Nuclear energy harnesses the energy released during the splitting orfusing of atomic nuclei. This heat energy is most often used to convert waterto steam, turning turbines, and generating electricity. However, nuclear energy also has many disadvantages.
An event thatdemonstrated 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 andcaused the world’s worst known reactor disaster to date. An experiment that wasnot 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 millioncuries of radionuclides were released into the atmosphere. Some of theradiation spread across northern Europe and into Great Britain. Sovietstatements indicated that 31 people died because of the accident, but the numberof radiation-caused deaths is still unknown.
The same deadly radiation that was present in this explosion is alsopresent in spent fuels. This presents special problems in the handling, storage,and disposal of the depleted uranium. When nuclear fuel is first loaded into areactor, 238U and 235U are present. When in the reactor, the 235U is graduallydepleted and gives rise to fission products, generally, cesium (137Cs) andstrontium (90Sr). These waste materials are very unstable and have to undergoradioactive disintegration before they can be transformed into stable isotopes.
Each radioactive isotope in this waste material decays at its characteristicrate. 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 neutronradiation).
Exposure to large doses of ionizing radiation causes characteristicpatterns of injury. Doses are measured in rads (1 rad is equal to an amount ofradiation that releases 100 ergs of energy per gram of matter). Doses of morethan 4000 rads severely damage the human vascular system, causing cerebral edema(excess fluid), which leads to extreme shock and neurological disturbancescausing death within 48 hours.
Whole-body doses of 1000 to 4000 rads cause lesssevere vascular damage, but they can lead to a loss of fluids and electrolytesinto the intercellular spaces and the gastrointestinal tract causing deathwithin ten days because of a fluid and electrolyte imbalance, severe bone-marrowdamage, and terminal infection. Absorbed doses of 150 to 1000 rads causedestruction of human bone marrow, leading to infection and hemorrhage death mayoccur after four to five weeks after the date of exposure.
Currently only theeffects of these lower doses can be treated effectively, but if untreated, halfthe 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 spentfuel continues to be a source of heat and radiation after it is taken from thereactor, it can be stored underwater in a deep pool at the reactor site. Thewater keeps the fuel assemblies cool and acts as a shield to protect workersfrom gamma radiation. The water is kept free of minerals that would corrode thefuel in tubes.
Fuel assemblies are kept separated in the pool by metal racks that leaveone foot between centers. This grid structure is made with metal containingboron, which helps to absorb neutrons and prevents their multiplication.
A problem with this type of storage is that in 1977, a federalmoratorium on reprocessing was instituted. This required the utility companiesto keep used fuel at the reactor site. This requirement was met by buildingcloser-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 fordisposal within five years of production. There are different approaches tosolidification. These include calcination, vitrification, and incorporation ofwaste into ceramics and synthetic materials. Calcination is a process in whichthe liquid waste is sprayed through an atomizer and then dried at a hightemperature. This results in calcine (which is highly radioactive) andtemporarily stored in bins for further processing. Vitrification consists of the mixing of calcined waste with borosilicateglass grit. This is melted in a specialized furnace and cast into a mold.
Borosilicate glass is considered a suitable matrix for nuclear waste because theglass has strong interatomic bonding but not a strict atomic structure. Becauseof this, it is able to contain a variety of different elements. Under runningor standing water, radioactive products leak out at a very slow rate. Inaddition, the glass is resistant to structural damage from radiation. Anotherway to encapsulate the waste is through crystalline ceramics. The ceramicmatrix is a substance that crystallizes into an ordered atomic structure thatcan be altered to suit specific types of wastes and geochemical condition.
Radioactive products leak very slowly from this type of structure as well, andthe 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. Designand safety considerations for these containers include radiation levels, effectsof temperature, wind, tornado, fire, lightning, snow and ice, earthquake, andaircraft crash. One of these containers is called the CASTOR V/21. This is acylindrical container is cast iron 16 feet tall, about 8 feet in diameter, andwith walls of 15 inches.
It has fins on its outside to help disperse thetemperature of decay. This container holds 21 fuel assemblies. These types ofcontainers are relatively low in cost compared to storage in a pool of water andcan be moved around if necessary. Another way to dispose of radioactive wastesis through geologic isolation. This is the disposal of wastes deep within thecrust of the earth. This form of disposal is attractive because it appears thatwastes can be safely isolated from the biosphere for thousands of years orlonger. Disposal in mined vaults does not require the use of advancedtechnologies, rather the application of what we know today. It is possible tolocate mineral, rock, or other bodies beneath the surface of the earth that willnot be subject to groundwater intrusion.
A preferred place would be at least1,500 feet below the earth’s crust, so that it may avoid erosion for thespecified period of time. None of the preceding methods offers a completesolution to the problem of nuclear waste. They only bury it, temporarilyshoving it out of our current view for a latter generation to solve. Maybe thefuture inhabitants of this world will find a solution to this problem, for as wechose to continue the use of nuclear power, more and more waste will beaccumulated, emitting deadly radiation long after we pass away.
