• Nuclear Power Plant

    Introduction

    A generating station in which nuclear energy is converted into electrical energy is known as a nuclear power station. A nuclear power station can produce huge amount of electrical energy from a relatively small amount of nuclear fuel as compared to other conventional types of power stations. It has been found that complete fission of 1 kg of Uranium (U235) can produce as much energy as can be produced by the burning of 4,500 tons of high grade coal. Although the recovery of principal nuclear fuels (i.e., Uranium and Thorium) is difficult and expensive, yet the total energy content of the estimated world reserves of these fuels are considerably higher than those of conventional fuels, viz., coal, oil and gas. At present, energy crisis is gripping us and, therefore, nuclear energy can be successfully employed for producing low cost (Rs 3.50 to Rs. 12) electrical energy on a large scale to meet the growing commercial and industrial demands.

     

    How a Nuclear Power Station works

    The key factor for a nuclear reaction in a power station is Uranium. Uranium is found in many forms, the most common of which is called Uranium-238, which makes up around 99% of total Uranium. The type used for generating nuclear power however, Uranium-235, only makes up around 0.7% of total Uranium. 

    Figure: A neutron strikes a Uranium-235 atom

    In a nuclear power station, Uranium-235(has a nucleus with 92 protons and 143 neutrons) is bombarded with thermal neutrons. When Uranium-235 is struck by a neutron, it absorbs it and becomes uranium-236: an unstable version of the same atom (a radioactive isotope of uranium) with 92 protons and 144 neutrons. Uranium-236 is too unstable to hang around for long so it splits apart into two much smaller atoms, barium and krypton, releasing quite a lot of energy and firing off three spare neutrons at the same time. The actual splitting is referred to as fission. Note that a single U-235 atom releases approximately 200 MeV and there are lots of uranium atoms in 1 kilogram of uranium.

    This reaction occurs underwater in a nuclear power station. This is for two reasons: firstly to control the speed of the neutrons, and secondly to keep the temperature of the Uranium under control. The energy released from the Uranium-235 splitting heats the water surrounding it – this heated water is then used as a heat source to boil another separate vessel of water and produce steam. This steam is then used to turn electricity generators, producing the electricity.

     

    Figure: Nuclear reactor

    The neutrons released from the reaction continue to travel onwards until they strike more Uranium-235, causing the same reaction over and over – a chain reaction. In an ideal situation, one of those three neutrons will strike another Uranium-235 atom, and one of the three neutrons released from that fission reaction will strike another. This situation, where only one out of three neutrons causes another fission reaction is very stable, for every neutron causing a reaction, there is only one resulting neutron causing another reaction, meaning the number of reactions happening does not increase or decrease.

    The major factor controlling how many resultant neutrons strike another Uranium-235 atom is the amount of Uranium-235 available to act as a target. If there is less Uranium-235 there are less targets for the neutrons to strike and it is called “subcritical”, the number of reactions will reduce and the energy production will diminish. If there is too much Uranium-235 there are many targets for the neutrons and it is called “supercritical”, more than one neutron may strike another Uranium-235, increasing the number of reactions occurring and increasing the amount of energy being released. If this is not controlled, the amount of energy can continue to increase until it causes an explosion. When only one neutron strikes another Uranium-235 it is called “critical mass”, with no increases or decreases of the energy produced.

    In practice, a nuclear power station will actually be supercritical, it will have more Uranium-235 than required to keep the one-to-one ratio. The levels of the reactions are controlled using control rods made from a mixture of metals. These control rods are pushed into the water and literally soak up the neutrons being released by the splitting of Uranium. By adding more control rods the number of follow-up reactions is reduced by removing more neutrons. Removing control rods has the opposite effect, with more neutrons being allowed to create reactions. Using these control rods the energy production can be closely controlled, keeping the nuclear reactions at a stable level, and also controlling the amount of steam produced and electricity output from the station. These control rods are capable of completely stopping the nuclear reaction, and can be used to shut-down the nuclear station almost instantly if the chain reactions increase out of control. This allows the station to increase or decrease electricity production in response to demand.

    As mentioned earlier, Uranium-235 only makes up around 0.7% of total Uranium. However, for use in a power station the amount of Uranium-235 needs to be around 2-3%. To increase the levels of Uranium-235, before being sent to the power station Uranium is enriched to increase the proportion of Uranium-235. After processing the Uranium is formed into pellets approximately 2.5 cm long, with the normal lifespan of these pellets in a nuclear power station around 3-5 years, after which the amount of Uranium-235 remaining in the pellet has reduced to a level which is no longer a critical mass. Following this time, the rods are removed from the power generator and become waste.

    As a point of comparison – nuclear weapons require at least 90% Uranium-235 to be effective. This means that Uranium power station fuel is woefully insufficient to be used as a weapon without extensive processing. Nuclear power stations are extremely safe when designed and operated correctly. However, they do produce toxic waste.

     

    Components of a Nuclear Power Plant

    Nuclear reactor:

    In a nuclear power plant, nuclear reactor (a cylindrical stout pressure vessel) is the part of the facility in which the heat, necessary to produce steam, is generated by fission of the nuclei of uranium (U235). There are several components common to most types of reactors:
     

    a. Fuel: Uranium is the basic fuel. Usually pellets of uranium dioxide (UO2) are arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core. In a new reactor with new fuel a neutron source is needed to get the reaction going. Usually this is beryllium mixed with polonium, radium or other alpha-emitter. Alpha particles from the decay cause a release of neutrons from the beryllium as it turns to carbon-12. Restarting a reactor with some used fuel may not require this, as there may be enough neutrons to achieve criticality when control rods are removed.

     

    b. Moderator:  A neutron moderator or simply moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235 or a similar fissile nuclide. Commonly used moderators include regular (light) water (roughly 75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium has also been used in some experimental types.

     

    c. Control rods: These are made with neutron-absorbing material such as cadmium (Cd48), hafnium (Hf72) or boron (B5), and are inserted or withdrawn from the core to control the rate of reaction, or to halt it.

     

    d. Coolant: A fluid circulating through the core so as to transfer the heat from it.  In light water reactors, the water moderator functions also as primary coolant. Except in BWRs, there is secondary coolant circuit where the water becomes steam.

     

    e. Pressure vessel or pressure tubes: Usually a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the surrounding moderator.

     

    f. Containment: The structure around the reactor and associated steam generators which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any serious malfunction inside. It is typically a meter-thick concrete and steel structure.

     

    2. Heat exchanger or steam generator: The coolant gives up heat to the heat exchanger which is utilized in raising the steam. After giving up heat, the coolant is again fed to the reactor.

     

    3. Steam turbine: The steam produced in the heat exchanger is led to the steam turbine through a valve. After doing a useful work in the turbine, the steam is exhausted to condenser. The condenser condenses the steam which is fed to the heat exchanger through feed water pump.

     

    4. Alternator: The steam turbine drives the alternator which converts mechanical energy into electrical energy. The output from the alternator is delivered to the bus-bars through transformer, circuit breakers and isolators.

     

    Types of Nuclear Reactor

    There are several types of nuclear reactors; some of them are discussed here.

    1. Pressurised water reactor (PWR):

    Pressurized water reactors constitute the large majority of the world's nuclear power plants and several are employed for naval propulsion. PWRs use ordinary water as both coolant and moderator. The design is distinguished by having a primary cooling circuit which flows through the core of the reactor under very high pressure, and a secondary circuit in which steam is generated to drive the turbine.

     

    A PWR has fuel assemblies of 200-300 fuel rods each of ≈ 13 ft in length, arranged vertically in the core, and a large reactor would have about 150-250 fuel assemblies with 80-100 tons of uranium. Water in the reactor core reaches about 325°C, hence it must be kept under about 150 times atmospheric pressure to prevent it boiling. Pressure is maintained by steam in a pressuriser (which forces the water to remain in the liquid phase and therefore ensure the most efficient heat transfer). In the primary cooling circuit the water is also the moderator, and if any of it turned to steam the fission reaction would slow down. This negative feedback effect is one of the safety features of the type. The secondary shutdown system involves adding boron to the primary circuit. The secondary circuit is under less pressure and the water here boils in the heat exchangers which are thus steam generators. The steam drives the turbine to produce electricity, and is then condensed and returned to the heat exchangers in contact with the primary circuit.

     

    2. Boiling water reactor (BWR):

    The boiling water reactor (BWR) is a simplest type of nuclear reactor for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor (PWR). BWR design has many similarities to the PWR, except that there is only a single circuit in which the water is at lower pressure (about 75 atm) so that it boils in the core at about 285°C. The reactor is designed to operate with 12-15% of the water in the top part of the core as steam, and hence with less moderating effect and thus efficiency there.  BWR units can operate in load-following mode more readily then PWRs. The steam passes through drier plates (steam separators) above the core and then directly to the turbines, which are thus part of the reactor circuit. Since the water around the core of a reactor is always contaminated with traces of radionuclides, it means that the turbine must be shielded and radiological protection provided during maintenance. BWRs use ordinary water as both coolant and moderator.

     

    The cost of this tends to balance the savings due to the simpler design. Most of the radioactivity in the water is very short-lived (with a 7 second half life), so the turbine hall can be entered soon after the reactor is shut down.  A BWR fuel assembly comprises 90-100 fuel rods each of ≈ 14.5 ft in length, and there are up to 750 assemblies in a reactor core, holding up to 140 tons of uranium. The secondary control system involves restricting water flow through the core so that more steam in the top part reduces moderation. Out of the 22 operational nuclear reactors of India, two are Boiling Water Reactors.

     

    3. Pressurised heavy water reactor (PHWR):

    The PHWR reactor design has been developed since the 1950s in Canada as the CANDU (which stands for Canadian Deuterium Uranium), and from 1980s also in India. PHWR generally use natural uranium (0.7% U-235) as fuel, hence needs a more efficient moderator, in this case heavy water (D2O). While heavy water is significantly more expensive than ordinary light water, it creates greatly enhanced neutron economy, allowing the reactor to operate without fuel-enrichment facilities. The PHWR produces more energy per kilogram of mined uranium than other designs, but also produces a much larger amount of used fuel per unit output.

     

    The moderator is in a large tank called a calandria, penetrated by several hundred horizontal pressure tubes which form channels for the fuel, cooled by a flow of heavy water under high pressure in the primary cooling circuit, reaching 290°C. As in the PWR, the primary coolant generates steam in a secondary circuit to drive the turbines. The pressure tube design means that the reactor can be refueled progressively without shutting down, by isolating individual pressure tubes from the cooling circuit. It is also less costly to build than designs with a large pressure vessel, but the tubes have not proved as durable.

     

     

    A CANDU fuel assembly consists of a bundle of 37 half meter long fuel rods (ceramic fuel pellets in zircaloy tubes) plus a support structure, with 12 bundles lying end to end in a fuel channel. Control rods penetrate the calandria vertically, and a secondary shutdown system involves adding gadolinium to the moderator. The heavy water moderator circulating through the body of the calandria vessel also yields some heat (though this circuit is not shown on the diagram above). Newer PHWR designs such as the Advanced CANDU Reactor (ACR) have light water cooling and slightly-enriched fuel. CANDU reactors can accept a variety of fuels. They may be run on recycled uranium from reprocessing LWR used fuel, or a blend of this and depleted uranium left over from enrichment plants. About 4000 MWe of PWR might then fuel 1000 MWe of CANDU capacity, with addition of depleted uranium. Out of the 22 operational nuclear reactors of India, eighteen are Pressurised Heavy Water Reactors.

     

    4. Advanced gas-cooled reactor (AGR):

    These are the second generation of British gas-cooled reactors, using graphite moderator and carbon dioxide or helium as primary coolant while water as a secondary coolant. The fuel is uranium oxide pellets, enriched to 2.5-3.5%, in stainless steel tubes. The carbon dioxide circulates through the core, reaching 650°C and then past steam generator tubes outside it, but still inside the concrete and steel pressure vessel (hence 'integral' design). Control rods penetrate the moderator and a secondary shutdown system involves injecting nitrogen to the coolant.

     

    5. Fast Breeder Reactor:

    Fast  reactors  generate  energy  from  nuclear  fuel  through  their  irradiation  with  fast neutrons. In a thermal reactor, neutrons produced as a result of neutron absorption in fuel  possess  high  kinetic  energy  of  the  order  of  MeV.  These  are  slowed  by  elastic collision  with  moderator  resulting  in  thermal  neutrons  with  energies  as  low  as  0.025 eV.  Since  the  fast  reactor  utilizes  fast  neutrons,  moderation  is  not  required.  To be precise, moderation is undesirable in a fast reactor. Hence fast reactors do not contain moderating materials like water, heavy water and graphite in the core.

    The fission cross section of U-235 in fast spectrum is low, compared to that of Pu-239. Hence Pu-239 is used as the main fissile isotope, though enriched U-235 is used at the start to initiate the chain reaction.

    Fast reactors are normally configured for breeding. This requires absorption of neutrons by a blanket of fertile material. Also neutron losses in structural components are to be minimized. Hence layers of blankets containing fertile material are used, to ensure that more fuel is breed than that burnt, qualifying the definition of a breeder.

    The most common coolants like water and heavy water cannot be used as coolants in a fast reactor. Non-moderating materials like Helium and liquid metals like sodium, lead, lead-bismuth eutectic qualify to be coolants owing to their non-moderating nature.

    Based on the coolant, fast (breeder) reactors are further classified as follows: (i) sodium cooled fast reactor (ii) lead cooled fast reactor (iii) helium cooled fast reactor Both sodium cooled fast reactor and lead cooled fast reactor are called Liquid Metal Cooled Fast Breeder Reactor (LMFBR).

    Due to better transport and neutronic properties, sodium is the most preferred choice for coolant. One of the advantages of using sodium as coolant is the possibility of achieving a high coolant (sodium) outlet temperature, while maintaining a pressure much lower than those maintained for light water and heavy water reactors. This is due to the high boiling point of sodium even at atmospheric pressure. Hence problems associated with high pressures are circumvented to a large extent.

     

    Sodium cooled fast breeder reactors use two cycles of coolant flows. The primary circuit involves the circulation of sodium through the core. Relatively low temperature sodium enters the core at the bottom and leaves at the top at higher temperature. This sodium, called primary sodium is radioactive due to exposure to neutrons while passing through the core.

    Another circuit involves heat transfer between the radioactive primary sodium and secondary sodium in separate heat exchangers. The secondary sodium in turn transfers heat to water in steam generator, thus producing steam. The use of secondary coolant between primary coolant and steam is aimed at preventing contact of radioactive sodium with water in case of leakage. While it is to be noted that sodium water reaction itself is exothermic and needs to be prevented, contact of radioactive sodium with water would also involve concerns with radioactivity. Hence preventing contact between radioactive sodium and water eliminates the radioactivity concerns.

     

    Advantages of Nuclear power Plant

    1. Space needed for operating a nuclear power plant is less as compared to other conventional power plants for production of same amount of energy.

    2. A nuclear power plant consumes very small quantity of fuel. Thus fuel transportation cost is less and large fuel storage facilities are not required. Further the nuclear power plants will conserve the fossil fuels such as coal, oil, gas etc. for other energy need.

    3. There is increased reliability of operation.

    4. Nuclear power plants are not affected by the unfavorable weather conditions.

    5. Nuclear power plants are well suited to meet large demands of power requirement. They give better performance at higher load factors of 80 to 90%.

    6. Materials expenditure on metal structures, piping, and storage mechanisms is much lower compared to a coal-burning power plant.

    7. It does not need large quantity of water.

    8. Efficiency ≈ 33 -36%

     

    Disadvantages of Nuclear power Plant

    1. Initial cost to set up nuclear power plant is higher as compared to hydro or fossil fuel based power plant.

    2. Nuclear power plants are not well suited for varying load conditions.

    3. Radioactive wastes if not disposed carefully may have an effect on the health of operators and the population nearby. In a nuclear power plant the major problem faced is the disposal of highly radioactive waste in form of solid, liquid and gas without any injury to the atmosphere. The preservation of radioactive waste for a long duration of time creates many difficulties.

    4. Maintenance cost of the nuclear power plant is high.

    5. Trained people are required to handle nuclear power plants.

     

    Fissile and fertile materials

    All heavy nuclides have the ability to fission when in an excited state, but only a few fission readily and consistently when struck by slow moving neutrons (low-energy or thermal neutrons). In nuclear engineering, fissile material (nuclide) is material that is capable of undergoing fission reaction after absorbing thermal (slow or low energy) neutron. These materials are used to fuel thermal nuclear reactors, because they are capable of sustaining a nuclear fission chain reaction and thus release enormous amounts of energy. In nuclear reactors, the fission process is controlled and the energy is harnessed to produce electricity. The most important fissile materials for nuclear energy and nuclear weapons are uranium-233 (233U), uranium-235 (235U), plutonium-239 (239Pu), and plutonium-241 (241Pu). Of these, only uranium-235 occurs in a usable amount in nature—though its presence in natural uranium is only some 0.7204 percent by weight, necessitating a lengthy and expensive enrichment process to generate a usable reactor fuel.

    As an alternative to processing and enriching uranium-235, it is possible to go through the process of generating quantities of other fissile nuclides that are not as prevalent as uranium-235. Prominent sources of these nuclides are thorium-232 (232Th), uranium-238 (238U), and plutonium-240 (240Pu), which are known as fertile materials owing to their ability to transform into fissile materials. Thus, fertile material is a material that, although not itself fissile (fissionable by thermal neutrons), can be converted into a fissile material by neutron absorption and subsequent nuclei conversions in a reactor. For example, thorium-232, the predominant isotope of natural thorium, can be used to generate uranium-233 through a process known as neutron capture. When a nucleus of thorium-232 absorbs, or “captures,” a neutron, it becomes thorium-233, whose half-life is approximately 21.83 minutes. After that time the nuclide decays through electron emission to protacttinium-233, whose half-life is 26.967 days. The protactinium-233 nuclide in turn decays through electron emission to yield uranium-233.

                   n + 232Th --> 233Th --> 21.83 min --> 233Po --> 26.967 days --> 233U

       Similarly,      n + 238U --> 239U --> 23.47 min --> 239Np --> 2.356 days --> 239Pu

     

    Thus Fertile materials are isotopes which are capable of becoming fissile by capturing fast moving neutrons possibly followed by radioactive decay. A power reactor contains both fissile and fertile material. The fertile materials partially replace fissile materials that are destroyed by fission, thus permitting the reactor to run longer before the amount fissile materials decreases to the point where criticality is not no longer manageable. The energy released from 1 kilogram of U-235 is 22.5 GWh.

     

    Shielding

    An operating reactor is a powerful source of radiation, since fission and subsequent radioactive decay produce neutrons and gamma rays, both of which are highly penetrating radiations. A reactor must have specifically designed shielding around it to absorb and reflect this radiation in order to protect technicians and other reactor personnel from exposure. In a popular class of research reactors known as “swimming pools,” this shielding is provided by placing the reactor in a large, deep pool of water. In other kinds of reactors, the shield consists of a thick concrete structure around the reactor system referred to as the biological shield. The shield also may contain heavy metals, such as lead or steel, for more effective absorption of gamma rays, and heavy aggregates may be used in the concrete itself for the same purpose.

     

    Nuclear fusion and nuclear fission

    Nuclear fission is a nuclear reaction (or radioactive decay process) in which a neutron collides with a nucleus of a large atom such as Uranium and neutron is absorbed by this large atom causing the nucleus to become unstable and thus split into two smaller stable atoms with the release of two or three neutrons and a considerable amount of energy. Nuclear fission can occur naturally with the spontaneous decay of radioactive material or it can be initiated by bombarding the fuel consisting of fissionable atoms with neutrons.

    figure: Nuclear Fission

    Nuclear fusion is a nuclear reaction or process in which the nuclei of two light atoms combine to form a single-bigger nucleus of a new atom and releasing large amounts of energy as well as some sub-atomic particles (neutron and/or proton) a consequence. The energy released in nuclear fusion is 3 to 4 times more than that of nuclear fission.

    Figure: Nuclear Fusion

     

    Comparison between Nuclear Fission and Nuclear Fusion

     

    Nuclear Fission

    Nuclear Fusion

    Definition

    Fission is the splitting of a large atom into two or more smaller ones.

    Fusion is the fusing of two or more lighter atoms into a larger one.

    Natural occurrence of the process

    Fission reaction does not normally occur in nature.

    Fusion occurs in stars, such as the sun.

    Byproducts of the reaction

    Fission produces many highly radioactive particles. So, health hazard is high.

    Few radioactive particles are produced by fusion reaction, but if a fission "trigger" is used, radioactive particles will result from that.

    Conditions

    Critical mass of the substance and high-speed neutrons are required.

    High density, high temperature environment is required.

    Energy Requirement

    Takes little energy to split two atoms in a fission reaction.

    Extremely high energy is required to bring two or more protons close enough that nuclear forces overcome their electrostatic repulsion.

    Energy Released

    The energy released by fission is a million times greater than that released in chemical reactions, but lower than the energy released by nuclear fusion.

    The energy released by fusion is three to four times greater than the energy released by fission.

    Nuclear weapon

    One class of nuclear weapon is a fission bomb, also known as an atomic bomb or atom bomb.

    One class of nuclear weapon is the hydrogen bomb, which uses a fission reaction to "trigger" a fusion reaction.

    Energy production

    Fission is used in nuclear power plants.

    Fusion is an experimental technology for producing power.

    Fuel

    Uranium is the primary fuel used in power plants.

    Hydrogen isotopes (Deuterium and Tritium) are the primary fuel used in experimental fusion power plants.

     

    Selection of Site for Nuclear Power Station

    The following points should be kept in view while selecting the site for a nuclear power plant:

    1. Availability of water: As sufficient water is required for cooling purposes, therefore, the plant site should be located where ample quantity of water is available, e.g., across a river or by sea-side.

    2. Disposal of waste: The waste produced by fission in a nuclear power plant is generally radioactive which must be disposed properly to avoid health hazards. The waste should either be buried in a deep trench or disposed off in sea quite away from the sea shore. Therefore, the site selected for such a plant should have adequate arrangement for the disposal of radioactive waste.

    3. Distance from populated areas: The site selected for a nuclear power station should be quite away from the populated areas as there is a danger of presence of radioactivity in the atmosphere near the plant. However, as a precautionary measure, a dome is used in the plant which does not allow the radioactivity to spread by wind or underground waterways.

    4. Transportation facilities: The site selected for a nuclear power station should have adequate facilities in order to transport the heavy equipment during erection and to facilitate the movement of the workers employed in the plant.

    From the above mentioned factors it's clear that ideal choice for a nuclear power plant must be away from thickly populated areas.