Masahide Suzuki

In situ magnetic measurements under neutron radiation in Fe metal and low carbon steel

Magnetic minor hysteresis loops of low carbon steel and Fe metal have been measured during neutron radiation at 563K in a 50MW nuclear reactor. For investigation of nucleation mechanism of copper precipitates and dislocation loops during neutron radiation, special attention was paid to minor-loop coefficients, which are deduced from simple relations between minor-loop parameters and are very sensitive to lattice defects such as dislocations, copper precipitates, and grain boundaries. We found that with increasing neutron fluence, the minor-loop coefficients of low carbon steel sharply increase and show a maximum at the fluence of 1×1019cm−2, followed by a slow decrease. The appearance of the maximum suggests the presence of two mechanisms of internal stress; while copper precipitates and dislocation loops in the matrix make the internal stress increase, those grown in the vicinity of dislocations compensate the internal stress of dislocations so as to minimize the elastic energy. On the other hand, the mi...

Kinetics of irradiation-induced Cu precipitation in nuclear reactor pressure vessel steels

The followup of the embrittlement of nuclear power reactor pressure vessels (RPVs) is of critical importance for the safety assessment in the nuclear industry. The prediction of their future degradation is based on the extrapolation of the past testing of surveillance materials irradiated in the power reactor and in material testing reactors with accelerated dose rates. Using positron annihilation spectroscopy, however, we here reveal a kinetics of irradiation-induced precipitation, i.e., very low dose rate can significantly enhance Cu nanoprecipitation. The mechanism results in the embrittlement in practical RPVs, occurring at a much earlier stage than that found from accelerated tests, suggesting that accelerated tests are not enough for prediction of the embrittlement from Cu nanoprecipitation.

Analysis of Minor Hysteresis Loops for Neutron Irradiation and Plastic Deformation in Pure Fe

In situ magnetic measurements of pure polycrystalline Fe have been performed at 563 K during neutron irradiation in a 50 MW nuclear reactor. Magnetic properties that are sensitive to lattice defects were obtained from measurements of minor hysteresis loops with low applied fields less than 2 kA/m. The minor-loop properties show a monotonic increase with increasing neutron fluence up to 5times1019 cm-2, which is due to the creation and growth of dislocation loops during neutron irradiation. The results of in situ measurements were compared with those of Fe single crystals at room temperature where dislocations were induced by tensile deformation

Stress Intensity Factor Estimation for Embedded and Surface Cracks in an RPV Subjected to Yielding of Cladding

The cladding of an RPV (Reactor Pressure Vessel) at the inner surface may be subjected to a plastic yielding due to a high thermal stress under some severe overcooling events, while the stress in base metal remains elastic. The stress intensity factor estimation at the deepest crack tip of an embedded crack (EC) or a surface crack (SC) under such loadings is essential in the integrity assessment of an RPV. However, an elasto-plastic FE analysis is required to obtain the stress intensity factor on this problem generally. A solution for an under-clad crack (UCC) was developed by EDF based on numerous 2D FE computations. This paper proposes a simplified estimation scheme which takes the yielding of cladding into account. This scheme estimates the stress intensity factor at the deepest crack tip of an embedded crack or a surface crack. It is assumed that the stress singularity does not exist at the shallowest crack tip in the cladding. To estimate the stress intensity factor of an embedded crack, the crack shape is replaced by a semi-elliptical one to utilize existing solutions of SC. Case studies to examine the proposed estimation scheme were carried out for an UCC and SC subjected to the SBLOCA, SLB and PTS transients, which were defined by a international round robin PROSIR (Pro babilistic S tructural I ntegrity of a PWR R eactor Pressure Vessel) project being conducted by OECD/NEA/IAGE-WG. It was found that the proposed scheme gives a reasonable estimation of the stress intensity factor for an UCC and a SC.Copyright