- This chapter was published on “Inuitech – Intuitech Technologies for Sustainability” on November 24th, 2010.
- The chapter was updated on November 25, 2015.
NUCLEAR POWER REACTORS
The world’s first nuclear reactors operated naturally in a uranium deposit about two billion years ago. These were in rich uranium orebodies and moderated by percolating rainwater. The 17 known at Oklo in West Africa, each less than 100 kW thermal, together consumed about six tonnes of that uranium. It is assumed that these were not unique worldwide.
Today, reactors derived from designs originally developed for propelling submarines and large naval ships generate about 85 percent of the world’s nuclear electricity. The main design is the pressurized water reactor (PWR) which has water at over 300°C under pressure in its primary cooling/heat transfer circuit, and generates steam in a secondary circuit. The less numerous boiling water reactor (BWR) makes steam in the primary circuit above the reactor core, at similar temperatures and pressure. Both types use water as both coolant and moderator, to slow neutrons. Since water normally boils at 100°C, they have robust steel pressure vessels or tubes to enable the higher operating temperature. (Another type uses heavy water, with deuterium atoms, as moderator. Hence the term ‘light water’ is used to differentiate.)
Here are eight components common to most types of nuclear power reactors, which illustrate how each component functions:
Fuel can be used two different ways to produce nuclear energy. Fuel is, burned in the case of chemical reaction, whilst alternation takes place in the nuclear reactions. Both these processes for producing nuclear heat are workable but the latter leads to much more release of thermal energy as compared to chemical reactions for similar quantities of fuel.
In order to form fuel rods, pellets of uranium oxide (UO2) arranged in tubes. The rods are arranged into fuel assemblies in the reactor core. In a new reactor with new fuel, a neutron source is needed to get, the reaction going. Usually this is beryllium mixed with polonium, radium or other alpha-emitter. Alpha particles from the decay cause a release of neutrons from the beryllium as it turns to carbon-12. Restarting a reactor with some used 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 the reactor 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 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; and
- A heterogeneous reactor on the other hand 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 Aluminum, Stainless Steel 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 former case used, 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.
A moderator is one of the important components of a nuclear power reactor helping to maintain neutron population in the thermal energy range from a controlling point of view.
The fact of the matter is that whenever a thermal neutron causes fission it also leads to the release of fast neutrons. These fast neutrons have to be stalled 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.
A moderator is a medium that 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.
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;
- 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; and
- Miscellaneous – Several materials such as Graphite, Beryllium, Lithium, are, used in different types of reactors as moderators.
It is important to keep it in mind that although moderators are necessary in most nuclear reactors this does not mean 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.
The chain reaction inside a nuclear reactor is what sustains “combustion” of the fuel that in turn depends on an 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.
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 thus serving no useful purpose. This might hinder the progression of a chain reaction that 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 that helps to reflect these escaping neutrons back towards the core 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 and whatever 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 the simple reason that most of the moderators also possess the properties mentioned above 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 mentioned properties.
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.
A nuclear reactor is a source of intense heat that is generated through the fission reactions taking place inside the core. Therefore, a coolant is needed to ensure that this heat 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 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 a 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.
There are some properties of the coolant, which are necessary to ensure the safety of the reactor and as 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;
- 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;
- 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 is not 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; and
- Since a coolant needs to circulate using a pump it should be capable of being pumped easily so that the least amount of energy is spent in pumping the coolant.
Nuclear fission is a source of tremendous energy that 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 with 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.
Here are some basic reasons to explain why proper control is necessary within nuclear power reactors:
- 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;
- 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;
- 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; and
- Fuel rods inside the reactor should be prevented from melting or being disintegrated and therefore a control mechanism is necessary.
In order for controlling and taming the wild nuclear power, the best method to achieve this goal 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.
One property that 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 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 reaction despite the heavy absorption of neutrons. In fact, the function of a control rod is just like a blotting paper that sucks the extra ink that has spilled somewhere but does not let it spread in a wider region.
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.
When a nucleus splits 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”, 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, moderated and reflected in order to contain them within the reactor core so that a sustained and controlled chain reaction takes place.
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 that 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.
A nuclear reactor consists of various parts that carry out different functions related to heat generation by “burning” of nuclear fuel, but housing, is needed to contain all these parts and act as a covering for all these paraphernalia.
In addition to generating electricity, it performs the following functions:
- It acts to enclose the various parts inside the reactor including the core, shield, reflector etc.;
- 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;
- 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; and
- Control of the nuclear reaction is, 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.
Although the reactor vessel is 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. The material used for the construction of a nuclear vessel is usually steel that 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 more so important in the case of nuclear reactor vessels that house source of intense radiations and heat energy.
Steam generators are heat exchangers, used to convert water into steam from heat produced in a nuclear reactor core. They are, used in Pressurized Water Reactors between the primary and secondary coolant loops.
In commercial power plants, steam generators can measure up to 70 feet in height and weigh 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 (treated water), which, is maintained at high pressure to prevent boiling, is pumped through the nuclear reactor core. Heat transfer takes place between the reactor core and the circulating water. The coolant is then pumped through the primary tube side of the steam generator by coolant pumps before returning to the reactor core. This is referred to as the primary loop.
That water flowing through the steam generator boils water on the shell side to produce steam in the secondary loop that is delivered to the turbine to make electricity. The steam is subsequently condensed via cooled water from the tertiary loop and returned to the steam generator, to be heated once again. The tertiary cooling water may be re-circulated 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, or ocean. This primary, secondary, tertiary cooling scheme is the most common way to extract usable energy from a controlled nuclear reaction.
These loops also have an important safety role because they constitute one of the primary barriers between the radioactive and non-radioactive sides of the plant as the primary coolant becomes radioactive from its exposure to the core. For this reason, the integrity of the 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 steam could 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.
- World Nuclear Association – Nuclear Power Reactors;
- Right Hub Engineering: Components of Nuclear Power Plant – Fuel;
- Bright Hub Engineering: Components of Nuclear Power Plant – Moderator;
- Bright Hub Engineering: Components of Nuclear Power Plant – Reflector;
- Bright Hub Engineering: Components of Nuclear Power Plant – Coolant;
- Bright Hub Engineering: Components of Nuclear Power Plant – Control Rods;
- Bright Hub Engineering: Components of Nuclear Power Plant – Shielding;
- Bright Hub Engineering: Components of Nuclear Power Plant – Reactor Vessel; and
- Wikipedia: Steam Generator.
Dr. Mir F. Ali