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Nuclear Energy Paper

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Paper

Nuclear Power

Nuclear fusion is the process by which two nuclei join to form a new larger nuclei. Nuclear fission is the process by which a single large nuclei splits into two smaller nuclei. Within the nuclei exists the strong nuclear force, which holds the atom together. Both neutrons and protons contribute to the strong nuclear force of the nuclei. Although, protons have a positive charge which pushes them apart. The strength of the strong nuclear force is able to overpower the repulsion of the protons, but only for a short distance. In the larger nuclei, many more neutrons are needed to provide the extra strong nuclear force needed to hold the nuclei together. Smaller nuclei have nearly equal ratios of neutrons to protons.

binding_energy.jpg A nuclei will always have less mass then the sum of all the neutrons and protons make up the nuclei. This missing mass is stored in the bonds that make up the nuclei, which is called the binding energy. Iron and nickel have some of the highest binding energies. Nuclei with fewer nucleons then Iron release energy as they are fused, and nuclei larger then nickel release energy when they fission (split). This concept is the basis to nuclear energy.

Nuclear Fission

There are three main types of nuclear fission. The first being spontaneous fission. The most common forms of radioactive decays are alpha, beta, and gamma. Although spontaneous fission is not as common as these, the principles are the same. An unstable nuclei undergoes a nuclear reaction while trying to reach a stable state. Spontaneous fission normally occurs in larger isotopes where the unstable nuclei splits into two small stable isotopes. U235 and P239 are among the most common isotopes that are known to spontaneous fission. These natural fissions help provide the source neutrons needed to start up nuclear reactors.

The second form of nuclear fission occurs when an isotope is bombarded by a high energy particles. The energy transfer from the particle can destabilize the nuclei and cause the nuclei to fission. Nuclear fission bombs rely on fast neutrons reactions to produce fission. Secondary cosmic rays can also induce fission when high energy mesons collide with nuclei.

The third type of nuclear fission occurs when an isotope absorbs a thermal neutron. Thermal neutrons have low kinetic energy, such as room temperature energy levels. Since neutrons are not repelled by the positive charge of the nuclei, they can easily collide with a nuclei and become absorbed. In the example of uranium-235, after absorbing a thermal neutron and turning into U-236, there is about a 79% chance that the U-236 will fission. Thermal neutron absorption is what drives nuclear power plants today. As compared to fast neutron absorption, thermal neutron reactions are much easier to control because changes in reaction rates occur in the hundreds of milliseconds, verses the microsecond reaction rate of fast neutron reactions.

Nuclear reactors have been built to harness the power of nuclear fission. The two most commonly used nuclei in these reactors are Uranium-235 and Plutonium-239, which form the nuclear fuel. The process of nuclear fission involve a continuous cycle. This cycle starts with the presence of thermal neutrons. Thermal neutrons are then absorbed fuel nuclei. These nuclei then undergo nuclear fission. This produces energy, two waste nuclei, and several fast neutrons. The fast undergo a process known as thermalization. Thermalization occurs when fast moving neutrons collide with other atoms gives up some of their energy until they reach ambient temperature. Once they are thermalized they can be absorbed by more fuel and the process repeats. Because these reactors rely on thermal neutrons, they are easier to control. The whole cycle takes several milliseconds to complete, as where fast neutron reactions occur in the microseconds.

A cloud of available thermal neutrons in the reactor is called a neutron flux. Maintaining the neutron flux is the key to maintaining the reactor. A moderator is used to thermalize the fast neutrons and produce this flux, while the control rods are used to reduce and shape the flux when needed. Pools of water surrounding the reactor core as provide a good neutron reflector to keep the flux in the core. Materials used in the reactor core must not absorb the neutron flux. There is a delicate balance between the density of fuel, density of the neutron flux, density of the moderator, rate of thermization, and the final rate of fission. This is important as to maintain critically in the reactor. A reactor is called critical when the fission reaction rate is stable.

Pressurized light water moderated reactors use H20 as the moderator. Since the density of H2O changes with temperature, it can be used to self regulate the reaction cycle. It's actually the hydrogen in H2O that provides that thermalization of the fast neutrons. Water moderated reactors have been used for power plants since the 1960's. Other types of fission power plants have also been build and used. Carbon is another suitable material for neutron moderation. Carbon is actually more efficient then H20 because the oxygen in H2O tends to absorb neutrons becoming O17. Thus, reactors with lower fuel densities can still reach criticality by using a carbon moderator. Carbon does not have the self regulating characteristic of H20. Most H20 moderated reactors also circulate the H2O through heat exchangers to cool the reactor core and transfer the reactor's power to a steam plant for making electricity. Carbon moderated reactors must rely on H2O or another fluids to cool the reactor.

Using H20 as a moderator has a favorable safety advantage. In the event of an accident, the H2O will turn to steam, thus shutting down the reaction. Chernoble was a carbon moderated reactor. When Chernobyl's operators took the reactor outside its designed operating limits, the fission reaction could not be stopped and the core melted while remaining critical. This type of accident is impossible in a water moderated reactors. There are several known accidents with water moderated reactors where the operators removed the control rods too fast causing the reactor to become super critical. In this state the reaction rate increase exponentially every millisecond. In all cases where H2O reactors have reached super criticality the H2O turned to steam and shut down the reaction. In the case of SL1, a Navel test reactor, the resulting steam produced an explosion destroying the reactor and killing the operators.

H2O based reactors have some design limitations. H2O is the most chemically active substance known to man. Thus, water chemistry can add a whole range of challenges to managing a nuclear reactor. Decay heat is another challenge to designing a nuclear reactor. After shutting down a reactor (stopping the fission process) the reactor core can continue to produce 1 to 7% of total reactor power from decay heat. Decay heat comes from alpha, beta, and gamma parictale released from the unstable fission fragments and other radioactive isotopes on the reactor's core. Three Mile Island was an example where decay heat caused the reactor's core to melt.

New fission reactor designs attempt to overcome these issues. Modern material science offers a many technologies that did not exist in the early days of nuclear science. Newer reactor designs use materials that do not become long lived radioactive isotopes. Thus, the amount of radioactive waste has been greatly reduced. The use of ceramic materials has allowed reactor cores that can withstand extremely high temperatures. Thus, problems with decay heat and the possibility of core meltdowns have been overcome. Water coolants have been replaced by inert gases. Thus, corrosions and radioactive contamination issues have been overcome.

One of the leading fission reactor designs to use all of these new technologies is the Pebble Bed Reactor (PBR). In the PBR design the fuel and the moderator are packed together into a tennis ball size pebble. The pebble is made out of pyrolytic carbon, which is very strong and has a melting point around 3000c. Since these reactors will operator at around 2000c, the pyrolytic carbon will be able to withstand temperatures higher then the reactor is able to produce. The carbon in the pebbles acts as the moderator for the fast neutrons. Unlike earlier carbon moderated designs, PBR reactors will operate at a temperature range where thermal expansion of the carbon will slow the rate of fission. Thus, changes in coolant temperature will result in corresponding changes in reactor power. Thus, external control methods, such as control rods, are not needed to continuously regulate a PBR. PBR's will use an inert gas such as helium to cool the reactor and transfer heat to a secondary system for electrical power generation. Some PBR designs use the high temperatures on the reactor to split H2O and create hydrogen as a byproduct. This hydrogen could be used to power cars. The pebble design allow for easy refueling and storage of the spent fuel cells. The pebbles will be rotated within the reactor to ensure even burning. Currently a number of countries are already working on projects to build PBR's.

Nuclear Fusion

On the other end of the nuclear energy spectrum is nuclear fusion. Nuclear fusion is the same process that powers the sun. Fusion has the promise in producing cheap abundant energy without generating any significant radioactive waste or harmful environmental effects. Hydrogen, which is abundant in water, could be fused to produce helium. The resulting reaction gives off hundreds of times more energy then fission without the radioactive fission fragments. The problem with fusion is that is takes a lot of energy to get atoms to fuse. The positively charged nuclei naturally repel each other. Thus, to create fusion scientists have to create a super heated plasma cloud. The plasma needs to be contained, so an extremely strong magnetic containment field is ued to contain the plasma.

Producing a few fusion events is fairly easy in the scientific community. Although, producing a sustained fusioning plasma cloud is another story. For many of the different fusion reactions that can occur, many of them have an extremely high starting energy. The fusion reaction with the lowest input energy requirement turns out to be deuterium + tritium.

:projects:fusion2.png

Tritium fusion can produce up to three times more energy than fission of U235. Test reactors have already been produced that can sustain a deuterium/tritium reaction for several hundred msecs. The British JET reactor was able to reach a peak power of 16MW.

This reaction has a downside. The single neutron released contains much of the reaction's energy. Free neutrons can be absorbed by another nuclei, thus potentially creating radioactive isotopes. Materials used in the construction of the fusion reactors will have to be carefully chosen which do not readily become radioactive isotopes.

255px-iter_tokamak_cutout.jpg The International Thermonuclear Experimental Reactor project ITER attempts to overcome these challenges. ITER will use a Tokamak reactor design which was first pioneered by the Russians. Scheduled to be complete by 2016, ITER will be able to produce 500MW of power. Of the challenges that ITER must overcome are materials that can stand up to the harsh environment of the plasma chamber. With the plasma operating at a temperature of 100,000,000 K anything it touches would vaporize. Thus, the powerful magnetic fields generated by the Tokamak donut shaped magnet will keep the plasma contained. The walls of the plasma chamber will need to withstand extremely high temperatures, powerful magnetic forces, neutron bombardment, and photon radiation; all while allowing heat energy to be collected for producing electrical power. This is a pretty tall order. That is why accompanying ITER will be International Fusion Materials Irradiation Facility (http://en.wikipedia.org/wiki/IFMIF).

Fusion reactors are inherently safer than fission reactors because of the difficulty in sustaining the fusion reactions. Fusion reactors are incapable of getting out of control. If their magnetic fields were to fail the plasma would simply disburse and all fusion would stop. Many other characteristics of the fusion process must be perfect, else the reactor will simply shut down. Combine this will unlimited cheap fuel and neg liable amounts of radioactive waste, fusion power promises the perfect energy source. Fusion power will be achieved, it's only a matter of time and funding.

MLA References

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