تجزیه و تحلیل حساسیت هیدرو -حرارتی - مکانیکی کاملا توأم از یک مخزن تحت فشار بتن پیش تنیده
|کد مقاله||سال انتشار||مقاله انگلیسی||ترجمه فارسی||تعداد کلمات|
|26950||2014||16 صفحه PDF||سفارش دهید||10995 کلمه|
Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Engineering Structures, Volume 59, February 2014, Pages 536–551
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. 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. 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. 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. 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.
Until recently, in many parts of the world, such as in the UK and other parts of Europe nuclear energy has fallen out of favour. But, with an ever increasing world demand for energy and the threat of global warming, nuclear power is now seen again by many as a reliable, plentiful and most importantly low carbon supply of electricity  and . As a result of this, there is currently a worldwide resurgence in the development and application of nuclear power with several countries including China, India and the UK recently approving the development of new build plants . However, while the international treaties to reduce carbon emissions must be enacted in relatively short timescales  and  the previous stagnation of the nuclear industry has resulted in a lead in time of many years before new nuclear plants can be commissioned . To fill this gap, the lives of the existing stock of nuclear power plants in the UK and Europe, many of which are reaching the end of their original design lives, will need to be extended. At the same time, the incidents following the Great East Japan earthquake in 2011, where a loss of cooling at the Fukushima Dai-ichi plant led to the overheating of several reactors and the release of radioactive material, have again raised questions as to the safety of nuclear power plants . The main line of defence around a reactor to prevent escape of radioactive material is the containment vessel. To ensure that there is not a repeat of some of the past nuclear accidents, it is important that structural integrity of the containment vessel is maintained under all conceivable circumstances and conditions. To achieve this, it is essential that not only their mechanical properties are understood but, as they are subjected to extreme environments, also how these change over the operational life of the reactor vessel. This paper presents an assessment of the long term behaviour of pre-stressed concrete pressure vessels (PCPV), typical of designs currently employed in the UK, that have been operating at elevated temperatures for periods in excess of 30 years. This assessment is important as it evaluates the current mechanical properties of the concrete which may then be used to ensure the structural integrity and containment of facilities whose life is to be extended. The current state of these vessels cannot be identified without accounting for the full history of the operating conditions experienced by the structure and consideration of the effects of thermal and mechanical loads on the concrete including transport of moisture and the development of gas pressures within the pore structure of the material. To achieve this, a fully coupled hygro-thermo-mechanical (HTM) model for concrete, originally developed during the EU FP5 Euratom MAECENAS (Modelling of Ageing in Concrete Nuclear Power Plant Structures) project, was employed to examine the behaviour of a typical UK PCPV over the course of its 30+ year life span under various operating conditions. In the first instance analysis of a typical PCPV is carried out to determine the current condition of the concrete structure. A parametric analysis is then presented in which consideration is given to variations in the concrete with respect to less well characterised properties such as porosity, moisture content, permeability and thermal conductivity. Although the records from the original design and construction of UK PCPV plants are well documented and maintained, little is known about some of these properties and examination of the literature shows that very wide ranges may exist (see for example Fig. 1). Full-size image (14 K) Fig. 1. Values for porosity vs. permeability for ordinary concretes (OC) and high performance concretes (HPC) described in the literature (, , , , , , , , ,  and ). Figure options This work demonstrates that the long term behaviour of these safety–critical structures is very susceptible to small changes in material properties and a good understanding of this sensitivity is therefore vital. Finally, a parametric study is presented in which the influence of operational conditions on the mechanical properties of the PCPV is demonstrated. This study shows how planned or unplanned events may affect the overall structural behaviour and how this may affect the safety of the PCPV. It is furthermore concluded that the numerical tool applied in this paper may be used to predict the behaviours of existing or new build concrete power plant structures (including but not limited to PCPV).1
نتیجه گیری انگلیسی
This work has shown the modelling of the hygro-thermo-mechanical behaviour of a typical PCPV, generically similar to several currently in operation in the UK, over its full 30+ year life time. Through extensive parametric and sensitivity studies 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 if current structures are to be considered for life extension and new build designs are to be optimised. The capabilities of the fully coupled, HTM model have been demonstrated and the following conclusions in relation to the current state of existing structures and the design and operation of new build structures may be drawn: • Analysis of the typical structure over its full life time showed that some damage had gradually developed due to outwards bending of the walls under normal operating conditions. This was largely a result of the changes to the concrete and the diminishing pre-stress loads and was not considered to be significant in terms of the structural integrity or safety of the structure at present. However, the findings suggest that further detailed analysis is required in order to provide a rational assessment for life extension. • The parametric study considering material properties that are typically less well characterised showed that under certain conditions gas pressures in the concrete (which may contribute adversely to the stress state in the structure, ultimately leading to the development of damage) may continue to increase over the whole life-time of the structure, representing a worsening case over the operating life time. It is feasible that these pressures may exceed 1 MPa. Permeability, which may be the least well characterised property of the concrete, was found to have the largest and most significant effect on the gas pressures, while the porosity, moisture content and thermal conductivity were found have more limited and respectively decreasing effects. The ‘worst case’ conditions, potentially leading to gas pressures of over 1.3 MPa, were found to be concrete of low permeability, high porosity, high moisture content and low thermal conductivity. These factors and the control of these properties should be considered when designing new build structures. • The sensitivity study found that only minor changes were seen in the development of gas pressures as a result of changes to the operating procedures. Outages of any length had minimal effect at low pressures but acted to interrupt and delay the development of persistently increasing gas pressures. In that sense, outages were found to be beneficial for the long term prevention of damage. Changes to the environmental conditions outside the PCPV had the potential to beneficially or adversely affect the behaviour depending on the properties of the concrete. • Temperature excursions, as may occur as a result of a loss of cooling, were found to potentially have very significant effects for the PCPV. Under analyses representing very extreme cases it was found that very high gas pressures could develop within the concrete potentially damaging the structure and the steel liner. Extensive thermal and mechanical damage were found to occur in the concrete walls of the structure affecting its strength and stiffness and certainly requiring further detailed analysis in order to provide a rational assessment for continued operation.