A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. • The most common use of nuclear reactors is for the generation of electric energy and for the propulsion of ships. • Fermi’s “atomic pile” is not actually the first nuclear reactor on Earth. In 1972, physicists measuring samples of uranium ore in Gabon, West Africa stumbled upon the existence of a natural nuclear reactor. • Uranium ore deposits are economically recoverable concentrations of uranium within the Earth’s crust. Uranium is one of the more common elements in the Earth's crust, some 40 times more common than silver and 500 times more common than gold. • As is the case with all natural uranium, the material under study contained three isotopes— that is to say, three forms with differing atomic masses: U-234, the rarest variety; uranium 238, the most abundant variety and uranium 235, the isotope that is coveted because it can sustain a nuclear chain reaction. U-235 and U-238 have the ratio of 1:40. • The Gabon sample contained an unusually low proportion of U-235. • It is believed that water trapped in the surrounding sandstone may have acted as moderator and that the natural reactors operated for a relatively short time.
What made such a thing possible was that in the distant past uranium was naturally enriched in U-235, that is, less of it had decayed away by nuclear fission. About 1.7 billion years ago, to be more precise, a natural deposit of uranium ore was radioactive enough to generate about 100 kilowatts of heat, off and on, for more than a million years. • Physicists learn from the natural reactors in Oklo, Gabon that nuclear reactor that radioactive wastes take a very long time to disappear. • The Oklo reactor operated for only a million years and took another 2000 million years before the radioactive wastes escaped from the reactor site. • At Oklo there was a lot of water, probably a river flowing above the buried ore body. The water allowed the nuclear interactions to reach the critical point, and the reactor began to work. But as it heated up, the water turned to steam and flowed away. This simple feedback cycle kept the Oklo reactors (there were at least a dozen of them) active until the U-235 was depleted. That took about a million years. • The fact that large parts of the reactors containing the original waste were still largely intact after 2000 million years, this provides some hope that nuclear fission waste may be stored in suitable geological repositories.
Possible Nuclear Accidents • Nuclear accidents may still occur if the controls, interlocks, and warning systems malfunction. • The prime example of a "major nuclear accident" is one in which a reactor core is damaged and large amounts of radiation are released, such as in the Chernobyl Disaster in 1986. • It is considered the worst nuclear power plant accident in history, and is one of only two classified as a level 7 event on the International Nuclear Event Scale. • The disaster began during a systems test on Saturday, 26 April 1986 at reactor number four of the Chernobyl plant. There was a sudden power output surge, and when an emergency shutdown was attempted, a more extreme spike in power output occurred, which led to a reactor vessel rupture and a series of explosions.
These events exposed the graphite moderator of the reactor to air, causing it to ignite. The resulting fire sent a plume of highly radioactive smoke fallout into the atmosphere The plume drifted over large parts of the western Soviet Union and Europe • This estimates the number of deaths potentially resulting from the accident: Thirty one deaths are directly attributed to the accident, all among the reactor staff and emergency workers. • The World Health Organization (WHO) suggests it could reach 4,000 civilian deaths, a figure which does not include military clean-up worker casualties. • Since then, there have been up to ten deaths from thyroid cancer due to the accident although it is estimated that this number could increase.
Routine Radioactive Emissions • A nuclear power plant routinely passes some radioactivity to the air and to the water outside it. • Some fission products are released into the atmosphere. • The emissions of the most common forms of spontaneous radioactive decay are the alpha (α) particle, the beta (β) particle, the gamma (γ) ray, and the neutrino. • According to studies, current levels of routine emissions from nuclear power plants may shorten one’s life span, just as other causes do: being overweight, smoking, living in the city.
Radioactive Wastes • are wastes that contain radioactive material. Radioactive wastes are usually by-products of nuclear power generation and other applications of nuclear fission or nuclear technology, such as research and medicine. • The period of time waste must be stored depends on the type of waste. Low-level waste with low levels of radioactivity per mass or volume (such as some common medical or industrial radioactive wastes) may need to be stored for only hours, days, or months, while high-level wastes must be stored for thousands of years. • A typical 1000 MV thermal nuclear reactor produces around 30 tons of spent fuel rods.
Three general principles in the management of radioactive wastes: delay and decay, dilute and disperse and concentrate and contain. • Waste disposal and the radioactive curve • Current major approaches to managing radioactive waste have been segregation and storage for short-lived wastes, near-surface disposal for low and some intermediate level wastes, and deep burial or transmutation for the long-lived, high-level wastes. • The decay portion may need to wait up to 250,000 years.
Thermal Pollution • Thermal pollution is the degradation of water quality by any process that changes ambient water temperature. • A common cause of thermal pollution is the use of water as a coolantby power plants and industrial manufacturers. When water used as a coolant is returned to the natural environment at a higher temperature, the change in temperature decreases oxygen supply, and affects ecosystem composition. • When a power plant first opens or shuts down for repair or other causes, fish and other organisms adapted to particular temperature range can be killed by the abrupt change in water temperature known as "thermal shock.“
Submitted by: KathrinaMaliño Diana Mamawal Joanne Ngojo