How do fusion and fission reactors compare as viable energy sources? Fusion Reactors

Transcription

1 How do fusion and fission reactors compare as viable energy sources? Fusion Reactors Method of energy generation: Nuclear fusion reactors function through the fusing of two small atoms or particles into a single larger one, triggering an energy release. The source of this energy comes from the fact that, when combining two particles into one, the product will have a mass slightly lower than the sum of the masses of the original particles. Due to Einstein s theory on energy-mass equivalence, E=mc 2, this loss of mass in accounted for as a release of energy, allowing nuclear fusion to generate energy. The mass lost is an incredibly small amount compared to the full atom, but even tiny amount of mass can create massive amounts of energy, as the square of the speed of light is an incredibly large number. The specific energy differences between certain atoms can be seen in a binding energy graph (fig.1), with energy changes for fission moving to the left with decreasing nucleus size, and fission towards the left. However, electrostatic repulsion generally prevents particles from fusing unless they already have incredibly high energies, which makes it difficult to initiate a fusion reaction. Figure 1: Binding energy of different elements. (Fastfission) There are two types of fusion reactions between small particles which should be considered for energy generation purposes; proton-proton and deuterium-tritium. Proton-proton fusion is technically the most basic form and the primary fuel source for sun-like stars. It is a chain reaction, which through the entire process results in individual protons being combined into a helium atom, with the release of radiation and energy throughout the reaction, which results in an overall mass loss, and therefore energy gain. Initially, two 1 H atoms combine to form a 2 H atom, electron, and energy, producing a neutron in the nucleus. A third 1 H atom then joins, forming a 3 He atom and additional energy. To complete the process, two of these 3 He atoms combine, forming a 4He atom and two 1H atoms. The process is described in the following equations, where v is a neutrino and λ is a gamma ray. This process releases approximately 25 MeV per complete reaction.

2 A representation of this process is seen in fig.2. Deuterium ( 2 H) Tritium ( 3 H) reactions an alternate fusion reaction, using different Hydrogen isotopes. These reactions can be written simply as seen in the following formula: 3 H + 2 H 4 He + 1 n + ~17.6MeV This produces less energy than proton-proton reactions, but the reactants are far more obtainable than lone protons, and would therefore be more suited to fuelling a reactor. Inputs and Outputs: Nuclear fusion is an excellent energy source. While hydrogen supplies are limited and generally gained through electrolysis, requiring energy input, it is a safe material to gain and store, and there will be a total positive energy output after the reactions. The primary product of fusion reactors is helium, a harmless gas which can have further useful purposes. Given that this reaction uses very common reactants, which do not need to be mined like Uranium or coal, this appears here to be a viable energy source. Fusion produces 4.75*10 11 J/g for deuterium-tritium fusion. Risks, Damages, Challenges Figure 2: Representation of Deuterium-Tritium fusion. (S. EGTS) Fusion reactors will have a minimal environmental impact due to the finding of reactants or storing of the products, when compared with all alternate energy sources. The main challenge involved in fusion is with the design and maintaining of the reactors. A very high energy input is required to initiate nuclear fusion, and a reactor will maintain incredibly high temperatures, since it is essentially a replica of a star. Therefore, fusion is essentially an ideal green energy source, although it is technically nonrenewable, and the primary challenge with implementing them is in the design of a functional, stable reactor. There is also a theoretical risk of fusion reactors exploding and causing massive damages, although a failure will not leave radioactive residue as seen with fission reactors. Another challenge with fusion reactors is the conversion of the energy produced into electricity. Due to the incredibly high temperatures sustained by nuclear fusion, current attempts to sustain fusion in stellarators such as the Wendelstein 7-X suspend the plasma produced with the aid of magnetic fields (Fig.3). Therefore, it is difficult to extract the heat energy to fuel a standard water-boiling generation system. Fission Reactors Method of energy generation: Fission reactors involve taking heavy, radioactive substances and using the energy released in their decay to boil water and power generators. Fission reactions are triggered and controlled by neutrons colliding with the atoms, releasing additional neutrons as a substance undergoes fission to sustain a chain reaction. When a Figure 3: Representation of the magnetic coils (blue) and plasma (yellow) used in the Wendelstein7-X to suspend the plasma and prevent the system from melting. (Max-Planck Institute)

3 neutron is first emitted, it exists as a fast neutron, with relatively high energies. These neutrons are able to trigger fission in more isotopes than the lowerenergy slow neutrons, although slow neutrons are more likely to be captured and cause a decay, and thus it is slow neutrons used in most reactors. 235 U absorbs slow neutrons far better than fast neutrons. When released, fast neutrons are generally slowed down by certain moderating substances, such as heavy water, until they are able to cause decay in a nucleus, releasing more fast neutrons and thus maintaining the reaction. Two common decays in reactors are 235 U and 239 Pu. These reactions can be represented as follows: 0nn UU 92UU KKKK + 56BBBB nn + eeeeeeeeeeee 0nn PPPP 56BBBB SSSS + aa 1 0 nn + eeeeeeeeeeee, where a is an integer. Fig.4: Decay of a radioactive substance through fission (NSDL) In order to produce 239 Pu, breeder reactors will use 238 U, a more common isotope, and use fast neutrons to convert this to 239 Pu, which then powers the reactor. This is represented by the equation The criticality of a substance refers to the balance of neutron production in the mass. A critical mass will overall produce 1 new neutron for every neutron lost, thus sustaining a reaction. A subcritical mass will produce fewer neutrons than are lost, and therefore lose neutrons over time. A supercritical mass will produce neutrons faster than they are lost, therefore continuing a reaction. A critical mass is the minimum mass of a substance which is capable of sustaining a critical reaction. Criticality is nnnnnnnnnnnnnn pppppppppppppppppppp rrrrrrrr represented as k eff, where kk eeeeee =. If k eff < 1, the reactor is subcritical. If k nnnnnnnnnnnnnn llllllll rrrrrrrr eff=1, reactor is critical. If k eff>1, reactor is supercritical. Shapes with larger surface areas will emit more neutrons away from the substance, reducing their k eff. A certain mass of substance is also required to sustain criticality, with larger masses having higher K eff values. Inputs and Outputs: Fission works on the opposite end of the binding energy curve to fusion, and it is therefore seen that far greater masses are required to gain the same amount of energy, due to the energy difference and high mass of products. A single fission reaction will have more energy released per instance of a reaction, but since the reactants for fusion weigh far less, more energy is produced per gram of reactant used. Fission produces 8.06*10 10 J/g for 235 U decays. One important output from fission reactors is the large amounts of radiation produced, from the initial fuels and also the products. The products of reactors have extraordinarily long half-lives, and are therefore very difficult to safely dispose of.

4 Risks, Damages, Challenges The main challenge with fission reactors is to do with the safe disposal of nuclear waste. Since the products of fission reactors have very long half-lives, it is difficult to find longterm ways to store such waste where it will not cause damage into the future. Additional environmental damage is caused in the mining of radioactive ores. Reactor meltdowns are growing increasingly unlikely with constant improvements to safety measures in place, but the public still perceives nuclear reactors as unsafe and a threat, and therefore many individuals will also try to resist the implementation of nuclear reactors. Fig. 5: Underground nuclear waste storage at Onkalo (J. Bradbury) Comparison Relative energy productions: While fission reactors produce more energy per reaction, fusion reactors are able to produce more energy per gram of product used, making fusion preferable in relation to energy output per mass. Benefit/Risk Assessment Fusion reactors are safer for the environment and people in every way. They do not produce radioactive waste, nor do they require mines to be dug in order to gain hydrogen. However, fusion reactors are generally not a viable source of energy at present. There will be huge costs in designing and creating these reactors, and there is no guarantee that it is even possible to safely sustain a fusion reaction while converting the energy it produces to electricity. Conclusion: At the present time, the best means of gaining energy is to produce additional fission reactors as a substitute for coal, while continuing to research fusion reactors and implementing those as soon as they can be safely and cost-effectively created. Word Count: 1500

5 Bibliography EG and G Rocky Flats, Inc. (1991, December 6). Reference Handbook: Nuclear Criticality. Retrieved from SchiTech: Encyclopedia Britannica. (n.d.). Proton-Proton Cycle. Retrieved from Encyclopedia Britannica: European Nuclear Society. (n.d.). Fusion. Retrieved from European Nuclear Society: Lackey, R. (2010, July 25). What releases more energy per unit mass, fission or fusion? Retrieved from Quora: Moran, G. (2016). Heinemann Physics 11. Melbourne: Pearson. Phys.org. (2016, June 7). Start of scientific experimentation at the Wendelstein 7-X fusion device. Retrieved from Phys.org: Radioactivity.eu.com. (n.d.). Slow and Fast Neutrons. Retrieved from Radioactivity.eu.com: Skinner, J. (n.d.). Nuclear Criticality Safety. Retrieved from qmplus: nal)_jackskinner.pdf uwaterloo. (n.d.). Fission Energy. Retrieved from uwaterloo: V, S. (2015, December 15). Calculate the quantity of energy produced per gram of reactant for the fusion of H-3 with H-1. Retrieved from Socratic: World Nuclear Association. (2016, September). Physics of Uranium and Nuclear Energy. Retrieved from World Nuclear Association: