Physics & Politics

Nuclear Power

Introduction

Nuclear power generation is very similar to other forms of power generation. (Currently, the only operating nuclear power generators use fission, but scientists and engineers are working on three other types of processes that use fusion as the reaction source. These include Tokamaks, Laser Energetic Devices, and, most recently, Sonoluminescence Cells. Also, see Physics Today article.) In all of these power plants some type of fuel reaction results in creation of "heat" and, of course, spent by products. Often, the heat is used to turn a turbine that drive an engine in order to generate electricity, which is much easier to transport from the power station to the end user. So, to better understand nuclear power generation, let us first study how a conventional hydro, wind, gas, diesel fuel, or coal power plant generates electricity.

Electric Power Plants

The physics of electric power generation goes back to the discovery, by Henry and Faraday, that changing magnetic flux creates an electric potential that can cause electricity - flow of electrons (electric current) in a conductive wire. In the physics circles, this is known as Faraday's Law of Induction. Basic electric engines, dynamos, use permanent magnets to create the magnetic flux. A collection of interconnected coils are made to rotate in the region of the magnetic field. In this way an electric current is generated that could be directed out of the electric engine using conductive wires connected to the coils. Of course to rotate this coil some other device needs to be involved. For example, in the hydroelectric plants water pressure created by a natural or artificial water fall is used to rotate a turbine coupled to the rotating coils of the electric generator. In the case of wind mills, the wind energy rotates the blades connected to the coil's shaft and thus rotates the coils and generates electricity.

In power plants that "burn" fuel in order to cause the electricity generating coils rotate in the magnetic field the heat generated by the fuel is used to generate steam. The steam is in turn used to turn the turbine, same as in hydroelectric plants. Once the steam passes over the blades of the turbine it cools. In most plants the steam is further cooled via heat exchange with another water source (often a river, pool, or the sea) and condensed back into its liquid water form and recirclated So, the only running water use is for the cooling of the steam. As a result, the used water receives a modest increase in its temperature. Plants of this type are often built near natural water sources. They pump cool water into the plant and return it back to its source with a few degrees increase in the water temperature. These plants, then, do not use the water; they just increase water temperature. (Of course this could temperature increase itself clearly affects the environment; good or bad.)

Nuclear power plants are not much different from the conventional fuel burning ones. Their major differences are: 1) they are much more efficient, and 2) they have the potential of being far more dangerous to the environment both in terms of disastrous accidents and in terms of creating dangerous waste. To appreciate some of these, let us examine how these plants operate.

Nuclear Power Plant Operation

There are many different types of nuclear power plants, so let us consider one of the basic types.

In all of these reactors the "burning" fuel is uranium. This element comes in many different isotopes. The isotope of uranium that is most efficient for nuclear power generation is uranium 235. Only about 0.3% of naturally occurring uranium is of this isotope. Almost 99% of mined uranium oar is of mass 238. So, in order to generate useful fuel for the nuclear reactor the natural oar needs to be enriched in its concentration of isotope 235. There are different ways that this could be done, but the most common methods involve chemical reactions and mass separations using centrifuges.

The interesting physics about uranium 235 (235U) is that it readily undergoes nuclear fission when it combines (collides) with a slow neutron. Interestingly enough, fast neutrons do not cause a fission reaction. Also interesting, and important in this process, is that the product of uranium 235's fission is more neutrons, as well as daughter fissile products. It is these extra by product neutrons that cause other 235U nuclei to undergo fission, and thus create a chain reaction. Of course, in a power plant the ideal chain reaction is one that is not only sustainable, but also controllable.

To create a sustainable chain reaction neutrons that are the reaction by products need to get slowed down so that they, in turn, effectively cause other 235U nuclei to undergo fission. To accomplish this the enriched nuclear fuel is embedded in a so called moderator. Three types of moderators are often used in nuclear power reactors. These are graphite, water, and heavy water. Graphite is made of carbon atoms, same as the so called "lead" in writing pencils. Heavy water is same as water, i.e. H2O, but one of the hydrogen atoms has an extra neutron in its nucleus (i.e. the hydrogen is replaced by a deuterium atom). Both heavy water and graphite are more effective moderators, but they are more expensive. So, in many reactors, especially in the US, pressurized water is the moderator of choice. To control the chain reaction neutron absorbing material, such as cadmium, are inserted in the moderator along side of the fuel. By raising and lowering control rods of cadmium the rate of chain reaction is decided by the power plant operators.

The uranium fuel embedded in pressurized water causes the water to reach very high temperatures. Through heat exchange with this high temperature water a secondary water source is turned into steam and is used to drive a turbine for electric power generation. This secondary water is then cooled further through heat exchange with a tertiary water source, which is often circulating river or sea water. Because neither the tertiary or the secondary waters mix with the primary water, which is highly radio active, under normal operation conditions nuclear power plants do not release any harmful radio activity.

Problems

There are two very serious problems associated with nuclear power reactors: short term and long term safety. In the short term accidents (intentionally caused or not) could release radio active elements, some with very long life times, into the environment. This has already happened in 1986 at the Chernobyl site. More serious accidents in poorly designed reactors could even create uncontrollable nuclear chain reactions. Newer designed reactors do not allow for this possibility. In the long term, the primary problem is the issue of nuclear waste. The spent fuel, and other unusable by products, remain highly radio active. To this day there are no full safe methods of dealing with this waste. Also, check out the NRC site.

For further reading on Nuclear Power see the Virtual Nuclear Tourist site by Joseph Gonyeau. Also, check out mini-reactors and their eign at Hyperion site.

Questions on Nuclear Energy

Last Modified May 20, 2010 malekis@union.edu

 A uranium Chain Reaction & its beta decay products

n + 235 92U --------->236 92U* --------> 144 56Ba + 89 36Kr + 3 n

both Ba and Kr undergo a series of beta decays, as follows:

144 56Ba  -------->144 57La ------->144 58Ce------->144 59Pr-------->144 60Nd

89 36Kr -------->89 37Rb --------> 89 38Sr --------> 89 39Y

In all of the above the superscript gives the nucleon (mass) number, A, and the subscript indicates the proton (charge) number, Z. The actual beta decays are of the type that results in increasing the proton number, i.e.:        n (in parent nucleus) ----------> p (in daughter) + b- + ne , without changing the mass number.

For an animation of this reaction check out the appelet: Nuclear Decay Animation!

For further reading on power generation methos using fusion please also see the site at the Contemporary Physics Education Project (thanks in parts to Union Alumni Dr. Ted Zaleskiewicz)