Facts of the Matter
Creating nuclear energy is a complex task
Energy is released during nuclear fission because the mass of all of the fragments does not equal the mass of the original atom.
According to Einstein's famous "E equals mc-squared" equation, the amount of energy released per unit mass is huge.
One kilogram (2.2 earth pounds) of matter converted to energy would yield 25 billion kilowatt-hours, enough to power the entire state of Hawaii for nearly two years.
Uranium is the most massive of naturally occurring atoms, a mixture of three isotopes: Uranium-238 accounts for about 99.3 percent; uranium-235 is about one atom of every 140; uranium-234 is one of every 18,000.
When a uranium atom splits, or fissions, the missing mass equates to a relatively large amount of energy.
The energy available from fission of a gram of uranium-235 is equivalent to 145 barrels of crude oil. Put in more familiar terms, that is equivalent to 66,000 barrels of crude for 1 pound of uranium, the explosive energy of 18 million pounds of TNT.
The story of fission began with the discovery in December 1938 that neutrons bombarding uranium can cause fission. Shortly thereafter, scientists studying the reaction made two important discoveries.
Fission had occurred only in the U-235 isotope; the U-238 and U-234 had not taken part in the reaction.
During fission, each U-235 atom absorbs a neutron and releases two or three neutrons. This makes possible a chain reaction in which the neutrons released from one fission lead to multiple subsequent fissions.
Not all absorbed neutrons cause fission, and the weights of the fission fragments are not always the same, even for the same nucleus.
Fission results from altering the stability of the nucleus, where neutrons acts as mediators against the repulsive forces exerted on one another by positively charged protons.
Uranium nuclei are in a quasi-stable state because of the interaction of large number of protons (92 in U-238) and neutrons (146).
They are radioactive and will eventually emit electrons, positrons, or alpha particles, and gamma radiation in a regular and specific sequence.
A captured neutron in the U-235 nucleus upsets the balance between protons and neutrons, and so the nucleus flies apart into two smaller nuclei. These pick up roving electrons to be atoms of barium and krypton, or strontium and xenon.
Whether the nucleus absorbs a neutron depends on the speed of the neutron and the neutron absorption cross-section of the nucleus.
The cross-section is a description of how big the nucleus "looks" to a neutron. It is different for different nuclei and is larger for slow-moving neutrons than for fast ones.
U-238 has a small cross-section and fissions only if it captures a fast, energetic neutron.
If the captured neutron's energy is below the fission threshold for U-238, the nucleus absorbs it and then spits out two electrons, leaving 94 protons and 145 neutrons in the new nucleus of plutonium-239.
A neutron with energy above the threshold fissions the U-238 atom, but the neutrons it releases are too slow to induce further fission.
No chain reaction is possible using U-238 alone.
U-235 has a larger capture cross-section, and unlike U-238 it fissions by absorbing slow neutrons and also emits slow neutrons.
A chain reaction can sustain itself only if sufficient neutrons are available for capture. This requires a critical mass of uranium, the amount of which depends on the U-235 concentration and the geometry of the uranium material.
In an uncontrolled chain reaction, emitted neutrons and energy increase exponentially until the reaction releases energy in a huge explosion.
By contrast, the reactor in a nuclear power plant uses a controlled chain reaction to produce heat from uranium that is slightly enriched in U-235.
The reactor must be able to sustain a chain reaction, and so it requires a higher than natural concentration of U-235 to provide enough neutrons, around 5 percent.
The 95 percent U-238 captures slow neutrons and has two effects.
It reduces the number of neutrons that are available for capture by U-235 atoms, which slows the chain reaction and transmutes them into plutonium-239, which can, like U-235, sustain chain reactions with slow neutrons.
This breeder reactor actually produces nuclear fuel as a waste product.
The process of enriching U-235 requires sophisticated equipment, a lot of energy and lots of nuclear engineering knowledge and experience.
Gaseous diffusion and centrifuge are the two most common enrichment methods. Both require uranium compounded with fluorine as uranium hexafluoride (UF6), which is solid at room temperature but sublimes directly to the gas at 134 degrees Fahrenheit.
Despite its obnoxious properties, UF6 is the only uranium compound that is volatile enough for the gaseous diffusion process, and yet can be shipped as a solid.
The diffusion plant must be kept above that temperature to retain the gaseous state during processing. Gaseous UF6 reacts strongly with water and is a powerful corrosive and oxidizing agent.
U-235 and U-238 have nearly identical properties, which vary minimally by atomic weight, and there is only about 1 and one-fourth percent difference in atomic weight between the two isotopes.
Both common enrichment methods use a number of identical stages in a cascading sequence. Each stage produces slightly enriched U-235 by concentrating the product of a previous stage before it moves to the next stage.
In the gaseous diffusion process, UF6 under pressure diffuses through a semipermeable membrane. Gaseous UF6 molecules that contain the lighter U-235 have a higher thermal velocity and so collect in a higher concentration on the downstream side of the membrane.
The stream that is slightly enriched in U-235 is withdrawn and fed into the next higher stage, while the depleted stream is recycled back into the next lower stage to be reprocessed.
Enrichment cascades can require thousands of stages, depending on the desired amount of enrichment. It takes as many as 1,750 stages to produce a 5 percent U-235 concentration.
The gaseous diffusion has been largely replaced by the gas centrifuge, which uses far less energy to produce the same enrichment.
The gas centrifuge uses a large number of rotating cylinders in series and parallel formations that are interconnected to form trains and cascades.
In this process, UF6 gas is rotated at a high speed in cylinders, forcing the heavier gas molecules (containing U-238) toward the outside of the cylinder, while the lighter gas molecules congregate closer to the center.
The depleted gas is returned to the previous stage while the enriched gas moves to the next stage.
Although the capacity of a centrifuge stage is much smaller than that of a single diffusion stage, it can separate isotopes with greater efficiency and might require only 15 stages to reach 5 percent U-235 concentration.
Nuclear weapons typically contain 85 percent or more of U-235, but a crude, inefficient weapon might work at 20 percent. It could be that even less is sufficient, but then the critical mass would be proportionally greater.
Even with the high efficiency of the gaseous centrifuge method, enrichment for even a crude 20 percent bomb would require 75 stages, and an ideal 90 percent would need 340.
The technology to link that many centrifuge stages is complex and can take many years to achieve.
Yet nuclear engineering is a rapidly evolving field, and it is the speed at which the technology travels that is the concern of those who fear the aggressive, vindictive, defensive or terroristic use of nuclear weapons.
picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be reached by e-mail at firstname.lastname@example.org