Fatigue crack growth prediction in nuclear piping using Markov chain Monte Carlo simulation

In this paper, we present and demonstrate a methodology to improve probabilistic fatigue crack growth (FCG) predictions by using the concept of Bayesian updating using Markov chain Monte Carlo simulations. The methodology is demonstrated on a cracked pipe undergoing fatigue loading. Initial estimates of the FCG rate are made using the Paris law. The prior probability distributions of the Paris law parameters are taken from the tests on specimen made of the same material as that of pipe. Measured data on crack depth over number of loading cycles are used to update the prior distribution using the Markov chain Monte Carlo. The confidence interval on the predicted FCG rate is also estimated. In actual piping placed in a plant, the measured data can be considered equivalent to the data received from in-service inspection. It is shown that the proposed methodology improves the fatigue life prediction. The number of observations used for updating is found to leave a significant effect on the accuracy of the updated prediction.     

Impact of DEM accuracy and resolution on topographic indices

Topography is an important land-surface characteristic that affects most aspects of the water balance in a catchment, including the generation of surface and sub-surface runoff; the flow paths followed by water as it moves down and through hillslopes and the rate of water movement. All of the spatially explicit fully distributed hydraulic and hydrological models use topography (represented by the DEM of the area modelled) to derive bathymetry. DEM is also used to derive some other key information critical in fully distributed hydraulic and hydrological models.With high-resolution DEMs such as LiDAR (Light Detection and Ranging) becoming more readily available and also with the advancements in computing facilities which can handle these large data sets, there is a need to quantify the impact of using different resolution DEMs (e.g. 1 m against 10 m or 25 m) on hydrologically important variables and the loss of accuracy and reliability of the results as we move from high resolution to coarser resolution.The results from statistical analysis carried out to compare field survey elevations with the LiDAR DEM-derived elevations, show that there are small differences between the two data sets but LiDAR DEM is a reasonably good representation of the actual ground surface compared to other commonly used DEMs derived from contour maps.The results from the analysis clearly show that the accuracy and resolution of the input DEM have serious implications on the values of the hydrologically important spatial indices derived from the DEM. The result also indicates that the loss of details by re-sampling the higher resolution DEM to coarser resolution are much less compared to the details captured in the commonly available coarse resolution DEM derived from contour maps. Topographic indices based on contour derived DEMs should be used with caution and where available, the higher resolution DEM should be used instead of the coarse resolution one.     

Cooling precipitation and strengthening study in powder metallurgy superalloy U720LI

The excellent mechanical properties of powder metallurgy (P/M) superalloys strongly depend on the microstructure, such as grain size, and morphology and size distribution of the γ’ precipitates. The microstructure is, in turn, determined by the heat treatment, viz., solution annealing, quenching, and subsequent aging. To study the effect of the quenching process, two types of quenching methods were used to produce different quenched microstructures in a UDIMET 720LI (U720LI) alloy. One was a continuous quenching method, where samples were colled along linearly controlled cooling profiles, each at a fixed cooling rate. This test studied the effect of cooling rate on the size of cooling γ’ precipitates (formed during quenching) and the consequent strengthening effect. The other test was the interrupted quenching test, which allowed tracking the growth of cooling γ’ precipitates with decreasing temperature during quenching at a given cooling rate. The strengthening response at each interrupt temperature was also studied. Results from the continuous cooling tests showed that the relationship between the size of the cooling γ’ precipitate and the cooling rate obeys a power law, with an exponential being about 0.35. The tensile strength was found to increase linearly with the cooling rate. Strengthening due to the subsequent aging treatment occurred regardless of cooling rates. The interrupted cooling tests showed that γ’ growth is a linear function of decreasing temperature for a given cooling rate. A nonmontonic degradation of tensile strength against interrupt temperature was discovered.     

Assessment of integrity of components in piping of 500 MWe PHWR: using R-6 method

The primary heat transport system of 500 MWe Indian PHWR comprises of straight pipes, elbows and headers. A study was conducted to qualify piping system for leak before break. R-6 method was used to assess integrity of system for leakage size crack (LSC), the margins on crack initiation load and unstable crack growth loads. Option 2 (material specific failure assessment diagram), Category 3 (ductile tearing) analysis was used for straight pipes, elbows and header. In order to enhance the confidence in the analytical results, detailed sensitivity analysis was also performed. For sensitivity analysis, variation in material properties, LSC was considered. The effect of variation in temperature on material properties was also considered. Tensile and fracture properties used for base and weld material data were generated from pipe material obtained from 220 MWe Nuclear Power Plant, under construction.     

Predictions for fatigue crack growth life of cracked pipes and pipe welds using RMS SIF approach and experimental validation

The objective of the present study is to understand the fatigue crack growth behavior in austenitic stainless steel pipes and pipe welds by carrying out analysis/predictions and experiments. The Paris law has been used for the prediction of fatigue crack growth life. To carry out the analysis, Paris constants have been determined for pipe (base) and pipe weld materials by using Compact Tension (CT) specimens machined from the actual pipe/pipe weld. Analyses have been carried out to predict the fatigue crack growth life of the austenitic stainless steel pipes/pipes welds having part through cracks on the outer surface. In the analyses, Stress Intensity Factors (K) have been evaluated through two different schemes. The first scheme considers the ‘K’ evaluations at two points of the crack front i.e. maximum crack depth and crack tip at the outer surface. The second scheme accounts for the area averaged root mean square stress intensity factor (K_RMS) at deepest and surface points. Crack growth and the crack shape with loading cycles have been evaluated. In order to validate the analytical procedure/results, experiments have been carried out on full scale pipe and pipe welds with part through circumferential crack. Fatigue crack growth life evaluated using both schemes have been compared with experimental results. Use of stress intensity factor (K_RMS) evaluated using second scheme gives better fatigue crack growth life prediction compared to that of first scheme. Fatigue crack growth in pipe weld (Gas Tungsten Arc Welding) can be predicted well using Paris constants of base material but prediction is non-conservative for pipe weld (Shielded Metal Arc Welding). Further, predictions using fatigue crack growth rate curve of ASME produces conservative results for pipe and GTAW pipe welds and comparable results for SMAW pipe welds.     

Adsorptive recovery of water soluble essential oil components

Many essential oils are produced by the process of steam distillation. During this process valuable oxygenated components of the oil which have relatively higher solubility in water are preferentially lost in the distillate water. This loss is directly reflected in a lower yield and poor quality of the oil. Adsorption is extensively used for recovery of such valuable organics from dilute aqueous streams. This paper describes a preliminary study on the feasibility of adsorptive recovery. The following synthetic polymeric adsorbents were used in this work: Amberlite XAD-2, XAD-4 and XAD-7. Equilibrium and column exahaustion studies were carried out for the adsorption of phenyl ethyl alcohol (PEA), linalool (LL) and eugenol (EG) and also of mixtures of PEA and LL, and EG and LL from dilute aqueous solutions. The results indicate that XAD-4 is a relatively better adsorbent for all the three solutes studied.     

Development and implementation of a generic pasture growth model (CLASS PGM)

This paper describes the development, testing and implementation of a generic pasture growth model (CLASS PGM) which can be used to simulate growth of composite pasture types of multiple species that may be summer or winter active, perennial or annual. The model includes carbon assimilation through photosynthesis and respiration followed by tissue growth, turnover and senescence. Environmental conditions as well as soil water, nutrient and salinity status influences pasture growth and tissue dynamics. The model allows the user to simulate a range of grazing management strategies. Concepts and theoretical basis of the pasture growth model is based upon the detailed technical report on pasture and crop growth modules (Johnson, 2003). For water balance computations, PGM is internally linked to the Richards’ equation - based hydrology tool, Unsaturated Moisture Movement Model U3M-1D (Vaze et al., 2004b) PGM is supported by a windows based user friendly graphical users interface (GUI). The model can be downloaded free from the Catchment Modelling Toolkit website supported by the eWater Cooperative Research Centre (http://www.toolkit.net.au/class). This paper gives an overview of the model structure, model inputs and outputs and the soils related inbuilt database. Results from model validation using long term observed data for soil moisture, pasture herbage mass and grazing for a grazing experiment at Wagga Wagga, New South Wales, Australia are discussed. When compared with herbage mass and soil moisture data from the experiment, PGM was found to adequately simulate the patterns and amplitudes of pasture growth and soil moisture recorded in the experiment.     

A Comparative Evaluation of Short-Term Streamflow Forecasting Using Time Series Analysis and Rainfall-Runoff Models in eWater Source

Over the past few decades, many numerical streamflow prediction techniques using observed time series (TS) have been developed and widely used in water resources planning and management. Recent advances in quantitative rainfall forecasting by numerical weather prediction (NWP) models have made it possible to produce improved streamflow forecasts using continuous rainfall-runoff (RR) models. In the absence of a suitable integrated system of NWP, RR and river system models, river operators in Australia mostly use spreadsheet-based tools to forecast streamflow using gauged records. The eWater Cooperative Research Centre of Australia has recently developed a new generation software package called eWater Source, which allows a seamless integration of continuous RR and river system models for operational and planning purposes. This paper presents the outcomes of a study that was carried out using Source for a comparative evaluation of streamflow forecasting by several well-known TS based linear techniques and RR models in two selected sub-basins in the upper Murray river system of the Murray-Darling Basin in Australia. The results were compared with the actual forecasts made by the Murray River operators and the observed data. The results show that while streamflow forecasts by the river operators were reasonably accurate up to day 3 and traditional TS based approaches were reasonably accurate up to 2?days. Well calibrated RR models can provide better forecasts for longer periods when using high quality quantitative precipitation forecasts. The river operators tended to underestimate large magnitude flows.     

A proposed process scheme for recovering metal values from pigment manufacturing waste streams

 Effluents from pigment manufacturing operations contain highly toxic substances. These substances in their pure form are highly valuable industrial raw materials and their loss in the effluents is not only a pollution problem but also one implying economic loss. This article describes an approach, which highlights a policy that can allow elimination of a pollution problem with simultaneous economic benefits through recovery of the pollutants in readily marketable forms. This approach converts serious "liabilities into substantial assets" and is worth adopting in many cases. The case considered here is that of the effluent from a plant manufacturing the pigment, phthalocyanine green (PG). This effluent is highly acidic and contains copper and aluminium in chloride form. There is a serious disposal problem concerning this effluent. The work reported here was focussed on recovering the metal values in this effluent in a form which can be readily marketed. Particularly important is the fact that the aluminium value can be recovered in the form of zeolite 4A, which is an environment friendly additive in modern detergent formulations. Received: 1 July 1998 / Accepted: 30 September 1998