Mitsuyuki Sagisaka

Effects of Carbide Precipitate Formation on the Change in Ultrasonic Velocity in Austenitic Stainless Steels

Ultrasonic tests were conducted for 304 type austenitic stainless steels with different annealing conditions and effects of carbide precipitate formation on the velocity changes were evaluated. The velocity increased with higher annealing temperature and/or longer annealing time. SEM observations indicated that carbide precipitates were formed mainly on grain boundaries. Results show that it is not the precipitation itself but the removal of carbon from the matrix that determines the velocity change.Copyright

Ultrasonic NDE for Irradiation-Induced Material Degradations

We have developed a prediction model of ultrasonic wave changes caused by microstructural changes in 304 stainless steel based on the theory of ultrasonic wave propagation and experimental results. The correlation between microstructure and ultrasonic waves was simulated in the model with microstructure components such as grain boundaries, voids, precipitates and dislocations taken into account. Material parameters used for the prediction model were determined from the literature, microscopic observations, and experimental results obtained from archive materials.Calculations were performed assuming respective and integrated changes of irradiation-induced microstructures and spectrum changes of ultrasonic signals due to microstructural changes were evaluated. Calculations for archive material were also performed for validation, and the results were reasonably consistent with experimental data. It was found that by selecting appropriate indicators one can identify the nature of microstructures and quantify the microstructural changes due to irradiation.© 2013 ASME

Development of Electrical Resistivity Measurement Systems for Inspecting Void Swelling in Irradiated Microscopy Discs

Results are presented of resistivity change measurements made on a model Fe-Cr-Ni-Ti-C austenitic alloy irradiated in the Fast Flux Test Facility to doses ranging from 1.87 to 67.8 dpa. Two different electrical resistivity measurement systems were developed to overcome problems associated with examination of small microscopy specimens in order to investigate small changes in resistivity arising from voids and other radiation-induced microstructural features. A correlation is shown between resistivity changes arising primarily from void swelling. However, contributions arising probably from radiation-induced redistribution and perhaps precipitation of carbon and titanium can dominate the resistivity at low dpa levels.Copyright

Economic evaluation of maintenance strategies for steam generator tubes using probabilistic fracture mechanics and a financial method

As an application of probabilistic fracture mechanics (PFM) and a financial method, a risk-benefit model was developed for the purpose of optimizing maintenance activities of steam generator (SG) tubes used in pressurized water reactors (PWRs). To justify whether or not it is worth while implementing the selected maintenance strategy in terms of an economic point of view, net present value (NPV) was calculated as an index which is one of the most fundamental financial indices for decision-making based on the discounted cash flow (DCF) method.

Maintenance Optimization of LWRs Based on PFM Analysis

We have been developing an integrated simulator for the maintenance optimization of LWRs (Light Water Reactors) based on PFM (Probabilistic Fracture Mechanics). As a typical example of the analysis, .an attempt was made to develop an economic evaluation model for the purpose of optimizing the maintenance activities of SG tubes in PWRs based on PFM approach, In the model, the probabilities of these risks were evaluated first and then the risk-benefit analysis was carried out to evaluate costs and profits as well during 60 years lifetime under various maintenance strategies. The model would be applied to pipes which are critical in terms of safety, availability and economy. The concept of the simulator is to provide a method to optimize maintenance activities for representative components and piping systems in nuclear power plants totally and quantitatively in terms of safety, availability and economic efficiency (both from cost and profit). The simulator will also provide a guideline regarding social acceptance of risk-based decision makings. This study has been conducted under “Innovative and Viable Nuclear Energy Technology (IVNET) Development Project” financially supported by Japanese METI.Copyright