Materials Used in the Construction of Heat Exchangers

With increase in global population, technological growth and general modernization of almost all aspects of human life, there has been a resultant exponential growth in energy needs in the world. There have also emerged environmental concern issues. To satisfy this ever increasing energy debt, it is of huge importance to not only tap into nuclear energy source but to ensure effectiveness, sustainability and overall efficiency in the energy generation process. With this knowledge, it is extremely vital to not only design nuclear reactors properly, but to also select wisely the materials used in the construction of the reactor components.

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Heat exchanger is one of the most important parts of a nuclear reactor. Its main role is to transfer heat from one fluid to another without need for the two coming into contact. In nuclear power plants, heat exchangers should be designed to ensure maximum heat transfer between the primary circuit which is the pressurized water reactor and the secondary circuit which is where water is converted in to the wanted steam.

This project looks to compare the materials used to construct the heat exchanger for purposes of increasing both the efficiency and durability of the stated components. It performed a thorough analysis of the qualities of the materials used to construct the heat exchanger. This comparison of their properties included the advantages and limitations, differences and similarities, effectiveness, efficiency and durability among others. The materials were studied under varied conditions that simulate the operating conditions of a nuclear reactor. This conditions can be mildly exaggerated to establish the suitability of the materials during normal operations and also during fault conditions that are characteristic of the nuclear plant.

BackgroundHeat exchangers in nuclear reactors are prone to corrosion, fatigue, mechanical breakdown and overall damage reducing its life span and efficiency. Due to this it is of utmost importance to select the best materials which will be used for heat exchangers. The materials should increase thermal efficiency and be able to meet the environmental and economic factors. The loss in capacity due to the heat exchanger is estimated at 3.2% which is high.

Heat exchanger design and its analysis involve the use of conduction and convention in heat transfer. The materials used should therefore be able to allow for conduction to take place. The major issues considered in the heat exchange material selection are the physical, chemical and mechanical properties of the materials in question.

In this research I looked at the properties of different materials and compared them in order to select that which has the best properties to act as a heat exchanger material.

The research was carried out between February 20eth 2016 to March 20eth 2016. I targeted experts from the nuclear sector and also use of library resources and the web.

PURPOSE

The main aim objective of this project is to compare various materials used in the construction of heat exchangers today in order to select the best and most suitable one so to increase and overall efficiency. The aim is to bring the efficiency of the heat exchange process as close as possible to 100%. Heat exchangers play a huge role in many processes. These processes include chemical, nuclear power plants and fossil, refrigeration, fertilizers and desalination. They are of utmost importance in the industries and due to the increase in the energy needs thy are of paramount importance.

Other objectives included reviewing the properties of the materials used in order to suggest future improvements and to select materials that are both durable, strong and that will suit the operational needs of the heat exchanger fully.

In addition, the project aimed to acquire knowledge on next generation heat exchanger materials that will not only fit the future generations of nuclear reactors with the purpose of identifying unsuitable materials that are currently in use and suggesting their replacement.

I looked to identify tubing materials that will help improve the heat exchanger. I studied two aspects when looking into the tubing materials, that is, the resistance to corrosion when exposed to conditions in the environment that are as a result of a steam or water leak in the sodium. The other aspect was the loss of mechanical strength as a result of decarbulisation of 2.25Cr-1Mo in Sodium.

Irrespective of the reactor types, the key to efficiency will be achieved by use of highly effective heat exchanger. There is high demand for construction integrity since the combination of the temperature and pressure in a reactor requires this. For the next generation reactors to be successful they need highly effective and highly compact heat exchangers.

Scope of the project

The project scope was the analysis of existing materials being currently used and gained insight as to why they are in use. Durability, efficiency and capability of improvement were covered. The project covered their general impact on the environment. The project did not however provide the cost analysis of the various materials. The overall focus of the project was only based on the effectiveness, efficiency, suitability and availability of improvements and not the economic impact of the above named aspects.

Milestones

The encountered milestones during the project were thee acquisition of information relevant to the project, starting and completing the project in due time and writing the project report.

LITERATURE REVIEWGeneration of electrical energy from nuclear energy is a direction the world is not only choosing, but becoming inevitable as times progresses. Over 11% of the worlds electricity is produced from nuclear energy CITATION Wor16 \l 1033 (World Nuclear Association, 2016). It is therefore extremely important to design a nuclear reactor to achieve high efficiencies. In the design it is also important to use the appropriate materials for construction.

Energy in nuclear reactors is attained by a process called fission. Fission involves splitting of a heavy nucleus into two smaller nuclei with release of energyCITATION Jew10 \l 1033 (Jewett, 2010). A nuclear reactor is used to ensure a self-sustained chain reaction.

A nuclear reactor works by uranium fuel fission. One of the fission products is heat. The heat produced during fission heats up the pressurized water in the primary loop. The water cannot however boil as it is pressurized. The pressurized water also works to slow down the neutrons with energy level of 2MeV which are usually produced as a result of the fission from the uranium. The heat exchanger then transfers the hot water to another loop the secondary loop where the heat exchanger transfers heat by conduction. The heat boils the water in the secondary loop to produce steam. The steam is then used to turn turbines which then produce electricity. The water in the primary and secondary loop is normally separated in order to prevent the contamination of the water in the secondary loop by the nuclei which are produced during fission.

A nuclear reactor has various components. The nuclear reactors use fuel in which most reactors in the world use uranium. The reactors use fuel rods that are made up of uranium pellets that are arranged in a tube. The rods are then assembled in the reactor core. Nuclear reactors have moderators. In a CANDU the moderator is the pressurized water which slows down the neutrons produced. This is done so that the neutrons may cause more fission. The reactor also contains control rods which are usually made by materials that can be able to absorb neutrons such as boron, cadium or hafnium. The neutrons are either withdrawn from the core or they are inserted. This then controls the fission reaction in the reactor core. During a fission reaction, the majority of the neutrons are usually released at once, some however take longer. This is necessary in order to allow f...