![Power Plant Engineering
Effects of Nuclear Radiation (Radiation Hazards):-
1] Effects on the tissues:-
Radiations affect the tissues in 3 ways,
a) Ionization: Ion pair in the tissue causes complete damage of tissues of man,
animals, birds.
b) Displacement: Displacement of an atom of the tissue from its normal lattice
position causes adverse effects on the tissues.
c) Absorption: This result in formation of a radioactive nucleus in the cell thus
altering its chemical nature .It causes cell damage and genetic modifications.
2] Effects on the cells (biological effects):-
a) Somatic effects:
This results blood cancer, lung cancer, thyroid cancer, bone cancer.
b) Genetic effects:
This results still births, growth and developmental abnormalities
Radioactive Waste Disposal Systems:-
The main objective in managing and disposing of radioactive (or other) waste is to protect
people and the environment Seal it inside a corrosion-resistant container, such as stainless steel. A
possibility for long term storage on the earth is burial in the sea bed. The rock formations in the sea
bed are generally more stable than those on dry land reducing the risk of exposure from seismic
activity. As well there is little water flow under the sea bed reducing the possibility of radioactive
material escaping into the ground water. High level radioactive waste is generally material from the
core of the nuclear reactor or nuclear weapon. This waste includes uranium, plutonium, and other
highly radioactive elements made during fission. There are three types of radioactive wastes.
a] Disposal of low level solid waste:
Primarily the low level solid waste is cast in cement in steel drum.
After it is buried few meters below from the soil or kept on ocean bed.
It gets diluted as it disperses.
b] Disposal of medium level solid waste:
These wastes mainly contaminated with neutron activation product isotopes.
This type of waste is primarily put in a cement concrete cylinder.
Then it is buried few meters below from the soil or kept on ocean bed.
c] Disposal of high level liquid waste:
High level liquid waste is stored in steel cylinder tanks with concrete.
It is water cooled to keep the temperature at 50ºc.then this cylinder is stored in salt mine.
The ocean is used for permanent storage of high level waste disposal.
Long-term storage of radioactive waste requires the stabilization of the waste into a form
which will not react, nor degrade, for extended periods of time. One way to do this is through
vitrification.](https://image.slidesharecdn.com/unit-3ppe-220829083608-15e0e4ab/85/Unit-3_PPE-pdf-9-320.jpg)
Unit-3_PPE.pdf
- 1. Unit – 3 Nuclear Power Plant Syllabus:- Nuclear Power Plant: Principles of release of nuclear energy Fusion and fis sion reactions. Nuclear fuels used in the reactors. Multiplication and thermal utilization factors. Elements o f the Nuclear reactor, Moderator, control rod, fuel rods, coolants. Brief description of reactors of the followi ng types - Pressurized water reactor, boiling water reactor, Sodium graphite reactor, Homogeneous graphite reactor and gas cooled reactor, Radiation hazards, Radioactive waste disposal. General Structure of Nuclear power plant:- The main components of the power plant: 1. Reactor vessel (Shiel ding) 2. Moderator 3. Control rod 4. Fuel rod 5. Coolant 6. Reflector 1. Reactor vessel (Shielding ): •It is a strong steel container in which the fuel rods, moderator, control rods and the reflector are arranged properly. •It forms a strong structu ral support for the reactor core.
- 2. Power Plant Engineering 2. Moderator: It is used to reduce the kinetic energy of fast neutrons into slow neutrons and to increase the probability of chain reaction. Graphite, heavy water and beryllium are generally used as moderator The hydrogen moderator would slow the neutron from 2MeV to 0.025eV. A moderator should possess the following properties: i. It should have high thermal conductivity ii. It should be available in large quantities in pure form iii. It should have high melting point in case of solid moderators and low melting point in case of liquid moderators. iv. Solid moderators should also possess good strength and machinability. v. It should provide good resistance to corrosion. vi. It should be stable under heat and radiation. vii. It should be able to slow down neutrons. 3. Control rod: Control rod is to regulate the rate of a chain reaction. They are made of boron, cadmium or other elements which absorb neutrons. Control rods should posses the following properties, i. They should have adequate heat transfer properties. ii. They should be stable under heat and radiation. iii. They should be corrosion resistant. iv. They should be sufficient strong and should be able to shut down the reactor almost instantly under all conditions. v. They should have sufficient cross sectional area for the absorption. 4. Fuel rods: Fuel rod tube like structure containing Nuclear Fuels . Nuclear fuels are made in the form of capsules & inserted in the tubes. During nuclear reaction nuclear fuel will release energy to produce power. Important properties of Fuel rods, i. It should withstand high temperature. ii. It should have high corrosion resistance. iii. It should have good thermal conductivity. iv. It should not absorb neutrons. v. It should withstand radiation effects. 5. Coolant: •Coolants are used to cool the reactor by carrying away the heat generated by the reactor. •There are many number of coolants are used some of them are water (H2O), heavy water (D2O), carbon-di-oxide (CO2), liquid sodium (liq. Na), organic liquid etc. •The coolants should have high latent heat of absorption.
- 3. Power Plant Engineering 6. Reflector: Function of the reflecto r is to minimize the neutron leakage by reflecting them back into the reactor. Graphite and Beryllium are generally used as reflectors. The important propertie s of good reflectors material are: i. It should have good thermal conductivity ii. It should have good corrosion resistance iii. It should have high stability under high temperature and pressure conditions iv. It should not abs orb neutrons v. It should have good reflectivity. Nuclear Energy:- Nuclear energy is the energy trapped inside each atom. Heavy atoms are unstable an d undergo nuclear reactions. Nuclear reactions are of two types, 1. Nuclear fission…the splitting of heavy nucleus 2. Nuclear fusion… the joining of lighter nuclei 1. Fission: Fission may be defin ed as the process of splitting an atomic nucleus into fission fragments. The fission fragments are generally in the form of smaller atomic nuclei and neutrons. Large amounts of ener gy are produced by the fission process. For eg. When neutron is bom barded into Uranium-235 (92U 235 ) it will split into smaller nuclei Xenon-140 & Strontium-94 w ith release of high energy in the form of neut ron.
- 4. Power Plant Engineering 2. Fusion:- It is defined as nuclear reaction whereby two light atomic nuclei fuse or combine to form a single larger, heavier nucleus. The fusion process generates tremendous amounts of energy. For fusion to occur, a large amount of energy is needed to overcome the electrical charges of the nuclei and fuse them together. Fusion reactions do not occur naturally on our planet but are the principal type of reaction found in stars. The large masses, densities, and high temperatures of stars provide the initial energies needed to fuel fusion reactions. The sun fuses hydrogen atoms to produce helium, subatomic particles, and vast amounts of energy. Comparison of fission and fusion:- Sl. No. Fission Fusion 1 Splitting of heavy nucleus Joining of light nuclei 2 Is a chain reaction Is not a chain reaction 3 Can be controlled cannot be controlled 4 Radiations are very harmful Will not emit harmful Multiplication Factor & Thermal Utilization Factor:- 1. Multiplication Factor, k: It is the ratio of neutrons in generation to the preceding generation. . i.e k= . If k<1, subcritical k=1, critical k>1, supper critical 2. Thermal Utilization Factor: It is the thermal Neutrons absorbed in the fuel to the thermal Neutrons absorbed in the entire core.
- 5. Power Plant Engineering Some of the reactors are, 1. Pressurized Water Reactor 2. Boiling Water React or 3. Liquid metal Fast breeder reactor or Sodium graphite reactor 4. Gas Cooled Reactor or Homogeneous graphite reactor 1. Pressurized Water Rea ctor(PWR):-
- 6. Power Plant Engineering In a PWR the primary coolant (natural and highly enriched fuel or water) is pumped under high pressure to the rea ctor core where it is heated from 275 °C to 315 °C by absorbing the energy generated by the fission of atoms. Then it moves to a pressurizer to maintain pressure of about 155bar in Primary circuit. The heated water then flows to a heat exchanger (steam generator) where it transfers its thermal energy to a seco ndary system where steam is generated and flo ws to turbines which, in turn, spins an electric generator. In contrast to a boiling water reactor, pressure in the primary coolant loo p prevents the water from boiling within the reactor. All PWRs use ordinary light water as both coolant and neutron moderator. PWRs are the most common type of power producing nuclear reactor, a nd are widely used in power stations such as s hips and submarines all over the world. More than 230 of them are in use in nuclear power plants to generate electric power, and several hundred more for marine propulsion in aircraft carriers, submarines and ice breakers. Advantages: 1. PWR reactors are very stable since they produce low power. 2. Less fissile material can be used hence safe. 3. Ordinary water as coola nt is easily available. 4. Small number of control rods is required. 5. Fission products remain contained in the reactor and are not circulated. Disadvantages: 1. Cannot be refueled whil e operating it will take long period of time (some week). 2. Severe corrosion proble m. 3. High initial cost. 4. High maintenance cost. 2. Boiling Water Reactor (BWR):- Boiling water reactor ( BWR) is the simplest of all facilities. Water a bsorbs heat from the reactions in the core and is directly driven to the turbines. After condensing the water is pumped back to the reactor core. In a B oiling Water Reactor enriched fuel is used. The BWR uses de miner alized water (light water) as a coolant and moder ator. Heat is produced by nu clear fission in the reactor core, and this causes the cooling water to boil, producing steam. The steam is directly used to drive a turbine, after which is cooled in a condenser and converted back to liquid water.
- 7. Power Plant Engineering Advantage: 1. Uses ordinary water as coolant, moderator which is easily available. 2. High thermal efficiency . 3. No need of Pressurizer. 4. Thicker vessel is not required. 5. Metal Temperature rem ains low. 6. Outlet temp of steam is very high. Disadvantages: 1. Higher cost due to larg e pressure vessel. 2. Possibility of radioactiv e contamination in the turbine. 3. The possibility of “burn out” of fuel is more 4. More safety required. 5. Lower thermal efficien cy. 3. Liquid metal Fast breed er reactor (Sodium Graphite reactor, LMFBR):- It is used to Produce, breed or generate fissile material like plutonium-239 from non fissionable uranium-238. This reactor consists of dou ble circuit coolant. Enriched uranium is used a s the fuel. Liquid metal and alloys such as sodium (and NaK) as coolant. Graphite as t he moderator. Here primary circuit uses s odium as coolant absorbs the heat from core and transfer to the Na or NaK (uses an alloy as coola nt) in the secondary circuit. Na or NaK transfers heat to the boiler to raise steam, which is used to run th e turbine to generate power. Sodium has high thermal conductivity which is more than 100 times t hat of water, hence increases the thermal efficie ncy of the plant. Advantage: 1. The moderator is not re quired. 2. High breeding is possible. 3. Small core is sufficient. 4. High burn-up of fuel is achievable. 5. Absorption of neutrons is low. 6. Thermal efficiency of the plant is very high.
- 8. Power Plant Engineering Disadvantages: 1. Requires highly enriche d fuel. 2. It is necessary to provid e safety against - melt-down. 3. Neutron flux is high at t he centre of the core. 4. The specific power of reactor is low. 5. There is a major problem of handling sodium as it becomes hot and radioactive. 6. Plant cost is more due to costly coolants. 4. Gas Cooled Reactor (G CR) or Homogeneous graphite reactor:- The fuel used is natural uranium, clad with an alloy of magnesium called Magnox. Gas-cooled reactor (GC R) system uses graphite – moderated systems. The GCR uses Uranium oxide UO2 as the fuel clad in stainless steel tu bes with CO2 gas as coolant and graphite as moderator. The gas-cooled reactor i s designed to use U-233 as the fissile material a nd thorium as fertile material. Initially, the system wo uld have to be fuelled with U-235 until sufficie nt U-233 is available for makeup fuel. Becau se of the very high melting point of graphite, the se fuel elements can operate at very high temperatures, and it is possible to generate steam at conditions equivalent to those in modern coal-fired power plant. The basic fuel forms a re small spheres of fissile and fertile material as carbides, UC2 or ThC2. The fissile spheres are 0 .35 to 0.50 mm in diameter and the fertile sphe res are 0.6 to 0.7 mm in diameter. Each sphere is coated with two to three layers of carbon and silicon carbide to prevent fission products from escaping from the particles. Advantage: 1. The processing of fuel is simpler. 2. No corrosion problem. 3. Graphite remains stable under irradiation at high temperature. 4. The use of CO2 as coola nt completely eliminates the possibility of explo sion in the reactor. Disadvantages: 1. Power density is low (du e to low heat transfer coefficient), therefore larg e vessel is required. 2. If helium is used instead of CO2, the leakage of gas is a major problem. 3. More power is required for coolant circulation (pumping cost is more). 4. Fuel loading is more elaborate and costly.
- 9. Power Plant Engineering Effects of Nuclear Radiation (Radiation Hazards):- 1] Effects on the tissues:- Radiations affect the tissues in 3 ways, a) Ionization: Ion pair in the tissue causes complete damage of tissues of man, animals, birds. b) Displacement: Displacement of an atom of the tissue from its normal lattice position causes adverse effects on the tissues. c) Absorption: This result in formation of a radioactive nucleus in the cell thus altering its chemical nature .It causes cell damage and genetic modifications. 2] Effects on the cells (biological effects):- a) Somatic effects: This results blood cancer, lung cancer, thyroid cancer, bone cancer. b) Genetic effects: This results still births, growth and developmental abnormalities Radioactive Waste Disposal Systems:- The main objective in managing and disposing of radioactive (or other) waste is to protect people and the environment Seal it inside a corrosion-resistant container, such as stainless steel. A possibility for long term storage on the earth is burial in the sea bed. The rock formations in the sea bed are generally more stable than those on dry land reducing the risk of exposure from seismic activity. As well there is little water flow under the sea bed reducing the possibility of radioactive material escaping into the ground water. High level radioactive waste is generally material from the core of the nuclear reactor or nuclear weapon. This waste includes uranium, plutonium, and other highly radioactive elements made during fission. There are three types of radioactive wastes. a] Disposal of low level solid waste: Primarily the low level solid waste is cast in cement in steel drum. After it is buried few meters below from the soil or kept on ocean bed. It gets diluted as it disperses. b] Disposal of medium level solid waste: These wastes mainly contaminated with neutron activation product isotopes. This type of waste is primarily put in a cement concrete cylinder. Then it is buried few meters below from the soil or kept on ocean bed. c] Disposal of high level liquid waste: High level liquid waste is stored in steel cylinder tanks with concrete. It is water cooled to keep the temperature at 50ºc.then this cylinder is stored in salt mine. The ocean is used for permanent storage of high level waste disposal. Long-term storage of radioactive waste requires the stabilization of the waste into a form which will not react, nor degrade, for extended periods of time. One way to do this is through vitrification.
- 10. Power Plant Engineering Nuclear fuel:- It is a material that can be 'burned' bynuclear fission or fusion to derive nuclear energy. Nuclear fuel can refer to the fuel itself, or to physical objects (for example bundles composed of fuel rods) composed of the fuel material, mixed with structural, neutron-moderating, or neutron-reflecting materials. Most nuclear fuels contain heavy fissile elements that are capable of nuclear fission. When these fuels are struck by neutrons, they are in turn capable of emitting neutrons when they break apart. This makes possible a self-sustaining chain reaction that releases energy with a controlled rate in a nuclear reactor or with a very rapid uncontrolled rate in a nuclear weapon. The most common fissile nuclear fuels are uranium-235 ( 235 U) and plutonium-239 ( 239 Pu). The actions of mining, refining, purifying, using, and ultimately disposing of nuclear fuel together make up the nuclear fuel cycle. Not all types of nuclear fuels create power from nuclear fission. Plutonium-238 and some other elements are used to produce small amounts of nuclear power by radioactive decay in radioisotope thermoelectric generators and other types of atomic batteries. Also, light nuclides such as tritium can be used as fuel for nuclear fusion.