Fourth Generation Of Nuclear Reactors Research Paper Examples

Nuclear Reactor and its Basic Structure

Ever since the development of the first “generation” of commercially produced nuclear reactors in the 1950s, scientists and nuclear physicists have strived to improve the safety and efficiency standards of nuclear energy to make it less costly but more viable to those who want to harness this type of technology.
The World Nuclear Association (2015) defined an nuclear reactor as the one that is splits the atoms of certain radioactive elements or the fission of the atoms, control the reaction caused by nuclear fission, and harness the energy produced by the splitting and control to convert it to electricity. The energy is formed through thermal heat, which allows the boiling of either normal water (light water) or deuterized water (heavy water or water that is converted from hydrogen to duterium) to produce steam, which goes through pipes to power up generators that convert mechanical energy to electrical energy. The electrical energy produced is then sent to the power grid and is used by households that are connected to the electrical grid.
There are several components in a nuclear reactor that are common to all reactors regardless of generation. They are the fuel, the moderator, control rods, coolant, pressure tubes, steam generators, and containment facilities. Each play an important role in the operation of the reactor.
Common fuel sources are the oxide forms of uranium and plutonium, with the former as the most common. Depending on design, uranium oxide in its natural form (which is to say that it is more in the form of uranium-238 and less of uranium-235) is used for nuclear fission and control. However, there are some cases in which enrichment is required to improve efficiency in the reaction process. This means that some of the uranium-238 is reacted to form more uranium-235 or even to plutonium-239. However, this is only done in some reactor designs.
The fuel is usually in the form of pellets that are placed in fuel rods and arranged in a lattice pattern for maximum neutron reactivity and for easy access going in and out of the core.

First generation Nuclear Reactors

The first generation of nuclear reactors was made in the 1950’s and 1960’s. These reactors were considered prototypes for civil use of nuclear energy, as previous uses for it were more of a destructive nature during the last few years of the Second World War and eventually the Cold War. The reactors themselves were only built to show that nuclear energy is feasible and can easily be harnessed. Despite that premise, they operated and produced electricity to many households nearby. Most of these reactors were built in the United States and Great Britain and are now discontinued.

Second generation of nuclear reactors

The second generation of nuclear reactors was first built in the 1960s and is the one that are commonly found in the world at the present time. From their knowledge gathered of the first generation of nuclear reactors, the second generation was designed to be economical and reliable with an operational lifetime of approximately forty years. These include the pressurized water reactors and the boiling water reactors, shown in Figure 1a and Figure 1b, respectively.
Figure 1a. Schematic diagram of a pressurized heavy water reactor. (Diagram taken from the Institution of Electrical Engineers, 2005)
Figure 1b. Diagram of a boiling water reactor. (Diagram taken from the Institution of Electrical Engineers, 2005)
The two designs use both light and heavy water as coolant. However, in the PWR, heavy water is favored. As water enters the core, heat is absorbed and the water is evaporated into steam, where it is carried to the pressure tubes to the steam generator. The steam powers up the turbines turning heat into mechanical energy. The movement of the turbines convert mechanical energy into electrical energy, and is sent to a series of transformers for distribution. The steam loses heat and vaporizes back into liquid water, and the process starts all over. This is done repeatedly during the whole process unless overused which in that case is processed for release into a large body of water particularly on the sea.
This generation now composes the bulk of the world’s nuclear reactors. The major improvement from the first generation includes safety features that are either triggered by operators or by automated sensors. However, these require high electrical grids and therefore more fuel to operate. Efficiency is also deemed low as only the radioactive uranium-235 and the enriched plutonium-239 are reacted that cause the chain reaction inside the core. This leaves the nonreactive uranium-238 and the final components of the fission reaction of the two radioactive isotopes as nuclear waste . Currently, nuclear waste is difficult to dispose and require years of storage in order for us to safely handle it; requiring specially protected suits to do so. In addition, ever since the meltdown of the Fukushima Daiichi Nuclear Plant that belongs to the second generation of reactors, safety standards are being questioned by nuclear agencies around the world.

Third generation of nuclear reactors

Although only a few are operating under this generation, the third generation of nuclear reactors are said to be more compact and more efficient than their second generation counterparts. This is because the designs are more standardized, more compact, and safer than previous plants. The most common design for this reaction is the advanced boiling water reactor. Third gen reactors are known to last for sixty years and even longer.
Third gen reactors are also undergoing modification in designs in safety and are known as the Generation III+ reactors, with newer designs requiring more automated systems with less human interference. However, these designs still require large electrical grids and fuel intake, and acceptance from the general public after Fukushima.

A summary and timeline of all the generations are shown in Figure 2.

Figure 2. Timeline of the generations of nuclear reactors both built and on design.

Fourth generation of nuclear reactors

The fourth generation of nuclear reactors is being drawn up by a consortium of nations that are known as the Generation-IV International Forum or GIF.
These generators are not expected to be built and be operational before the year 2040, however current research and development has started for new designs. These are done by cooperation and contribution of the countries under the GIF, composed of the Canada, China, France, Japan, South Korea, Russia, South Africa, Switzerland, the United States and the European Union.
Current reactor designs would be revolutionized in the fourth generation of nuclear reactors as well as the introduction to new concepts that may be used for fuel efficiency, more energy output, and more safety measures. Lessons learned from the second and third generation reactors were included in the design in the fourth generation from the design of the core, cooling systems, power generation and waste disposal.

Fast reactor technology

Most of the three previous generations of reactors are known as “thermal” reactors. These reactors use slower neutrons that are needed to maintain the fission reaction of uranium-235. However, they do not react with the more stable uranium-238 and may require enrichment. This means more processes are required with the possibility of non-reacting uranium-238 remaining and end up as nuclear waste which includes the use of plutonium-239.
In the fourth generation, the process of fast reactors is being closely developed. In this principle, the fuel used would be plutonium that is surrounded by uranium-238 that remains from enrichment.
The plutonium would then convert the uranium-238 to plutonium while in the reaction core, forming more plutonium in the process. This means that there would be an increase of the starting material during reaction than the heat produced. This would mean that less nuclear waste and more energy is produced.
However, there is a challenge on how to design this properly. To make this happen, it is believed that the moderator would have to be removed, heat transfer would no longer be in the form of water but through materials of high conductivity, there should be a high heat absorption and dissipation in the processes involved, there should be easier maintenance and hazard warnings, and there should be little impact to the environment as a whole. In one of the fast reactor processes, sodium in its liquid form is used. Fortunately enough, heated liquid sodium does not boil at the temperatures at which heat is released, thereby requiring less or no pressure and is safely contained using current containment facilities.

A diagram of a fast reactor is shown in Figure 3.

Figure 3. Diagram of a fast reactor with liquid sodium as coolant. (taken from the Institution of Electrical Engineers, 2005)
In the sodium-cooled fast reactor, sodium is heated up as a liquid and is passed through the cooling tubes towards the core, where it absorbs the heat produced from the fission reaction. The heated sodium is then brought by the cooling tubes back to its original container where it heats up another container filled with light water. The light water converts in to steam, which in turn goes through another set of cooling tubes back to the steam generator. The steam powers the generator turning thermal heat to mechanical energy. This in turn converts mechanical energy into electrical energy and is distributed to the grid.
The challenge of the process may be due to the probability of solidifying sodium at the cooling tubes when not all of the heat is absorbed properly. This would cause a blockage of the tubes and eventually cause a difficulty in the transfer of heat from the core to the generator resulting to maintenance problems.
Another type of fast reactor is the lead fast reactor. This design uses the lead-bismuth or Pb-Bi eutectic alloy as coolant instead of liquid sodium . The diagram for this design is shown in Figure 4.
Figure 4. Lead fast reactor diagram. (taken from Gen IV International Forum, 2014)
For this reactor system, the combined lead and bismuth alloy is liquefied as it enters the cooling tubes towards the core. The mixture then absorbs the heat from the fission reaction and travels toward the generator to turn the turbines and produce electricity. The alloys are then cooled further before sending it back to the core.
The problem; however for the use of lead is that if the temperature is not maintained to its eutectic point, one of the components may be solidified and may cause blockage in the cooling system. The opposite may also happen as there may be excess heat that may have not been transferred to the turbines cause inefficiency in total heat transfer. In addition, lead or bismuth may react radioactively to form new elements that may also cause blockage in the system . These problems have yet to be addressed.
Another fast reactor design is the gas fast reactor, with the use of helium gas from a mixture of helium and nitrogen as the coolant. The design is shown in Figure 5.
Figure 5. Diagram of a gas fast reactor. (taken from Gen IV International Forum, 2014)
Helium gas is heated at the core. The heated gas moves towards the turbines in the steam generator, converting thermal energy to mechanical energy to electrical energy. The slightly cooled gas is then cooled further using a series of chambers filled with light water and goes back through cooling tubes back to the core.
The design is unique in the sense that the possibility of blockage in the cooling tubes is none for this design since helium gas does not solidify at cooler temperatures. However, challenges such as loss in pressure in the said tubes may cause loss in heat going the steam generators, as well as heat that is not properly cooled and is sent back into the core may also cause less absorbance of heat. Also, the design may be prone to leaks and that with the use of a gas instead of a liquid or a liquid mixture, the design may also be prone to fires. These problems are being addressed as of the moment.

Another design under the Fourth Generation is the Very High Temperature Reactor, with the design shown in Figure 6.

Figure 6. Diagram of a very high temperature reactor. (taken from the Gen IV International Forum, 2014)
In this system, helium gas is heated to high temperatures due to the fission reaction at the core, and goes through the cooling tubes to a container filled with light water, which heats up as well and travels to a hydrogen production reactor for hydrolysis reactions, forming hydrogen gas to be used for energy purposes, including electricity generation.
The concept uses a helium coolant, similar to the gas fast reactor. However, as the operating temperature is aimed to be between 800-1000°C, additional cooling is done with light water, which can be processed further into hydrogen gas by electrolysis once outside the core and be used for any future energy use.
The disadvantages of this method are due to several matters. First, due to the high operating temperature, some of the coolants may not be cooled completely and may contain excess heat which could cause less absorption of heat at the core, leading to inefficiency in the heat transfer. In addition, depressurization and leaks may also develop. Another matter is that with the formation of hydrogen may also be prone to problems as well as hydrogen gas is extremely combustible and may require a lot of energy to produce large amounts of the substance.
However, if this technology would be greatly feasible, it is hoped that two energy sources would then be developed – both electrical power from the reactor and the hydrogen gas.
Lastly, the molten salt reactor is considered also under the Fourth Generation nuclear reactors. The design is shown in Figure 7.
Figure 7. Diagram of the molten salt reactor (taken from the Gen IV International Forum, 2014)
The molten salt reactor uses flourides of beryllium and lithium as primary and secondary coolant, with temperatures reaching as high as 1400°C and requires high pressures. These salts are liquid at these temperatures and as such are feasible as coolants in this process. . The salts are first purified to ensure high heat absorption, and then are transferred to the core where the salts are heated up and melted. The melted salts are then sent to a tank that heats up light water that travels the steam generator and turns the turbines to generate electricity. The light water is then cooled and travel back to the tank. The cooled salts are then sent back to the chemical processing plant for purification.
However, the drawbacks are on the purity of the coolants after many cycles. Any mix in the solution by any radioactive material may cause the formation of isotopes of lithium and beryllium that may cause difficulties in maintenance and increases the possibility of corrosion in the long term. In addition, beryllium isotopes are considered toxic and would endanger lives should it leak.

The future of nuclear reactors: What happens next?

Although the fourth generation nuclear reactor designs are still in its early stages, the basis of such concepts are from sound principles that could aid in the efficiency of nuclear power in general. However, we must consider that there are many complicated processes involved, the dangers that may happen while in process, and the hazards and potential inefficiencies in all the designs involved. Safety issues would definitely be of concern as well to ensure that the general public would be confident in its usage. However, the scientific and engineering bases of these designs seem to be sound in theory. All it needs is its large scale execution.

References

European Commission. (2013, March 21). The different "generations" of nuclear technology. Retrieved from European Commission.
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Gen IV International Forum. (2014). Technology Roadmap Update for Generation IV Nuclear Energy Systems. Washington: Gen IV International Forum.
Gen IV International Forum. (2014). Very High Temperature Reactor. Retrieved March 23, 2015, from Gen IV International Forum: https://www.gen-4.org/gif/jcms/c_42153/very-high-temperature-reactor-vhtr
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