|
|
|
| SCIENCE & TECHNOLOGY |
|
|
|
What light water nuclear reactor is
Prof Yash
Pal PROF YASH PAL
Cape Canaveral, Florida: Space shuttle Atlantis left the International Space Station on Wednesday after a seven-day stay to deliver gear to keep the outpost operating after the shuttle program is retired next year. |
|
|
What light water nuclear reactor is
Nuclear power is power (generally electrical) produced from controlled (i.e., non-explosive) nuclear reactions. Commercial plants in use to date use nuclear fission reactions. Electric utility reactors heat water to produce steam, which is then used to generate electricity. In 2007, 14 per cent of the world’s electricity came from nuclear power, despite concerns about safety and radioactive waste management.
The Central government and state governments are strongly advocating the setting up of Light Water Nuclear Reactors (LWNR) in the country. Besides, nuclear energy is promoted as the ultimate solution not only to power demand but also to counter the polluting coal based thermal power plants. Just as many conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear power plants convert the energy released from the nucleus of an atom, typically via nuclear fission. When a relatively large fissile atomic nucleus absorbs a neutron, fission of the atom often results. Fission splits the atom into two or more smaller nuclei with kinetic energy (known as fission products) and also releases gamma radiation and free neutrons. A portion of these neutrons may later be absorbed by other fissile atoms and create more fissions, which release more neutrons, and so on. This nuclear chain reaction can be controlled by using neutron poisons and neutron moderators to change the portion of neutrons that will go on to cause more fission. Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if unsafe conditions are detected. A cooling system removes heat from the reactor core and transports it to another area of the plant, where the thermal energy can be harnessed to produce electricity or to do other useful work. Typically the hot coolant will be used as a heat source for a boiler, and the pressurized steam from that boiler will power one or more steam turbine driven electrical generators. There are many different reactor designs, utilizing different fuels and coolants and incorporating different control schemes. Some of these designs have been engineered to meet a specific need. A number of new designs for nuclear power generation, collectively known as the Generation IV reactors, are the subject of active research and may be used for practical power generation in the future. Many of these new designs specifically attempt to make fission reactors cleaner, safer and/or less of a risk to the proliferation of nuclear weapons. Passively safe plants are available to be built and other designs that are believed to be nearly fool-proof are being pursued. The nuclear fission reactors used in the United States for electric power production are classified as “light water reactors” in contrast to the “heavy water nuclear reactors (LWNR)” used in elsewhere. The light water nuclear reactor (LWNR) is a type of thermal reactor that uses light water as a coolant and neutron moderator (water, H2O) as opposed to heavy water as a coolant/moderator (deuterium oxide, 2H2O). Thermal reactors are the most common type of nuclear reactor, and light water nuclear reactors are the most common type of thermal reactor. There are three varieties of light water reactors; the pressurized water nuclear reactor (PWNR), the boiling water nuclear reactor (BWNR), and the supercritical water nuclear reactor (SWNR). The light water reactor produces heat by controlled nuclear fission. The nuclear reactor core is the portion of a nuclear reactor where the nuclear reactions take place. It mainly consists of nuclear fuel and control elements. In the boiling water reactor, the heat generated by fission turns the water into steam, which directly drives the power-generating turbines. But in the pressurized water reactor, the heat generated by fission is transferred to a secondary loop via a heat exchanger. Steam is produced in the secondary loop, and the secondary loop drives the power-generating turbines. In either case, after flowing through the turbines, the steam turns back into water in the condenser. The light water nuclear reactor also uses ordinary water to keep the reactor cooled. The use of water as a moderator is an important safety feature of LWNR, as any increase in temperature causes the water to expand and become less dense; thereby reducing the extent to which neutrons are slowed down and hence reducing the reactivity in the reactor. Therefore, if reactivity increases beyond normal, the reduced moderation of neutrons will cause the chain reaction to slow down, producing less heat. This property, known as the negative temperature coefficient of reactivity, makes LWNR reactors very stable. In event of a loss of coolant accident, the moderator is also lost and the reaction will stop. PWNR mode of LWNR is widely used in western countries with following advantages and disadvantages being associated with it. Advantages w PWR reactors are very stable. w
PWR reactors can be operated with a core fissile material with a delayed neutron decay factor which extends the effective neutron lifetime. Because PWR reactors use enriched uranium as fuel they can use ordinary water as a moderator. w
PWR turbine cycle loop is separate from the primary loop, so the water in the secondary loop is not contaminated by radioactive materials. Disadvantages w
The coolant water must be highly pressurized to remain liquid at high temperatures. This requires high strength piping and a heavy pressure vessel and hence increases construction costs. w
Most pressurized water reactors cannot be refuelled while operating. w
The high temperature water coolant with boric acid dissolved in it is corrosive to carbon steel (but not stainless steel); this can cause radioactive corrosion products to circulate in the primary coolant loop. w
Because water acts as a neutron moderator, it is not possible to build a fast neutron reactor with a PWR design. |
|
PROF YASH PAL
Was it necessary to have life, or it is sheer coincidence?
You have asked me whether it was necessary for the universe to have life. It is tempting to believe that this large universe with all stars and galaxies was made for us. But that is tremendous arrogance on the part of a species occupying a tiny planet. Perhaps there are others like us but it seems rather unlikely. Of course all the birds, horses and cockroaches can also think that the universe was really created for them. None of them would be wrong or right. I do not know whether development of life in the universe was inevitable, but I would also oppose those who might contend that this was impossible. After all, all the ingredients are available and given enough time some thing like what happened was quite likely. I know that this argument can never end. I would like to close it by saying that we are fortunate that we came to be, and even more grateful that we developed a power of comprehension. It would be marvellous if we could all delight in being a part of the grand and marvellous drama all around us, even it was not written only for us. What is the reason that lizards can move on the roof without falling? I think that lizards can stay stuck to vertical walls and ceilings because of the construction of the pads on their feet. These pads of thousands upon thousands narrow hair like needled structure allow van der Wall forces, that are atomic in nature, to come into play. In a simple way I understand such forces could be called proximity forces —they basically arise from electromagnetic interaction. I could also use a term not used in science books: When some molecules get close to each other, they form friendships and hold hands. Fine hair on those pads, when pointed right, allow intimate contact with the molecules of the surface on which lizard moves. This might be the reason for the magical capability of lizards to defy gravity. What is a time machine? Is it is possible to travel the time? The phrase time machine came in currency through the work of a great fiction writer. I think the only effective time machine lies in the power of our imagination. That is good enough for me. Will an ice age follow global warming? Coming in of an ice age following global warming is an idle fancy in which I have also engaged. If global warming were to bring the travelling belt of ocean currents to stop, the temperate region of the Earth might get very cold because they would be deprived of the heat transported by the Ocean currents like the Gulf Stream. This is not a well worked out argument; it relies on the surmise that a change in salinity of the sea might interfere with the organised movement of the travelling belt of currents. It might be a wrong argument and even if correct it is not possible to make a prediction of the time scale. This was not a serious well worked out argument, though fun to throw it in the ring of possibilities. |
|
|
Atlantis leaves station as NASA eyes shuttles’ end
Cape
Canaveral, Florida: Space shuttle Atlantis left the International Space Station on Wednesday after a seven-day stay to deliver gear to keep the outpost operating after the shuttle program is retired next year.
Space station flight engineer Nicole Stott joined the six Atlantis
astronauts for the return trip home. She is expected to be the last station
crewmember to catch a ride on the shuttle. Atlantis is due back at the Kennedy
Space Center in Florida on Friday. |
