga, Motoyuki; Kaburaki, Hideo; Matsuzawa, Hiroshi*
no journal, ,
First-principles method is applied to assess the embrittlement mechanism of bcc iron (Fe) grain boundaries due to the segregation of various solutes (C, Si, P, S, Mn, Cr, Cu, Mo, etc.) that are contained, for example, in the nuclear reactor pressure vessel steel. The simulation is performed for the bcc Fe 3(111) symmetrical tilt grain boundary to evaluate the effects of solute segregation on the strength of the interface separation, in particular, focusing on the mechanism and degree of embrittlement. We discuss the relative effects of these solutes on the embrittlement mechanism and segregation energy.
Yamaguchi, Masatake; Ebihara, Kenichi; Kaburaki, Hideo; Matsuzawa, Hiroshi*
no journal, ,
We investigate the migration mechanism of phosphorous in nickel using first-principles calculations. The results are very different from the previous parameters in Murhpy's simulation of irradiation-induced phosphorous segregation. This indicates that the simulation should be reconsidered using parameters determined from first-principles calculations.
Onizawa, Kunio; Matsuzawa, Hiroshi*
no journal, ,
Concerning the structural integrity analysis method for aged reactor pressure vessel (RPV) during pressurized thermal shok (PTS) event, a probabilistic analysis approach has been investigated. Based on the survey on the proposed revision of the PTS rule in USA and the revision of radiation embrittlement prediction equation in Japan, probabilistic analyses of RPV integrity were performed using probabilistic fracture mechanics (PFM) analysis code PASCAL2. The effects of major factors affecting the integrity such as crack existing probability, radiation embrittlement prediction equation, fracture toughness equation have been studied. From the comparison of analysis results by deterministic and probabilistic methods, it was shown that the transient loading had little effect on the correlation between deterministic safety margin and corresponding probabilistic fracture probability.
Nishiyama, Yutaka; Yamaguchi, Masatake; Onizawa, Kunio; Matsuzawa, Hiroshi*
no journal, ,
In order to predict the degree of irradiation embrittlement of reactor pressure vessel (RPV) steels exposed to high neutron fluences, it is necessary to study on the likelihood of intergranular embrittlement, which decreases the grain boundary strength due to the segregation of impurities such as phosphorus at grain-boundaries and promotes the increase in ductile-brittle transition temperature (DBTT). Using highly irradiated RPV steel samples, it was found through scanning Auger microprobe analyses that the irradiation-induced P segregation increased with increasing neutron fluence and bulk levels of P. In addition, results from the comparison between this results and DBTT, and theoretical analysis on the embrittlement mechanism by first principle calculation are presented.
Ebihara, Kenichi; Yamaguchi, Masatake; Nishiyama, Yutaka; Onizawa, Kunio; Matsuzawa, Hiroshi*
no journal, ,
In order to evaluate numerically the grain-boundary segregation of phosphorus in reactor pressure vessel steels, the neutron irradiation-induced segregation was simulated under the condition of the irradiation experiments using the bcc lattice model incorporating the parameters which are obtained by the first principles calculations. The following results were obtained from the simulation results: (1) The grain-boundary P coverage becomes close to experimental results by increasing the sink strength of vacancies and interstitial atoms. (2) The irradiation-rate dependency of the grain-boundary P coverage is not observed. (3) The irradiation temperature dependency of the grain-boundary P coverage is different from the result reported previously.
Yamaguchi, Masatake; Ebihara, Kenichi; Nishiyama, Yutaka; Onizawa, Kunio; Matsuzawa, Hiroshi*
no journal, ,
no abstracts in English
Nishiyama, Yutaka; Ebihara, Kenichi; Yamaguchi, Masatake; Onizawa, Kunio; Matsuzawa, Hiroshi*
no journal, ,
The effect of dose-rate on grain-boundary phosphorus (P) segregation in neutron-irradiated reactor pressure vessel steels has been determined by scanning Auger electron microprobe. The neutron flux ranged from 110
to 110
n/cm/sec up to a fluence of 110
n/cm at 290
C. The comparisons made between the model prediction using rate equation and experimental observations indicate that the grain-boundary P is not affected by the dose-rate in the present range.
Yamaguchi, Masatake; Ebihara, Kenichi; Nishiyama, Yutaka; Onizawa, Kunio; Matsuzawa, Hiroshi*
no journal, ,
From first-principles (using VASP), we calculated the energy parameters such as the formation and migration energies of point defect (vacancy or interstitial) and the defect-phosphorous (P) complexes in bcc Fe and fcc Ni. For bcc Fe system, the stability of a Fe-P mixed dumbbell pair and an interstitial P are similar with each other. Although there are some differences, our results are in overall agreement with the other previous results using SIESTA code. For fcc Ni system, on the other hand, the Ni-P mixed dumbbell cannot be stabilized and thus only interstitial P is formed.
Nishiyama, Yutaka; Yamaguchi, Masatake; Ebihara, Kenichi; Onizawa, Kunio; Matsuzawa, Hiroshi*
no journal, ,
no abstracts in English
Ebihara, Kenichi; Yamaguchi, Masatake; Nishiyama, Yutaka; Onizawa, Kunio; Matsuzawa, Hiroshi*
no journal, ,
The irradiated-induced phosphorous segregation in a reactor pressure vessel steel was simulated using the reaction-diffusion equations which include the diffusion constants evaluated by the first principles calculation. The influences of the dose and the dose rate on the phosphorous grain boundary coverage were estimated from the simulation results. Furthermore, by including the effect of carbon on the migration of the radiation defects, the influence of carbon on the phosphorous grain boundary coverage was also estimated.
Ebihara, Kenichi; Yamaguchi, Masatake; Nishiyama, Yutaka; Onizawa, Kunio; Matsuzawa, Hiroshi*
no journal, ,
The experimental results on neutron-irradiated reactor pressure vessel (RPV) steels have revealed grain boundary (GB) segregation of phosphorous(P) causing GB embrittlement. Since the dependence of the segregation on dose and dose-rate is unclear due to the lack of experimental database, the estimation of the irradiation-induced GB P segregation by the rate theory model is desired. In this presentation, we incorporated the effect of carbon(C) into the model by considering C as the trap of vacancies(Vs) and self-interstitial atoms (SIAs), and evaluated the GB P coverage in the RPV steels. As a result, by selecting the proper sink strength of V and SIA, the simulation reproduced the experimental GB P coverage. It was confirmed that C hardly influences the dose rate dependency of the GB P coverage. It was found that C influences the GB P coverage by mainly slowing the V migration. Therefore it is considered that V is necessary for simulating GB P coverage even though V hardly transports P.
