The full nuclear fuel cycle shows that nuclear is a renewable energy source, because the spent fuel can be reprocessed to recover unburned uranium and plutonium that can be fabricated into new reactor fuel. At present, the U.S. nuclear is once through, going from spent fuel to interim storage and then longer-term storage.



It would take 2 million grams of oil or 3 million grams of coal to equal the power contained in 1 gram of uranium fuel.2 Unlike oil and coal, nuclear fuel is recyclable and, in a breeder reactor, can actually produce more fuel than is used up! For these reasons, nuclear energy is by far the best means now available to power a modern industrial economy.

Nuclear power is a gift to humanity, and only the propaganda of Malthusian extremists, dedicated to stopping human progress and reducing the worlds population, has created public fear and skepticism.

The best way to overcome irrational fear is through knowledge. To this end, reviewed here is the process by which natural uranium ore is turned into fuel for a nuclear reactor, how it is used, and how it can be recycled, such that the reader will come to understand that there is really no such thing as nuclear waste.

The Nuclear Fuel Cycle

DOE

An overhead view of rows of centrifuge units at a U.S. enrichment plant in Piketon, Ohio.

To understand the renewability of nuclear fission fuel, we have to look at the complete fuel cycle. At the beginning of the nuclear age, it was assumed that nations would complete the fuel cycleincluding the reprocessing of spent nuclear fuel from reactors, to get as near to 100% use of the uranium fuel as possible. Here we very briefly review the seven steps of this cycle. Keep in mind that the brevity of description leaves out details of the complex chemical processes, which were initiated during the Manhattan Project and are still being improved on.

1. First, natural uranium is mined. There are enough sources of uranium worldwide for todays immediate needs, but once we begin an ambitious nuclear development program (to build 6,000 nuclear reactors in order to provide enough electricity to bring the entire world population up to a decent living standard), we would have to accelerate the development of fast breeder nuclear reactors, which produce more fuel than they consume in operation.

2. Next, the uranium is processed and milled into uranium oxide (U 3 O 8 ), called yellowcake, which is the raw material for fission fuel. Yellowcake became infamous in the political fabrication that Saddam Husseins Iraq was trying to import yellowcake from Niger, in order to use it for bomb-making.

It is basically natural uranium ore, which is crushed and processed by leaching (with acid or carbonate) to dissolve the uranium, which can then be extracted and concentrated to 75% uranium, in combination with ammonium or sodium-magnesium.

3. The concentrated uranium is then converted into uranium hexafluoride (UF 6 ), which is heated into a gas form suitable for enrichment.

Uranium Enrichment

4. Natural uranium has one primary isotope, U-238, which is not fissionable, and a much smaller amount of U-235, which fissions. Because most uranium (99.276%) is U-238, the uranium fuel must go through a process of enrichment, to increase the ratio of fissionable U-235 to the non-fissionable U-238 from about 0.7% to 3 to 4%. (Weapons uranium is enriched to about 93% U-235.)

The technology of enrichment was developed during the World War II Manhattan Project, when the object was to create highly enriched uranium (HEU) to be used in the atomic bomb. Civilian power reactors use mostly low-enriched uranium (LEU). (Canada has developed a type of reactor, the CANDU, which uses unenriched, natural uranium in combination with a heavy water moderator to produce fission.)

Frank Hoffman/DOE

The huge Gaseous Diffusion Plant in Oak Ridge, Tenn., the first such facility in the world. The U-shaped building, constructed during the Manhattaan Project, began operation in 1945. Later, the facility was expanded to produce enriched uranium for plants around the world.

The gaseous diffusion method of enrichment, which is still used by the United States, was developed under the Manhattan Project. Uranium hexafluoride gas is pumped through a vast series of porous membranesthousands of miles of them. The molecules of the lighter isotope (U-235) pass through the membrane walls slightly faster than do the heavier isotope (U-238). When extracted, the gas has an increased content of U-235, which is fed into the next membrane-sieve, and the process is repeated until the desired enrichment is reached. Because the molecular speeds of the two uranium isotopes differ by only about 0.4%, each diffusion operation must be repeated 1,200 times.

The Manhattan Project devised this method of gaseous diffusion with incredible speed and secrecy. It was not finished in time to produce all the uranium for the uranium bomb dropped on Japan, but it produced most of the enriched uranium for the civilian and military programs in subsequent years. Although a successful method, it required a tremendous amount of energy and a huge physical structure to house the cascades of separate membranes. Four power plants were built in Oak Ridge, Tenn., to power the process, producing as much electric power as the consumption of the entire Soviet Union in 1939! Almost all the power consumed in the diffusion process is used to circulate and compress the uranium gas.

Technological pessimists take note: At the time the gaseous diffusion plant was being built, scientists had not yet figured out how to make a membrane to be used in the processbut they did it in time to make it work!

The centrifuge system, used in Europe and Japan, is 10 times as energy efficient. The strong centrifugal field of a rotating cylinder sends the heavier isotope in uranium hexafluoride to the outside of the cylinder, where it can be drawn off, while the U-235 diffuses to the inside of the cylinder. Because of the limitations of size of the centrifuge, many thousands of identical centrifuges, connected in a series called a cascade, are necessary to produce the required amounts of enriched uranium.

A centrifuge plant requires only about 4% of the power needed for a gaseous diffusion plant, and less water is needed for cooling.

Other methods of enrichment are possibleelectromagnetic separation, laser isotope separation, and biological methods.

Fabrication Into Fuel Rods

U.S. AEC

A cylinder of uranium hexaflouride enriched in U-235 is readied for shipment to a conversion facility, where it will be converted to uranium dioxide for use in fuel rods. The cylinder weighs 2.5 tons. Westinghouse Photo

A partially completed nuclear fuel assembly. The long tubes guide the control rods in the reactor, which regulated its operation. The grids that hold the guide sheaths also align the fuel rods containing uranium pettets. When the fuel rods are inserted through the grids, parallel to the guide sheath, the fuel assembly will be completed.

5. Once the enriched uranium is separated from the depleted uranium, it is converted from UF 6 into uranium dioxide and fabricated into uniform pellets. The pellets are loaded into long tubes made out of a zirconium alloy, which captures very few neutrons. This cladding prevents the release of fission products and also transfers the heat produced by the nuclear fission process in the fuel. The fuel is then transported to the reactor site.

Different types of reactors require different designs of fuel rods and fuel bundles. In a light water reactor, the fuel rods are inserted into the reactor to produce fission, which creates steam, which turns a turbine that creates electricity.

The fuel for the next-generation high-temperature gas-cooled reactors is different: The enriched uranium is formed into tiny pebbles which are coated with graphite and special ceramics that serve as individual containment buildings for the fuel pebbles.

6. Fuel rods are used for about four and a half years before replacement, and usually a reactor replaces about a third of its fuel at one time. The fuel is considered spent when the concentration of fissile uranium-235 becomes less than 1%. When removed from the reactor, the spent fuel is put into cooling pools, which shield it as its short-lived nuclides decay. Within a year, the total radioactivity level is only about 12% of what it was when the fuel rod came out of the reactor.

At present, the United States does not reprocess spent fuel, and so the spent fuel rods sit in cooling pools at the reactor. After the spent fuel has cooled, it is stored in dry casks, waitingfor burial or reprocessing.