Chapter 02_Nuclear Power Reactors _ Components

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Chapter 02:Nuclear Power Reactors - Components

Edited byDr. Mir F. Ali 1

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1. NUCLEAR ELECTRICITY GENERATION:A nuclear power reactor is a mechanism, device, or system that is, designed to allownuclear fission chain reactions to initiate, control, and sustain at a contained velocity,releasing energy at a controlled rate in a nuclear power reactor or at a very rapiduncontrolled rate in a nuclear weapon.

Here is a simplified description of how a nuclear power reactor functions: Uranium,naturally occurring radioactive chemical element found in the earths crust, serves as fuelto operate nuclear power reactors. An infinite natural energy stored in uranium atomsmakes it possible for nuclear power reactors to function. More specifically, the interactionbetween three heavy elements two types of uranium and a form of plutonium (A solidsilvery grey radioactive transuranic-element, whose atoms split when bombarded withneutrons, found in minute quantities in uranium ores but is usually, synthesized in

nuclear reactors.) creates a chain reaction that helps to generate electricity. The nuclearreaction generates heat that becomes the source for boiling water to create steam in orderto drive a turbine, and to generate electricity. The nuclear chain reaction referred torepresents nuclear fission.

The graph presented under Figure 2-1, illustrates an induced fission reaction.


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A slow-moving neutron absorbed by the nucleus of a uranium-235 atom, which in turnsplits into fast-moving lighter elements (fission products) and releases three freeneutrons.

As an explanation, nuclear fission is a nuclear reaction in which the nucleus of an atom

splits into smaller parts, often producing free neutrons and photons. Fission of heavyelements is an exothermic reaction that can release large amounts of energy both aselectromagnetic radiation and as kinetic energy of the fragments (Heating the bulkmaterial where fission takes place). For fission to produce energy, the total bindingenergy of the resulting elements has to be lower than that of the starting element. Fissionis a form of nuclear transmutation because the resulting fragments are not the sameelement as the original atom.

2. COMPONENTS OF NUCLEAR POWER REACTORS:In general, a nuclear power reactor is not much different from a conventional power plant

when it comes to generating electricity, except for the manner in which heat is generatedusing nuclear reactions chain. Here are eight components common to most types ofnuclear power reactors, which illustrate how each component functions:The following seven major components are, described here:
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2.1 The Fuel Component:Fuel can be used two different ways to produce nuclear energy. Fuel is, burned in case ofchemical reactions, whilst alternation takes place in the nuclear reactions. Both theseprocesses for producing nuclear heat are workable but the latter leads to much morerelease of thermal energy as compared to chemical reactions for similar quantities of fuel.

In order to form fuel rods, pellets of uranium oxide (UO2) are arranged in tubes. The rodsare, arranged into fuel assemblies in the reactor core. In a new reactor with new fuel, aneutron source is, needed to get, the reaction going. Usually this is beryllium mixed withpolonium, radium or other alpha-emitter. Alpha particles from the decay cause a releaseof neutrons from the beryllium as it turns to carbon-12. Restarting a reactor with someused fuel may not require this, as there may be enough neutrons to achieve criticalitywhen control rods are, removed.

Fuel rods are, placed within the reactor core, which are fabricated and placed within thereactor in such a manner so that it leads to a uniform production of heat within the

reactor. There are the following two types of reactors based on the manner in which thefuel and moderator are, placed within the core as follows:a. The homogeneous 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 insidethe reactor core; and

b. A heterogeneous reactor on the other hands has pure fuel in the form of rods orplates while the moderator surrounds the fuel elements separately. In this case, thefuel rods are often clad with different materials including Aluminum, StainlessSteel or Zirconium, which help to prevent oxidation of Uranium.

The fuel cycle includes 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. In the formercase, fuel is not thrown or dumped away at any random place but is placed and packagedproperly in order to prevent contamination of the biosphere.

2.2 The Moderator Component:A moderator is one of the important components of nuclear power reactor helping tomaintain neutron population in the thermal energy range from a controlling point ofview.

The fact of the matter is that whenever a thermal neutron causes fission it also leads tothe release of fast neutrons. These fast neutrons have to be stalled and brought to lowerenergy levels if they have to cause successful fission in turn. It is here that the concept ofa moderator comes in the picture.

A moderator is a medium that is, used to absorb a portion of the kinetic energy of fastneutrons so that they come in the category of thermal neutrons, which help to sustain a
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controlled chain reaction. The mechanism of speed control works in such a way that fastmoving neutrons strike the nuclei of moderator material which is not efficient atabsorbing them but simply slows them down with repeated collisions thus bringing theminto the thermal zone.

There are several materials, which are, used for the purpose including the following:a. Normal or Light Water is, used in majority of the reactors simply because of its

cheap and abundant availability. The only flip side of using light-water is thatthe fuel has to be enriched to use with water;

b. Deuterium also known as heavy water in common terminology, Deuterium iscostly to manufacture as compared to light water but gives the option of usingun-enriched fuel in the reactor which is a big advantage; and

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


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It is important to keep it in mind that although moderators are necessary in most nuclearreactors this does not mean all reactors require moderators. There is a special class ofreactors known as fast reactors, which do not use moderators but depend on the use offast moving neutrons for causing fission.

2.3 The Reflector Component:The chain reaction inside a nuclear reactor is what sustains Combustion of the fuel thatin turn depends on ample supply of thermal energy neutrons within the core. A reflectormaterial is, used to ensure that neutrons do not simply fly off the reactor leaving littleroom for the chain reaction to continue.

The reactor consists of the fission process, which occurs when a thermal energy neutronis absorbed by the target nucleus leading to its division into two nuclei and emission of 2or 3 neutrons apart from the heat energy. These neutrons fly randomly in all directionsand are usually in the region of fast moving energy neutrons. The moderator is, used tocontrol the speed of these neutrons so that they act usefully in creating more fission, but
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many of these neutrons may simply get lost, by flying off the reactor core, thus serving nouseful purpose. This might hinder the progression of a chain reaction that is verynecessary 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 that helps to reflect these escaping neutrons back towards thecore of the reactor and these materials are, known as reflecting materials.There are, a variety of materials, which are, used as a reflecting medium for neutrons andwhatever material is, used for the process, it must possess these properties: LowAbsorption, High Reflection, Radiation Stability, and Resistance to Oxidation.In actual practice, there may not be a different material for moderator and reflector forthe simple reason that most of the moderators also possess the properties mentionedabove of a good reflector as well. Hence, they serve the dual purpose of a reflector and amoderator as well. There light water, heavy water and carbon are mostly used asreflectors since they possess the mentioned properties.

The use of a proper reflector helps to reduce the size of the reactor core for a given poweroutput since the number of neutrons leaking are lesser and help to propagate the fissionprocess instead. It also reduces the consumption of the fissile material.

2.4 The Coolant Component:A nuclear reactor is a source of intense heat that is, generated through the fissionreactions taking place inside the core. Therefore, a coolant is, needed to ensure that thisheat is captured and utilized in a proper manner.

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 helpsto keep the working temperature of the core within safe limits for the materials used inthe construction of the reactor. Hence, a coolant plays an important role in componentsof nuclear power plant and serves the dual purpose of removing the heat from the reactoras well as transferring it to the electricity generation circuit either directly or indirectlydepending on the type of nuclear reactor being, used for the purpose.

There are some properties of the coolant, which are necessary to ensure safety of thereactor and as well as proper performance of the coolant for the intended purpose. Someof the desired properties of an ideal coolant are as follows:

a. A coolant should not absorb neutrons or should have a minimum neutronabsorption cross section. The reason for this is obvious since this function shouldbe left to the moderator and not the coolant;

b. Since a coolant is exposed to high temperatures and well as severe levels ofradiation, it is obvious that it should posses excellent resistance to both hightemperatures as well as high levels of radiation;
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c. 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 properremoval of heat;

d. Coolants used in nuclear reactors could be either in the liquid state or in the solidstate. In case the coolant is, a liquid it should have a high boiling point so that, it isnot evaporated due to the high heat inside the reactor. But in case it is a solid itshould have a relatively low melting point due to obvious reasons; and

e. 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.

2.5 The Control Rods Component:

Nuclear fission is a source of tremendous energy that could be either, used for destructivepurposes such as nuclear weapons or constructive purposes such as a nuclear reactor forproducing electrical energy. Even though a nuclear reactor in a power plant with peacefulintentions, the tremendous power, heat and energy which is, associated with nuclearfission cannot be left on its own but needs to be controlled in a predictable manner. It ishere that controls rods come in the picture and form an important part of thecomponents of nuclear power plant.


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Here are some basic reasons to explain why proper control is necessary within nuclearpower reactors:

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

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

c. 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 disasterlike say Chernobyl which could be very costly in terms of loss to life andenvironment; and

d. Fuel rods inside the reactor should be prevented from melting or beingdisintegrated and therefore a control mechanism is necessary.

In order for controlling and taming the wild nuclear power, the best method to achievethis goal is through the use of control rods which can be inserted or withdrawn from thecore and help to control the nuclear reactions taking place inside the reactor.

One property that is, a must for control rod material is the heavy absorption capacity forneutrons so that they can carry out the control function effectively. The commonly used


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materials that satisfy these criteria include cadmium, boron, iridium, silver and hafnium.Another property of control rods is that the material should not start a fission reactiondespite the heavy absorption of neutrons. In fact, the function of a control rod just like ablotting paper that sucks the extra ink that has spilled somewhere but does not let itspread in a wider region.

2.6 The Shielding Component:A nuclear reaction is a source of intense radiation apart from the heat generated in theexothermic process. Because of the risk, radiation shielding is required to prevent thisharmful radiation from leaving the reactor and affecting the outside men and materials.

When a nucleus splits into two parts during the fission process it results in theproduction 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 otherby-products such as alpha rays, beta rays, gamma rays and of course the fast movingneutrons. The fast moving neutrons are controlled, moderated and reflected in order to


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contain them within the reactor core so that a sustained and controlled chain reactiontakes place.

These by-products in the form of different kinds of radiation would simply leak out intothe 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 thenearby flora and fauna.

This makes clear the case for having a proper shield so that these radiations get absorbedwithin the reactor without having a chance to escape into open air. This is, done by usingmaterials that are good absorbents of the same. Concrete and steel are very good atabsorbing radiation and they are equally strong as well, hence used in forming theshielding material.

2.7 The Vessel Component:


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A nuclear reactor consists of various parts that carry out different functions related toheat generation by burning of nuclear fuel, but housing, is needed to contain all theseparts and act as a covering for all these paraphernalia.In addition to generating electricity, it performs the following functions:

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

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

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

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

Although the reactor vessel is, compared to a cookery vessel in the common usage of theterm, technically speaking it is more of a pressure vessel. There are legal implicationsassociated with defining a pressure vessel and these vary with the country in which it isbeing used or manufactured. Different countries have different authorities, which governrules and regulations regarding pressure vessels. The material used for the construction ofa nuclear vessel is usually steel that would be expected, as the material has to be verystrong and resilient.

Pressure vessels of all kinds are subject to various tests to check for their strength againstlaid down standards, which, is very important to ensure safety of these vessels. This is

more so important in the case of nuclear reactor vessels that house source of intenseradiations and heat energy.

2.8 The Steam Generator Component:Steam generators are heat exchangers, used to convert water into steam from heatproduced in a nuclear reactor core. They are, used in Pressurized Water Reactors betweenthe primary and secondary coolant loops.

In commercial power plants, steam generators can measure up to 70 feet in height andweigh as much as 800 tons. Each steam generator can contain anywhere from 3,000 to

16,000 tubes, each about three-quarters of an inch in diameter. The coolant (treatedwater), which, is maintained at high pressure to prevent boiling, is pumped through thenuclear reactor core. Heat transfer takes place between the reactor core and thecirculating water. The coolant is, then pumped through the primary tube side of thesteam generator by coolant pumps before returning to the reactor core. This is, referredto as the primary loop.
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That water flowing through the steam generator boils water on the shell side to producesteam in the secondary loop that is, delivered to the turbine to make electricity. Thesteam is subsequently condensed via cooled water from the tertiary loop and returned tothe steam generator, to be heated once again. The tertiary cooling water may be,recalculated to cooling towers where it sheds waste heat before returning to condense

more steam. Once through tertiary cooling may otherwise be provided by, a river, lake, orocean. This primary, secondary, tertiary cooling scheme is the most common way toextract usable energy from a controlled nuclear reaction.

These loops also have an important safety role because they constitute one of the primarybarriers between the radioactive and non-radioactive sides of the plant as the primarycoolant becomes radioactive from its exposure to the core. For this reason, the integrity ofthe tubing is essential in minimizing the leakage of water between the two sides of theplant. There is the potential that, if a tube bursts while a plant is operating, contaminatedsteam could escape directly to the secondary cooling loop. Thus during scheduledmaintenance outages or shutdowns, some or all of the steam generator tubes areinspected by eddy-current testing.


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This chapter was published on Inuitech Intuitech Technologies for Sustainability onNovember 24th, 2010:http://intuitech.biz/?p=7803

Resources:1. Wikipedia Nuclear Fission:http://en.wikipedia.org/wiki/Nuclear_fission2. Bright Hub Components of Nuclear Power Plant Fuel:

http://www.brighthub.com/engineering/mechanical/articles/2602.aspx3. Bright Hub:

http://www.brighthub.com/engineering/mechanical/articles/2602.aspx
http://intuitech.biz/?p=7803http://intuitech.biz/?p=7803http://intuitech.biz/?p=7803http://en.wikipedia.org/wiki/Nuclear_fissionhttp://en.wikipedia.org/wiki/Nuclear_fissionhttp://en.wikipedia.org/wiki/Nuclear_fissionhttp://www.brighthub.com/engineering/mechanical/articles/2602.aspxhttp://www.brighthub.com/engineering/mechanical/articles/2602.aspxhttp://www.brighthub.com/engineering/mechanical/articles/2602.aspxhttp://www.brighthub.com/engineering/mechanical/articles/2602.aspxhttp://www.brighthub.com/engineering/mechanical/articles/2602.aspxhttp://www.brighthub.com/engineering/mechanical/articles/2602.aspxhttp://en.wikipedia.org/wiki/Nuclear_fissionhttp://intuitech.biz/?p=7803

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Chapter 02_Nuclear Power Reactors _ Components