So far we have focused on uranium and plutonium in our discussion of nuclear fuel and reactors because they are the fuels for most of the world's reactors. There are other nuclear fuels used in existing reactors and atomic batteries or suggested for use in new designs. One of these alternative nuclear fuels that holds great promise is thorium.
When spent fuel rods are removed from a nuclear reactor, they are giving off heat and emitting radiation, primarily from fission products. They are stored in special pools of water or boric acid to allow the heat and radiation to diminish. The cooling fluid absorbs the radiation and is circulated through heat exchangers to get rid of the heat. It can take several years for the heat and radiation to drop to a safer level.
After manufacture, nuclear fuel is transported from the production facility to a nuclear power plant for use in a reactor. Specialized transport companies transport nuclear fuel assemblies which release little radioactivity and do not require special shielding.
In a typical nuclear reactor, sets of fuel rods called cells surround a control rod which can be inserted or withdrawn to control the neutron flux and thus, the rate of the chain reaction.
U-235 atoms are bombarded by neutrons and fission which produces heat and more neutrons. Some of the U-235 transmutes into plutonium which also undergoes fission producing about one third of the heat in the reactor core. The heat from the core is used to produce steam which drives the turbines that produce the electricity.
As the nuclear fuel in the rods fissions, the heat generated causes thermal expansion which can cause cracking. The nuclear fuel reacts with cladding materials such zirconium alloy which forms the shell of the fuel rod. The chemical composition of the fuel near the edge of the pellet changes as does its thermal conductivity. The purer uranium oxide in the center of the pellet will reach higher temperatures than the fuel near the outer edge of the pellet.
One ton of natural uranium can generate about fourty four million kilowatt hours. It would require over twenty thousand tons of coal or eight million cubic meters of natural gas to generate the same amount of electricity.
The rate at which the fuel is consumed is measured in gigawatt-days per ton of fuel and it is proportional to the level of concentration of U-235 in the nuclear fuel contained in the rods. The level of heat generation that can be safely handled by the current reactors limits the enrichment to about four percent which will yield a burn up rate of fourty gigawatt-days per ton. With improvements in materials and design, enrichment as high as five percent can be utilized, ultimately producing seventy gigawatt-days per ton.
Only a third of the heat produced by the core is captured in steam production. The other two thirds of the heat is passed to the water of the cooling system and either released in into a body of water such as a large river or the ocean. Alternatively, the water may be sent into cooling towers for evaporative cooling. Normally, a small amount of radioactivity is released into the cooling water.
The progression of nuclear fuel through a series of stages of production, use and disposal is referred to as the nuclear fuel cycle. When the spent fuel is used once and then disposed of, the cycle is called open or once-through. When the spent fuel is reprocessed and used again, the cycle is called closed.
The common nuclear fuel uranium oxide also known as UOX is usually compressed and cooked into cylindrical ceramic pellets. These pellets manufactured to exacting standards and are machined to precise dimensions. The pellets are then sealed into long metal tubes called fuel rods
There are a number of different types of nuclear fuel in terms of what isotope is used, what other materials are mixed with the isotope and the physical configuration of the fuel. Nuclear reactors are designed to use a specific isotope in a specific shape
There are a number of different types of nuclear fuel in terms of what isotope is used, what other materials are mixed with the isotope and the physical configuration of the fuel. Nuclear reactors are designed to use a specific isotope in a specific shape
In order to control how much power is generated in a nuclear reactor, the number of neutrons available to cause nuclei to fission must be controlled. Control rods which absorb neutrons must be inserted into or withdrawn from the core in order to control the chain reaction.
A nuclear reactor is a complex device that is designed to start and sustain a controlled nuclear chain reaction. They are usually utilized to generate electrical power or to provide propulsion for ships and submarines. Controlled nuclear fission is used to heat either water or a gas which is then passed through a turbine. The turbine in turn either spins the propellers of a ship or the generators of an electrical power station.
The stagnation of the nuclear power industry extended into the 1990s with few new plants being ordered and many ordered plants being cancelled. However, during the 90s a third generation of power plants were being designed to replace the second generation plants constructed in the 70s and 80s. This new design moved the moderator rods to a different location in the plant in order to minimize leaks. Plants with the new design were ordered in the late 90s.
The Kyoto Protocols first signed by several nations in 1997 called for the reduction of carbon dioxide emissions by to 5% below the emissions in 1990. This demand for carbon dioxide reduction much of which comes from oil and coal fired power plants and the need to replace old second generation reactors which were scheduled for decommissioning led to pressure for the adoption of nuclear power. While nuclear power stations do not emit carbon dioxide, their construction involves huge quantities of cement which does emit a great deal of carbon dioxide.
The election of George W. Bush, an enthusiastic supporter of nuclear energy to the US Presidency stimulated renewed interest in the expansion of nuclear power generation. In 2004, Bush signed a resolution to allow the US Department of Energy to move forward on the construction of a long term nuclear waste depository at Yucca Mountain in Nevada. In 2005, Bush signed the first new US energy bill in more than a decade. The bill included funding for nuclear research, decommissioning of old nuclear power plants, tax credits and loan guarantees for the nuclear industry, and a liability cap in case of serious nuclear accidents. A number of consortia of companies were created to build more nuclear power plants.
In the first decade of the Twenty First Century, design evolution continued with the development of new passive nuclear power plants that relied on natural processes such as gravity, convection, evaporation and condensation to achieve cooling instead of active systems of pumps. Emergency cooling water pools were built above the core so that in an emergency, the water could be easily dropped into the core. Other reactor designs relied on balls of graphite instead of graphite rods or water and were gas cooled.
By 2007, there were over four hundred nuclear power reactions in the world in thirty one countries. France, Japan and the U.S. accounted for over fifty percent of nuclear generated electricity. As of 2010, China had over twenty five reactors under construction with plans to build many more. In the United States almost half of the operating reactors have had their licenses extended to sixty years.
Worldwide, there are currently four hundred and thirty six nuclear reactors generating three hundred and seventy gigawatts of electrical power. Another sixty three are under construction with a combined capacity of sixty gigawatts.
The terrible nuclear accident at Fukushima where a tsunami caused by an earthquake destroyed four reactors a coastal nuclear power plant has caused many countries to reevaluate their reliance on nuclear power generation and their plans for expansion. Most of the power plants in Japan have been shut down for maintenance and then not restarted. Germany has decide to phase out all nuclear power plants by 2022 and Italy has banned nuclear power. The International Atomic Energy Commission has cut its estimated of new nuclear power generation by 2035 in half. However, in the United States, two new reactors were recently licensed for construction in Georgia and there are orders for more reactors.
