Nonrenewable Sources With Short Service Life Burner Reactor Nuclear Energy

The current nuclear power plants produce energy by causing uranium atoms to break into two atoms of elements each about half the atomic weight of uranium. This process is called fission. When uranium atoms fissions into two lighter elements the nuclear binding energy of the two lighter elements is greater than the binding energy of the single uranium atom. This difference in energy is converted to heat. U2,113

The uranium is consumed in the fission reaction. Energy generation by burning fossil fuels consumes fossil fuel chemicals and converts them to harmful combustion products. Nuclear reactors "burn" uranium and convert it to harmful fission products. Unlike fossil fuel materials, uranium has little other use than for the production of energy. Like fossil fuels, there is a finite supply of the minerals used to produce the uranium for the fuel cycle. The worldwide amount of potential energy available by use of the burner reactor cycle is similar to that available from oil. If used at a high level the supplies of burner reactor uranium could be depleted in the middle of the next century. 114

There are many different types of reactors. In the United States, the majority of the reactors are pressurized water reactors with graphite moderators. The Canadians built the CANDU reactor using heavy water as both moderator and coolant. Naval ship reactors are graphite moderated liquid metal cooled reactors. The detailed differences between the reactor types will not be examined, but the operating principal common to all will be discussed. 115

Uranium occurs as two types of atoms (isotopes), U235 and U238. Isotopes are atoms of a chemical element that have different atomic weights. The superscript gives the atomic weight. From natural uranium, only U235 can be fission to release energy. Isotopes that can fission to produce energy are termed a "fertile" isotope. Natural uranium contains only 0.7% fertile U235. To make uranium suitable for a power reactor the amount of U235 must be increased, or enriched. Because U235 and U238 are isotopes of the same element, they have identical chemical properties. The enrichment requires the use of subtle separation processes that act on the slight difference in weight of the two atoms.

Enrichment is a very complex, energy consuming process. The amount of enrichment required is dependent on the design of the reactor; it can range from 1.5% to 85%. Three processes have been used for enrichment. Some of the very first separations were made with a device called a Calutron. The uranium was ionized in a vacuum chamber. The ions were fired through a magnetic field where they were separated based on their mass. This process was very energy intensive and slow. It was abandoned in favor of gaseous diffusion and centrifugal separation. Both processes use the slight

110 Editors, "Power Buoys", The Economist, May 19, 2001, Page 78

111 Hogerton, John F., "The Arrival of Nuclear Power", Scientific American, Vol. 218, No. 2, February 1968, Page 21

112 I-ester, Richard K., "Rethinking Nuclear Power?" Scientific American, Vol. 254, No. 3, March 1986, Page 31

113 Lewis, Harold W., "Safety of Fission Reactors", Scientific American, Vol. 242, No. 2, March 1980, Page 53

114 Bethe, Hans A., " The Necessity of Fission Power", Scientific American, Vol. 234, No. 1, January 1976, Page 21

115 Mclntyre, Hugh C., "Natural Uranium Heavy-Water Reactors", Scientific American, Vol. 233, No. 4, October 1975, Page 17

difference between the atomic weight of the two isotopes to effect separation. Light atoms diffuse more rapidly through small holes than heavy atoms. When gaseous uranium hexafluoride is allowed to diffuse through a porous barrier the U235 passes through the barrier more rapidly. Each time the hexafluoride diffuses through the barrier the U235 is enriched. In a high-speed centrifuge, the lighter atoms are concentrated in the center and the heavier at the rim. In both of these processes, it requires many stages of separation to achieve relatively pure U235. When a large enough quantity of U235 is assembled it becomes critical mass and energy is released by a chain reaction. This places severe constraints on the design of the separation hardware. At all stages of the separation, great care must be taken to ensure that a critical mass of U235 is not accumulated in one place.

Several years ago, there was a lot of excitement concerning a method of using lasers to separate isotopes. A laser was adjusted to a very narrow wavelength that would excite only one of the uranium isotopes. With only one isotope excited it was projected that a near single stage separation method was possible. Little has been said about this method in recent years. The lack of the need for uranium separation has placed a damper on the developments. 116

The heart of the energy generation process is the same in all reactors. A critical mass of uranium is assembled in a tank with a moderator. One U235 atom fissions into two lighter atoms and several high energy (high velocity) neutrons. A moderator slows down the neutrons without reacting with them. U235 reacts best with slow neutrons. The neutrons strike other U235 atoms, causing further fission, and the chain reaction is sustained. The tank is equipped with a heat exchanger for removing the heat produced by the uranium fission. The amount of uranium required depends on the power rating of the reactor, the enrichment level of the uranium and the types of control system used.

The rate of the reaction is modulated by controlling the number and energy of the neutrons allowed to stay in the uranium filled core of the reactor. Control rods are used to modulate the nuclear reaction rate. Control rods are made from an element (cadmium metal is often used) that strongly adsorbs neutrons. The rods are installed in channels in the reactor. When the rods are fully inserted in the reactor, so many neutrons are adsorbed that little reaction can occur. As the rods are withdrawn, more and more neutrons can react and the reactions begin. The reaction rate is controlled by the depth and number of rods inserted in the reactor.

When the uranium atom is broken apart, two new atoms and several neutrons are released. The new atoms and neutrons, taken together, weigh slightly less than the weight of the original uranium atom. The mass involved in the weight change is converted to energy. The energy is recovered as heat to drive the generators. The new atoms remain in the fuel elements. The neutrons go on to cause further fissions or are adsorbed by the atoms of the control rods or the reactor structure. Some of the atoms produced are of the same type of strongly neutron adsorbing elements used in the control rods. Virtually all are radioactive. The atoms of the construction materials also become radioactive when they adsorb neutrons. Because of the consumption of U235 and the build-up of neutron adsorbing fission products, the fuel elements are ultimately unable to produce further useful energy. When they reach this state, they are removed and replaced with new fuel elements. The used fuel elements are the source of many of the problems with fission nuclear power.

Used fuel elements still contain significant amounts of potentially useful U235. They contain plutonium (several different isotopes), a synthetic element that can be used to produce energy in essentially the

116 http://www.llnl.gov/str/Hargrove.html

same manner as U235. The fission products in the fuel elements are intensely radioactive and will remain so for thousands of years. Storage of the used fuel elements is costly because of the high level of protection required. It also wastes the valuable U235 and plutonium. Handling and disposition of waste fuel elements presents a difficult environmental protection problem. The fuel element problem is a major barrier to more widespread utilization of nuclear power.117

Nuclear fission energy of the type currently in use has the potential to provide enough energy for the operation of civilization, but it presents much the same supply lifetime problem as fossil fuels. The waste products present a severe environmental problem. The problem is very different from that presented by fossil fuels but possibly more dangerous. Despite much criticism of the use of fission nuclear power, its use may be preferred to fossil fuels because of the lack of other peaceful use for uranium and the fact that the waste products can be confined. Remember, fossil fuels wastes are not confined. They are dispersed through the ecosphere as acid rain and carbon dioxide.118

Despite the barriers to the increase use of fission energy, it is being given serious consideration by energy planners. James A. Lake, of the United States Department of Energy Idaho National Engineering & Environmental Laboratory says, "The energy crisis has shined a spotlight right on us. We are sitting at a point where the potential for future contributions is enormous. There are 103 reactors in the United States and 438 Worldwide, and people are thinking there should be 4,000 in the next 20 years. 119

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