Overview of ReactorsIn a commercial power plant, the conversion of energy is usually from thermal energy to mechanical energy to electrical energy. More specifically, the steam from heating water (thermal energy) is used to turn the blades of a steam-turbine engine (mechanical energy) which in turn, generates electricity. The source of heat energy has traditionally been the combustion of fossil fuels. In a nuclear power plant, the source of heat energy is the fission reactions of heavy nuclei in a nuclear reactor. Uranium-235 is the most common form of heavy nuclei used. The energy produced by the fission of 1 kilogram of uranium is comparable to that produced by the combustion of 10 tons of oil(1). Uranium-235 is the less abundant of the two naturally occurring uranium isotopes (uranium-238 is the other isotope). Since only uranium-235 is fissionable, natural uranium is sometimes enriched ( that is the percentage of uranium-235 is increased) before it used as fuel in a nuclear reactor.

HOW NUCLEAR REACTORS WORK

Source: Environmental Science, Lancaster University webpage, http://www.es.lancs.ac.uk/casestud/jp/image4.gif

Fuel (that is, fissionable nuclei such as uranium-235 or plutonium-239), is contained in fuel rods in the reactor core. It is usually in the form of pellets. The fissionable nuclei are bombarded with neutrons. If fission results, the nuclei split into lighter nuclei (the fission products). Energy and neutrons are also released in the process. These neutrons can then be directed to bombard even more fissionable nuclei in the fuel. A chain reaction develops and energy production continues.

A gas or liquid functions as the coolant. The coolant prevents the overheating of the system. It flows around the reactor core and absorbs the heat energy produced by the fission reactions. This heat energy is then transferred to the heat exchanger for steam generation. The steam is used to drive turbines in electricity generation. It is then condensed, pumped back to the heat exchanger and the entire cycle begins again. The higher the temperature the coolant can be heated to, the more heat energy that will be transferred to the heat exchanger, the more steam that will be generated and thus the more power that will be produced. Ideally, a coolant will not undergo a phase change in the reactor due to an increase in temperature. Phase changes require energy and will reduce the thermal efficiency of the reactor. Good coolants are also inert.

The neutrons that are produced as fission products are very energetic and so move very quickly. To increase the likelihood that a fission chain reaction is set up, these neutrons can be slowed down by a substance called the moderator. The moderator ensures that the neutrons are moving slow enough to be able to collide with the nuclei. Common moderators are light water (H2O), heavy water (D2O) and graphite. A good moderator is one whose atoms are similar in mass to neutrons so that when collisions occur between the two, energy will be efficiently transferred from the neutrons to the moderator material. Gases are not good moderators because they are too low in density and would not allow for regular collisions with the neutrons. A good moderator is also a poor neutron absorber - we want a moderator that slows down neutrons not absorbs them. Light water is not the best moderator because it tends to absorb neutrons. The less enriched the fuel, the less fissionable material it contains, the more efficient the moderator must be to ensure that the neutrons are slowed down and directed to bombard the limited fissionable nuclei.

Reactors with moderators are called Thermal Reactors. There are reactors without moderators. These are termed Fast Neutron Reactors because the neutrons’ speed is not controlled. Unlike Thermal Reactors, highly enriched fuel must be used to increase the probability that the neutrons are captured by the fissionable nuclei.

To ensure that the fission reaction does not explode out of control, control rods can be periodically inserted into the reactor core. These rods contain materials (such as cadmium, hafnium and boron) that absorb neutrons and thus limit the fission reaction rate.

The reactor core is shielded to prevent the release of harmful radioactive material to the environment.

REACTOR TYPES

Nuclear reactors are characterized by their type of coolant and moderator and by their design in general. Table 1 gives an account of some of the more common nuclear reactor types. All of the listed reactors are thermal reactors.

Table 1: Types of Nuclear Reactors

Source: What is a Nuclear Reactor? European Nuclear Society webpage, http://www.euronuclear.org/info/energy-uses.htm

LIGHT WATER REACTOR (LWR)

Currently, the LWR is the main reactor type employed in the U.S. In LWRs, light water (H2O) is used as both the coolant and the moderator. There are two main types of LWRs; pressurized water reactors (PWRs) and boiling water reactors (BWRs)

PWR

Energy from the fission reactions can cause the temperatures in the reactor core to reach 300 degrees Celcius (2). This is way above the temperature that would cause the light water coolant to boil if the pressure were that of the atmosphere (101kPa). To prevent the water from boiling and thus vaporizing, the coolant is forced to flow around the reactor core at an exceedingly high pressure of approximately

15 to 16 MPa (See Primary water in Figure 3) (3). A disadvantage of the PWR is that it must shut down regularly for routine check ups - under such high pressure, it is wise to regularly check that there is no wear and tear on the system’s components.

The coolant also functions as the moderator. If the moderator were to vaporize, the fission reactions would be less efficient. The gas would be too low in density to have effective collisions with the neutron fission products to cause them to slow down.

The coolant then transfers the heat to the Secondary water. The Secondary water can boil and generate steam because this part of the system is held under less pressure (6MPa). The steam is used to turn the blades of the turbine and generate electricity. It is then condensed and the resulting water is pumped back to steam generator to begin the process all over again.

Figure 3: Nuclear power plant with pressurized water reactor

Source: What is a Nuclear Reactor? European Nuclear Society webpage, http://www.euronuclear.org/info/energy-uses.htm

BWR

In these reactors, the reactor cores are held under a pressure of approximately 7MPa (4) - a much lower pressure than that of the PWR. This pressure is close enough to atmospheric pressure to allow the light water coolant/moderator to begin to boil. Unlike the PWR, because the steam is generated directly in the reactor core of the BWR, the core is bigger and more expensive.The steam produced (at the top part of the reactor core) is delivered directly to the turbine to aid in electricity generation (See Figure 4). The BWR system is much simpler than that of the PWR because no heat exchangers/steam generators are needed.

In the reactor core, the steam is separated from the water below by steam separator plates. The neutron efficiency of BWR is less than that of the PWR because 12-15% of the moderator is vaporized (5). The coolant/moderator is in contact with the turbine. Since the coolant/moderator is radioactive (due to its contact with the fuel rods), the turbine is contaminated and must also be shielded. In a PWR however, the turbine is only in contact with the Secondary water which does not contain significant amounts of radioactive material. The turbine in the PWR plant does not need to be shielded.

PRESSURIZED HEAVY WATER REACTOR (PHWR)

The PHWR design is also called the CANDU design because it originated in Canada in the 1950s. It is very similar to the PWR with respect to the pressure system. However, the main difference is that unlike the LWRs, these reactors use natural uranium as the fuel. There is thus, not a lot of fissionable uranium-235 in the fuel. A moderator that will control the speed of the neutron fission products but will not absorb the neutrons is needed. As many neutrons as possible are needed to bombard the limited uranium-235 in order to continue the fission chain reaction. Light water, though it slows down neutrons efficiently, has a tendency to also absorb the neutrons. These reactors therefore use heavy water (deuterium oxide, D2O) as the moderator. Deuterium oxide is less likely to absorb neutrons because deuterium already has twice the amount of neutrons as the hydrogen in light water. However, a large quantity of heavy water is needed to have an influence on the neutron speed. Another disadvantage of heavy water is that if it absorbs neutrons it produces the radioactive isotope of hydrogen called tritium.

Though heavy water is expensive, costs are cut down since natural uranium rather than processed enriched uranium can be used as the fuel. Furthermore, the PHWR can be refueled while in operation.

GAS-COOLED REACTORS (GCR)

GCRs are of European origin. The coolant is carbon dioxide and the moderator is graphite. Carbon dioxide is an effective coolant because it can be heated to high temperatures and can thus maximize heat transfer to the steam generation process. Also, because it is gaseous, carbon dioxide will not absorb the neutrons that are involved in the fission chain reaction. However, as explained in the PWR section, gases are poor moderators. Graphite is used as the moderator because it is inexpensive, readily available and not damaged by high temperatures (6). The thermal efficiency of these reactors is very high but the fuel efficiency is low.

Figure 6: Nuclear Power Plant with Gas Cooled Reactor

Source: Joseph’s Gonyeau’s The Virtual Nuclear Tourist! webpage, http://www.nucleartourist.com/

LIGHT WATER GRAPHITE REACTORS(LWGR)

The LWGR is a Soviet invention. It was uniquely designed to generate power and produce plutonium. The coolant is light water and the moderator is graphite. This coolant/moderator combination is unique to the LWGR. THe light water vaporizes as it

passes through the reactor core. This steam is used to drive the turbines. One major advantage of the LWGR is that it can be refueled while in operation.

NUCLEAR FUSIONIn Nuclear Fusion the nuclei of atoms fuse together, causing much more energy being released than in Nuclear Fission. This is the process which powers the Sun. Here the Deuterium and Tritium atoms (these are the isotopes of Hydrogen:- these have extra neutrons in the nucleus than normal) fuse together to form He. Energy is released due to a difference in mass between He atoms compared with the sum of the Hydrogen atoms therefore it is conclusive evidence that some mass is converted into energy via Einstein’s relationship of E=mc2; where E=Energy, m=mass of nuclei, and c=speed of light (3x108). This is why the energy (E) released is greater in fusion than in fission because of a very large mass deficit (m).

 PROS AND CONS OF NUCLEAR FUSION

PROS CONS

There is a lot more energy released during fusion than fission so would be more profitable once set up.

Fusion will only occur at temperatures of 10-15 million Kelvin (K), hence there is no point of producing energy since it is not profitable.

Hardly any waste produced compared to Nuclear Fusion, so no money wasted in disposing Nuclear waste.

Even if fusion did take place, it would be far ahead of it’s time since no material exists which can cope with temperatures of 10-15 million K.

Fusion could power the entire world at a very low cost compared with power sources today. Fusion would not be depleted like non-renewable sources today since it would be possible to make hydrogen nuclei manually as it is a very simple element (requires only 1 proton and 1 neutron).

At the moment the only profitable way for fusion to occur is using cold fusion*, but this has not yet been discovered. (*cold fusion is fusion at low enough temperatures to make profits because this would mean that the output energy would be greater than input energy ).

CONCLUSION: For the moment the only way to produce energy using nuclear power is via nuclear fission. This is because Physicists are still trying to discover ways in which to produce energy using cold fusion. If discovered all our energy crisis would be solved.