Yoshiyuki Kaji

In-Core SCC Growth Behavior of Type 304 Stainless Steel in BWR Simulated High-Temperature Water at JMTR

Irradiation-assisted stress corrosion cracking (IASCC) is one of the critical concerns when stainless steel components have been in service in light water reactors for a long period. In-core IASCC growth tests have been carried out using the compact tension-type specimens of type 304 stainless steel that had been pre-irradiated up to a neutron fluence level around 1 × 1025 n/m2 under a pure water simulated boiling water reactor (BWR) coolant condition at the Japan Materials Testing Reactor (JMTR). In order to investigate the effect of synergy of neutron/gamma radiation and stress/water environment on SCC growth rate, we performed ex-core IASCC tests on irradiated specimens at several dissolved oxygen contents under the same electrochemical potential condition. In this paper, results of the in-core SCC growth tests are discussed and compared with the results obtained by ex-core tests from a viewpoint of the synergistic effects on IASCC. From results of in-core and ex-core tests using pre-irradiated specimens, the effect of synergy of neutron/gamma radiation and stress/water environment on SCC growth rate was considered to be small, because the in-core data under the same ECP condition were similar to the ex-core data under the DO = 32 ppm condition.

Fundamental role of Σ3(1¯11) and Σ3(1¯12) grain boundaries in elastic response and slip transfer

The techniques of grain boundary engineering are rapidly gaining significance microstructural design. To understand individual grain boundary characteristics, the influence of grain boundaries on the elastic and plastic deformation behaviors of copper bicrystals with Σ3(1¯11) twin and Σ3(1¯12) grain boundaries were investigated by large scale molecular statics simulation. These grain boundaries were chosen as examples of coherent and incoherent grain boundaries. Nanoindentation tests perpendicular to the grain boundary plane were used to investigate local deformation properties. Our results showed that an incoherent boundary experiences a reduction in elastic resistance due to the increase in excess free volume and structure-dependent local indentation modulus, while a coherent boundary has little effect on the elastic deformation. The propagation of plastic deformation is strongly blocked by the dissociation into a displacement shift complete (DSC) lattice dislocation which explains the superficial absor...

Nonempirical prediction of impurity segregation in α-Fe from first principles

The segregation and clustering of impurities in α-Fe were investigated by first principle density functional theory calculations. The segregation tendencies of various elements observed in reactor pressure vessels were considered and the interaction characteristics between Fe and each impurity element were estimated by mean field approximation. Stable N-atom impurity clusters were subsequently chosen to evaluate the changes in free energy for clustering. These calculations show that Cu and Mn impurities embedded in α-Fe are more stable when they are in the segregated state. Conversely, Nb and Ta are stable in the separately solute state. The present estimates provide reliable suggestions for the segregation characteristics, and the tendencies are in good agreement with the recent atom probe observations. We suggest that the segregation tendency is derived from the d-orbital interaction and that the solubility limit is not necessarily correlated with the tendency of clustering formation.

Thermodynamic properties of neptunium nitride: a first principles study

The thermal and mechanical properties of neptunium nitride (NpN) were investigated by first principles calculations. From the Helmholtz free energy equilibrium lattice constants, thermal expansion coefficients, bulk moduli and specific heat capacities were calculated for temperatures up to 1500 K. The electronic specific heat capacity was also calculated from the electronic density of states at the Fermi energy. The obtained specific heat capacity reproduced the experimental data well. It was thus clarified that the specific heat capacity of NpN consists of the lattice and electronic specific heat capacities and the contribution of the lattice dilatation to the specific heat capacity.

Study of Weld Residual Stress Field in the Girth Seam H6A of Core Shroud of Boiling Water Reactor

Many accidents have occurred in nuclear power plants due to the intergranular stress corrosion cracking (IGSCC) in the heat affected zone (HAZ) of welded joint of the core shroud of boiling water reactors (BWRs) in past years. The IGSCC is considered to be caused by the synergistic roles of corrosion environment, neutron irradiation and the welding residual stress. After several decades, the degradation of Type 316L low carbon stainless steel used in the core shroud occurs due to the neutron irradiation and thermal cycles. The degradation can be referred to the irradiation hardening, segregation of the local chemical composition at grain boundaries and swelling. The synergistic effects of those eventually lead to the initiation and propagation of the irradiation-assisted stress corrosion cracking (IASCC) in core shroud for long operation. The HAZ of the girth seams H6a in the core shroud are sensitive to the stress corrosion cracking. We are focusing on the weld residual stress field around the girth seam H6a in the core shroud as weld. The analysis work adopted different approaches in ABAQUS to simulate the weld residual stress, and they are Static General Analysis (SGA) and Fully Coupled Temperature-Displacement Analysis (FCTDA) respectively. The former is much simple to finish the progress while cannot obtain much accurate results at the boundaries of beads due to the discontinuous temperature field in the model. The later analysis gave the much accurate results comparing with the experimental results. The axial stress field in the crossing section of the wall of the core shroud was also clarified.Copyright

Heterogeneous plastic deformation and Bauschinger effect in ultrafine-grained metals: atomistic simulations

The effect of the dislocation density on yield strength and subsequent plastic deformation of ultrafine-grained metals was investigated in large-scale atomistic simulations. Polycrystalline models were constructed and uniaxial tension and compression were applied to elucidate the heterogeneous plastic deformation and the Bauschinger effect. The initial yield becomes heterogeneous as the dislocation density decreases owing to a wide range of Schmid factors of activated slip systems in each grain. A different mechanism of the Bauschinger effect was proposed, where the Bauschinger effect of ultrafine-grained metals is caused by the change in dislocation density in the process of forward and backward loadings.

Remaining life prediction of the core shroud due to stress corrosion cracking failure in BWRs using numerical simulations

Stress corrosion cracking (SCC) of the welded joints in a reactor core shroud is the primary result of the residual stresses caused by welding, corrosion and neutron irradiation in a boiling water reactor (BWR). Therefore, the evaluation of SCC propagation is important for the safe maintenance of the core shroud. This paper attempts to predict the remaining life of the core shroud due to SCC failures in BWR conditions via SCC propagation time calculations. First, a two-dimensional finite element method model containing H6a girth weld in the core shroud was constructed, and the weld processing was simulated to determine the welds residual stress distribution. Second, using a basic weld residual stress field, the SCC propagation was simulated using a node release option and the stress redistribution was calculated. Combined with the J-integral method, the stress intensity factors were calculated at depths of 2, 3, 4, 8, 12, 16, 19, 22, 25 and 30 mm in the crack setting inside the core shroud; then, the SCC propagation rates were determined using the relation between the SCC propagation rate and the stress intensity factor. The calculations show that the core shroud could safely remain in service after 9.29 years even when a 1-mm-deep SCC has been detected.

Effects of hydrazine addition and N2 atmosphere on the corrosion of reactor vessel steels in diluted seawater under gamma-rays irradiation

Seawater was injected into the reactor cores in the Fukushima Daiichi Nuclear Power Station. Corrosion of primary containment vessel (PCV) steel and reactor pressure vessel (RPV) steel is considered to progress until the molten fuel debris is removed. To evaluate durability of the PCV and RPV steels, corrosion tests were conducted in diluted seawater at 50 °C under gamma-rays irradiation of dose rates of 4.4 and 0.2 kGy/h. To evaluate the effect of hydrazine (N2H4) as an oxygen scavenger under gamma-rays irradiation, 10 and 100 mg/L N2H4 were added to the diluted seawater. Without addition of N2H4, weight loss in the PCV and RPV steels irradiated with the 0.2 kGy/h dose rate was comparable with those without irradiation and weight loss in the vessel steels irradiated with the 4.4 kGy/h dose rate was higher than those without irradiation. Under irradiation, weight loss in the PCV and RPV steels in diluted seawater containing N2H4 was comparable with that in diluted seawater without N2H4. When gas phase in the flask was replaced with N2, weight loss in the PCV and RPV steels, and O2 and H2O2 concentrations in the diluted seawater decreased.

Variation in the surface morphology of polycrystalline UO2 powder induced by helium precipitation

Abstract This report addresses the precipitation of helium in polycrystalline UO2, which deforms the morphology of the grains and their surfaces. The formation of intragranular gas bubbles by the diffused helium can adversely affect the sintering process of ceramic-type nuclear fuel pellets. Helium was injected into pulverized UO2 particles at 1473 K by hot isostatic pressing (HIP). The specific surface area measured by volumetric gas adsorption instrument implied that small pores should exist on the as-helium-treated sample surface. Field-emission scanning electron microscopy observations showed that numerous shallow basins (approximately 500 nm in radius) with hexagonal fringe were formed on the surface. The basin resembles a ruptured blister whose lid has been forced open. SEM observations showed a uniform polygonal-shaped section of the gas bubble on the fracture surface; this implies that precipitated helium forms a negative crystal in the grain. These interesting results suggest the possibility that the rupture of the negative crystal formed in the vicinity of the surface is related to the formation of the basin with a hexagonal fringe.

Simulation on Nanostructured Metals Based on Multiscale Crystal Plasticity Considering Effect of Grain Boundary

Ultrafine-grained metals whose grain size is less than one micron have attracted interest as high strength materials. Whereas nanostructured metals produced by severe plastic deformation express remarkably peculiar behavior in both material and mechanical aspects, its mechanism has been clarified by neither experimental nor computational approaches. In this study, we develop a multiscale crystal plasticity model considering an effect of grain boundary. In order to express release of dislocation from grain boundaries, information of misorientation is introduced into a hardening law of crystal plasticity. In addition, carrying out FE simulation for FCC polycrystal, the stress-strain responses such as increase of yield stress due to existence of grain boundary are discussed. We investigate comprehensively the effect of dislocation behavior on the material property of nanostructured metal.