Generation of Electrical Energy | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) PDF Download

Generation of Electrical Energy.

ELECTRIC ENERGY SOURCES

The major sources of electric energy, in India, are the fossil fuels and water. The present contribution by different types of plants are :

Coal Fuels There are following important types of coal fuels used in thermal power plants.

Peat ¡ It is the first s tage in the conversion of vegetation into coal. It has a high percentage of moisture (60 to 90%) and low carbon content (5 to 20%). It is not suitable for use in power plants.

Lignite 

• Lignite : It has about 30 to 50% moisture and 20 to 40% carbon. When exposed to air its moisture content decreases appreciably.

It can be used in pulverized form for generation of electric energy.

 Bituminous Coal 

• It has low moisture content, about 60 to 80% carbon and 6 to 10% ash.
• Semi-bituminous coal has properties in between those of bituminous and anthracite coal and is widely in power plants.

Anthracite Coal 

• It is very hard and has the highest percentage of carbon. It is difficult to pulverize this coal.

Liquid fuels The liquid fuels are obtained by refining the crude oil (petroleum) which contains about 84 to 87% carbon, 11 to 16% hydrogen and oxygen, nitrogen, sulphur etc.

Advantages Over Coal Fuels 

• They are easy to handle, store, transport and burn. 
• There is no problem of ash disposal and plants using liquid fuels can meet the load changes very quickly.

Types of Liquid Fuels Naptha 

• Naptha is a higher end petroleum product and is very commonly used in petro-chemical industries.
• Its advantages includ e low cost, low emissions, good availability and good turbine efficiency.
• The cycle efficiency is about 41%.

High Speed Diesel (HSD) 

• It is also known as gas oil.
• It has high calorific value but is also pretty expensive.
• It is cleaner than fuel oil.

Orimulsion 

• Orimulsion is a fossil fuel and is basically an emulsion consisting of natural bitumen dispersed in fresh water in the 70:30 ratio.
• Its calorific value is good (about 7000 k cal/ kg) and ash content is low about (0.2%).
• The overaal composition is : oxygen 26.4%, carbon 60.1%, hydrogen 10.1%, sulphur 2.8%.

Gas Turbine Fuel Oil 

• It is a low sulphur fuel and is cheaper than HSD.
• Gas turbines using this fuel need more frequent maintenance.
• It is being widely used now-a-days.

Gaseous Fuels Gaseous fuels are either natural gas or manufactured gas (e.g., water gas or produce gas).

Natural Gas 

• It is very easy to burn and it mixes well with air.
• It can be easily trasported in pipe lines.
• Natural gas is a clean and eco-friendly fuel.

Nuclear Fuels This fuels used in nuclear reactors are natural uranium, slightly enriched uranium dioxide and plutonium.

Natural Uranium 

• It contains about 0.7% uranium-235.
• 1 ton of natural uranium produces the heat equivalent of 10,000 tons of coal.

Remember: 

• The capital cost of a nuclear reactor is so high that the cost of generation in nuclear plants is higher than that in coal fired power to plants.

Uranium Dioxide 

• It is more stable than natural uranium and presents less problems of oxidation.

Uranium Carbide 

• It has high melting point, possesses good thermal conductivity and is free from the trouble of phase changes. It is not economical to use this fuel.

Plutonium-239 

• It is most l;ikely to be used in fast reactors. It is highly toxic.

LOADS AND LOAD CURVES

Maximum Demand 

• The maximum demand of a consumer means the maximum power that his circuit is likely to draw at any time.

Remember: 

• If all the devices and outlets were used simultaneously to the full extent, the maximum demand of the consumer would equal his connected load.

Demand Factor 

• The demand factor indicates the contribution of the device towards the maximum demand of the consumer.

Group Diversity Factor

• The contribution of the maximum demand of a consumer to the power requirements of the group depends on the group diversity factor.

Group Diversity Factor

Remember: 

• The group diversity factor is always greater than unity.

Note: 

• The consumers maximum demand divided by the group diversity factor will determine his effective demand at the distribution transformer.

Peak Diversity Factor 

• The diversity facrtors between transformers, between feeders and between sub-station can be combined into a single term commonly referred to as peak diversity factor.

Peak Diversity Factor.

Remember: 

• Peak diversity factor gives an indication of the effective demand of the consumer group at the time of system peak demand and is the product of the diversity factors between transformers, between feeders the between sub-stations.

Load Curves 

Chronological load curve 

• A graph showing the variation of the system load during the 24 hours of the day is known as the system chronological load curve.

Remember: 

• The area under a chronological load curve gives the energy consumed (i.e. kWh) during the 24 hours.

Load Duration Curve-Energy Load Curve 

• A load duration curve is a re-arrangement of all the load elements of a chronological curve in a descending order.
• The area under the load duration curve is equal to the area under the chron

 Mass Curve

• It is used in the study of variations between the rate of water flow and the electrical load and the determination of the necessary storage.
• It gives the total energy used by the load upto each hour to the day.

Load Factor

• Load factor for a system or a plant is the ratio of the average load to the peak load, for a certain period of time.

Load factor =

• Load factor can also be defined as the ratio of the energy consumed in a certain times to the energy which would be consumed if the load is maintained at the maximum value throughout that time.

Load factor = 

• Load factor has an effect on power plant design, operation and cost of generation.
• 100% load factor means a rectangular load duration curve with constant load during the whole period of the time considered.

 Capacity Factor

• The plant capacity factor (also known as plant factor) is the ratio of the average annual load to the power plant capacity.

Capacity factor = 

It can also be defined as the ratio of the energy produced by the plant in a year to the maximum energy that the plant could have produced.
Note: 

• If the plant is always run at its rated capacity, then the capacity factor will be 100%.
• The capacity factor depicts the ext
• Rated capacity of each plant is always greater than the expected maximum load.

Capacity Factor =

Note: 

• If the rated plant capacity equals the maximum load, the capacity factor and load factor become identical.

Utilization Factor

• It is defined as the ratio of the maximum demand to the rated capacity of plant.

Utilization factor =

• A low utilization factor means that the plant is either a standby plant or has been installed to take into account the future increase in the load.
• A high utilization factor shows that the plant is probably the most efficient in the system.

Capacity Factor = (Load Factor) × (Utilization Factor) Base Load and Peak Load Plants

• A base load plant operates at a high load factor and should be one which has low operating costs.
• The peak load plant operates at a low load factor.

Depreciation

• The value of the power plant decreases from its initial value to the salvage value at the end of its useful life.
• This depreciation is due to ageing, wear and tear of machinery, corrosion, weathering, inadequacy and obsolescence of equipment etc. At the end of the useful life of the plant, funds must be available to replace the equipment.
• The depreciation charge represents the amount which is set aside from income every year and placed in depreciation reserve. For calculating this charge, it is necessary to estimate the useful life of plant.

DIESEL PLANT

ADVANTAGES

• They can be quickly procured, installed and commissioned. 
• They can be started and put on load quickly without any standby losses and have good efficiency (about 43-45%). 
• They are free from ash handling problems. 
• Their capital cost per kW of capacity is low. 
• The cooling water requirements for diesel plants are less than for steam plants. 
• They can be located very near the load.

DISADVANTAGES

• Their useful life is very short, only about 5 years. 
• Their repair and maintenance costs are high. 
• Their overload capacity is small. 
• They are more polluting.

APPLICATIONS
There are following fields of applications :

• Emergency Plant 
• To maintain essential services when supply from the grid is not available. 
• They are used for starting auxiliaries in steam power stations.

Mobile Plant 

• Mobile diesel electric plants mounted on trailers are used for temporary and emergency purposes.

Peak Load Plant 

• A diesel plant can be started and loaded quickly.

As such these plants can be used as peak load plants. 

• They can be used in remote locations where supply from grid is not available. The use of diesel electric plants during the construction stages of thermal and hydro electric power plants is very common.

GAS TURBINE PLANT

Gas turbine plants can use a variety of fuels–solid, liquid and gas. 

• Natural gas which has 80% methane and small fractions of other gases is very widely used in plants situated near the gas fields. 
• Used for auxiliary power generation in oil fields.
•  Present day gas turbine plants generally used natural gas and liquid petroleum fuels.

APPLICATIONS
 Peak Load Plants 

• Gas turbine plants are very suitable for use as peak load plants because they can be started and loaded quickly.

Base Load Plants 

• The operating cost of a gas turbine plant is very high. As such it is rarely used as base load plant.

Auxiliary Power Plant for Thermal Stations 

• Gas turbine plants of about 25 MW size are used in coal fired stream power plants for starting the auxiliaries of the plant.

THERMAL POWER PLANT

Thermal power plants convert the heat energy of coal into electrical energy. Coal is burnt in a boiler which converts water into steam. The expansion of steam in turbine produces mechanical power which drives the alternator.

MAIN EQUIPMENT
The main and auxiliary equipment in thermal plant are :

Coal Handling Plant 

• The function of coal handling plant is automatic feeding of coal to the boiler furnace. 
• A 200 MW plant may require around 2000 tons of coal daily.

Pulsverising Plant 

• Pulsverisation is a means of exposing a large surface area to the action of oxygen and consequently helping the combustion.

Advantages of Using Pulverised Coal

• the rate of combustion can be controlled and changed quickly to meet the varying load.
• The banking losses are reduced.
• The percentage of excess air required is low.
• Automatic combustion control can be used.
• Preheated air can be used successfully.
• A wide variety of even low grad coals can be used.
• The boiler can be started from cold conditions very rapidly.

Disadvantages

• Skilled personnel are required.
• Investment cost of plant is increased.
• Auxiliary power consumption is increased.

Draft System 

• The circulation of air is caused by a difference in pressure, known as draft. 
• Draft is the differential in pressure between the two points i.e. atmosphere and inside the boiler. 
• A differential in draft is needed to cause flow of gases through the boiler setting. This required differential is proportional to square of the rate of flow.

Natural Draft

• A natural draft is provided by the chimney or stack.
• Chimney serves two purposes
  (i) to produce a draft so that air can flow into the boiler and products of combustion are discharged to the atmosphere.
  (ii) to deliver the products of combustion and fly ash to a high altitude so that air pollution is reduced.
• The gases within the chimney are at a higher temperature than that of the surrounding air.
• Natural draft is used only for small boilers.

Mechanical Draft

• In a mechanical draft (or artificial) system the movement of air is due to the action of a fan.
• A mechanical draft may consist of forced draft or induced draft or both.
• In a forced draft system the fan is installed near the base of the boiler.

BOILER
A boiler (or steam generator) is a closed vessel in which water, under pressure, is converted into steam.
It is always designed to absorb maximum amount of heat released in the process of combustion. This heat is transferred to the boiler by all the three modes of heat transfer i.e. conduction, convection and radiation.

Fire Tube Boiler 

• This boiler is so named because the products of combustion pass through the tubes which are surrounded by water. 
• A fire tube boiler is simple, compact and rugged in construction. Its initial cost is low. 
• They are economical only for low pressures and are, therefore, available in small sizes having steam capacity of about 15000 kg per hour.

Water Tube Boiler 

In this boiler, water flows inside the tubes and hot gases flow outside the tubes.

SUPERHEATER AND REHEATER
Superheated steam is that steam which contains more heat than the saturated steam at the same pressure i.e. it has been heated above the temperature corresponding to its pressure.
Superheated steam causes lesser erosion of turbine blades and can be transmitted for longer distances with little heat loss.
The heat of combustion gases from the furnaces is used for superheating.
The function of reheater is to superheat the partly expanded steam from the turbine. This ensures that steam remains dry through the last stage of turbine.

STEAM TURBINE
A steam turbine converts heat energy of steam into mechanical energy and drives the generator. They are of two types, impulse and reaction.
The standard speeds are 3000 rpm and 15000 rpm for coupling to 50 Hz generators. Governors are used to maintain speed constant when load changes.

mpulses Turbine 

• In impulse turbine steam is expanded in turbine nozzle and attains a high velocity.

Reaction Turbine 

• In a reaction turbine only partial expansion takes place in the nozzle and further expansion takes places as the steam flows over the rotor blades.

CONDENSER

• The condenser does the job of condensing the steam exhausted from turbine. 
• It helps in maintaining low pressure (below atmospheric) at the exhaut, thereby permitting expansion of steam in the turbine to a very low pressure. 
• This improves the plant efficiency. 
• Maintenance of high vacuum in the condenser is essential for efficiency operation. 
• M odern powe r p lants m os tly us e s urface condenser.

COOLING TOWERS
Roughtly one kilogram of steam needs 100 kilogram of cooling water for the condenser. A 2000 MW plant needs about 1500 × 106 gallons/day of cooling water.

ECONOMIZER

• Flue gases coming out of the boilers carry lot of heat. An economizer extracts a part of this heat from the flue gases and uses it for heating feed water. 
• The use of an economizer results in saving in coal consumption and higher boiler efficiency but needs extra investment and increase in maintenance costs and floor area required for the plant.

AIR-PREHEATER
After the flue gases leave economizer, some furhter heat can be extracted from them and used to heat the incoming air for combustion. 

• Cooling of flue gases by 20°C (by extracting their heat) raises plant efficiency by about 1%. 
• A regenerative air heater uses a cylinderica rotor made of corrugated steel plants.

Generation of Electrical Energy.

HYDRO ELECTRIC PLANTS

WATER POWER

•  When water drops through a height, its energy is able to rotate turbines which are coupled to alternators. 

The electric power

Where Q = Discharge, m³/sec H = Waterhead, m h = Overall efficiency of turbine alternator set

ADVANTAGES

• The useful life of a hydro-electric plant is around 50 years as compared to around 25-30 years for a steam station. 
• The hydro plants do not require any fuel. Their operating costs are, therefore, low. 
• There are no standby losses in hydro plants. 
• They can be run up and synchronized in a few minutes. The load can be adjusted rapidly. 
• Hydro plants are more robust as compared to steam plants. 
• The maintenance cost of hydro plants is very low as compared to that of steam and nuclear plants. 
• Efficiency of hydro plants does not reduce with age. On the other hand efficiency of steam plants decreases with age.

DISADVANTAGES

• The capital cost per kW of hydro plants is considerably higher than that of steam plants. 
• H ydro pow er genera tion is dep endent on availability of water. In a dry year, the power generation is very small. 
• The firm capacity of hydro plants is low and needs to be backed up by steam plants.

HYDROLOGY

Run-off 

• Only a small part of rainfall can be used for power generation. A significant part of rain water evaporates, another part seeps into soil and forms the underground storage and some portion is taken up by the vegetation. The remaining water flows on the ground surface of the catchment area to form the steam and is known as run-off.
• The factors affecting run-off are – rainfall pattern, geology of the area, shape and size of catchment area the topography and nature of soil in the catchment area, amount of vegetation and weather conditions in the catchment are.

Stream Flow 

• A knowledge of the quantity of water flowing in the stream and its variation with time over a period of many years is necessary to estimate the available energy. 
• The stream flow meafurements are generally done by stream gauging at site and expressing m3/sec.

Hydrograph 

• Hydrograph shows the variation of stream flow in  m³/sec with time for a particular river site. 
• It is similar to the chronological load curve.

Flow Duration Curve 

• Flow duration curve is a re-arrangement of all the stream flow elements of a hydrograph in a descending order.

Mass Curve 

• A mass curve indicates the total volume of runoff in cubic meters upto a certain time. 
• The slope of the curve at any point shows the rate of flow at that time. 
• If rain fall is uniform throughout the year, the mass curve would be a straight line having uniform slope.

Storage 

• The main function of storage is to make water available during deficient flow times thus increasing the firm capacity of the station. 
• Storage also means that the plant would produce more energy.

Pondage 

• If the power is away from the storage reservoir, a smalll pond is needed near the power plant to meet the hourly changes in demand. 
• The capacity of pond should be sufficient to cope with hourly change of 24 hours.

Site Selection The following factors have to be considered in the selection

Availability of Water 

• The river run-off data pertaining to many years should be available so that an estimate of the power potential of the project can be made. 
• The data should include the average flows, minimum and maximum flows and their periods.

Water Storage 

• Because of wide fluctuations in stream flows, storage is needed in most hydro-projects to store water during high flow periods and use it during lean flow periods. 
• The storage reservoir may be located near the power plant or some distance away from it.

Head of Water 

• An increase in effective head reduces the quantity of water to be stored and handled by penstocks, screens and tubrines and therefore the capital cost of the plant is reduced.

Water Pollution 

• Polluted water may cause excessives corrosion and damage to metallic structures. 
• It is necessary to see that the water is of good quality and will not cause such troubles.

Sedimentation 

• The capacity of storage reservoir is reduced due to the gradual deposition of silt. 
• Silt may also cause damage to turbine blades.

Access to Site 

• Construction of a hydro project involves transport of huge amount of cement, steel, other building material and heavy machinery. 
• The site selected should be such that the railway line and roads can be constructed and material and machinery transported.

CLASSIFICATION

Run off River Plants Without Pondage

• These plants are located such that they use water as it comes, without any pondage or storage.
• There is no control on flow of water.
• During periods of high flows or low loads, water is wasted. 
• During lean flow periods, the plant capacity is very low.
• During the periods of high flows, these plants can supply a substantial portion of base load.

Run off River Plant With Pondage. 

• Pondage refers to storage at the plant to take care of hour to hour fluctuations in load on the station.

Remember: 

• Such plants can serve as base load or peak load plants depending on the stream flow.
• When plenty of water is available, these plants can be used as base load plants.
• When stream flow decreases, these plants can be made to work as peak load plants.
• These plants offer maximum conservation of coal when operated in conjunction with steam plants

Reservoir Plants 

• Most of the hydro-electric plants, belong of this category.
• When water is stored in a big reservoir behind a dam. it is possible to control the flow of water and use it most effectively.
• Storage increases the firm capacity of the plant.

Remember: 

• The plant can be used as a base load plant or as a peak load plant depending on the water stored in the reservoir, the rate of inflow and the system load.
• Bhakra Plant in India are notable examples of reservoir plants.

According to Load Ba

Base Load Plants

• They feed the base load of the system and they supply almost constant load throughout and operate on a high load factor.

Note: 

• Base load plants are usually of large capacity.
• Run off river plants without pondage and reservoir plants are used as base load plants.

Peak Load Plants

• Run off river plants with pondage can be used as peak load plants during lean flow periods.
• Reservoir plants can also be used as peak load plants.
• Peak load plants have large seasonal storage.

Classification According to Head Low Head Plants

• When water head is less than 30 m, the plant is called a low head plant.
• The power plant is located near the dam and therefore, no surge tank is needed.

Remember: 

• Francis of Kaplan turbines are used.

Medium Head Plants

• When water head is between 30 m to 100 m, the plant is called a medium head plant.
• An open channel brings water from main reservoir to the fore-bay from where penstocks carry water to the turbines.

Remember: 

• Francis or Kaplan turbines are used.

High Head Plants

• The plants operating at heads above 100 m are generally classified as high head plants.

Remember: 

• Generally Francis turbines are used for heads below 200 m and Pelton turbines for still higher heads.

Storage Reservoir 

• it is necessary to store water during excess flow periods so that the same may be used during lean flow periods. 
• The storage reservoirs help in supplying water to the turbines according to the load on the plant.

Remember: 

• Low head plants require very high storage reservoir. 
• The capacity of storage reservoir depends on thedifference between run off during high and lean flows. 
• Detailed hydrological studies over long periods are necessary to arrive at a suitable figure.

DAM

• The function of dam in a hydro electric project, is to create an artificial head and storage. 
• It is the most expensive and important part of a hydro project.

Forbey 

• It is an enlarged body of water at the intake to store water temporarily to meet the hourly load fluctuations.

Intake

• The function of intake is to provide a passage to water to flow into the water conduit, channel or penstock.

Surge Tank 

• The function of the surge tank is to absorb these sudden changes in water requirements so as to prevent water hammer and vacuum.

Remember: 

• The surge tank helps in stabilizing the velocity and pressure in the penstock and reduces water hammer and negative pressure (i.e. vacuum). 
• It should be located as near the power house as possible. Some times it may be located underground also.

Penstock 

• A penstock carries water from the water storage system to the turbine. 
• A low pressure penstock may be a canal, flume or a steel pipe and the high pressure penstock consists of thick steel pipes.  Each turbine has a separate penstock.

Spillway

• Every dam is provided with an arrangement to discharge excess water during floods this arrangement may be a spillway or a by-pass tunnel or conduit.

Tailrace 

• A tail race is required to discharge the water leaving the turbine, into the river. 
• It is necessary that the draft tube must remain water sealed all the time.

Remember: 

• Impulse turbines do not need a draft tube and discharge water directly. 
• The design and size of the tailrace should be such that water has a free exit and the jet of water, after it leaves turbine, has unimpeded passage.

HYDRAULIC TURBINES

• Hydraulic turbines convert the energy of water into mechanical energy which drives the alternators. 
• They are highly efficient (efficiency exceeding 90° at full load), simple in construction, can be controlled easily and pick up load in a very short time. 
• Hydraulic turbines can be classified into impulse and reaction turbines.

Specified Speed 

• Specific speed of a turbine is the speed of a scale model of turbine which develops 1 metric h.p. under a head of 1 metre.

Where Ns = Specific speed in metric units N = Speed of turbine in rpm Pt = Output in metric h.p.
H = Effective head in metres Types of Turbines A classification of turbines according to range of head and specific speed is as under : Remember:

Pelton Turbine

• It works under large head and low quantity of water.
• This turbine is not suitable for heads below 200 m.
• Its specific speed is in the range of 10-50.

Francis Turbine 

• It develops power partly due to the velocity of water and partly due to the difference in pressure acting on the front and back of the runner buckets.

Remember: 

• It is a reaction turbine suitable for medium heads and medium flows.

Propeller and Kaplan Turbines

• Propeller turbine is a reaction turbine suitable for low head and large quantity of water. It is suitable for heads below 30 m.
• A Kaplan turbine is a propeller turbine with adjustable blades, the advantage of adjustable blades being that a Kaplan turbine operates at high efficiency even under part load conditions.

Remember: 

• The speed of Kaplan turbines is more than that of Francis turbines and lies in the range of 400-1500 rpm.

Bulb Turbines Remember: 

• It is particularly useful for the low heads is the bulb turbine unit and tidal schemes.
• A bulb turbine is an axial flow turbine, so called because water flows through the machine coaxial with the turbine shaft.

Advantages:

• They can be used at very low and widely varying heads and have high efficiency due to axial straight flow.
• The cos t of civil engineering w orks is considerably reduced when bulb turbines are used.
• They are smaller, faster and easier to build than other types and are cheaper in capital cost than Kaplan turbines.
• The bulb turbines are universally used for small hydro schemes.

Disadvantage :

• It is difficult to access the generator for maintenance and the generator has low inertia

PUMPED STORAGE PLANTS

• A pumped storage plant is a special type of plant meant to supply peak loads. 
• During peak load period, water is drawn from the head water pond through the penstock and generates power for supplying the peak load. 
• During the off-peak period, the same water is pumped back from the tail water pond to the head water pond so that this water may be used to generate energy during the next peak load period.

Remember: 

• Peak loads can be supplied at a lesser cost than that incurred when these loads are supplied by steam and nuclear plants.  Pumped storage plants need a starting time of only 2 to 3 seconds and can be loaded fully in about 15 seconds. 
• The load factor of steam and nuclear plants is increased thus ensuring their efficient and economic operation. 
• They can be used for load frequency control.

NEUCLEAR POWER PLANTS

The nuclear power is the only source which can supply the future energy demands of the world.

MAIN ADVANTAGES

• The amount of fuel used is small. Therefore, the fuel cost is low. 
• There are no problems of fuel transportation storage etc. 
• Nu clear pl ant s need less area than the conventional steam plants. 
• Greater nuclear power production leads to conservation of coal, oil etc. 
• The initial capital cost of nuclear plants is very high.

ENERGY MASS RELATIONSHIP

• Einstein’s theory of relativity shows that mass and energy are interchangeable. If mass is destroyed, energy is produced and mass can be produced by expenditure of energy.
• Energy mass relation E = mc2 where E = Energy is Joules m = Mass in kilogram c = Velocity of light (3 × 108 m/sec) 
• Mass is represented in atomic mass units (amu).
• One amu equals one-twelfth the mass of a carbon atom. 
• Mass of proton and neutron is each equal to 1 amu. 
• 1 amu = 1.6604 × 10–24 grams. 
• One amu mass is equivalent to 931.1 MeV.

ISOTOPES

• Elements, having same atomic number but different mass numbers, are called isotopes. 
• Uranium exists as 92U235, 92U235 and 92U238.

The natural uranium contains 99.23% of 92U238.

RADIOACTIVITY

• Isotopes of thorium, radium and uranium are unstable. They disintegration of an unsable nucleus is called radioactivity. 
• Artificial radioactivity is often produced by subjecting nuclei to neutron bombardment. The radiations emitted in the process of radioactive decay are :
  • a particles, b particles, g arays and neutrons.
  • a particles are nuclei of the belium atom of the helium atom 2He4.

Examples
92
U238 ® 2He4 + 90Th234 94
Pu239 ® 2He4 + 92Th235 CANNING
The canning materials are aluminium, magnesium, beryllium and stainless steel. 

• The fuel element, in a nuclear reactor, is canned so that the fuel does not contaminate the coolant. 
• Canning eliminates radiation hazards. 
• The choice of canning material depends largely on the fuel used in the reactor.

COOLANT

• Coolant removes heat from the fuel elements and transfers it to water. 
• A good coolant should not absorb neutrons, should be non-oxidizing non-toxic and non corrosive and have high chemical and radiation stability. It should have good heat transfer capability.

Remember: 

• The materials used as coolants are carbon dioxide, liquid metal, water and heavy water.

MODERATOR

• The purpose of moderator material is to slow down the fast neutrons. 
• The fast neutrons collide with the nuclei of moderator material, loose their energy and get slowed down.

Properties required for a good moderator :

• It must not react with neutrons because neutrons captured in nuclear reactions are lost to the fission process and this leads to an inefficient reactor.
• It should be inexpensive
• It should be chemically inert and should not corrode or erode.
• It should not undergo harmful physical or chemical changes when bombarded by neutrons.
• ¡ The average neutron-nucleus collision should lead to large neutron energy loss.

FISSILE AND FERTILE MATERIALS

• The materials which are known to undergo neutron fission are U235, U233 and Pu239. These are fissile materials U235 is the only one occurring in nature (as 0.72% of the natural uranium) and has served as the basis of nuclear energy programme.
• U238 and Th232 are not fissionable. However these two materials can be converted into Pu239 and U233 resepctively.  These two materials (i.e. U238 and Th232) are known as fertile materials.

NUCLEAR REACTORS

Advanced Gas Cooled Reactor (AGR) 

• This reactor uses uranium dioxide as fuel. 
• The reactor can be refuelled on load and this is an operational advantage. 
• The coolant is CO2. 
• A gas cooled reactor is economical when load factor is more than 75%. 
• Its overall efficiency is about 40%.

Magnox Reactor 

• This is also a gas colled reactor and similar to AGR.
• It uses natural uranium as fuel. 
• The overall efficiency is about 30%.

Pressurised Water Reactor (PWR) 

• The fuel used in PWR is enriched uranium oxide, clad in zinc alloy. 
• Pressurized water is used both as coolant and moderator. 
• One of the main drawbacks of this reactor is the design of high strength pressure vessel. 
• The advantage is that steam supplied to the turbine is completely free from contamination. 
• The overall efficiency is about 33%.

Boiling Water Reactor (BWR) 

• BWR also uses enriched uranium oxide as fuel. 
• The advantages of this reactor include a small size pressure vessel,high steam pressure and simple construction. 
• Ordinary water is used both as coolant and moderator. 
• The overall efficiency is about 33%.