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Executive Summary

The demand for electricity in todays world is on the rise. As a result, many technologies are being tested to come up with the most appropriate one to supplement the current electric power generation. Over the years, the use of pressurized water reactors has become common. The process is carried out in nuclear power stations. Radioactive fuel is used as a source of energy. The plants use three distinct water systems to generate electricity. The three are the primary, the secondary, and the condenser systems. Only the primary and the secondary systems play an active role in the functioning of the plant. The condenser system plays a passive role in that it only helps in cooling the feed water. Secondary and primary water systems share some similarities. For example, both are closed. As such, their contents are physically separated. The primary system cools the reactor fuel after it has undergone fission, which is a heat-generating process. The primary system carries the heat it has drawn from the reactor to the steam generator. Here, the primary and the secondary systems exchange heat. Water in the secondary system is superheated to produce steam. The steam is dried. It is then directed towards the main turbine. The turbine is under high pressure. From there, it is dried once again and reheated to boost its pressure. It is then directed towards the low-pressure turbines. The turbines are linked to a generator. Electricity generated from these nuclear plants is used in industrial and residential areas. It can also be used in naval vessels, such as warships and submarines. However, it is important to carefully monitor the plant to avoid accidents, which may be catastrophic.

Introduction

With demand for electricity on the rise across the world, countries have to come up with additional means of boosting their power production (Krepper 2013). Many alternatives are currently under trial in different nations. They include wind, solar, and hydroelectric stations. Even so, the use of steam to generate power has become popular, especially in developed countries. There are a lot of deliberations with regards to the best source of steam for power generation. Traditionally, heat sources, such as coal, were used for the purposes of heating water. However, with technological advancements, other sources of heat, such as nuclear reactors, are being used.

There are a number of commonly used nuclear power reactors. One of them is the Light Water Reactor (LWRs). Under this category, the Pressurised Water Reactor (PWR) is the most commonly used. PWR plants rely on nuclear fission to generate heat. Radioactive materials, usually uranium and plutonium, are used as fuel in the reactor (Lips 2005). The basic functioning of the PWRs involves the transfer of heat generated in the nuclear core to water.

It is important to note that PWR has three separate water systems. The three are primary, secondary, and condenser. In the first one, water is heated under very high temperatures. Heat is passed on to the water in the first system. The exchange takes place as the water circulates around the heads of the reactor vessels. Here, it acts as a coolant to moderate the heat generated (Krepper 2013). The water in the system comes in direct contact with uranium fuel. The temperatures in the first system are high. However, the water is not allowed to boil. To ensure this, it is kept under high pressure. It is important to note that the system is contained. Water is held within a piping loop. It is highly pressurized. It also passes a series of tubes. The piping is within the steam generator.

The tubes are held inside a second water system. Its main purpose is to generate steam. As a result, it is referred to as the steam-generating system. The heat from the first system is transferred to the second. In the secondary system, it is transformed to steam which is then pumped to the turbine chamber (Nakath, Schuster & Hurtado 2013). The second water system is also closed. As such, it does not come into contact with water from the other systems. The third water system is referred to as the condenser. It is used for the purpose of cooling the steam that escapes from the turbine chambers. The condensed water is returned into the steam generator and the cycle is recurring.

In this paper, the author seeks to explore the processes involved in the generation of power in PWRs. All aspects of PWR will be taken into consideration. The components of PWR plants will also be highlighted and their functions discussed. The advantages and disadvantages of the system will also be analyzed. The author will also look into the safety issues associated with the use of these plants. In conclusion, waste management will be discussed. Tables, graphs, pictures, and schemes will be used to illustrate the various aspects of PWR.

The Pressurised Water Reactor System

Overview

The setting up of nuclear power plants requires strict procedures to be followed. The plants are complex and are composed of many systems performing different functions. For the plant to be successful, it is important for these systems to be effective and efficient in performing their various functions (Krepper 2013). Two major systems play a major role in the production of electricity in the PWRs. The two are the primary and secondary systems. The two systems are linked together through the steam generator. Although water is used in both systems, it is important to note that their contents do not mix. Only heat is exchanged between the two systems but not water. There is a third one, which is also referred to as the condenser. It plays an important role in the recycling of water within the system. However, it does not directly impact on electricity generation.

The diagram below illustrates the three different systems of PWR and their components:

The Primary System

The system is also commonly referred to as the reactor coolant. Its main role is to pass heat to the generator. The energy is generated from the reactor fuel. It also helps in the containment of any fission waste products that may escape from the fuel reactor. It consists of five major components which include the reactor vessel, the steam generator, the coolant pump, pressuriser, and connecting pipes (Madasamy et al. 2011). It is noted that the steam generator only acts as the site for heat transfer between the primary and the secondary systems. As such, the contents of the two systems never come into contact. The basic components of the system are interconnected by a series of piping. It is also tightly enclosed to avoid spilling its content. Since the system works under high pressure, great care should be taken to avoid any faults which could be catastrophic. The water in this compartment is also contaminated with radioactive products emanating from the fuel. As such, leakages within the system would expose individuals working in the plants to radiations. When such occurrences take place on a large scale, the effect is often catastrophic. To this end, radiations can be experienced over a wide area.

The major components of all PWR systems are similar. However, the arrangement of the components may vary from one plant to another (Madasamy et al. 2011). For example, it may have between 2 and 4 loops. The number of loops within a plant depends on the amount of power it is expected to generate.

The two-loop PWR plant

The two-loop PWR plant is unique in that it has two steam generators. In addition, it has a pair of reactor coolant pumps. However, the plants have a single fuel reactor and pressuriser. Another major characteristic of the two-loop plants is the components of their reactor vessel. In most cases, it has 121 fuel assemblies (Lee et al. 2005). Each of these fuel assemblies is arranged in 14 x 14 arrays. They are 132 inches in diameter. The plants are considered to have a slightly lower power output compared to those with three and four-loops. On average, the installation produce 500 megawatts of power.

The diagram below is an illustration of the two-loop PWR plants major components and their arrangement:

Figure 2: Components of a two-loop PWR plant

The three-loop PWR plant

The three-loop PWR plant has three steam generators. It also has a total of three reactor coolant pumps and a single pressuriser. The PWR plants of this kind have 157 fuel assemblies. Some plants may have fuel assemblies are arranged in 15 x 15 arrays while others are17 x 17 (Krepper & Schaffrath 2013). The reactor vessels are also of a larger diameter compared to those used in the two-loop system. They are 156 and 159 inches. The reason behind this is that they accommodate more fuel assemblies which are required to provide additional heating. The power output of the installation varies. It ranges from 700 to slightly over 900 megawatts.

The diagram below is an illustration of the three-loop PWR plants major components and their arrangement:

Figure 3: The components of a 3-loop PWR plant

The four-loop PWR plant

The four-loop PWR plant is the largest of the three systems. It has four steam generators. It also has a total of four reactor coolant pumps and a single pressuriser. Most plants that are of this kind use 193 fuel assemblies. They are all fitted in a reactor vessel that is 173 inches in diameter. The fuel assemblies are arranged in 17 x 17 arrays (Krepper & Schaffrath 2013). However, in rare cases, the 15 x 15 arrays may be used. The plants produce the highest amount of power among the three. On average, they produce between 950 and 1250 megawatts.

The diagram below is an illustration of the four-loop PWR plants major components and their arrangement:

Figure 4: Components of a 4-loop PWR plant

Components of the primary system and their functions

Pressuriser

The pressuriser is also one of the basic components of the primary system. Its main role is to regulate the pressure of the system. In order to perform this function, it has four major components which include an electrical heater, a safety valve, and a pressuriser spray. It also contains a series of relief valves. The primary system works with steam and water. The two are in equal ratios. The main reason behind this is to prevent boiling. A deviation from this is quickly returned to norm. There are various sources of pressure variations within the system. One is an increase in temperature. A change in pressure is directly proportional to temperature (Hohne & Kliem 2007). As such, an increase in temperatures will automatically lead to a rise in pressure and vice versa.

The reactor coolant system is hooked to the pressuriser. The link is established through a surge line. The temperature of the network and the density of the cooling agent are related. For example, when one rises, the other decreases. Consequently, the coolant will occupy more space (Choi, Park & Song 2013). The reason behind this is the formation of steam. Its volume is 600% more than that of water. As such, it closes the surge line and enters the pressuriser. It tries to stabilise the pressure. To achieve this, the spray line will pass water into the steam area. The water is relatively cold. It is drawn from the reactor coolant. The steam will condense to form water, decreasing the pressure of the system. In the event that the pressure of the system continues to increase above normal levels, then the relief valve is deployed. It opens transferring the steam to a pressuriser relief tank. In the event that the temperatures of the system continue to rise rapidly, the relief valve is not effective enough (Choi et al. 2013). In this case, the pressuriser automatically deploys the safety valves. The valve also opens into the pressuriser relief tank.

On the other hand, a decrease in temperature within the reactor coolant system will result in a reverse reaction. First, the density of the coolant will rise. As such, the volume occupied by the coolant will be significantly reduced. Its level in the pressuriser will be subsequently reduced. The reason behind this is a pressure reduction within the reactor coolant system. In this case, the electrical heater is deployed. Its main purpose is to heat water in order to generate steam (Jung & Yeon 2010). Since steam occupies close to six times the volume required to hold water of the same mass, the pressure of the system will be raised. If there is no increase in the pressure of system, the plant will continue to run under low capacity until a predetermined set point is reached. The pressuriser in such a situation signals the plants protection system to trip the reactor.

The pressuriser relief tank is not regarded as a major component of the reactor coolant system. However, it plays a major role in ensuring that it functions efficiently. It is usually a large tank with water. The liquid is set at nitrogen atmosphere. Its main role is to condense any steam that is released from the reactor coolant system through the safety and relief valves in an attempt to reduce pressure (Jung & Yeon 2010). The nitrogen atmosphere increases it efficiency. The reason behind this is that nitrogen is a better coolant compared to water. It also provides an inert atmosphere thus preventing cases of explosion. The reason behind this is that one of the components of the reactor coolant is hydrogen. If ejected into the atmosphere at high temperatures and pressure, it would easily lead to an explosion.

Reactor coolant pumps

The main purpose of a reactor coolant pump is to speed up the process of removing the heat generated within the reactor vessel after fission has occurred. It achieves this by forcing the coolant through and around the fuel assemblies within the reactor vessel (Lips 2005). It is important to note that even without the use of the pumps, circulation could still occur within the reactor coolant system. However, the rate of circulation would not be efficient enough to remove the heat from the fuel assemblies. As such, natural circulation is only used when the plant is shutting down.

The cooling agent passes through to the pump. It uses the steam generators valve to achieve this. The pump impeller works on the water. It increases its velocity. An increase in the velocity of the reactor coolant automatically translates to a raise in pressure at the discharge volute of the pump (Lee, Yoo & Kim 2005). At the outlet point, the pressure of the cooling agent is high. It is about 90 psi more compared to that in the inlet (Lee et al. 2005). The increase in pressure within the primary system resulting from the action of the primary reactor pump plays a number of roles in promoting the efficiency of the entire plant. To begin with, it hastens the process of heat transfer within the reactor vessel. As a result, more power will be generated within the plant. The pump also prevents the stagnation or backflow of the reactor coolant.

The number of reactor coolant pumps required per PWR plant depends on the number of steam generators it has. The reason behind this is that the major role of the pumps is to speed up the movement of the coolant from the steam generator to the reactor vessel. A reactor coolant pump has three major components. The first is the hydraulic segment. The second is the motor. Lastly, there is the seal. The pumps and their components are large in size. The motor used in the reactor coolant pump is powered by electricity. Most pumps have a rating of between 6,000 and 10,000 horsepower (Madasamy et al. 2011). The power generated is enough to pump over 100,000 gallons of the reactor coolant per minute.

The hydraulic section contains two main parts. One is the discharge volute. The second is the impeller. The latter is linked to the electric motor. The connection is via a long shaft. As such, the impeller is said to be powered by the motor. It helps push out the water flowing through the pump at high velocity (Lee et al. 2005). The water exits the reactor coolant pump through the discharge volute. The third element is located between the hydraulic section and the motor. It major role is preventing thee leakage of the coolant up, past the shaft and into the plants containment atmosphere. Any coolant that manages to leak through the shaft is collected and taken to the seal leak-off system.

Reactor vessel

The reactor vessel houses the reactor core and its barrel. Usually, it is cylindrical in shape. At both ends, it contains heads that are hemispherical in nature. Usually, the top head is removable which enables the operators of the plant to refuel the reactor. The reactor vessel is constructed using steel, manganese, and molybdenum (Kim & Lee 2011). The material used to assemble the container is placed on the outside. It is contained in specimen holders. The materials are removed on a regular basis and tested to establish the effect radiations from the fuel have had on their strength. As such, the safety status of the reactor vessel can be determined. It is important to prevent corrosion. To this end, all surfaces that are in touch with water are covered with stainless steel. The barrel of the core stores the reactor fuel. It slides from top. Near the bottom, the container has a plate. A fuel assembly rests on this section. The entire core container and its internal components are suspended inside the reactor container. They are held in place by a support ledge.

Water in the primary system is also commonly referred to as the reactor coolant. It enters the reactor vessel through the inlet nozzle that is located at the top head. Here, it hits against the central barrel. The motion makes it move downwards. The cooling agent flows within the system. It passes between the nucleus container and the sides of the reactor. After reaching the bottom of the reactor vessel, the direction of the water flow changes (Kim & Lee 2011). The cooling agent starts to flow in an upward direction. It does this inside the core barrel. It also passes through the fuel combinations. Water passes over and inside the assemblies. It draws out heat generated through fission. The effective number of neutrons that have undergone fission can be determined using the formula below:

Formula

The rate of reaction is usually a function of the density of neutrons, their speed, and microscopic cross section of the medium for the reaction type x. It can be determined using the formula below:

Formula

The system controlling the reactor maintains chain reaction at a desired constant state. It achieves this by keeping track of the ratio between the number of neutrons of a single generation and that of the next are at a reasonable range. The ratio is also commonly referred to as the multiplication factor. It is denoted as k. It is calculated using the formula below:

k=Number of neutrons in current generation/ Number of neutrons in the preceding generation.

After leaving the fuel assemblies, the now hot water continues to flow upwards towards the outlet headed for the steam generators.

Reactor safety

PWR plants play an important role in the generation of electricity. However, they also pose potential harm to human existence. Radiations from the fuel used in the reactors can cause serious health problems, such as cancer and other genetic disorders. In order to minimise the exposure to the public, countries are urged to use nuclear power responsibly (Kim 2011). A number of principles have been adopted and incorporated to the designs of the PWR plants across the world. They serve as the guiding principle and help improve on reactor safety. The principle states that the public and the environment are put at minimum risk from the fuel used in the PWR reactors provided that the following guidelines are adhered to:

The power of the reactor is controlled. To achieve this, the reactor should be closely monitored at all times (Kim 2011). The functioning of all the support systems, such as the pressuriser should also be monitored in order to ensure that all the parameters are maintained at norm.
The fuel should also be cooled adequately. For this to happen, all the components of the PWR plant should be working efficiently. The reactor coolant system plays the greatest role in lowering the temperatures of the fuel. The reactor coolant pump should be functioning in the right manner in order to ensure that there is adequate circulation of the reactor fuel at all times.
Radioactivity should also be contained. All the radioactive material should be safely stored within the facility. Leakages should also be avoided, especially within the reactor coolant system since this would result in unwanted exposure to radioactive elements. During maintenance and waste processing, the products generated should also be stored or disposed safely to reduce the risk of future exposure. Members of the public should also be discouraged from occupying the land adjacent to the PWR plants to lower their risk of being harmed by radiations emanating from it.

The guideline is often simplified as control, cool, and contain. There are many ways in which the three guiding principles can be achieved. Together, they have been used to come up with a concept referred to as defence in depth. They are adhered to in all aspects of PWR plants (Kim 2011). They guide the process of designing, constructing, commissioning, as well as operating PWR plants. The concept of defence in depth can be illustrated in a five model chart.

The chart below is an illustration of the five models of the defence-in-depth concept:

The concept operates under three major assumptions. They include:

The design of the PWR plant will have some flaws. As such, problems should always be anticipated. Consequently, the personnel working in these plants must always be on the lookout. They must be able to anticipate these changes and deal with them effectively when they occur.
Equipments used to construct the PWR plant will occasionally fail. In this case, constant monitoring will be required to determine the effect of the radioactive fuel on the strength of the material used to construct the reactor vessel (Kim & Lee 2011). As such, regular testing is needed. All employees should also be aware of the worst case scenario and know how to respond to it in the event that it occurs.
Operating personnel are bound to make mistakes occasionally. As a result, the design should make it easy to reverse such actions without much damage. Warning systems should also be installed to detect malfunctioning caused by human error.

The key to ensuring defence-in-depth is ensuring that the flaws, failures, and mistakes that occur within a PWR plant is can be accommodated without raising the risk of an accident occurring (Li et al. 2006). The model shown above can be used to achieve this.

To begin with, the process systems should be reliable. They are the systems that perform a function on the plant continuously. Good examples include the primary reactor coolant system and the steam generators. In this case, the primary system will ensure that there is constant cooling of the reactor fuel. The steam generator too ensures that there is continuous transfer of heat from the primary to the secondary system (Nakath et al. 2013). In this case, reliability means that these systems will perform their intended functions as long as the plant is operational. As such, electricity generation will be on a continuous basis.

Reliable safety systems should also be put in place. They compensate for any failures that may occur within the process systems. A good example is the shutdown system. The personnel working in a PWR plant can be in a position to achieve this by deploying the emergency core cooling system (Bahn 2013). Reliability in this case means that in rare cases when the systems are required to intervene, they will be effective.

Multiple barriers, on the other hand, are aimed at preventing the release of radiations to the public. There are five barriers aimed at preventing the escape of radiations to the environment from the PWR plants (Bahn 2013). They include:

The reactor fuel is moulded to ceramic pellets of very high melting points. As such, they lock in most of the products released following the fission process thus preventing them from entering the reactor coolant system.
The fuel sheaths are made from zircaloy. It is a metal of high integrity. It houses the ceramic fuel.
The primary reactor coolant system is made up of tubes that are of high strength. Its leak proof nature reduces the chances of spilling of the coolant.
The system that is relatively leak proof is maintained below the atmospheric pressure. As such, it is a partial vacuum. It encourages air to leak into the system than out thus preventing the release of radioactive material.
There is always an exclusion zone round the reactor. The zone has a radius of at least a kilometre. Radiations released from the PWR plant are diluted by the time the one kilometre barrier is crossed. As such, no harm will be caused to the public.

The technicians charged with the responsibility of operating and maintaining PWR plants should also be competent. It is noted that the plants are designed to operate automatically. However, the personnel present should not rely on the systems put in place to operate the plant. The reason is that occurrences of accidents cannot be ruled out completely when automation is used (Bahn 2013). They should be knowledgeable about the working of the system. They should also be in a position to anticipate risks and act promptly before accidents occur.

Failures in the system should also be detected and corrected promptly. Procedures and processes to be followed in times of system failure should also be always known to the personnel. In order to detect cases of failure, it is important to carry out routine testing (Li et al. 2006). The operations of the entire plant should also be put under surveillance to ensure that the systems and equipments repairs and replacement of components is done even before accidents arise.

Steam generators

After leaving the outlet nozzle of the reactor vessel, the coolant flows to the steam generators. It acts as the link between the primary and the secondary coolant systems. The number of steam generators present in a PWR plant depends on its kind, that is, whether it is two, three, or four-loop. Each steam generator is made up of many connecting tubes. The reactor coolant flows inside these tubes at high pressure and velocity (Madasamy et al. 2011). The secondary coolant, also commonly referred to as the feed water flows outside the tubes. In the process, it absorbs heat from the primary coolant. Up on absorbing sufficient amounts of heat, it boils to generate steam. Most variations are in terms of the mechanisms used to separate water and steam. The water content in the steam should be as low as possible. The aim is to avoid damaging the blades of the turbines.

There exist slight differences in the mechanism used to generate steam based on the design of the plant. The three most common are the Westinghouse, combustion engineering, and the Babcock & Wilcox designs (Madasamy et al. 2011). The functioning of the steam generators in the Westinghouse and the combustion engineering designs is basically the same. Up on heating, a mixture of water and steam is generated. The two are then taken through a series of moisture alienation phases. In the first stage, the mixture is spun. As a result, water is slung to the outside. The water is drained back to the first stage and is used to generate more steam (Lee et al. 2005). The steam collected in the first stage proceeds to the second. The second stage involves changing orientations. The mixture is made to shift direction rapidly. Water is heavier than steam. As such, it cannot change its direction as fast as the gas. It is collected and returned to the first stage. Steam on the other hand is light and is able to make the directional changes. It eventually escapes the steam generator. The system is said to be highly efficient. Every 100 pounds of steam that is generated contains less than 0.25 pounds of water.

On the other hand, the Babcock & Wilcox design is popular for applying a Once Through Steam Generator (OTSG). The primary coolant flows from the top to bottom of the steam generator down (Nakath et al. 2013). As the primary cooling agent flows, it gives out heat to the secondary system. The heat transfer is so great that the secondary coolant is heated above boiling point. As a result of superheating, the steam that is generated in this design is dry. As such, no separation is required to be carried out.

The manner in which the primary coolant and the steam leave the steam generator also depends on the type of PWR plant design. A case in point is the Westinghouse steam generator. It has only one outlet. The primary coolant also exits the generator via one outlet. On the other hand, the Combustion Engineering and the Babcock & Wilcox designs have two steam outlets (Krepper 2013). They also have a pair of primary coolant outlets. The steam generated is channelled into the central turbine. The primary coolant on the other hand is routed the primary coolant pump. The cycle is recurring throughout the period that the PWR plant is on.

The Secondary System

Once the secondary coolant leaves the steam generator, it is routed towards the main turbine. Unlike the reactor coolant, the contents of the secondary system contain little or no radioactive particles. The reason behind this is that the two are physically separated. The piping network within the secondary coolant system mainly carries steam (Krepper & Schaffrath 2013). As such, it is often referred to as the steam system. Once the steam enters the turbine compartment, it is of very high pressure. As such, it is directed to the central turbine. From there, it heads for the lower pressure turbines. However, since its pressure has considerably lowered after driving the main turbine, it has to be reenergised first. In order to achieve this, its water

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