Fully coupled, hygro-thermo-mechanical sensitivity analysis of a pre-stressed concrete pressure vessel

Following a recent world wide resurgence in the desire to build and operate nuclear power stations as a response to rising energy demands and global plans to reduce carbon emissions, and in the light of recent events such as those at the Fukushima Dai-ichi nuclear power plant in Japan, which have raised questions of safety, this work has investigated the long term behaviour of concrete nuclear power plant structures.<p></p> A case example of a typical pre-stressed concrete pressure vessel (PCPV), generically similar to several presently in operation in the UK was considered and investigations were made with regard to the extended operation of existing plants beyond their originally planned for operational life spans, and with regard to the construction of new build plants.<p></p> Extensive analyses have been carried out using a fully coupled hygro-thermo-mechanical (HTM) model for concrete. Analyses were initially conducted to determine the current state of a typical PCPV after 33+ years of operation. Parametric and sensitivity studies were then carried out to determine the influence of certain, less well characterised concrete material properties (porosity, moisture content, permeability and thermal conductivity). Further studies investigated the effects of changes to operational conditions including planned and unplanned thermal events.<p></p> As well as demonstrating the capabilities and usefulness of the HTM model in the analysis of such problems, it has been shown that an understanding of the long-term behaviour of these safety–critical structures in response to variations in material properties and loading conditions is extremely important and that further detailed analysis should be conducted in order to provide a rational assessment for life extension.<p></p> It was shown that changes to the operating procedures led to only minor changes in the behaviour of the structure over its life time, but that unplanned thermal excursions, like those seen at the Fukushima Dai-ichi plant could have more significant effects on the concrete structures.<p></p&gt

Modelling of transport processes in concrete exposed to elevated temperatures – An alternative formulation for sorption isotherms

There is a significant need to understand, analyse and assess moisture transport in cementitious materials exposed to elevated temperatures in order to confidently predict the behaviour and ultimately the development of damage in safety critical applications such as nuclear reactor vessels, structures exposed to fire and well bore grouts. In view of this need a rigorous and robust formulation to describe water retention curves (sorption isotherms) as a function of temperature based on the evolution of physical parameters is presented. The model formulation is successfully validated against independent sets of experimental data up to temperatures of 80 °C. It is then further validated under isothermal drying conditions and then high temperature conditions through the numerical reproduction of laboratory experiments following implementation in a fully coupled hygro-thermo-mechanical finite element model. The new formulation is found to work well under a variety of conditions in a variety of cementitious material types

Aspects of permeability in modelling of concrete exposed to high temperatures

The development of heat and moisture transport in concrete is critical to the development of pore pressures, which are thought to be a primary driver of damage and thermal spalling in concrete exposed to elevated temperatures. In the light of uncertainty and variation in the value of certain material properties and constitutive or parametric descriptions found in the literature, various sets of numerical experiments were conducted to investigate the significance of the intrinsic permeability, the evolution of permeability related to temperature and the relative permeability of the fluid phases as functions of saturation in predicting and analysing the behaviour of concrete drying under normal, low temperature, isothermal conditions and under exposure to very high temperature conditions as might be encountered during a fire. A fully coupled hygro-thermo-mechanical finite element model for concrete was employed with the permeability values and parametric functions altered in the model as required. Results of mass loss and the development of gas pressures with time were considered in relation to the potential for the occurrence of damage and thermal spalling, which is thought to be variously related to these processes. The analyses showed that permeability, and its variation with temperature, are very important in controlling the predicted behaviour at both low and high temperatures. Most significant of all were the relationships chosen to define the relative permeabilities. These were shown to strongly control the results of analyses of both low and high temperature problems and to potentially imply apparently different permeability values for the same concrete

Tensile behaviour of unsaturated compacted clay soils — A direct assessment method

This paper presents a new method for testing the behaviour of soils placed under tensile load and demonstrates its suitability for testing a number of soil types under various conditions including saturation, compaction and stabilisation. Validation of the results obtained for the soils at relatively low saturation has been conducted using the established Brazilian (indirect) test for measuring the tensile strength of brittle materials. A fair comparison has been found and the results highlight the limited applicability of the Brazilian method to soils at very low water contents at which the tensile failure criterion has been assumed using this methodology. Optical characterisation of the performance of both testing methods has also been conducted using Digital Image Correlation. The consistent, accurate measurement of directly induced tensile strains using the proposed new method has been confirmed, verifying its capability to apply a direct tensile stress in the absence of shearing, a problem commonly associated with other tensile testing methods. The developed technique has then been used to investigate the water content–tensile strength relationship for compacted, unsaturated soils and offers significant advantages in the characterisation of clay soils subjected to variable climatic loading