Nuclear Power Plant Components

Components of Nuclear Power Plant – Reactor Vessel A nuclear reactor consists of various parts which carry out different functions related to heat generation by “burning” of nuclear fuel, but a housing is needed to contain all these parts and act as a covering for all these paraphernalia

Introduction Just imagine if your beautiful body did not have the cover of the skin, and when you met any individual you could simply see through their various organs and into their "dirty" workings. This would certainly be not a very pleasing sight and would take out the very charm of human personality. This is not much different in the case of nuclear reactors as well. I cannot imagine going to a nuclear power plant just to find that the reactor core, fuel rods, control rods etc are all lying bare bones without any proper cover of enclosure. Hence the outside component of nuclear power plant is very important and is known as the reactor vessel.

Reactor Vessel Vessels are often used to cook food, and though a nuclear reactor may not be cooking food directly for you, it certainly provides a source of an equally valuable food for the society: electrical energy. But apart from the cooking business there are a lot of functions which a nuclear reactor vessel has to perform and some of these are as follows. •

It acts to enclose the various parts inside the reactor including the core, shield, reflector etc.

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The coolant needs a passage to flow through the reactor so that it can be used to transfer the heat to the working fluid or the turbine directly, as the case may be, and this passage is provided by the reactor vessel.

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To withstand the high pressure with exists inside the reactor and could be of the order of 200 kgf/cm2, to provide a safe working environment for all concerned.

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Control of the nuclear reaction is absolutely necessary and this is done with the help of control rods. The reactor vessel provides a place to insert these control rods in the nuclear reactor and move them in or out of the reactor core depending on the requirements of power.

The Pressure Vessel Although the reactor vessel has been compared to a cookery vessel in the common usage of the term, technically speaking it is more of a pressure vessel. There are legal implications associated with defining a pressure vessel and these vary with the country in

which it is being used or manufactured. Different countries have different authorities which govern rules and regulations regarding pressure vessels and in the US this is done by the American Society of Mechanical Engineers Boiler and Pressure Vessel Code. The material used for the construction of a nuclear vessel is usually steel which would be expected as the material has to be very strong and resilient. Pressure vessels of all kinds are subject to various tests to check for their strength against laid down standards which is very important to ensure safety of these vessels. This is moreso important in the case of nuclear reactor vessels which house source of intense raditaions and heat energy. Hence we see that though a nuclear reactor vessel may not be performing any useful function direcly in the generation of electrical energy, it acts to hold together all major components of the power plant

Components of Nuclear Power Plant – Shielding A nuclear reaction is a source of intense radiation apart from the heat generated in the exothermic process. Because of the risk, radiation shielding is required to prevent this harmful radiation from leaving the reactor and affecting the outside men and materials.

Introduction As you know when a nucleus gets split into two parts during the fission process it results in the production of large amounts of heat energy since the reaction is exothermic in nature. But this is not the only product of nuclear fuel "combustion" but there are several other by-products such as alpha rays, beta rays, gamma rays and of course the fast moving neutrons. The fast moving neutrons are controlled, moderation and reflected in order to contain them within the reactor core so that a sustained and controlled chain reaction takes place but what do you think happens to the other by-products? Just read on to find out why a shielding is one of the important components of nuclear power plant.

The Radiation Yes, you guessed it right. These by-products in the form of different kinds of radiation would simply leak out into the atmosphere in the absence of proper arrangements to prevent this. Radiation leakage would be very harmful for the personnel working in the nuclear plant as well as the nearby flora and fauna. This makes clear the case for having a proper shield so that these radiations get absorbed within the reactor without having a chance to escape into open air. This is done by using materials which are good absorbents of the same. Concrete and steel are very good at absorbing radiation and they are equally strong as well, hence used in forming the shielding material.

The Shield The question now arises that how much thickness of these materials should be used to prevent radiation from leaking out into the atmosphere? If you just compare it with the amount of thickness of typical steel plate required for preventing a powerful bullet from going across it you are in for a surprise. Although I am not a weapons expert but I know for sure that for stopping ordinary bullets a few mm of steel plate should be sufficient and a few cm of plate should be sufficient to stop even the most powerful of guns. You might wonder that if such a thickness is required for a bullet which is quite bulky and dangerous, then only a couple of mm should be sufficient for humble intangible rays and neutrons but if you think so you are utterly wrong. A typical reactor core would require an inner lining which is of the order of nearly half a meter thickness of steel (don't gasp for breath). The icing on the cake is that even this much thick steel is not considered entire safe. It is further reinforced by using a few meters of concrete to make it safer. This should give you an idea about how powerful these radiations are and their penetrating capability. It is also interesting to note that the amount of radiation to which human beings could be exposed safely without causing any harm to the body is expressed in units of rad and rem which give the amount of absorbed radiation from different perspectives.

Components of Nuclear Power Plant – Coolant A nuclear reactor is a source of intense heat which is generated through the exothermic fission reactions taking place inside the core. Therefore a coolant is necessary to ensure that this heat is taken away and utilized in a proper manner.

Introduction The immense amount of heat energy present in the nuclear reactor core needs to be transferred in some manner so that it is converted into electrical energy. This also helps to keep the working temperature of the core within safe limits for the materials used in the construction of the reactor. Hence a coolant plays an important role in components of nuclear power plant and serves the dual purpose of removing the heat from the reactor as well as transferring it to the electricity generation circuit either directly or indirectly depending on the type of nuclear reactor being used for the purpose.

Properties of an Ideal Coolant There are some properties of the coolant which are necessary to ensure safety of the reactor and well as proper performance of the coolant for the intended purpose. Some of the desired properties of an ideal coolant are as follows •

A coolant should not absorb neutrons or should have a minimum neutron absorption cross section. The reason for this is obvious since this function should

be left to the moderator and not the coolant. •

Since a coolant is exposed to high temperatures and well as severe levels of radiation, it is obvious that it should posses excellent resistance to both high temperatures as well as high levels of radiation.

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A coolant should be non-corrosive in nature otherwise it might tend to damage and corrode the very core which is meant to be protected by it through proper removal of heat.

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Coolants used in nuclear reactors could be either in the liquid state or in the solid state. In case the coolant is a liquid it should have a high boiling point so that it does not get evaporated due to the high heat inside the reactor. But in case it is a solid it should have a relatively low melting point due to obvious reasons.

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Since a coolant needs to circulate using a pump it should be capable of being pumped easily so that least amount of energy is spent in pumping the coolant.

It can be well imagined that the above list is quite extensive and therefore there is hardly any material which satisfies all the above criteria to the maximum possible extent. Therefore different types of coolants are used in different types of reactors depending on various factors and parameters.

Commonly Used Coolants Since no single material qualifies as an ideal coolant, different coolants are used in different circumstances and some of the commonly used coolants are light water, heavy water, carbondioxide, helium, nitrogen, sodium, sodium-potassium mixture and so on. It can be seen that the coolants used vary from solids, liquids and gases and depending on the type of the reactor, the appropriate coolant is preferred. It must be also kept in mind that sometimes a coolant is used to transfer heat to the working fluid in the secondary circuit through a heat exchanger while in other cases it is directly used in the turbine to rotate the blades and then fed back to the reactor after cooling in the condenser.

Components of Nuclear Power Plant – Reflector The chain reaction inside a nuclear reactor is what sustains “combustion” of the fuel which in turn depends on ample supply of thermal energy neutrons within the core. A reflector material is used to ensure that neutrons do not simply fly off the reactor leaving little room for the chain reaction to continue

Introduction The principle of reflection is fairly simple and we come across it in our everyday lives. It is the same principle of reflection which lets you see how you look in a mirror by reflecting the light waves. In fact the term reflection refers to any wave or particle being

thrown back after hitting a reflecting surface. This principle is extremely useful in the reactor core and helps to maintain an ample amount of thermal energy neutrons, the lack of which could simply extinguish the fission process, rendering the device useless for producing power. Hence a reflector holds an important position amongst the components of nuclear power plant.

The Reflection Process As we know the reactor consists of the fission process which occurs when a thermal energy neutron is absorbed by the target nucleus leading to its division into two nuclei and emission of 2 or 3 neutrons apart from the heat energy. These neutrons fly randomly in all directions and are usually in the region of fast moving energy neutrons. The moderator is used to control the speed of these neutrons so that they act usefully in creating more fission, but many of these neutrons may simply get lost by flying off the reactor core and thus serving no useful purpose. This might hinder the progression of a chain reaction which is very necessary for the nuclear reactor. In order to reduce this process of neutron loss the inner surface of the reactor core is surrounded by a material which helps to reflect these escaping neutrons back towards the core of the reactor and these materials are known as reflecting materials.

Materials used as Reflectors There are a variety of materials which are used as a reflecting medium for neutrons and whatever material is used for the process, it must possess these properties. Low absorption - this is necessary since if the reflecting material itself starts to absorb the very neutrons it is supposed to reflect back, then the purpose of installing the reflector material would itself be defeated and it would be better not to install any reflector at all. High reflection - this is an obvious property and does not need any explanation for that is the very purpose for which the reflector exists in the core Radiation stability - since the reflector material will be exposed to high levels of radiation, it is but natural to assume that it should have a high stability towards radiation Resistance to Oxidation - the material should not get oxidized otherwise it will fail to serve the requisite purpose In actual practice there may not be a different material for moderator and reflector for the simple reason that most of the moderators also possess the above mentioned properties of a good reflector as well. Hence they serve the dual purpose of a reflector and a moderator as well. There light water, heavy water and carbon are mostly used as reflectors since they possess the above mentioned characteristics. The use of a proper reflector helps to reduce the size of the reactor core for a given power

output since the number of neutrons leaking are lesser and help to propagate the fission process instead. It also reduces the consumption of the fissile material.

Components of Nuclear Power Plant – Moderator Moderation is necessary in all aspects of life if one has to achieve success. Usually extreme of anything is bad, no matter whether it is good or bad. It is no wonder the same principle applies to nuclear reactions as well. Just learn few basic concepts about moderation and moderators in context of nuclear power plants

Introduction The nuclear fission reaction consists of bombarding fuels such as Uranium with energetic neutrons. This makes the target unstable and makes it split into two parts accompanied with the release of energy which is utilized to generate electricity. There is a certain threshold below which the neutron will not be absorbed by the target nucleus, but that does not mean that above that threshold any neutron can cause fission. Infact there is a range of energy within which they can cause fission. Neutrons which fall above that range are known as fast neutrons and they are not readily absorbed by the target nucleus and hence not useful in sustaining a chain reaction. A moderator is one of the important components of nuclear power plant helping to maintain neutron population in the thermal energy range.

The Moderator The problem lies in the fact that whenever a thermal neutron causes fission it also leads to the release of fast neutrons. Now these fast neutrons have to be slowed down and brought to lower energy levels if they have to cause successful fission in turn. It is here that the concept of a moderator comes in the picture. As you must have understood above, a moderator is a medium which is used to absorb a portion of the kinetic energy of fast neutrons so that they come in the category of thermal neutrons which help to sustain a controlled chain reaction. The mechanism of speed control works in such a way that fast moving neutrons strike the nuclei of moderator material which is not efficient at absorbing them but simply slows them down with repeated collisions thus bringing them into the thermal zone.

Materials for Moderator There are several materials which are used for the purpose including the following •

Normal or Light Water is used in majority of the reactors simply because of its cheap and abundant availability. The only flipside of using light-water is that the fuel has to be enriched to use with water

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Deuterium - also known as heavy water in common terminology, Deuterium is costly to manufacture as compared to light water but gives the option of using un-

enriched fuel in the reactor which is a big advantage •

Miscellaneous - Several materials such as Graphite, Beryllium, Lithium are used in different types of reactors as moderators

Is it always necessary? Although moderators are necessary in most nuclear reactors this does not mean to say that all reactors require moderators. There is a special class of reactors known as fast reactors which do not use moderators but depend on the use of fast moving neutrons for causing fission. Even otherwise it must be remembered that fast moving neutrons have lesser probability of getting absorbed and causing fission but it does not mean that they are incapable of causing the fission reaction. Just to give you a relative idea a fast moving neutron travels with a speed which is nearly in the region of 10% of the speed of light, while a thermal neutron travels with a speed which is typically of the order of a few kilometers per second. There are also other categories of neutrons based on their energy levels such as slow neutrons, cold neutrons, ultra cold neutrons and so forth.

Components of Nuclear Power Plant – Fuel A nuclear power plant is not much different from a conventional power plant except for the manner in which heat is generated using nuclear reactions. We will study one very important component of nuclear power plants here, namely the fuel rods used in the reactor core, and the nuclear fuel cycle.

Introduction Fuel is needed for any energy producing process and refers to the material which is either burned or altered in order to produce energy. Burning takes place in case of chemical reactions, whilst alternation takes place in the nuclear reactions. Both these processes are exothermic but the latter leads to much more release of thermal energy as compared to chemical reactions for similar quantities of fuel. No doubt fuel rods top the list in the components of nuclear power plants for there would be no "fire" without the fuel.

What is a Fissile Nuclide? As you must have surely guessed, fissile has something to do with fission and you are right about this. A fissile material is that which attains fission when hit by a neutron of any energy level. The commonly known fissile nuclides are isotopes of Uranium and Plutonium namely U-233, U-235, Pu-239 and Pu-241.

What is a Fertile Nuclide? Whereas a fissile nuclide can achieve fission with any neutron, a fertile nuclide is one which requires neutrons of more than a certain level of energy to achieve the same

usually in the range of 1-MeV. If you are wondering why the name fertile is given to a seemingly infertile nucleus, let me explain the reason to you. Actually a fertile nuclide becomes fissile upon absorption of the appropriate neutron hence the name fertile. We all know fertile nuclide include the U-235 isotope of Uranium and Thorium Th-232.

Shape of Fuel Used Fuel is usually placed within the reactor core in the form of fuel rods which are fabricated and placed within the reactor in such a manner so that it leads to a uniform production of heat within the reactor. There are two types of reactors based on the manner in which the fuel and moderator are placed within the core as follows. The homogenous reactor is one in which the fuel and moderator are mixed to form a uniform mixture which is then placed in the form of rods and plates inside the reactor core. A heterogeneous reactor on the other hands has pure fuel in the form of rods or plates while the moderator surrounds the fuel elements separately. In this case the fuel rods are often clad with different materials including Aluminium, Stainless Steel or Zirconium which help to prevent oxidation of Uranium.

The Fuel Cycle The fuel cycle with regards to the nuclear power plant refers to the total process of preparation of fuel, burning of fuel and final disposal. If the fuel from the last stage is recycled to be used again in the nuclear reactor, it is known as a closed fuel cycle otherwise it is known as open fuel cycle. Of course in the former case, fuel is not thrown or dumped away at any random place but is placed and packaged properly in order to prevent contamination of the biosphere.

Components of Nuclear Power Plant – Control Rods What do control rods in a nuclear power plant do? It has been often said that power corrupts and absolute power corrupts absolutely. This maxim is usually associated with politics but applies equally to a nuclear reactor as well. Unless there is something to control the immense power that a nuclear reaction wields, the reactor would simply go haywire.

Introduction Nuclear fission is a source of tremendous energy which could be either used for destructive purposes such as nuclear weapons or constructive purposes such as a nuclear reactor for producing electrical energy. Even though a nuclear reactor in a power plant has got peaceful intentions, the tremendous power, heat and energy which is associated with nuclear fission cannot be left on its own but needs to be controlled in a predictable

manner. It is here that controls rods come in the picture and form an important part of the components of nuclear power plant.

Why the Need to Control? It does not require much reflection to imagine why proper control is necessary with in nuclear reactor. Some of the basic reasons are as follows. •

A nuclear chain reaction should be started when a reactor fires from the cold condition. In the absence of such a reaction the process would soon die out.

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It is not only necessary and sufficient to start the chain reaction but it is equally necessary to ensure that the reaction is sustained in the long run as long as the power requirements are present.

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In case of emergency situations such as a sudden mechanical or structural damage, the reactor needs to be shut down quickly in order to prevent any major disaster like say Chernobyl which could be very costly in terms of loss to life and environment.

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Fuel rods inside the reactor should be prevented from melting or getting disintegrated and therefore a control mechanism is absolutely necessary.

We have seen the reasons for controlling and taming the wild nuclear power and the best method to achieve this is through the use of control rods which can be inserted or withdrawn from the core and help to control the nuclear reactions taking place inside the reactor.

What do Control Rods Do? One property which is a must for control rod material is the heavy absorption capacity for neutrons so that they can carry out the control function effectively. The commonly used materials which satisfy these criteria include cadmium, boron, iridium, silver and hafnium. Another property of control rods is that the material should not start a fission reaction despite the heavy absorption of neutrons. Infact you can imagine the function of a control rod just like a blotting paper which sucks the extra ink that has spilled somewhere but doesn't let it spread in a wider region. The mechanism of control consists of arranging control rods in assembles which are usually mounted vertically within the reactor core and are inserted into the guide tubes with the fuel elements. For purposes of safety of a reactor in case the lifting mechanism also suffers a failure, the control are arranged in such a way that they will get into the stop position and shut down the reactor completely in such a case. Hence we see that control rods tend to provide a mechanism wherein the immense nuclear energy can be tamed within reasonable limits and ensure safety and security of the reactor as well the outside environment.