Nuclear Waste Management

Nuclear Waste management Nuclear energy harnesses the energy released during the splitting or fusing of atomic nuclei. This heat energy is most often used to convert water to steam, turning turbines, and generating electricity. However, nuclear energy also has many disadvantages. An event that demonstrated this was the terrible incident at Chernobyl'. Here on April 26, 1986, one of the reactors of a nuclear power plant went out of control and caused the world's worst known reactor disaster to date. An experiment that was not properly supervised was conducted with the water-cooling system turned off. This led to the uncontrolled reaction, which in turn caused a steam explosion. The reactor's protective covering was blown off, and approximately 100 million curies of radionuclides were released into the atmosphere. Some of the radiation spread across northern Europe and into Great Britain. Soviet statements indicated that 31 people died because of the accident, but the number of radiation-caused deaths is still unknown. The same deadly radiation that was present in this explosion is also present in spent fuels. This presents special problems in the handling, storage, and disposal of the depleted uranium. When nuclear fuel is first loaded into a reactor, 238U and 235U are present. When in the reactor, the 235U is gradually depleted and gives rise to fission products, generally, cesium (137Cs) and strontium (90Sr). These waste materials are very unstable and have to undergo radioactive disintegration before they can be transformed into stable isotopes. Each radioactive isotope in this waste material decays at its characteristic rate. A half-life can be less than a second or can be thousands of years long. The isotopes also emit characteristic radiation: it can be electromagnetic (X-ray or gamma radiation) or it can consist of particles (alpha, beta, or neutron radiation). Exposure to large doses of ionizing radiation causes characteristic patterns of injury. Doses are measured in rads (1 rad is equal to an amount of radiation that releases 100 ergs of energy per gram of matter). Doses of more than 4000 rads severely damage the human vascular system, causing cerebral edema (excess fluid), which leads to extreme shock and neurological disturbances causing death within 48 hours. Whole-body doses of 1000 to 4000 rads cause less severe vascular damage, but they can lead to a loss of fluids and electrolytes into the intercellular spaces and the gastrointestinal tract causing death within ten days because of a fluid and electrolyte imbalance, severe bone-marrow damage, and terminal infection. Absorbed doses of 150 to 1000 rads cause destruction of human bone marrow, leading to infection and hemorrhage death may occur after four to five weeks after the date of exposure. Currently only the effects of these lower doses can be treated effectively, but if untreated, half the perso ns receiving as little as 300 to 325 rads to the bone marrow will die. To store the nuclear waste products that give off this deadly radiation, many precautions must be taken. Spent fuel may be stored or solidified. The primary way of storing the nuclear waste is storage. Since spent fuel continues to be a source of heat and radiation after it is taken from the reactor, it can be stored underwater in a deep pool at the reactor site. The water keeps the fuel assemblies cool and acts as a shield to protect workers from gamma radiation. The water is kept free of minerals that would corrode the fuel in tubes. Fuel assemblies are kept separated in the pool by metal racks that leave one foot between centers. This grid structure is made with metal containing boron, which helps to absorb neutrons and prevents their multiplication. A problem with this type of storage is that in 1977, a federal moratorium on reprocessing was instituted. This required the utility companies to keep used fuel at the reactor site. This requirement was met by building closer-packed racks to store more fuel in the same amount of space. An alternative way of storing spent fuel is through solidification. Federal regulations require that liquid reprocessing waste be solidified for disposal within five years of production. There are different approaches to solidification. These include calcination, vitrification, and incorporation of waste into ceramics and synthetic materials. Calcination is a process in which the liquid waste is sprayed through an atomizer and then dried at a high temperature. This results in "calcine" (which is highly radioactive) and temporarily stored in bins for further processing. Vitrification consists of the mixing of calcined waste with borosilicate glass grit. This is melted in a specialized furnace and cast into a mold. Borosilicate glass is considered a suitable matrix for nuclear waste because the glass has strong interatomic bonding but not a strict atomic structure. Because of this, it is able to contain a variety of different elements. Under running or standing water, radioactive products leak out at a very slow rate. In addition, the glass is resistant to structural damage from radiation. Another way to encapsulate the waste is through crystalline ceramics. The ceramic matrix is a substance that crystallizes into an ordered atomic structure that can be altered to suit specific types of wastes and geochemical condition. Radioactive products leak very slowly from this type of structure as well, and the crystalline structure continues to exist even if the ceramics break down. Dry storage of spent fuel has the advantage of avoiding the need for water pools. Containers are easily made, and very little maintenance is required. Design and safety considerations for these containers include radiation levels, effects of temperature, wind, tornado, fire, lightning, snow and ice, earthquake, and aircraft crash. One of these containers is called the CASTOR V/21. This is a cylindrical container is cast iron 16 feet tall, about 8 feet in diameter, and with walls of 15 inches. It has fins on its outside to help disperse the temperature of decay. This container holds 21 fuel assemblies. These types of containers are relatively low in cost compared to storage in a pool of water and can be moved around if necessary. Another way to dispose of radioactive wastes is through geologic isolation. This is the disposal of wastes deep within the crust of the earth. This form of disposal is attractive because it appears that wastes can be safely isolated from the biosphere for thousands of years or longer. Disposal in mined vaults does not require the use of advanced technologies, rather the application of what we know today. It is possible to locate mineral, rock, or other bodies beneath the surface of the earth that will not be subject to groundwater intrusion. A preferred place would be at least 1,500 feet below the earth's crust, so that it may avoid erosion for the specified period of time. None of the preceding methods offers a complete solution to the problem of nuclear waste. They only bury it, temporarily shoving it out of our current view for a latter generation to solve. Maybe the future inhabitants of this world will find a solution to this problem, for as we chose to continue the use of nuclear power, more and more waste will be accumulated, emitting deadly radiation long after we pass away.