Rik Moorthy

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.

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.

Applicability of neutron noise technique for diagnostics of in-core components for heavy water reactors

Abstract Neutron flux signal is composed of a steady or mean component resulting from the flux produced by power operation of the reactor and a very small fluctuating component called ‘noise’ component. Analysis of neutron noise from suitably located sensors is a proven technique to monitor the in-core components of light water reactors (LWRs). However, the use of neutron noise has been rare, if any, for heavy water reactors (HWRs) as it was generally felt that the unfavourable transfer function characteristics of the reactors would limit its applicability. To assess the applicability of technique in pressurised heavy water reactors (PHWRs), experiments were carried out using in-core and out-of-core neutron sensors in a research reactor. This paper discusses the measurement details and results of the experiment. This paper concludes that the neutron noise technique can be effectively utilised for diagnostics/characterisation of the in-core components of heavy water reactors.