Uranium, Plutonium, and Nuclear Energy Conversion Process

Generating electricity through nuclear power is an enormously complex technical feat. It takes the combined skills of geologists, mine operators, engineers, and scientists, as well as large numbers of highly trained and skilled plant operators. The federal government oversees the construction and operation of these plants to make sure that they are built and operated to the very highest standards. Uranium and plutonium are the main radioactive component for nuclear power, which we will discuss bellow.

Uranium

Producing nuclear power begins with the fuel, uranium. In the Earth, radioactive decay of uranium is the planet’s main source of internal heat. Uranium is used primarily in the nuclear industry, but it has other uses as well. Because it is a dense, heavy element (18.7 times as dense as water), it is sometimes used in the keels of boats as a weight to keep them upright. Its density also makes it useful as a counterweight in such applications as airplane rudders, and it makes a good radiation shield.

Uranium Atom Fission

Uranium is the heaviest naturally occurring element. It has sixteen different isotopes, although the most common ones are U235 and U238.U234 is found in trace amounts and results from the decay of U238. From the standpoint of nuclear energy, the important of uranium isotopes is U235.

The nucleus of a U235 atom consists of 92 protons and 143 neutrons. This is the uranium isotopes whose atoms can be split relatively easily. When a U235 atom is struck by a neutron, the atom splits and then releasing energy. In a nuclear reactor, the released energy is at first in a form of potential kinetic energy. Kinetic energy is the energy contained in anything (such as water, wind, or a neutron) that is in motion. But sub-microscopic particles travel only tiny distances, so the potential kinetic energy is rapidly converted to heat (similar to the way the brakes on a car get hot when they stop the kinetic energy of a moving car). This heat is then used to produce steam, which turns a generator to produce electricity. Heat makes up about 85 % of the energy released.

Further, not every neutron that hits a uranium atom causes fission. Sometimes the neutrons are absorbed by the atoms they strike, so no fission takes place. Other neutrons simply escape and do nothing.

The challenge for nuclear engineers is to keep the ongoing fission reaction in precise balance. When the reaction is in balance, scientists say that it has reached ‘‘criticality.’’ At criticality, the neutrons are doing their work in balance, meaning that their numbers remain constant and under control. The pace of the reaction can be speeded up or slowed down by increasing or decreasing the number of neutrons. If the increase is too rapid, the reaction can almost instantaneously get out of control.

Uranium: From the ground to the reactor

While uranium can be found in seawater, it is found most commonly in rocks and is as common as the elements tin and gold. It exists in concentrations of about two to four parts per million. Uranium is mined in at least two ways. One is to dig up the ore that contains it, crush the ore, and then treat it with acid, which dissolves the uranium to remove it from the ore. The other is a process called in situ leaching (in situ is Latin for ‘‘in place’’). In this process, the uranium is dissolved from rock and pumped to the surface of the Earth. Either way, the end result is a compound called uranium oxide, or U3O8. This material is often referred to as ‘‘yellowcake.’’

The uranium, though, cannot be used as fuel in this form. It first has to be ‘‘enriched,’’ so mine operators sell the yellowcake to uranium enrichment plants. The first step in converting it into a usable fuel is to convert it into a gas, uranium hexafluoride, or UF6. This increases the amount of uranium from its natural level of 0.7 percent to 3 to 4 percent, so the uranium is said to be ‘‘enriched.’’ The next step is to convert the uranium hexafluoride to uranium dioxide, or UO2. Uranium dioxide can then be processed into pellets that are about the size of a knuckle on a person’s finger. The pellets are then inserted into thin, 12-foot-long (3.5- meter-long) metal tubes, called fuel rods. Bundles of these tubes are then inserted underwater into the core of the nuclear reactor.

Plutonium and Conversion Process

Plutonium is an element that forms in a reactor core as the isotope Pu239.It forms when U238, which is also present in nuclear fuel, absorbs a neutron. Now the atom has an odd number of particles in the nucleus, making it fissile in the same way that U235. But like uranium isotopes—U235, it sometimes just absorbs the neutron, creating the isotope Pu240, which is not fissile. Over time, the amount of Pu240 builds up in the fuel rods. When the rods are ‘‘spent,’’ or no longer usable as fuel, this plutonium can be recycled. It undergoes a conversion process that makes it usable as nuclear fuel. Not all nuclear reactors are designed to allow this recovery and conversion process. Those that do are called ‘‘breeder reactors,’’ for they ‘‘breed,’’ or produce, additional fuel.

Plutonium is perhaps the most highly toxic substance that exists. The smallest amount can cause such diseases as lung cancer. Workers who handle plutonium observe the strictest safeguards to avoid exposure.