Anil Kakodkar

A database to evaluate stress intensity factors of elbows with throughwall flaws under combined internal pressure and bending moment

Abstract The advent of the leak-before-break (LBB) concept has widely replaced the traditional design basis event of a double-ended guillotine break (DEGB) in the design of primary heat transport (PHT) piping. The use of the LBB concept requires postulation of the largest credible cracks at highly stressed locations and demonstration of their stability under the maximum credible loading conditions. Stress analysis of PHT piping in nuclear power plants shows that the highly stressed piping components are normally elbows and branch tees. This necessitates detailed fracture mechanics evaluation of such piping connections by computing their stress intensity factors (SIF) and/or J -integral. Simple analytical solutions for evaluation of the SIF and J -integral for cracks in straight pipes are readily available in the literature. However the same type of solution for elbows and tees is limited in the open literature. In the present work a database is generated to evaluate the SIF for throughwall circumferential and longitudinal cracks under combined internal pressure and bending moment. Different parameters used to characterise a cracked elbow are pipe factor ( h ), pipe bore radius to thickness ratio ( r t ) and crack length. Another parameter (ϱ) is used to consider the relative magnitude of stresses due to internal pressure and remote bending moment. This database has been used to derive closed-form expressions to evaluate the SIF for elbows with cracks in terms of the aforementioned parameters.

On line fatigue life monitoring methodology for power plant components

Abstract Fatigue is one of the most important aging effects of power plant components. Information about fatigue helps in assessing structural degradation of the components and so assists in planning in-service inspection and maintenance. It may also support the future life extension programme of a power plant. In the present paper, the development of a methodology for on line fatigue life monitoring using available plant instrumentation is presented. The Greens function technique is used to convert plant data to stress-time data. Using a rainflow cycle counting method, stress-time data are analysed and the fatigue usage factor is computed from the material fatigue curve. Various codes are developed to generate Greens functions, to convert plant data to stress-time data, to find the fatigue usage factor and to display fatigue information. Using the developed codes, information about the fatigue life of various components of a power plant can be updated, stored and displayed interactively by plant operators. Three different case studies are reported in the present paper. These are the fatigue analyses of a thick pipe, of a nozzle connected to a pressure vessel and of a reducer connecting a heat exchanger to its piping system.

Limit load analysis and safety assessment of an elbow with a circumferential crack under a bending moment

The use of the leak-before-break concept in the design of a Primary Heat Transport (PHT) piping system for nuclear power plants requires postulation of the largest credible flaws at highly stressed points and demonstration of system stability under the most severe loading conditions. In the PHT piping system, the elbows and branch tees are normally found to be among the most highly stressed piping components. This necessitates detailed flaw evaluation of these components. In the present paper, safety assessment of a pump discharge elbow of 500 MWe Indian PHWR with a throughwall circumferential crack under a bending moment is carried out using the R6 approach. The stress intensity factors (SIF) have been computed by using a database of SIF for elbows with cracks which was generated in one of our recent studies. Limit loads at plastic collapse have been evaluated by carrying out non-linear finite element analysis. Finally, the safety assessment of the pump discharge elbow is performed by doing a sensitivity study on crack length, applied moment and material fracture toughness.

Optimization of the initial fuel loading of the Indian PHWR with thorium bundles for achieving full power

When attempting initial power flattening with thorium bundles, the placement of the bundles should take into account not only the need for power flattening, but also the fact that sharp flux depressions caused by the presence of thorium can alter the reactivity worth of shutdown systems. Since the safety assessment of the reactor is made under the assumption of certain shutdown worths, it is important that these worths are not disturbed by the fuel loading. We describe here a method by which this problem is tackled. The thorium loading that was worked out using this method was found to satisfy all the desired criteria of full power, and no loss of worth of the two independent shutdown systems. This loading has been used in the Indian PHWR at Kakrapar, the KAPS-1, which went critical on 3 September 1992.

Further considerations in modified crack closure integral based computation of stress intensity factor in bem

This paper deals with modified crack closure integral based computation of stress intensity factors in a boundary element method for problems with mechanical loading remote from the crack edges. The modified crack closure integral technique has been coupled with the local smoothing scheme to obtain simple relations for energy release rates for linear, quadratic and quarter point elements around the crack tip. Stress intensity factors calculated through the proposed formulation and the displacement method for a number of examples are compared, wherever possible, with data available in the literature. The results based on the proposed scheme are more accurate than those obtained by the displacement method.

Coupled shell-fluid interaction problems with degenerate shell and three-dimensional fluid elements

Discrete methods for practical coupled three-dimensional fluid-structure interaction problems are developed. A C° explicitly integrated two-dimensional degenerate shell element and a three-dimensional fluid element are coupled to study shell dynamics, fluid transient and coupled shell-fluid interaction problems. The method of partitioning is used to integrate the fluid and shell meshes in a staggered fashion in an optimum manner. Effective explicit-implicit partitioning is shown to achieve high computational efficiency for this type of problem.

Modified crack closure integral based computation of stress intensity factors for 2-D thermoelastic problems through boundary element method

A modified crack closure integral (MCCI) based computation of stress intensity factors (SIFs) for thermal loading through boundary element method (BEM) is presented. Simple relations are given for the determination of stress intensity factors (SIFs) using the BEM results for linear, quadratic and quarter point elements employed around the crack tip. Examples of crack under mode I, mode II and mixed mode thermal and/or mechanical loading are examined. The computed SIFs are compared wherever possible with solutions available in the literature. The agreement is good. The effect of crack tip element size on the accuracy of results is reported.

Effect of modelling of traction and thermal singularities on accuracy of SIFS computation through modified crack closure integral in BEM

The effect of simultaneous modelling of both strain and traction singularities, and temperature derivative and heat flux singularities at the crack tip, on the accuracy of computation of stress intensity factors (SIFs) based on the modified crack closure integral in boundary element method is presented. Simple relations are given for SIF calculations. Results on mode I, mode II and mixed mode, crack subjected to mechanical and/or thermal loading are studied to illustrate the difference between partial and total modelling of the singularities. The dependence of accuracy of the SIFs on the crack tip element size is examined. The effect of order of Gaussian quadrature on the accuracy is also reported.

Diagnostics of direct CT - PT contact of the coolant channels of PHWRs

It has now been realised that the garter springs which maintain the gap between the pressure tube (PT) and calandria tube (CT) of a PHWR can get displaced significantly from their deign position in many channels. It has also been recognised that the large unsupported span of the PT restricts the life of the channel due to premature contact of the PT with the CT making it susceptible to delayed hydrogen cracking. This paper reports the details of a non-intrusive diagnostic technique based on vibration measurement for detecting the contacting channels.

BEM based evaluation of SIFs using modified crack closure integral technique under remote and/or crack edge loading

This paper deals with the boundary element method based evaluation of stress intensity factors for mode I and mixed mode problems under remote and/or crack edge loading. The modified crack closure integral technique has been used to enhance the accuracy of the computed stress intensity factors. Simple, ready to use, relations for the strain energy release rate have been obtained corresponding to linear, quadratic and quarter point elements surrounding the crack tip. The boundary element method is employed by breaking a domain into two subregions for the mixed mode problems. Case studies involving remote or crack edge loading for both mode I and mixed mode problems are presented to demonstrate the accuracy of the new scheme. The stress intensity factors based on the proposed scheme show better agreement with the standard solutions available in the literature than those obtained directly through the displacement method.