RADAR stands for RAdio Detecting And Ranging. As the radar antenna turns, it emits extremely short bursts of radio waves, called pulses. Each pulse lasts about 0.00000157 seconds with a 0.00099843-second "listening period" in between. It may be reflected by objects in their path back to the radar. When these pulses intercept precipitation, part of the energy is scattered back to the radar. This concept is similar to hearing an echo. From this information the radar is able to tell where the precipitation is occurring and how much precipitation exists. The transmitted radio waves move through the atmosphere at about the speed of light.
A weather radar consists of a parabolic dish (it looks like a satellite dish) encased in a protective dome and mounted on a tower of up to five stories tall
Components of Radar
Radars in their basic form have four main components:
· A transmitter, which creates the energy pulse.
· A transmit/receive switch that tells the antenna when to transmit and when to receive the pulses.
· An antenna to send these pulses out into the atmosphere and receive the reflected pulse back.
· A receiver, which detects, amplifies and transforms the received signals into video format.
The received signals are displayed on a display system.
Radar output generally comes in two forms: reflectivity and velocity. Reflectivity is a measure of how much precipitation exists in a particular area. Velocity is a measure of the speed and direction of the precipitation toward or away from the radar. Most radars can measure reflectivity but need a Doppler radar to measure velocity.
Reflectivity: The German Heinrich Hertz discovered the behaviour of radio waves in 1887. He showed that the invisible electromagnetic waves radiated by suitable electrical circuits travel with the speed of light, and that they are reflected in a similar way. In the following decades these properties were used to determine the height of the reflecting layers in the upper atmosphere. This is why data received from the radar is called reflectivity.
Doppler: In 1842 the Austrian physicist Christian Doppler had discovered what is now called the Doppler effect. This is the theory that sound waves will change in pitch when there is a shift in the frequency. This theory is used by Doppler weather radar to determine the speed of precipitation in the atmosphere, toward or away from the radar. Since precipitation as it falls generally moves with the wind, you can determine the wind velocity with Doppler technology.
Although already invented, radar was further developed during World War II, with work on the technology stimulated by the threat of air attacks. Radar had many uses during the war - it was used for
· Locating enemy ships and aircraft
· To direct gunfire
· To aid ship and aircraft navigation.
Though the military continues to use radar, the technology was released to the public after World War II and was quickly used by many other industries. Radars are now used to help navigate ships in fog and airplanes in bad weather. Radar can detect a speeding car and track a satellite. Most importantly for meteorologists, radars can detect all sorts of atmospheric phenomena.
Radar Images Weather radar images are generally a map view of reflected particles for a specified area surrounding the radar. Depending on the intensity of the precipitation, different colours will appear on the map. Each colour on the radar display will correspond to a different level of energy pulse reflected from precipitation.
The strength of the pulse returned to the radar depends on the
· Size of the particles
· How many particles there are
· What state they are in (solid-hail, liquid-rain)
· What shape they are
After making many assumptions about these factors and others, the approximate rain rate at the ground can be estimated. In fact, the most reflective precipitation particles in the atmosphere are large and usually have a liquid surface (water-coated hailstones).
Radar images will not always accurately reflect what is occurring in the atmosphere and not everything you see on the radar will be precipitation.
· The radar sometimes detects precipitation that occurs higher in the atmosphere but doesn't reach the ground. That's why the radar may appear to show rain when rain isn't occurring. This is called virga.
· If the radar is close to the coast and the beam is broad enough, it may reflect off the sea and return strong reflectivity that is really just sea "clutter".
· At some wavelengths the radar beam is not fully reflected when passing through very heavy rain or hail, thus reducing or obscuring the echo intensity further out from the radar.
· The presence of mountains within the range of the radar can block part or whole of the radar beam, thus reducing the echo intensity from rain on the other side of the mountains. This is considered "ground clutter" and can also be produced by buildings and trees.
· Occasionally birds, planes, ships and even a dense enough swarm of insects can be detected by weather radar. This is even more common with Doppler radars due to their higher sensitivity.
· As you move further away from the radar, the returned echo becomes weaker. Because as the radar beam broadens with distance, the proportion of the beam that is filled with rain lessens and reduces the echo intensity. The radar beam is also further from the ground with distance (partly because of the Earth's curvature, and partly because the beam is angled upwards by a fraction of a degree), thereby missing the lower parts of the rain.
Holography is a photographic technique that records the light scattered from an object, and then presents it in a way that appears three-dimensional. The word holography comes from the Greek words "whole and grafē; meaning "writing" or "drawing". Photographs record only the amplitude of the light that hits the film, while holograms record differences in both amplitude and the phase. Various types of holograms have been made over the years, including transmission holograms, which allow light to be shined through them and the image to be viewed from the side; and rainbow holograms, which are used for security purposes — on credit cards and driver's licenses.
The development of hologram technology started in 1962, when Yuri Denisyuk, in the Soviet Union, and Emmett Leith and Juris Upatnieks at the University of Michigan developed laser technology that recorded 3D objects.
The holographic recording itself is not an image; it consists of an apparently random structure of either varying intensity, density or profile.Holograms can also be used to store, retrieve, and process information optically
Types of Holograms
· Transmission holograms, A later refinement, the "rainbow transmission" hologram, allows more convenient illumination by white light rather than by lasers. Rainbow holograms are commonly used for security and authentication, for example, on credit cards and product packaging.
· Reflection or Denisyuk hologram, can also be viewed using a white-light illumination source on the same side of the hologram as the viewer and is the type of hologram normally seen in holographic displays. They are also capable of multicolour-image reproduction.
· Specular holography is a related technique for making three-dimensional images by controlling the motion of specularities on a two-dimensional surface. It works by reflectively or refractively manipulating bundles of light rays,
· Gabor-style holography works by diffractively reconstructing wavefronts.
To make a hologram, the following are required:
· A suitable object or set of objects
· A suitable laser beam
· Part of the laser beam to be directed so that it illuminates the object (the object beam) and another part so that it illuminates the recording medium directly (the reference beam), enabling the reference beam and the light which is scattered from the object onto the recording medium to form an interference pattern
· A recording medium which converts this interference pattern into an optical element which modifies either the amplitude or the phase of an incident light beam according to the intensity of the interference pattern.
· An environment which provides sufficient mechanical and thermal stability that the interference pattern is stable during the time in which the interference pattern is recorded
The surface of the object is rough on a microscopic level, even if it looks smooth to the human eye, so it causes a diffuse reflection. It scatters light in every direction following the laws of reflection. This diffuse reflection causes light reflected from every part of the object to reach every part of the holographic plate. This is why a hologram is redundant -- each portion of the plate holds information about each portion of the object. When a hologram is cut in half, the whole scene can still be seen in each piece. This is because, whereas each point in a photograph only represents light scattered from a single point in the scene, each point on a holographic recording includes information about light scattered from every point in the scene.
Three important properties of a hologram
· An amplitude modulation hologram is one where the amplitude of light diffracted by the hologram is proportional to the intensity of the recorded light.
· A thin hologram is one where the thickness of the recording medium is much less than the spacing of the interference fringes which make up the holographic recording.
· A transmission hologram is one where the object and reference beams are incident on the recording medium from the same side.
When the hologram plate is illuminated by a laser beam identical to the reference beam which was used to record the hologram, an exact reconstruction of the original object wavefront is obtained. An imaging system (an eye or a camera) located in the reconstructed beam 'sees' exactly the same scene as it would have done when viewing the original. When the lens is moved, the image changes in the same way as it would have done when the object was in place. If several objects were present when the hologram was recorded, the reconstructed objects move relative to one another, i.e. exhibit parallax, in the same way as the original objects would have done.
· Data Storage: advantage of this type of data storage is that the volume of the recording media is used instead of just the surface. They can have write speeds of 1Gbps and read speed of 1Tbps
· Optical Computing
· Sensors and Biosensors
· Security: Security holograms are very difficult to forge, because they are replicated from a master hologram that requires expensive, specialized and technologically advanced equipment.
According to Allison MacFarlane, associate professor of environmental science and policy at George mason university and a member of the blue ribbon commission on America’s nuclear future, coming up with storage solutions for nuclear waste continue to be a last-minute decision in a number of countries besides Japan. It is surprisingly common for reactor sites to be overburdened with spent fuel with no clear disposal plan. In South Korea, for example storage at the nation’s four nuclear plants is filling up leading to a potential storage crisis within the next decade.
The United Arab Emirates broke ground for the first of four nuclear reactors in a march 2011 but has not prioritized storage. Hans blix former head of the international atomic energy agency and current chairman of the UAE’s international advisory board noted: The question of a final disposal plan is still open and more attention should be spent on deciding what to do.
Some very low level nuclear wastes can go into landfill-type settings. But low level wastes composed of low concentrations of long-lived radio-nuclides and high concentrations of short-lived ones must remain sequestered for a few hundred years in specially engineered subsurface facilities. Intermediate and high level wastes require disposal hundreds of meters below the ground for thousands or even hundreds of thousands of years to ensure public safety.
In all types of energy production money is made at the front end of the process rather than in waste management at the back end. Macfarlane argues however that a failure to plan for waste disposal can cause the more profitable front end of the operation to collapse.
Nuclear fuel is discharged from a light water reactor after about four to six years in the core. Because the fuel is extremely thermally and radioactively hot at discharge it must be cooled in a pool. Actively cooled with circulated borate water, spent fuel pools are about 40 feet (12 meters) deep. The water not only removes heat but also helps absorb neutrons and stops chain reactions. In a number of countries, including the united states, metal racks in spent fuel pools hold four times the originally intended amount of fuel. Plans to reprocess fuel have failed for both economic and policy reasons.
Japan’s fukushima diarchy plant has seven spent fuel pools, one at each reactor and a large shared pool, as well as dry cask storage for spent fuel on site. Initially Japan had planned a short period of spent fuel storage at the reactor site prior to reprocessing but Japan’s reprocessing facility has suffered long delays (scheduled to open in 2007, the facility is still not ready). This has caused spent fuel to build up at the plant’s reactor sites.
Countries should include additional spent fuel storage in their nuclear power plans from the start rather than creating ad hoc solutions after spent fuel has already begun to build up. Siting storage is a technical issue but importantly also a social and political one. Countries with nuclear power programs need a medium-term strategy for spent fuel storage prior to the long-term plan for spent fuel or high level waste disposal Macfarlane explains. Though difficult the disposal of high level nuclear waste is possible and a clear strategy to develop a repository combines both technical and societal criteria in a phased approach.
After fukushima the nuclear industry and nuclear regulators must redefine a successful nuclear power program. Safe electricity production will not suffice a nuclear power program must be safe secure and sustainable for its entire lifecycle, from mining uranium ores to disposing of spent nuclear fuel. Failure o plan ahead for nuclear waste management will lead the public in many countries to reject nuclear as an energy choice.
FINLAND FAR AHEAD IN NUCLEAR WASTE MANAGEMENT
Finland consumes nearly 17,000 units of electric power per capital annually; its share of nuclear electricity is about 28 per cent. Though its nuclear power programme is very modest compared to that of U.S. or U.K. it is far ahead in its universally applauded plans for nuclear waste management.
The general refrain of lay public (often reinforced by antinuclear rhetoric) is that there is no ultimate solution for managing high level nuclear waste. Finland demonstrates that it has in place a popularly accepted technological solution.
Finnish programme Currently Finland operates four nuclear power reactors with a total installed capacity of 2716 MWe. It produces about 70 tonnes of spent fuel annually. Finland has no plans to reprocess the spent fuel. Finland started its preliminary preparations for its nuclear waste management shortly before the first reactor started operation in 1977-1978. In 1978 the first lot of spent fuel entered the facility for interim storage at loviisa.
The nuclear energy act 990/1987 passed by its parliament stated that nuclear waste generated in connection with or as a result of the use of nuclear energy in Finland shall be handled stored and permanently disposed of in Finland. In 1983, Finland started screening of potential sites for spent fuel disposal. Within the next four years, Finnish scientists started field research in five municipalities for selecting the final disposal site.
Final repository In 2000, they chose Olkiluoto. They plan to dispose of spent fuel in an underground geological repository. Posiva, a finnish company which is entrusted with the job has drilled a 6.5 metre0high, 5 m- wide and 5000m long Okalo tunnel. It has removed over 100,000 cubic metre of rock.
The company successfully located the place where no one would ever be likely to dig a deep hole later for exploiting minerals because the place is not mineral rich. The idea is to abandon forever, the mostly natural, and party engineered underground repository after filling it.
Canister design After a few decades of interim storage, the levels radioactivity and heat of spent fuel reduce to about 0.1 per cent of the original values. It is then encapsulated in a cast iron insert which in turn is covered by a 5 cm thick copper canister. Each insert may carry up to 12 fuel bundles.
They will be placed in neatly bored holes a few metre apart in the underground repository. The gaps between each canister and the hold will be filled with bentonite clay, which swells by absorbing water. This clay provides cushioning to the canister in case of geological movements and ensures that there are no voids through which water can enter and corrode the container. Finland hopes to start filling the repository by 2012 and completing it by 2120. They can cover the mouth of the tunnel and forget about it.
Most of the radioactivity in the spent fuel is due to fission products. They have a half life of about 30y. In 100,000 years, the radioactivity remaining in the fuel will be negligible. Finnish scientists proved that 1.5 cm of copper cladding would last over 100,000 years. Evidently, 5 cm of copper cladding will be more than adequate.
During the period an ice age may come and cover the area under 2-3 km of ice. The pressure on the canister due to ice, tightly gripping bentonite clay and ground water way equal that experienced by it at an ocean depth of 4.5 km. finns proved that their copper cylinders well withstand a pressure three times that before failing.
Waste management cost is manageable. Finland collects a few percentage of the electricity cost per unit of power to manage the waste and deposits it in an independent national nuclear waste management fund controlled and administered by the ministry of trade and industry.
The agency estimates and assesses the liability annually. Finland’s nuclear waste management programme was accepted by people because the government took them into confidence at every stage. Finland demonstrates that nuclear waste can be managed safely. This issue need not come in the way of harnessing nuclear power.