Pressurizer spray line of PWR plant cools reactor coolant by injecting water into pressurizer. Since the continuous spray flow rate during commercial operation is considered insufficient to fill the pipe completely, there is a concern that a water surface exists in the pipe and may periodically sway. In order to identify the flow regimes in pressurizer spray piping and assess their impact on pipe structure, a two-phase flow experiment under steam-water condition was conducted. In the experiment, temperature fluctuation due to swaying was investigated. With a full-scale mock-up made of stainless steel, two-phase flow of 100°C steam and 60°C water under atmospheric pressure condition was investigated. The experiment revealed continuous temperature fluctuation of 5°C at edge of the water surface. Also, the fluctuation disappeared when concentration of non-condensable gas (Dissolved Oxygen) reached 1ppm. Even though DO concentration reached minimum of 20ppb and the gas phase could be considered almost 100% steam, no violent motion of the surface was observed which was concerned when steam condensation involved. Water levels and values of steady state heat transfer coefficient were estimated and compared with the values by Dittus Boelter correlation.
This paper describes residual stress measurements and analysis of austenitic stainless steel pipe with a butt-welded joint. The measurements were done with neutron diffraction and strain gauge techniques. The measured results had typical characteristics of butt-welded pipe regarding both the decline of stress along the axial direction and the bending distribution of axial stress along the radial direction. The measured residual stress distribution by neutron diffraction was shifted more to the tensile side than that by the finite element method simulation. However, the measured radial and axial strains, except for the hoop strain determined by neutron diffraction, coincided well with analysis strains. The hoop strain was actually equivalent strain converted by a correction method because a different lattice plane had to be used to measure hoop strain. This might be one reason why the difference occurred. Therefore, future study of the correction method would be desirable.
Thermal fatigue may initiate at a T-junction or a branched off line where high and low temperature fluids mix. These are common piping elements in nuclear power plants. To ensure structural integrity against thermal fatigue during the design phase, it is important to estimate thermal load from such design specifications as flow rate, temperature difference, pipe diameter, etc. IMAT-F, an evaluation method integrating thermal hydraulic and structural analysis, was developed in this study to precisely determine thermal load excluding safety margins or conservative engineering judgment. The method was validated by numerical flow simulations of high-cycle thermal fatigue experiment SPECTRA, conducted by Japan Atomic Energy Agency. Results confirmed that IMAT-F can accurately simulate fluid and pipe wall temperature fluctuation using fluid-structure coupled analysis. Thermal stress fluctuation resulting from distribution of temperature fluctuation in the pipe wall was then calculated. Fluctuation fatigue life was also estimated for comparison with the experimental results.
This paper reports measured and estimated results of residual stress distributions of butt-welded austenitic stainless steel pipe in order to improve estimation accuracy of welding residual stress. Neutron diffraction and strain gauge method were employed for the measurement of the welding residual stress and its detailed distributions on inner and outer surface of the pipe as well as the distributions within the pipe wall were obtained. Finite element method was employed for the estimation. Transient and residual stresses in 3D butt-welded joint model were computed by employing Iterative Substructure Method and also commercial FEM code ABAQUS for a reference. The measured and estimated distributions presented typical characteristic of straight butt-welded pipe which had decreasing trend along the axial direction and bending type distributions through wall of the pipe. Both results were compared and the accuracy of measurement and estimation was discussed.
In a PWR plant, on top of the pressurizer, there’s spray line that cools the reactor coolant. The amount of minimum flow rate is considered to be insufficient to fulfill the pipe completely during commercial operation of the plant, so it is concerned that there is a surface of water in the pipe and it may periodically sway due to condensation of the steam from the pressurizer during commercial operation. In order to identify the flow conditions in the spray line piping and assess its impact on thermal stress, flow visualization experiment were conducted. In the experiment, air is used in substitute of steam to simulate the gas phase of the pressurizer. With a full scale mock-up made of acrylic, flow under room temperature and atmospheric pressure condition was visualized and the locations where periodical swaying of water surface may be observed were identified. The period of swaying and its extent is roughly estimated based on the experiment. Based on the visualization experiment, 3 patterns of swaying of water surface were assumed and the amplitude of thermal stress fluctuation when the layer swayed instantaneously was calculated. Thus, with the assumed 3 patterns of swaying based on the visualization experiment, it was confirmed that the amplitude of thermal stress would not exceed the endurance limit defined in the Japanese design and construction code.
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