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Radiation damage of materials for fission and fusion reactors has been scaled in terms of the number of displacements per atoms (dpa). The method of evaluating the dpa has been established and standardized. However, it has become obvious that more detailed analyses are required, particularly for the nature and spatial distribution of the introduced point defects and their clusters. Such detailed nature of the defects introduced is thought to be governed by the primary knock-on atom (PKA) spectrum, A number of trials to elucidate the PKA dependent radiation effects by choosing the mass and energy of the incident ions have been explored. In some cases, defect formation by a single impinging ion has been observed. However, it has also been recognized that there are a number of artefacts arising from energy deposition distributions, existence of surface sinks together with radiation induced surface modifications and so on. In this paper, discussion will be made on how to establish irradiation correlation between neutron and heavy ion irradiations in a cascade damage formation regime. For the past fifty years, the correlation between neutron and ion irradiations from the view point of simulating the neutron radiation damage by ion irradiations has been discussed many times. However, the correlation itself has not been fully discussed separately. This is the major objective of this paper.

Radiation effects in materials for fission and fusion reactors are caused by fast neutrons which produce cascade damage, which has been known to be considerably different from simple Frenkel pairs produced by relativistic electrons. In-situ observation of radiation damage using a combined facility of electron microscope and heavy-ion accelerators has been a powerful tool to investigate the nature of the cascade damage. In this overview, brief historical survey of this experimental technique will be given, followed by the results and discussions on cascade damage mainly in gold will be reviewed. Careful considerations will be required to generalize the results to other metals as well as to other experimental conditions. Finally, there are a number of points to be developed for better understanding of cascade damage and for extending the “in-situ” techniques to other materials as non-metallic solids and to other intriguing phenomena. [doi:10.2320/matertrans.MD201304]

This report describes the status review of the JSPS Grant Team to develop advanced materials for the spallation neutron sources and modeling of radiation damage. One of the advanced materials is a toughness enhanced, fine-grained tungsten material (W-TiC) having four-times larger fracture toughness than ordinary tungsten and appreciable RT ductility in the recrystallized state. The other is an intergranular crack (IGC)-resistant austenitic stainless steel which was processed by the grain-boundary engineering (GBE). The experimental results are devoted to corrosion in a lead–bismuth eutectic, arrest of corrosion of weld-decay, radiation damage and creep rupture as well as new technique of GBE using a laser and annealing procedure. New technique seems to be applicable to large or complicated-shaped components. A series of the multi-scale models is built up from nuclear reaction between incident particles and medium nuclei to material property change due to radiation damage. Sample calculation is made on 3 mm-thick nickel bombarded by 3 GeV protons.

So far, we have found that the hardness of Al–Cu–Mg alloy (JIS2017, Duralumin) increases by energetic heavy ion irradiation at room temperature. Observations by using the three-dimensional atom probe (3DAP) have revealed that nano-meter sized precipitates are homogeneously distributed in the irradiated specimens, which are produced through the irradiation enhanced diffusion of solute atoms. The small precipitates contribute to the increase in hardness. In this report, we show the results for the hardness modification of Al–Cu–Mg alloy by the combination of energetic ion irradiation and thermal aging treatment. The hardness of the specimens pre-irradiated and thermally aged at 423 K is much larger than that without the pre-irradiation. The present result indicates that the combination of energetic ion irradiation and subsequent thermal aging can be used as an effective tool for the hardness modification of Al–Cu–Mg alloy.

Changes in mechanical property of Ni under irradiation by 3 GeV protons were estimated by multi-scale modeling. The code consisted of four parts. The first part was based on the Particle and Heavy-Ion Transport code System (PHITS) code for nuclear reactions, and modeled the interactions between high energy protons and nuclei in the target. The second part covered atomic collisions by particles without nuclear reactions. Because the energy of the particles was high, subcascade analysis was employed. The direct formation of clusters and the number of mobile defects were estimated using molecular dynamics (MD) and kinetic Monte-Carlo (kMC) methods in each subcascade. The third part considered damage structural evolutions estimated by reaction kinetic analysis. The fourth part involved the estimation of mechanical property change using three-dimensional discrete dislocation dynamics (DDD). Using the above four part code, stress–strain curves for high energy proton irradiated Ni were obtained.

Self-interstitial atom (SIA) type dislocation loop is one of the possible candidates of the so-called matrix damage that causes hardening and embrittlement of blanket structural materials of fusion reactors and/or pressure vessel materials of light water reactors. We present in this paper molecular dynamics computer simulation results on the interactions between an edge dislocation and a SIA loop with Burgers vectors of b = a 0 /2 [111] and b = a 0 /2 [111], respectively, which are introduced in bcc-Fe crystal. Then shear stresses of several different magnitudes are applied so that the dislocation moves to meet the SIA loop. General observation is that the SIA loops with diameter of ∼2 nm can be obstacles to dislocation motion, and the strength as obstacles to dislocation motion depends on applied stress. The origin of the stress dependent strength can be explained athermally using the elastic theory of dislocation interaction. In most cases, the SIA loops are absorbed by the edge dislocations to form a large super-jog after the interactions. This suggests a possibility of localized deformation of irradiated bcc-Fe due to the formation of dislocation channeling.

To study the mechanism of radiation-enhanced clustering of copper atoms in Fe–Cu alloys, in situ electrical resistivity measurements are performed during irradiation with 100 MeV carbon ions and with 2 MeV electrons at 300 K. Two kinds of highly pure Fe–Cu alloys with Cu content of 0.02 and 0.6 wt% are used. The results are summarized as follows: (1) Although there is a steep initial resistivity increase below about 10 μdpa, the resistivity steadily decreases after this initial transient in Fe–0.6wt%Cu alloy, while in Fe–0.02wt%Cu alloy, the resistivity either decreases slowly or stays almost constant. The rate of change in resistivity depends on copper concentration. (2) The rate of change in resistivity per dpa is larger for electron irradiation than for ion irradiation. (3) Change in dose rate from 10 −8 to 10 −9 dpa/s slightly enhances the rate of resistivity change per dpa. The decrease in resistivity with dose is considered to be due to clustering or precipitation of copper atoms. The initial abrupt increase in resistivity is too large to be accounted for by initial introduction of point defects before copper clustering. Tentatively the phenomenon is explained as due to the formation of embryos of copper precipitates with a large strain field around them. Quantitative evaluation of the results using resistivity contribution of a unit concentration of Frenkel pairs and that of copper atoms gives an important conclusion that more than one copper atom are removed from solid solution by one Frenkel pair. The clustering efficiency is surprisingly high in the present case compared with the ordinary radiation-induced or radiation-enhanced precipitation processes.

Kinetic Monte Carlo (KMC) simulations of defect accumulation in neutron-irradiated bcc-Fe are presented. Comparisons of the KMC simulations with the recent experimentals are made to discuss the validity of the proposed KMC model. Then, the effect of environmental variables such as irradiation temperature, dose rate, and neutron spectrum is studied using the technique. The magnitude of dose rate effect is irradiation-temperature dependent, and it is found that a threshold dose rate, below which defect accumulation is not affected by the difference of dose rate, exists when irradiated at 600 K. An evaluation of the number of vacancy jumps of both irradiation-induced vacancies as well as thermal vacancies is done to study a possible effect of dose rate on a solute diffusion in bcc-Fe. The results show that the total number of vacancy jumps is dose rate dependent at very low and high dose rates while there is a dose rate independent region in between.

Two kinds of model alloys, Fe–0.6 wt.% Cu and Fe–1.2 wt.% Cu, are irradiated with GeV heavy ions at 250 C and at room temperature. Vickers microhardness test at room temperature shows that the change in hardness due to irradiation strongly depends on the irradiation temperature and the copper content. For Fe–1.2 wt.% Cu alloy irradiated at 250 C, the hardness change is much larger than that for 2.5 MeV electron irradiation at the same dpa (displacements per atom through elastic collisions). The experimental result implies that the electronic excitation by GeV ions enhances the copper precipitation. This effect is, however, hardly observed for Fe–0.6 wt.% Cu alloy or for the irradiation at room temperature. 2002 Elsevier Science B.V. All rights reserved.

To study the mechanism of irradiation embrittlement in pressure vessel steels, it is necessary to sort out the irradiation parameter dependence, which is particularly complex in the neutron irradiation case. Irradiation with high energy heavy ions allows us to carry out single parameter experiments, thereby providing basic data for the modelling of irradiation embrittlement. In this study using Fe–Cu model alloys, three important parameters; ion fluence, irradiation temperature and copper concentration are selected and irradiation induced Vickers hardness change is measured. The hardness change shows a steep increase with fluence after a certain incubation dose. The irradiation temperature dependence of hardness change at fixed fluence shows a peak at around 250–300 °C. Shift of irradiation temperature dependence from neutron irradiation case may be ascribed to the difference in dose rate. Copper concentration dependence shows that the change in hardness seems to depend on the square root of copper concentration. This supports the dispersed barrier hardening model.

Direct formation of a large vacancy loop by displacement cascade in α-Fe has been observed for the first time in a molecular dynamics (MD) computer simulation study. This phenomenon occurred in one anomalous run out of 100 simulations of 50 keV primary knock-on atom energy cascades, in which one large displacement cascade was produced instead of the formation of smaller subcascades. Two large self-interstitial atom clusters were produced at the periphery of the cascade core, followed by the formation of a very high concentration region of vacancies at the centre of the cascade during the quenching of the thermal spike phase. Finally, one large vacancy loop with Burgers vector b = a 0 was formed by cascade collapse. A very-low-probability, one hundredth or probably less, for vacancy loop formation in the present MD simulation is consistent with the experimental observation of a low defect yield in irradiated α-Fe.

High-energy neutron irradiation on copper alloys will cause atomic displacements and nuclear transmutation, which will lead to degradation of thermal conductivity and mechanical properties. To study the synergistic effects, we have utilized Cu–Ni–Zn alloys to simulate the effect of transmutation. Thermal conductivity of these alloys has been derived from electrical resistivity measurements between 298 and 770 K. These alloys were also irradiated with 2.4 MeV copper self ions at room temperature and the mechanical properties were evaluated from micro-hardness measurements. The resistivity of the copper alloys depends linearly on nickel concentration. However, the effect of zinc is complex and may be less prominent than the nickel effect. It is suspected that dezincification has taken place. The change in micro-hardness shows saturation at a displacement dose of around 0.5 dpa. There is a slight influence of alloying elements, particularly zinc, on the fluence dependence of hardness change.

It has been reported that there are substantial differences in radiation effects in fcc copper and bcc iron. Whether these differences are due to the difference in crystal structure or not is the subject of the present paper. These differences have been discussed in terms of microstructure and mechanical property changes, whereas in the present paper, results of electrical resistivity measurements are discussed in terms of damage production cross sections, defect annihilation cross sections, damage efficiency and so on during and after various ion irradiations with wide energy ranges from 1 MeV to more than 100 MeV. For crucial discussion on the effect of the difference in crystal structure, nickel and iron are compared. These metals are allotted closely in the periodic table, with similar melting points and fairly strong electron–lattice coupling, both ferromagnetic and yet with different crystal structure. It may be concluded that as far as the damage production and defect annihilation cross sections and survival ratio are concerned, the difference in crystal structure is not an essential factor. Electronic energy deposition may play an important role even for low energy ions as well as for high energies. The effect of electronic energy deposition on defect clustering is discussed.

A number of recent problems in structural materials for nuclear energy systems require quantitative and reliable predictions of materials behaviour in as yet unrealized operating conditions. An example is found in pressure vessel steels of a light water reactor, where prediction of embrittlement of the steel for extended period of service must be done with confidence, because the integrity of the pressure vessel is of vital importance for the safety of the light water reactor. Light water reactor fuel, cladding and wrapper of a fast breeder reactor and first wall and blanket structural materials of a fusion reactor are such examples that are briefly discussed. In such problems, we have either scarce data or limited and rather irrelevant data of the materials performance for the service conditions of the materials in question. The method used to predict the irradiation behaviour of materials from incomplete existing data is called irradiation correlation. The correlation methodology is discussed. ...

Primary knock-on atom (PKA) energy spectrum extends up to several hundreds of keV in fusion reactor materials irradiated with 14 MeV neutrons. When we are going to evaluate materials behavior in an expected d-Li type intense neutron source, fundamental knowledge on effects of high energy PKA on formation of cascade damage, microstructural evolution and properties of irradiated materials is required. In this study, 170 MeV self-ion irradiation of thin foils of gold were performed to estimate effects of very high energy PKA on formation of defect clusters by cascade damage and its interactions. Defect clusters of vacancy type were observed in the thin foils of gold irradiated to 5 × 10 13 –1 × 10 15 ions/m 2 . In the case of irradiation with 170 MeV self-ions of which the projected range exists at 6.3 μm from the ion incident surface, films of gold, 1.1, 3.0, 4.3, 6.3 and 7.2 μm in thickness, were placed in front of the 50 nm thick specimens to change PKA energy spectrum within the specimens. The number of vacancy clusters within a cluster group formed by a PKA varied with the thickness of gold film. High energy PKA was found to increase the number of defect clusters. However, size distributions of defect clusters were not strongly dependent on PKA energies. Interactions of high energy cascades result in the appearance of new defect clusters near the existing defect cluster groups in the higher dose range. Dose dependence of defect cluster density was similar to that observed in 14 MeV and fission neutron irradiated specimens. The contribution of PKA higher than 400 keV to the interactions of cascades is estimated from the calculated PKA energy spectrum and low energy self-ion irradiation data.

High strength high conductivity copper alloys will be used for heat sink materials of a high heat flux component of a fusion reactor. High energy neutron irradiation on these materials will cause atomic displacements and nuclear transmutation, which will lead to degradation of thermal conductivity and mechanical properties of the materials. These effects will be briefly reviewed with special emphasis on synergistic effects of displacements and transmutation. It should be kept in mind that even pure copper will be transformed into Cu-Ni-Zn alloy by fusion neutron irradiations. This effect on the thermal conductivity and mechanical properties cannot be ignored for long term use of copper alloys in neutron irradiation environment.We have been utilizing Cu-Ni-Zn alloys to simulate the effect of nuclear transmutation. Displacement damage with fusion neutrons has been simulated by ion irradiation with 2.4MeV copper self ions. Results of these experiments will be summarized.

In situ transmission electron microscopy (TEM) observation of radiation damage with energetic heavy ions has been a powerful tool for the study of radiation effects since it provides means to observe evolution of cascade damage. The objective of the present paper is to summarize the status of this experimental technique, its major achievement and current problems and to suggest the future directions. Defect accumulation by irradiation with energetic heavy particles is quite different from that with electron irradiation. Spatial and temporal fluctuations occur in defect production and annihilation. These observations will be summarized together with the dependence of damage evolution on irradiation temperature, target materials, incident ions and specimen geometry. Self ion irradiation is particularly useful for the study of cascade damage. Successful results of self ion irradiation for ion-neutron correlation will be presented. The experimental issues in such in situ heavy ion radiation damage studies are that the samples have to be thin enough to be transparent to the TEM electrons and that the range of self ions to simulate a primary knock-on atom (PKA) is generally short. Moreover, damage distribution is strongly depth-dependent. We have been purposely utilizing ‘thin foils’, taking advantage of the strong sink effect of surfaces for mobile point defects. The issues will be discussed in terms of the potential use of this technique to derive cascade damage characteristics in the bulk. Additional techniques such as computer simulation of cascade should be incorporated in conjunction with the experiment in order to develop models of defect production, annihilation and evolution. Prospect of the experimental technique will be discussed.

Development of materials for first wall and blanket structures which are resistant to high fluence irradiation of fusion spectrum neutrons is a key issue of realizing fusion energy systems. Since fusion reactor environment has not been realized as yet, prediction of materials behavior must be based on fundamental radiation damage studies combined with the data from existing irradiation facilities. High fluence irradiation, high PKA energies and high nuclear transmutation rates are the important factors in considering the fusion radiation effects. Experimentally, various surrogate means have to be used, for which establishing irradiation correlation is important. The correlation should not merely be a phenomenological one but it should be based on physics-based mechanisms. The methodology of irradiation correlation will be discussed. This includes discussion of PKA dependent damage parameters, computer simulation, modeling and model integration. The methodology, once established, will also be applicable to lifetime prediction of materials of neutron interactive components in the fusion reactor as well as those in the existing nuclear energy systems.

Research activities on fusion materials by Japanese university researchers are reviewed. Organized research on fusion material has been initiated around mid 1970s under auspices of Monbusho (Ministry of Education, Science and Culture). Particularly effective was the Special Research Project on Fusion for fiscal year 1980–1989. At the same time, Japan/U.S. collaboration on fusion materials (1982–2000) has been very successful, yielding numerous useful results. The highlights of the technical achievement of these projects are briefly summarized. Both of these projects may be characterized to be composed of two major tasks, namely, fundamental aspects of alloy development for fusion and high fluence irradiation effects under fusion reactor environment. The basic philosophy of the project is discussed. The recent trend is to organize the university research activities into a comprehensive research network.

The chemical potential approach, making it possible for one to account for density effects in supersaturated solution of point defects, is applied to consideration of interstitial loop formation kinetics in metals under homogeneous irradiation. Quasi-chemical rate equations are reformulated in terms of chemical potential, rather than in terms of concentrations of species involved in reactions, following a procedure known in the theory of irreversible processes. The extension of rate equations is tested on a simpler model of bimolecular reactions. Then the density effect, such as radiation induced spontaneous clustering of interstitials, is investigated in irradiated metals on the background of interstitial loop formation as described by conventional rate equation solution. The phenomenon, that reveals itself by spontaneous formation of di-interstitials, would be expected to become noticeable at irradiation temperatures about 0.25 to 0.3 of the absolute melting temperature if the rate of defect generation is high enough. For a steady-state solution analyzed, the density effect brings about the higher number density of forming interstitial loops with reduced mean dimensions if compared with conventional based consideration. Possibilities of experimental investigations of the proposed mechanism of interstitial loop formation are discussed.

In the paper, an open system of point defects in metals under irradiation at temperatures below one third part of the melting point is considered. A consideration is based on an analysis of chemical potentials of interstitials and vacancies in their supersaturated solid solutions. Density effects in supersaturated solutions that can be naturally ascribed to terms of higher orders in an expansion of the chemical potential over the concentration of the solute species are tken into account. It is shown that the mean square values of spatial fluctuations in the supersaturated solution remarkably increase. Such a situation takes place when, for some irradiation conditions, the temperature of a sample is low enough to reduce mobilities of point defects. The density effects would be expected as to be able to cause a spontaneous clustering process at high supersaturations. Possible consequences of the proposed effect for irradiation experiments are discussed.

The spatial distributions of atomic displacement at the end of the collisional phase of cascade damage processes were calculated using the computer simulation code MARLOWE, which is based on the binary collision approximation (BCA). The densities of the atomic displacement were evaluated in high dense regions (HDRs) of cascades in several pure metals (Fe, Ni, Cu, Ag, Au, Mo and W). They were compared with the measured cascade collapse probabilities reported in the literature where TEM observations were carried out using thin metal foils irradiated by low-dose ions at room temperature. We found that there exists the minimum or “critical” values of the atomic displacement densities for the HDR to collapse into TEM-visible vacancy clusters. The critical densities are generally independent of the cascade energy in the same metal. Furthermore, the material dependence of the critical densities can be explained by the difference in the vacancy mobility at the melting temperature of target materials. This critical density calibration, which is extracted from the ion-irradiation experiments and the BCA simulations, is applied to estimation of cascade collapse probabilities in the metals irradiated by fusion neutrons.

A new mechanism of point defect clustering in irradiation environment is proposed. The process considered reflects a possibility for spontaneous (nondiffusional) clustering to exist in a highly enough supersaturated solution of point defects under certain conditions. The influence of the production of di-interstitial clusters by this mechanism on the kinetics of interstitial dislocation loop nucleation and growth is analyzed. It is shown that the intensity of irradiation must be high enough and the temperature of the sample must be correspondingly low enough for the effect of the spontaneous small interstitial clustering to prevail over the usual mechanism of diffusion driven nucleation and growth of interstitial clusters. Both analytical and numerical solutions of an extended version of conventional rate equations demonstrate the possibility of this phenomenon. This version takes into account the terms of higher order in an expansion of the chemical potential over the interstitial concentration. The new form of equations naturally follows from a canonical approach to chemical-type rate equations that is known in nonlinear nonequilibrium thermodynamics. When density effects are negligible, one gets the conventional kinetics of diffusion driven interstitial loop formation. A simple model of interstitial loop nucleation and growth through di-interstitial is used in the analysis of the clustering kinetics. A noticeable enhancement of interstitial loop number density with smaller radii is expected for temperatures below one third of the melting point, where the loop growth process due to diffusional processes is weak enough. For this novel phenomenon to be noticeable, a prolonged irradiation is necessary. Conditions and possibilities for experimental investigations of the predicted effect as well as the possible competitive processes are considered.

Defect distributions after the collisional phase of cascade damage processes were calculated using the computer simulation code MARLOWE, which is based on the binary collision approximation. The densities of vacant sites were evaluated in defect-dense regions at the end of the collisional phase in simulated ion irradiations of several pure metals (Au, Ag, Cu, Ni, Fe, Mo and W). The vacancy density distributions were compared to the measured cascade collapse efficiencies obtained from low-dose ion irradiations of thin foils reported in the literature to identify the minimum or “critical” values of the vacancy densities during the collisional phase corresponding to cascade collapse. The critical densities are generally independent of the cascade energy in the same metal. The relationships between physical properties of the target elements and the critical densities are discussed within the framework of the cascade thermal spike model.

To understand cascade damage formation as a function of primary recoil energy, thin foils of gold were irradiated with 20–400 keV self-ions to a fluence of 1.0 × 10 14 ions/m 2 at 300 K. The yield of groups of vacancy clusters per irradiated ion saturated at an ion energy higher than 100 keV. The number of clusters in a group was found to vary even for the same energy ions. The size distribution of the clusters was not strongly dependent on the number of clusters in a group and the ion energy. An attempt was made to predict the distribution of defect clusters in thin foils of gold irradiated with 14 MeV neutrons from the present data based on a primary recoil energy spectrum.

Presented at the IEA Workshop on the Use of Molecular Dynamics in Modelling Radiation Effects and Other Non-equilibrium Phenomena, May 6–8, 1991 at La Jolla, USA Molecular dynamics calculations wer...

原子力機器の高信頼性,長寿命化への要求がさらに高まるのに伴い,照射環境における材料挙動を機構論的理解の上に立って評価することに関心が持たれるようになっている。異なった照射条件下の材料挙動を関連づけてゆく場合のスケーリング則を「照射相関」と呼ぶが,圧力容器鋼の照射脆化予測も機構の理解の上に立ち,照射相関の考えに基づいて行うことが精度の向上につながると考えられる。本稿では,照射脆化機構研究についての現状を解説する。

D.J. Bacon ‘, D. Gavillet b, A. Horsewell ‘, S. Ishino d, L.K. Mansur +-, B.N. Singh C, H. Trinkaus f, H. Ullmaier f and M. Victoria b a Materials Sciences & Eng. Department, University of Liverpool, Liverpool L69 3BX, UK ’ Paul Scherrer Institut, CH-5232 Wigen PSL Switzerland ’ Materials Department, Rise National Laboratory, DK-4000 Roskilde, Denmark d Nuclear Engineering Department, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan ’ Metals & Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA f Forschungszentrum Jiilich, IFF, D-52425 Jz’ilich, Germany

Understanding of the defect production and annihilation processes in a cascade is important in modelling of radiation damage for establishing irradiation correlation. In situ observation of heavy ion radiation damage has a great prospect in this respect. Time and temperature dependence of formation and annihilation of vacancy clusters in a cascade with a time resolution of 30 ms has been studied with a facility which comprises a heavy ion accelerator and an electron microscope. Formation and annihilation rates of defect clusters have been separately measured by this technique. The observed processes have been analysed by simple kinetic equations, taking into account the sink effect of surface and the defect clusters themselves together with the annihilation process due to thermal emission of vacancies from the defect clusters. Another tool to study time and temperature dependence of defect production in a cascade is computer simulation. Recent results of molecular dynamics calculations on the temperature dependence of cascade evolution are presented, including directional and temperature dependence of the lengths of replacement collision sequences, temperature dependence of the process to reach thermal equilibrium and so on. These results are discussed under general time frame of radiation damage evolution covering from 10 −15 to 10 9 s, and several important issues for the general understanding have been identified.

The effect of hydrogen on cavity formation in vanadium was investigated using dual ion irradiation at 773,873 and 973 K with hydrogen injection of 0, 15, 30 and 60 appm/dpa doses up to 50 dpa. In some of the samples after dual beam irradiation with H and Ni ions, bimodal cavity size distribution was observed. The injected hydrogen may affect cavity nucleation in a similar manner as helium. There is a critical radius of the cavity under H dual irradiation which depends on the irradiation temperature, like He dual irradiation. The dependence of the critical radius on the amount of injected hydrogen is not clear, but it is suggested that the amount of hydrogen more than a certain value is necessary to promote the nucleation of cavities by injected hydrogen.

In-situ observation of defect clusters was performed to examine cascade damage produced by energetic heavy particle in gold. Thin foils of pure gold were irradiated with 400 keV Xe+ ions using a combined facility of a 400 kV heavy ion accelerator and a 200 kV transmission electron microscope (TEM). TEM images of defect clusters under irradiation were recorded on videotapes, and ion fluence dependence of the defect cluster density, production and annihilation efficiencies of defect clusters were measured on the video images. Defect cluster density under irradiation was determined by the balance between the production and the annihilation of defect clusters. A simple model of accumulation processes of cascade damage was proposed.

To study the effect of helium and lithium on cavity formation for V and V-Ti alloys in a fission reactor, V, V-5Ti and V-20Ti (both doped and undoped with 0.014 10 B) were irradiated at 365°C to 14 dpa (below-core) and at 406/520°C (level-2) to 42.5 dpa in the FFTF/MOTA. The He/dpa ratio was estimated to be about 22 at 365°C and 5 at 406 and 520°C. TEM observations following irradiation indicated that helium and lithium had little effect on swelling in the specimens. Cavity growth was assisted by helium in V-Ti alloys, though the number density for cavities was rather low compared to that of undoped alloys. Agglomeration of small dislocation loops, designated as “rafts”, was the main feature observed in V-5Ti, whereas tangling of dislocations was predominant in V-20Ti. Halos containing many cavities were formed around boride precipitates only in V-20Ti-0.014 10 B at 520°C, and arose from the effect of lithium.

Molecular dynamics calculations were performed to study the cascade damage evolution, especially paying attention to the cascade equilibrating process during the first several pico-seconds. The calculated region was surrounded by periodic boundary condition, and the interatomic potentials used here were either the pairwise potential or the many-body potential. From the thermodynamic characterization of the cascade evolution, it was concluded that during the cooling phase of the cascade the system was locally equilibrated. Temperature effects on the cascade damage evolution were also discussed and it was suggested that the damage parameter function must include the temperature of the target materials.

Collision cascades initiated from high energy PKAs produce defect clusters in crystalline solids irradiated with fast neutrons or energetic ions. They will affect not only an early stage of microstructural evolution but that at high fluence by changing free defect survival rate. To elucidate fundamental processes of cascade damage evolution, in situ observation of microstructure in FCC metals under heavy ion irradiation has been carried out using a combined facility of a 400 kV accelerator and a 200 kV transmission electron microscope installed in the University of Tokyo, Tokai. Defect clusters produced by individual energetic ions are observed during irradiation to examine subcascade formation, interaction of point-defects from cascade damage and related point-defect processes.

R&D programs on fusion reactor materials in Japan have been promoted mainly by universities, JAERI and NRIM. The historical development and highlights of the research of each of these sectors will be reported together with on-going programs and planning activities. The topics include the foundation of the National Institute of Fusion Science (NIFS), planning of an Energy Selective Neutron Irradiation Testing Facility (ESNIT), construction of SUBNANOTRON facility, and current International Cooperation Programs. Industrial involvement is also growing larger in cooperation with universities, JAERI and NRIM. Future research trend and coordination of the research among these organizations will be discussed based on the objectives and roles of these organizations for the national fusion program.

Austenitic model alloys were irradiated with fast neutrons in FFTF (Fast Flux Test Facility) to investigate fundamental mechanisms of microstructural evolution in simple austenitic steels under relatively high dose irradiation. Irradiations were performed in MOTA (Material Open Test Assembly) at 693, 793 and 873 K. Void swelling was reduced by the addition of minor elements such as titanium and carbon to a simple Fe-Cr-Ni ternary alloy at all the tested temperatures. In the titanium bearing alloys, very small cavities were detected on dislocation lines and MC precipitates. Fine dislocation loops were observed in the matrix of the irradiated materials and their size and density were not found to be affected by the solute additions.

After brief descriptions on critical requirements to materials databases, two new features of a material database for materials design under development by the present authors, are reported. The first feature concerns “geological or geographical maps”, i.e., generic files on typical but comprehensive data sets for positioning of each set of data. The second one concerns integration of relevant materials databases, each of which has its own set of abstract views on materials information reflecting its objective and constraints due to complexities of the materials and difficulties to describe them.

To link a molecular dynamic calculation to binary collision approximation codes to explore high energy cascade damage, time between consecutive collisions is introduced into the binary collision MARLOWE code. Calculated results for gold by the modified code show formation of sub-cascades and their spatial and time overlapping, which can affect formation of defect clusters.

Simple model alloys of Fe-16Ni-15Cr-xC-yTi-zNb were irradiated at 673 K up to 2.5 dpa in the experimental fast breeder reactor JOYO. TEM examinations were made to investigate the separate and synergistic effects of C, Ti and Nb additions on the development of microstructure. Additions of C decreased swelling resulting from decreased cavity number densities. Ti or Nb alone changes little swelling. Ti significantly decreased cavity number densities. C additions to the alloys with Ti and Nb increased the size and number density of cavities; the number density of smaller sized cavities (for example

Need for the intense neutron sources for the development of fusion materials has been recognized from the early stage of fusion development. A brief history including recent international collaborative activities is described.The most difficult problem in developing fusion materials is related to radiation damage produced during prolonged irradiation in a fusion energy system. Suitability to study this will define the requirements of the neutron source. Although there have been a number of proposals for the source, there seem to be no intense neutron sources satisfying all the suitability and feasibility requirements. Inherent problems and issues to be solved are summarized for various kinds of the sources. In particular, detailed integral descriptions on a d-Li stripping reaction source facility are given as a reference for considering the irradiation facility. It is emphasized that the intense neutron source facility for materials research is not merely an accelerator or a plasma machine but an integrated one composed of ion source, accelerator, target, irradiation cell, remote maintenance, shielding, hot laboratories and waste disposal.

Computer simulation calculations have been carried out of precipitate recoil resolution under energetic particle irradiations. Using MARLOWE code, recoil range distributions in complete cascade events from primary recoilswith energies between 1 to 100 keV are calculated in pure iron. From these results, precipitate shrinkage rate is estimated in various irradiation conditions. The resolution rate is found to be highly sensitive to projectile mass rather than projectile energy. This suggests a considerable difference of the resolution rate under heavy and light ion irradiations. Remarkable change in the precipitate size begins only after irradiations to several tens dpa. This implies that some precipitates, which are present at low to intermediate doses, can disappear at high doses by recoil resolution. It is shown, by comparison with fission neutron irradiation data, that the recoil resolution of tiny but numerous precipitates can be a factor to determine the incubation dose of void swelling.

Charged particle irradiations were originally expected to provide means to simulate the effect of neutron irradiations. However, it has been recognized that quantitative and sometimes even qualitative simulation of neutron radiation damage is difficult and the role of the charged particle irradiations has shifted to establishing fission-fusion correlation based on fundamental understanding of the radiation damage phenomena. We have been studying radiation effects in fusion materials using energetic ions from the latter standpoint. In this paper, we will review our recent results using a heavy-ion/electron microscope link facility together with sets of small heavy ion and light ion accelerators on cascade damage produced by energetic primary recoils and on the effect of helium on microstructural and microchemical evolution. Some of the other applications of the ion accelerators will also be mentioned.

The Fourth International Conference on Fusion Reactor Materials was held in Kyoto in December, 1989. Some of the statistic figures are shown to illustrate the overall activity of the world fusion materials community. Several topics presented in plenary sessions and poster sessions are highlighted. One of the specific features of this Conference is that the third and fourth days are entirely devoted to six discussion sessions. Some of the important outcomes from this first trial are presented.

This is a review of a series of experiments on formation, annihilation, structure or behaviour of cascade defects by in-situ observation using a 400 kV heavy ion accelerator/200 kV electron microscope link facility at the University of Tokyo. Most of the results described here are obtained in gold. The experiments take advantage of the high mobility of interstitials in gold and of the strong effect of surface sinks on the thin foil specimens, which reveal vacancy properties rather distinctively. The temperature of the in-situ observations ranges from 120 to 800 K. The cascade defect images are formed instantaneously during irradiation, being composed of several groups of images at lower temperatures especially below 470 K, reflecting sub-cascade structure. A new concept of sub-cascade energy is introduced. For irradiation at higher temperatures, the defect images become sharper and the majority are identified either as loops or stacking fault tetrahedra (SFT). The size of the defects shows that s...