[en] To avoid repeating an accident similar to the one that occurred at TEPCO’s Fukushima Daiichi plant, policy bills to reform regulatory organizations and systems were submitted to Congress. A new nuclear regulatory authority will be created by the Ministry of the Environment, aimed at obtaining a nuclear regulatory body capable of preserving total independence in its regulatory decision making processes. One of the focal points is enhancement of measures against severe accidents and reassessment of the potential effects of initiating external events. Implementation of these measures will be mandatory and defined by law. Discussions have already started to clarify the regulatory requirements to allow regulated implementation of these policies at the national level. One of the most important lessons learned is that operating plants in Japan lacked a vision for continuous safety improvement and the necessary related programmes for its implementation. The changes being proposed to enhance safety at the Japanese plants involve the basic regulatory safety framework necessary to implement any back-fitting programme and enact a comprehensive safety assessment of all the Japanese plants. The newly proposed regulatory system imposes an operation limit of 40 years to deal with aged reactors. Stress tests are being conducted to enhance public confidence in continuing nuclear power plant operation. In order to implement the safe decommissioning of the accident site in Japan, many issues are still uncertain or need to be investigated further. To that effect, and in the interest of all nuclear power operators around the world, international collaboration in R&D is strongly encouraged. The utilization of materials and components extracted from decommissioned reactors in research laboratories is strongly encouraged so as to meet the overall objective of achieving safe, LTO and effective PLiM of nuclear power plants around the world

[en] Model austenitic steels based on Fe-15Cr-16Ni with additions of 0.25Ti, 500 appm B, or 0.25Ti-500 appm B were irradiated in FFTF/MOTA over a wide range of dose rates at ∼400 deg. C. In addition to the effect of dose rate on swelling, it was desired to study the effect of boron addition to produce variations in He/dpa ratio. A strong effect of dose rate was observed, so strong that the relatively small distances separating the boron-free and doped alloys introduced a complication into the experiment. For specimens irradiated within the core, boron addition had no significant effect. For irradiations conducted near or outside the core edge, swelling appeared to be either enhanced or decreased by boron. The variability was a consequence of a strong dose rate effect overwhelming the influence of boron and helium. It is shown that helium exerted little influence relative to other important factors in these alloys

[en] Effects of dose rate on microstructural evolution in a simple model austenitic ternary alloy are examined. Annealed specimens are irradiated with fast neutrons at several positions in the core and above core in FFTF/MOTA between 390 deg. C and 435 deg. C in a wide range of doses and dose rates. In Fe-15Cr-16Ni, swelling seems to increase linearly with dose without incubation dose. Cavities are observed even in the specimens irradiated to 0.07 dpa at 1.9x10^-9 dpa/s. Both cavity nucleation and growth are enhanced by low dose rates. These are mainly caused by accelerated formation of dislocation loops at lower dose rates. Low dose rates enhance swelling by shortening incubation dose for the onset of steady-state swelling. In the specimens irradiated at higher dose rates to higher doses, high density of dislocation increases average cavity diameter, however decreases cavity density

[en] The Prime Candidate Alloy (PCA) for a near term fusion reactor is irradiated with dual ions to investigate the microchemical evolution in this alloy under fusion irradiation conditions. Radiation induced solute segregation or depletion, depending on the atomic size factor, is observed by EDS analysis on cavities and grain boundaries. At 830 K grain boundary migration is induced by irradiation and results in a row of cavities and Ni concentration peaks at the original position of the grain boundary. At 830 and 870 K, irradiation causes the precipitation of a Ni, Ti, Si and Al rich phase and the decrease of Ni concentration in the matrix, leading to another peak in the swelling variation with temperature. (orig.)

[en] Fe-15Cr-16Ni, -0.25Ti, -500appmB, and -0.25Ti-500appmB have been irradiated in FFTF/MOTA over a wide range of dose rate which covers more than two orders difference in magnitude, within the very limited temperature range of 387-444 C. The effects of dose rate and boron addition on swelling are examined. Lower dose rates increase the swelling by shortening the incubation dose for swelling. Addition of boron does not significantly change the swelling nor the dose rate dependence of swelling for both the ternary and Ti-modified alloy. The helium pressure of cavities is found to be much smaller than the surface tension at every irradiation condition including the lowest dose and dose rate, helium generated by boron transmutant does not play any role in cavity formation in this experiment. Cavities form without helium. The difference in cavity morphology by boron addition is most likely caused by formation of borides and by lithium

[en] The effect of interactions of cascade damage on clustering and point-defect behavior in α-iron at 563 K is simulated by the Monte-Carlo (MC) method. Cascade damage near the existing cluster increases recombination of point-defects, but freely migrating defects formation is not strongly modified by the overlapping. Growth and shrinkage of vacancy clusters depends on the distance between cascades. Small interstitial clusters tend to grow by the interaction of nearby cascades. Clustering, recombination and the production of freely migrating defects are quantitatively estimated for two 10 keV cascade interactions

[en] The stability of MC precipitates in the Japanese PCA is investigated by the following two methods. First, recoil resolution of the precipitates under heavy-ion irradiation was investigated using techniques for in-situ observation. The size and morphology of the precipitates (>20 nm) did not change for irradiation up to a dose of 100 dpa. Secondly, the distribution of alloying elements during irradiation at high temperature was measured by X-ray EDS Energy Dispersive Spectroscopy. Irradiation caused the enrichment of Ni and Si atoms around the precipitates, while in the case of simultaneous injection of helium, Si atoms were found to be absorbed into the precipitates. (orig.)

[en] A Fe-9Cr-2Mo-1NiVNb ferritic/martensitic steel designated JFMS has been selected as a reference material for future fusion reactor applications by the Japanese fusion reactor materials research group. To obtain fundamental information on the microstructural evolution during irradiation in the normalized and tempered JFMS, samples have been irradiated with 400 keV Al-ions at 773 K to 30 dpa followed by TEM observations. The microchemical characterization of the samples in the unirradiated and irradiated conditions has been carried out using an Energy Dispersive X-ray Spectroscopic (EDS) technique. The data show that irradiation up to 30 dpa does not cause saturation of the dislocation density nor any cavity formation. Precipitates of M₂₃C₆, M₆C. laves phase and NbC or NbN are found in the unirradiated sample. Irradiation accelerates precipitation of fine particles on the dislocations generated. EDS analysis reveals that the ferrite grains are richer in Mo than are the tempered martensite in the unirradiated sample and that the compositions of Fe, Cr and Mn in both phases remain unchanged by the irradiation. (orig.)

[en] Defect production and microstructural evolution in fusion materials should be evaluated based on the primary knock-on atom (PKA) energy spectrum. In this study, formation and interaction of cascade damage are evaluated as a function of PKA energy from irradiation experiments on thin foils of gold irradiated with fission neutrons and 21 MeV self-ions. These results are compared with fusion neutron irradiation data. The interaction of cascades at higher dose is found to result in the appearance of new clusters near the existing groups of vacancy clusters. These are considered to come from the conversion of invisible vacancy rich regions to visible clusters by near-by cascades. The minimum PKA energy to produce new visible clusters by interaction is estimated to be 165 keV, assuming that cascade zone size increases with damage energy. (orig.)

[en] Full text: The roadmap for light water reactor safety technology and human resource has been constructed by the Special Committee on Nuclear Safety Research Roadmap in the Atomic Energy Society of Japan (AESJ). Based upon the lessons learned from the Fukushima Daiichi accident, effective planning of research activities to improve safety can also contribute to enhance human resource and management of accumulated knowledge-base in the future domestic and international community. (author