Jyoti K. Sinha

The use of model updating for reliable finite element modelling and fault diagnosis of structural components used in nuclear plants

Reliable finite element (FE) modelling in structural dynamics is very important for studies related to the safety of structural components used in the nuclear power industry. FE model updating is a tool to produce these reliable models. The method uses an initial FE model and experimental modal data of the structural components to modify physical parameters of the initial FE model, and a number of approaches have been developed to perform this task. This paper presents an overview of model updating and its use in fault diagnosis, using typical examples. The paper concentrates on the usefulness of the updating method, rather than describing the different updating methods in detail.

Simulation of the dynamic response of a cracked beam

Abstract The experimental vibration behaviour of a free–free beam with a breathing crack is simulated for a sinusoidal input force using a simple FE model for a crack in beam. The present simulation was compared with an earlier study and found to be more realistic.

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.

Combined experimental and analytical method for a realistic seismic qualification of equipment

Abstract The seismic qualification of equipment/structures are, in general, carried out either exclusively by analysis or exclusively by testing using a shake table. The analytical methods have the risk of the model not being a true reflection of the structure unless very elaborate modelling techniques are used. Even with an elaborate model there are many idealisations made which may not actually be realised in practice. The shake-table testing, avoids the modelling deviations to a large extent, but is also not without drawbacks. The important ones are the cost and the availability of a shake table of the required size and capacity. The shake-table testing is also carried out on the isolated equipment without the piping/structural connections from other components. The present paper suggests a combined experimental and analytical method on the ‘as installed’ equipment as an attractive alternative which overcomes the above drawbacks. In contrast to the existing practice of using the experimental results just to validate the analytical model, the suggested method uses the experimentally obtained dynamic characteristics of the ‘as installed’ equipment to obtain the response to the design seismic load. The paper brings out through an example of a simple storage tank which is too heavy for a shake table, the large deviations in its actual behaviour vis-a-vis an idealised analytical model.

Added mass of submerged perforated tubes

There have been many studies on the added mass of submerged vibrating objects for common geometrical configurations of structures used in practice. However, adequate study has not been reported on the mass of surrounding fluid to be added to the vibrating perforated tubes which are common in nuclear reactors. Such a study is presented here based on the experiments conducted on a few of perforated tubes and an empirical formulation is suggested for the added mass which could be useful for undertaking structural dynamics evaluation at design stage.

Significance of analytical modelling for complete interpretation of experimental modal analysis: a case study

The reliability of the dynamic qualification of structural components of a nuclear power reactor totally depends on the dynamic characterization, i.e. identification of natural frequencies, mode shapes and damping of the components. Often, the correct identification of these parameters by the experiment or analysis alone may be difficult for many cases. In this paper, the strength of the analytical modelling in understanding the experimental results and interpreting them are presented through a case study.

Simplified method for the seismic qualification using measured modal data

The seismic response estimation by the response spectrum method using only the experimental modal data are presented here. The modal participation factors (MPFs) used for the response estimation are calculated using the experimental mode shapes only. This response is compared with the response estimated using the conventional MPFs but the experimentally extracted mode shapes are used along with the mass matrix estimated corresponding to the measured degree of freedoms (dofs) using physical dimensions of the structure. The presented study eliminates the uncertainty associated with analytical modelling for evaluating mass matrix.

A parameter identification technique for detection of spacer locations in an assembly of two coaxial flexible tubes

Assemblies of two horizontal coaxial flexible tubes with loosely held spacers to maintain the annular gap between the coaxial tubes, are generally used in nuclear reactor for carrying hot fluid inside the inner tube with an insulating gas filled annulus between the outer and inner tubes to reduce heat losses. The appropriate location of these spacers is important for maintaining coaxiality and preventing contact between inner and outer tubes due to bending creep of inner tube. Determination of spacer locations is therefore an important task. The conventional method of inspection may be costly and time consuming. This paper presents a non-intrusive technique based on vibration measurement, developed for the detection of such spacer spring locations in the assembly of the two coaxial tubes. The technique is based on a parameter identification approach. It involves the identification of spacer locations by updating the position parameters of the spacer in a Finite Element (FE) structural model through the optimization of an error criterion based on the difference between measured and computed natural frequencies of the assembly of the two coaxial tubes. A gradient-based method is used for optimization in the FE model updating problem. The proposed technique has been validated by numerical simulation as well as on a laboratory scale experimental setup.

Use of an unconventional technique for seismic qualification of equipments

There is a great deal of equipment in nuclear power stations which is required to withstand predefined levels of earthquakes. Such equipment is generally qualified analytically or experimentally by shake-table tests. However, some equipment is so complicated that an analytical simulation is very difficult. This equipment could also be so large and heavy physically that shake-table testing may not be possible in many cases. One typical example of such equipment is the Diesel Generator (DG) sets of Nuclear Power Plants (NPPs). For functional qualification of such equipment, the use of railway track unevenness to induce stationary random vibrations is being put forward as an economical and conservative alternative. This article also brings out the feasibility of using such a technique for all difficult to model and/or test equipment both in a passive and an active state.

Dynamic qualification of complex structural components of nuclear power plants

Abstract The safety requirements and the lack of accessibility for any future repair, impose the design requirement that the integrity of reactor components of nuclear power plants be assured for the lifetime of the plant. To meet this design requirement it is essential to qualify the component, i.e. prove its capability to perform the design function for the design life. In performing its design function, the component is subjected to both static and dynamic loads. The qualification for static loads is rather simple and reliable, but qualification for dynamic loads is complex and often uncertain. This is because analytical tools are often inadequate for a realistic dynamic qualification and exact structurally simulated experimental models are almost always difficult to build. In such a situation, methods using tests on simple experimental set-ups supplemented by conservative analytical back-ups must be evolved. This paper highlights the intricacies involved in the conservative dynamic qualification of the complex components by considering the example of the moderator sparger tube. This component is a perforated tube submerged in water and excited by flow. For such a case, a completely analytical or a totally experimental qualification is not possible. This paper describes a procedure by which the required dynamic characteristics such as added mass, damping and fluid forces are generated from simple experiments and the component is qualified by analysis using these data.