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Chapter 02: Nuclear Power Reactors - ComponentsEdited byDr. Mir F. Ali

1. Nuclear Electricity Generation:A nuclear power reactor is a mechanism, device, or system that is, designed to allow nuclearfission 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 rapid uncontrolled rate in anuclear 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 fuel tooperate nuclear power reactors. An infinite natural energy stored in uranium atoms makes itpossible for nuclear power reactors to function. More specifically, the interaction betweenthree heavy elements two types of uranium and a form of plutonium (A solid silvery greyradioactive transuranic-element, whose atoms split when bombarded with neutrons, foundin minute quantities in uranium ores but is usually, synthesized in nuclear reactors.) createsa chain reaction that helps to generate electricity. The nuclear reaction generates heat thatbecomes the source for boiling water to create steam in order to drive a turbine, and togenerate electricity. The nuclear chain reaction referred to represents nuclear fission.

The graph presented under Figure 2-1,illustrates an induced fission reaction. Aslow-moving neutron absorbed by thenucleus of a uranium-235 atom, which inturn splits into fast-moving lighter elements(fission products) and releases three freeneutrons.

As an explanation,nuclear fissionis a nuclearreaction in which the nucleus of an atomsplits into smaller parts, often producing freeneutrons and photons. Fission of heavyelements is an exothermic reaction that canrelease large amounts of energy both aselectromagnetic radiation and as kineticenergy of the fragments (Heating the bulkmaterial where fission takes place). For

fission to produce energy, the total bindingenergy of the resulting elements has to belower than that of the starting element.Fission is a form of nuclear transmutationbecause the resulting fragments are not thesame element as the original atom.

2. Components of Nuclear Power Reactors:
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In general, a nuclear power reactor is not much different from a conventional power plantwhen 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 of nuclearpower reactors, which illustrate how each component functions:

The following seven major components are,describedhere:2.1 The Fuel Component:

Fuel can be used two different ways to producenuclear energy. Fuel is, burned in case ofchemical reactions, whilst alternation takesplace in the nuclear reactions. Both theseprocesses for producing nuclear heat areworkable but the latter leads to much morerelease of thermal energy as compared tochemical reactions for similar quantities offuel.

In order to form fuel rods, pellets of uraniumoxide (UO2) arranged in tubes. The rods are,arranged into fuel assemblies in the reactorcore. In a new reactor with new fuel, aneutron source is, needed to get, the reactiongoing. 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 tocarbon-12. Restarting a reactor with someused fuel may not require this, as there may be

enough neutrons to achieve criticality when 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 the fuel andmoderator are, placed within the core as follows:

The homogenous reactor is one in which the fuel and moderator are mixed to form auniform mixture which is then placed in the form of rods and plates inside the reactorcore; and

A heterogeneous reactor on the other hands has pure fuel in the form of rods or plateswhile the moderator surrounds the fuel elements separately. In this case, the fuelrods are often clad with different materials including Aluminum, Stainless Steel orZirconium, which help to prevent oxidation of Uranium.

The fuel cycle includes the total process of preparation of fuel, burning of fuel and finaldisposal. If the fuel from the last stage is recycled to, be used again in the nuclear reactor, itis known as a closed fuel cycle otherwise it is known as open fuel cycle. In the former case,fuel is not thrown or dumped away at any random place but is placed and packaged properlyin order to prevent contamination of the biosphere.
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2.2 The Moderator Component:A moderator is one of the important components ofnuclear power reactor helping to maintain neutronpopulation in the thermal energy range from acontrolling point of view.

The fact of the matter is that whenever a thermal

neutron causes fission it also leads to the release offast neutrons. These fast neutrons have to be stalledand brought to lower energy levels if they have tocause successful fission in turn. It is here that theconcept of a moderator comes in the picture.

A moderator is a medium that is, used to absorb aportion of the kinetic energy of fast neutrons so thatthey come in the category of thermal neutrons,which help to sustain a controlled chain reaction.The mechanism of speed control works in such away that fast moving neutrons strike the nuclei ofmoderator material which is not efficient atabsorbingthem 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 flipside of using light-water is that the fuel has tobe enriched to use with water;

b. Deuterium - also known as heavy water in common terminology, Deuterium is costlyto manufacture as compared to light water but gives the option of using un-enrichedfuel in the reactor which is a big advantage; and

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

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 of fastmoving neutrons for causing fission.2.3 The Reflector Component:

The chain reaction inside a nuclear reactor is what sustains combustion of the fuel that inturn 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 little roomfor the chain reaction to continue.
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The reactor consists of the fission process, whichoccurs when a thermal energy neutron is absorbedby the target nucleus leading to its division into twonuclei and emission of 2 or 3 neutrons apart fromthe heat energy. These neutrons fly randomly in alldirections and are usually in the region of fastmoving energy neutrons. The moderator is, used to

control the speed of these neutrons so that they actusefully in creating more fission, but many of theseneutrons may simply get lost, by flying off thereactor core, thus serving no useful purpose. Thismight hinder the progression of a chain reactionthat is very necessary for the nuclear reactor.

In order to reduce this process of neutron loss theinner surface of the reactor core is, surrounded by amaterial that helps to reflect these escaping

neutrons back towards the core of the reactor andthese 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: Low Absorption,High Reflection, Radiation Stability, and Resistance to Oxidation.

In actual practice, there may not be a different material for moderator and reflector for thesimple reason that most of the moderators also possess the properties mentioned above of agood reflector as well. Hence, they serve the dual purpose of a reflector and a moderator aswell. There light water, heavy water and carbon are mostly used as reflectors 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 fission reactionstaking place inside the core. Therefore, a coolant is, needed to ensure that this heat iscaptured 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 helps tokeep the working temperature of the core within safe limits for the materials used in theconstruction of the reactor. Hence, a coolant plays an important role in components ofnuclear power plant and serves the dual purpose of removing the heat from the reactor aswell as transferring it to the electricity generation circuit either directly or indirectlydepending on the type of nuclear reactor being, used for the purpose.
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There are some properties of the coolant, whichare necessary to ensure safety of the reactor andas well as proper performance of the coolant forthe intended purpose. Some of the desiredproperties of an ideal coolant are as follows:

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

b. Since a coolant is exposed to hightemperatures and well as severe levels ofradiation, it is obvious that it shouldposses excellent resistance to both high

temperatures as well as high levels ofradiation;c. A coolant should be non-corrosive in

nature otherwise, it might tend todamage and corrode the very core which is meant to be protected by it throughproper removal 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 pointdue to obvious reasons; and

e. Since a coolant needs to circulate using apump it should be capable of being, pumpedeasily 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 energythat could be either, used for destructive purposessuch as nuclear weapons or constructive purposessuch as a nuclear reactor for producing electrical

energy. Even though a nuclear reactor in a powerplant with peaceful intentions, the tremendouspower, heat and energy which is, associated withnuclear fission cannot be left on its own but needsto be controlled in a predictable manner. It is herethat controls rods come in the picture and form animportant part of the components of nuclear powerplant.

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Here are some basic reasons to explain why proper control is necessary within nuclear powerreactors:

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 the powerrequirements 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 disaster likesay Chernobyl which could be very costly in terms of loss to life and environment; and

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

In order for controlling and taming the wild nuclear power, the best method to achieve thisgoal is through the use of control rods which can be inserted or withdrawn from the coreand 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 usedmaterials 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 it spreadin a wider region.2.6 The Shielding Component:A nuclear reaction is a source of intenseradiation apart from the heat generatedin the exothermic process. Because of

the risk, radiation shielding is required toprevent this harmful radiation fromleaving the reactor and affecting theoutside men and materials.

When a nucleus splits into two partsduring the fission process it results in theproduction of large amounts of heatenergy since the reaction is exothermicin nature. But this is not the only

product of nuclear fuel "combustion" butthere are several other by-products suchas alpha rays, beta rays, gamma rays andof course the fast moving neutrons. Thefast moving neutrons are controlled,moderated and reflected in order tocontain them within the reactor core so that a sustained and controlled chain reaction takesplace.

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These by-products in the form of different kinds of radiation would simply leak out into theatmosphere in the absence of proper arrangements to prevent this. Radiation leakage wouldbe very harmful for the personnel working in the nuclear plant as well as the nearby floraand 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 using

materials 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 the shieldingmaterial.2.7 The Vessel Component:A nuclear reactor consists of various parts that carryout different functions related to heat generation byburning of nuclear fuel, but housing, is needed tocontain all these parts and act as a covering for allthese paraphernalia.

In addition to generating electricity, it performs thefollowing functions:a. It acts to enclose the various parts inside the

reactor including the core, shield, reflectoretc.;

b. The coolant needs a passage to flow throughthe reactor so that it can be used to transferthe heat to the working fluid or the turbinedirectly, as the case may be, and this passageis provided by the reactor vessel;

c. To withstand the high pressure with existsinside the reactor and could be of the order of200 kgf/cm2, to provide a safe workingenvironment for all concerned; and

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

associated 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 of anuclear vessel is usually steel that would be expected, as the material has to be very strongand 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

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so important in the case of nuclear reactor vessels that house source of intense radiationsand heat energy.2.8 The Steam Generator Component:Steam generators are heat exchangers, used toconvert water into steam from heat produced in anuclear reactor core. They are, used in PressurizedWater Reactors between the primary and

secondary coolant loops.

In commercial power plants, steam generators canmeasure up to 70 feet in height and weigh as muchas 800 tons. Each steam generator can containanywhere from 3,000 to 16,000 tubes, each aboutthree-quarters of an inch in diameter. The coolant(treated water), which, is maintained at highpressure to prevent boiling, is pumped through thenuclear reactor core. Heat transfer takes place

between the reactor core and the circulatingwater. The coolant is, then pumped through theprimary tube side of the steam generator bycoolant pumps before returning to the reactorcore. This is, referred to as the primary loop.

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. The steamis subsequently condensed via cooled water from the tertiary loop and returned to the steamgenerator, to be heated once again. The tertiary cooling water may be, recalculated tocooling towers where it sheds waste heat before returning to condense more steam. Once

through tertiary cooling may otherwise be provided by, a river, lake, or ocean. This primary,secondary, tertiary cooling scheme is the most common way to extract usable energy from acontrolled 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 the plant.There is the potential that, if a tube bursts while a plant is operating, contaminated steamcould escape directly to the secondary cooling loop. Thus during scheduled maintenance

outages or shutdowns, some or all of the steam generator tubes are inspected by eddy-current testing.

The next chapter will cover the types of nuclear power reactors.
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