To learn more about Nuclear Reactors or just to get a good headstart on your education or to bolster your professional career, check out the following book which has been recommended by numerous industry professional and professors alike:

Different Types of Advanced Nuclear Reactors:

• Advanced Gas-cooled Reactors (AGRs),
• Pressurized Water Reactors (PWRs),
• Boiling Water Reactors (BWRs),
• Pressurised Heavy Water Reactors (PHWRs).

What Are Advanced Gas-cooled Reactors (AGRs) and How Do They Work? 

AGRs make use of carbon dioxide as a coolant and graphite as a moderator, thereby allowing them to operate at high temperatures, hence greater thermal efficiency, usually around 40%. This is because, unlike water, carbon dioxide is stable at high temperatures without causing phase change (e.g., boiling) under the conditions designed in AGRs. High operating temperatures allow a higher percentage of the heat generated by fission to be converted into mechanical and thus electrical energy therefore, higher thermal efficiency.

Graphite is an effective neutron moderator even at high temperature. Its characteristic moderation of the fast neutrons without a significant loss of moderating efficiency at high temperature makes it a highly suitable material for a reactor core where high-temperature operation is required such as AGRs.

This choice of coolant and moderator also allows a wider range of uranium fuels to be used. However the construction and maintenance of the said reactors is far more expensive compared to their counterparts due to the materials required for high-temperature operations. 

What Are Pressurized Water Reactors (PWRs) And How Do They Work?

Pressurized Water Reactors (PWRs) are one of the most widespread and most used types of nuclear reactors. They utilise water for two primary purposes: cooling the core of the reactor and moderating (slowing down) neutrons emitted during fission.

Within the reactor core, the uranium fuel undergoes nuclear fission, where the uranium atoms disintegrate and release heat. Heat is generated as fast neutrons are slowed down through collision in the water, which acts as a moderator. The moderated neutrons are more probable to trigger subsequent fission reactions, sustaining the chain reaction in a controlled manner.

The water circulated through the core serves a dual purpose. It removes the heat produced by fission, acting as a coolant, and slows down neutrons acting as a moderator. The pressuriser maintains pressure on the water. The high pressure, increases the boiling point of water, thus, preventing the water from boiling even at such temperatures as it reaches, allowing it to efficiently carry heat away from the reactor core.

After absorbing heat in the reactor core, the pressurised heated water travels through a heat exchanger known as the steam generator. In the steam generator, the heat from the primary water circuit is transferred to a separate water system. This secondary water is not under high pressure and is allowed to boil, creating steam.

The steam produced in the secondary loop is directed to turbines. As the steam expands and cools, it drives the turbine blades, which in turn rotates a generator. This mechanical energy conversion produces electricity. After passing through the turbine, the steam is usually condensed back into water and recirculated to be heated again, completing the cycle.

Overall, the design of a PWR; with its high-pressure primary water loop and a separate secondary steam cycle, provides a reliable and efficient means of harnessing nuclear energy while ensuring safety and operational stability.

What Is A Boiling Water Reactor (BWR) and How Does It Work?

A Boiling Water Reactor (BWR) works by placing uranium fuel assemblies within the reactor core, where nuclear fission generates both heat and fast neutrons. When these neutrons strike water molecules that are being circulated through the core, the water heats up and, at the same time, serves as a moderator, cooling the neutrons down enough to allow for a controlled chain reaction. Because the coolant water is allowed to boil when it comes into contact with the fuel, steam is produced directly in the reactor vessel; eliminating the need for a separate steam generator loop like in the PWRs.

Once produced, the steam flows directly from the reactor core to the turbine, causing its blades to spin and driving an electric generator. Upon exiting the turbine, the steam is channeled into a condenser, where it is cooled, condensed back to liquid, and pumped back into the reactor vessel to loop again. Internal recirculation variable speed motor pumps inside the reactor use the water flow rate through the core so that the boiling rate and hence the reactor’s power level can be adjusted by the operators without needing to shift control rods.

Control rods, made out of neutron-absorbing elements. Common types of material used to make the control rods are: Boron Carbide (B₄C) and Hafnium. The control rods are added from above to regulate the fission rate and provide for safe levels of reactivity. On a sudden decrease in pressure or water levels, a number of emergency core cooling systems can flood additional coolant into the system in order to prevent overheating. All this within a sturdy containment building designed to protect against the release of any radioactive material upon an accident occurring.

BWRs are relatively compact, since fewer mechanical and heat‑exchange loop elements are required. However, because the steam employed to drive the turbine is radioactive, rigorous water chemistry controls must be implemented to minimise corrosion and limit the formation of radioactive deposits. BWRs typically have thermal efficiencies of around 32–34%, with operating pressures of around 7 MPa and temperatures of around 285 °C. This combination of simple steam production and minimised circuitry makes BWRs cost-effective to build and simple to operate but still demanding fine control of materials and safety protocols.

How Is a BWR Different From a PWR Reactor?

Compared to the secondary loop PWRs, the steam that is generated in the BWR core is utilised to directly drive a turbine that is connected with a generator to produce electricity. The steam is thereafter condensed into water in a condenser and recycled back to the reactor core for reheating. Control rods, made of neutron-absorbing materials, are inserted into or withdrawn from the reactor core to control the rate of fission and, consequently, the quantity of steam produced. This direct cycle of heating water to produce steam in the reactor simplifies the design and operation of BWRs, without a separate steam generator and with a less cumbersome system.

Safety systems are in place to cool the reactor and contain radioactive materials in the event of an emergency, ensuring the safe operation of the reactor.

What Are PHWRs And How Do They Work ?

Pressurised Heavy Water Reactors (PHWRs) are characterised by their use of heavy water (D2O) for both cooling and moderating neutrons, enabling the efficient use of natural uranium as fuel. Heavy water has a much lower neutron‑absorption cross‑section. This means that when fast neutrons from fission collide with D₂O molecules, they are slowed (moderated) into the thermal energy range without being captured as readily as they would be in H₂O. Although heavy water behaves chemically like ordinary water, each molecule contains deuterium atoms (hydrogen isotopes with an extra neutron), making it about 11% denser.

This design allows for significant moderation of neutrons, increasing the probability of nuclear fission reactions without necessitating the enrichment of uranium. The heavy water circulates around the reactor core, collecting the heat caused by fission, and transfers it to a secondary water circuit via a heat exchanger, creating steam without direct contact between both circuits. The steam powers turbines that produce electricity, and the steam is afterwards condensed and recycled.

PHWRs are fueled with natural uranium dioxide pellets, which are contained in rods that are clustered together, and utilise control rods to regulate the fission rate and power. One of the distinctive advantages of PHWRs, particularly of the CANDU design, is that they have the capability for online refueling, i.e., they can be operated continuously without being shut down to replace the fuel. This operational flexibility, together with the possibility of using unenriched uranium, is what PHWRs provide economically despite the added cost and complexity of heavy water use.

Advantages and Disadvantages of Different Types of Nuclear Reactors

Hassan Ahmed

Hassan graduated with a Master’s degree in Chemical Engineering from the University of Chester (UK). He currently works as a design engineering consultant for one of the largest engineering firms in the world along with being an associate member of the Institute of Chemical Engineers (IChemE).

The post Exploring Nuclear Reactor Types | AGRs, PWRs, BWRs, and PHWRs Unveiled appeared first on Engineeringness.

Energy can exist in numerous forms;

• Thermal, 
• Mechanical, 
• Kinetic, 
• Heat,
• Potential, 
• Electrical, 
• Magnetic, 
• Chemical, 
• Nuclear. 

The sum of these energy is the total energy and the sum of all microscopic forms is called the internal energy of a system. 

Heat is a form of energy (sometimes called heat energy), that can be transferred between systems when there is a temperature difference. Heat transfer occurs from a higher temperature region to a lower temperature region, with heat transfer stopping when there is no longer a temperature difference. 

There are three types of basic model that describe heat transfer, these are; 

• Conduction 

• Convection 

• Thermal Radiation 

Conduction

Conduction involves the transfer of heat from more energetic particles of a substance to less energetic particles, due to the interaction between the two particles (Figure 1). In solids conduction occurs due to vibrations of molecules and the energy transport by free electrons. For gases and liquids conduction occurs due to collisions and diffusion of molecules during their random motion. 

The rate of heat conduction through a plane is proportional to the temperature difference across the layer and the heat transfer area, and is inversely proportional to the layer thickness: 

Rate of heat conduction ∝ (area x temperature difference) / thickness

Q_cond = kA x (ΔT/Δs)

k – thermal conductivity a measure of a materials ability to conduct heat

A – Area of the material

The heat transfer area is the thermal conductivity multiped by the material area. A high thermal conductivity means that the material is a good heat conductor, and low thermal conductivity shows that the material is a bad conductor or an insulator. Heat is conducted from the higher temperature regions to lower temperature regions, with the temperature gradient becomes negative when temperature decreases with increasing thickness. 

Convection 

Convection involves heat transfer between a solids surface and a moving liquid or gas, and involves both conduction and fluid motion, with the greater fluid motion the greater the convection heat transfer (figure 2). If there is no fluid motion, then heat transfer would be purely by conduction. 

There are two types of convection, forced convection and natural convection (figure 3). 

• Forced convection – forcing a fluid to flow over a solid surface,

• Natural convection – fluid motion is due to the buoyancy force that are induced by density differences due to temperatures variations in the liquid or gas. 

Furthermore, a change in phase of a fluid is classified as convection as the fluids are in motion such as vapour bubbles during boiling or liquid droplets during condensation. 

The equations for convection involved the surface area and convection heat transfer coefficient: 

q_conv = hA(Ts-Tf)

h – convection heat transfer coefficient (W/(m2-K))

A – surface area convection is taking place (m2)

Ts – surface temperature (K)

Tf – temperature of fluid away from the surface (K).

Thermal Radiation 

Heat transfer by thermal radiation is done by electrometric waves and doesn’t require any medium and can pass through a vacuum (figure 4). Thermal radiation is caused by the random movement of atoms and molecules, with the movement of the charged protons and electrons results in the emission of electromagnetic radiation.

The hotter an object the greater the thermal radiation, with the temperature of an object affecting the wavelength and frequency of the radiated waves. As temperatures increase the wavelengths of the emitted spectra decrease, resulting in a short wavelength high-frequency radiation. 

The Stefan-Boltzmann law is used to calculate thermal radiation:

P = e · σ · A · (Tr4 – Tc4)

• P = net radiated power; 
• A = radiating area; 
• Tr = temperature of the radiator; 
• Tc = temperature of surroundings; 
• e = emissivity; 
• σ = Stefan’s constant.

Emissivity is as object’s effectiveness in emitting energy as thermal radiation and is a ratio, at a given temperature, of the thermal radiation from a surface to the radiation from an ideal black surface as determined by the Stefan-Boltzmann law (machinedesign, 2015). Stefan’s constant is determined by constants of nature: 

σ = (2 · π5 · k4)/(15 · c2 · h3) = 5.670373 × 10^–8 W · m^–2 · K^–4

k = Boltzmann’s constant; 

h = Planck’s constants; 

c = speed of light in a vacuum.

Common materials have lower emissivity values than an ideal radiator which has a value of 1.

References 

claesjohnson. (2011). claesjohnson. Retrieved from Radiative Heat Transfer: Theory: http://claesjohnson.blogspot.com/2011/10/radiative-heat-transfer-theory.html

Engineering ToolBox. (2020). Conductive Heat Transfer. Retrieved from Engineering ToolBox: https://www.engineeringtoolbox.com/conductive-heat-transfer-d_428.html

machinedesign. (2015). What’s the Difference Between Conduction, Convection, and Radiation? Retrieved from machinedesign: https://www.machinedesign.com/learning-resources/whats-the-difference-between/document/21834474/whats-the-difference-between-conduction-convection-and-radiation

McGraw-Hill Higher Education . (2020). Convection Heat Transfer. Retrieved from McGraw-Hill Higher Education : http://www.mhhe.com/engcs/mech/cengel/notes/ConvectionHeatTransfer.html

Dr. Adam Zaidi

Dr. Adam Zaidi, PhD, is a researcher at The University of Manchester (UK). His doctoral research focuses on reducing carbon dioxide emissions in hydrogen production processes. Adam’s expertise includes process scale-up and material development.’

The post A Breakdown | Heat Transfer Mechanism appeared first on Engineeringness.

The first law of thermodynamics states that:

“Energy can be neither created nor destroyed but one form of energy can be converted to another form.”

For example, consider a ball is placed on the top of a table initially. It will have certain potential energy ( Energy possessed by virtue of its height ) as it is at a height from the ground. When it is allowed to fall from the table this potential energy will be converted into kinetic energy ( Energy possessed by virtue of its motion ). This kinetic energy will be converted to heat, sound, etc. when it touches the ground.
In the application of the first law to a given process, the sphere of influence of the process is divided into two parts namely system and surroundings, which is illustrated in the image below.

Figure 1: Diagram to show the difference between system, surroundings and boundary.

The region in which the process occurs is the System and everything which the system interacts is the surroundings. First law of thermodynamics applies to both system and surroundings. In general,

Δ Energy of system  +  Δ Energy of surrounding  = 0

For the above example if you consider ball as a system  initial energy is potential and final energy is kinetic,but the energy is gained by surroundings as heat and sound.

Systems are of two types.

• Open = System which exchange both mass and energy with surroundings.
• Closed = System which exchange only energy with surroundings.

For simplification here we are considering closed systems only. In general system contains some internal energy ( in the form of attractions and vibrations ) and this tend to change when the heat is added or removed, when work is done on the system or delivered by the system.For closed systems energy transfer between system and surroundings takes place in the form of work and heat. ( where as in open systems internal energy will be associated in transit also i.e., at entry and exit of the system ). For closed systems energy changes mostly occur in internal energy. So,

Δ Energy of system = Change in internal energy  = ± Q ± W

Only change in internal energies can be found as it is hard to know the energy associated with  attractions and vibrations. Q is heat and W is work.

Sign Convention For Heat And Work

• Heat given by the system, Heat produced by the system = -Q
• Heat given to the system, Heat supplied to the system = +Q
• Work done by the system, work produced by the system = -W
• Work done on the system, work given to the system = +W

Example:

Δ Internal energy  =  Q – W

Heat is given to the system and work is done by the system.

Adrian Michaels

Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).

The post Overview of The First Law of Thermodynamics and Sign Conventions of Work & Heat appeared first on Engineeringness.

Thermodynamics is a branch of physics that involves the relationship between heat, work and temperature and other forms of energy. The word thermodynamics can be split up in two: Thermo which refers to heat and dynamics which refers to motion and is the study of:

• The movement of heat.
• Heat and work.

Thermodynamics is expressed in terms of four laws that are universally valid and cannot ever be broken:

• 0^th law – Defines temperature (T)
• 1^st law – Defines energy (U)
• 2^nd law – Defines entropy (S)
• 3^rd law – The numerical value of entropy

Basic Thermodynamic Definitions

These are a list of some of the most important phrases used in thermodynamics and need to be remembered, they are straightforward and self-explanatory so won’t require a lot of effort to remember.

• System: Quantity of matter of fixed identity of the universe that we are studying.
• Surroundings: Rest of the universe.
• Boundary: Surface splitting the system from the surroundings.
• Open system: Mass and energy can move between the system and the surroundings.
• Closed system: Only energy can transfer between the system and the surrounding and not mass.
• Isolated system: No mass or energy can be transferred between the system and surroundings.

Dr. Adam Zaidi

Dr. Adam Zaidi, PhD, is a researcher at The University of Manchester (UK). His doctoral research focuses on reducing carbon dioxide emissions in hydrogen production processes. Adam’s expertise includes process scale-up and material development.’

The post Basic Thermodynamic Concepts And Definitions appeared first on Engineeringness.