Gas Thermal Helium Moderator Reactor

Fusion and Fission: A Comparison by Kieran Franklin 13Ts What is fission? What is fusion? Which is better?

Nuclear Fission There exist in nature several elements whose atoms are large and contain many neutrons. These atoms are unstable and prone to spontaneous decay. Normal radioactive decay (although damaging in the case of over-exposure) isn’t particularly dramatic, but fission is something a bit special. Clever scientists can artificially create unstable radioisotopes by chucking neutrons at already large nuclei. One of these nuclei is so unstable that rather than decaying slowly by alpha, beta or gamma emission, it splits spontaneously into two smaller, more stable nuclei, throwing out more neutrons at the same time. These neutrons hit more unstable nuclei and cause them to split, causing a chain reaction. Fission doesn’t have to be spontaneous though: when it was first discovered, it was triggered by bombarding uranium atoms with neutrons to cause them to split. This is called induced fission.

So where does the energy come from? Well… Here’s an equation for the induced fission of uranium 235 into caesium and rubidium: 235 92

1 138 96 U+ 2 01 n 0 n→ 55 Cs + 37 Ru +

To see where the energy comes from, we need to add up the masses on both sides of the equation: Left Hand Side: 235 92 U

Right Hand Side 138 2.2895 × 10 -25 kg 55 Cs

3.9014 × 10 -25 kg 1 0n

96 37

2 01 n

1.6750 × 10 -27 kg Total

mass

Ru

1.5925 × 10 -25 kg 2 × 1.6750 × 10 -27 kg

Total mass = 3.9155 × 10 -25 kg =

3.91815 × 10 -25 kg

The numbers show that the mass of the products of fission is less than the mass of the original nucleus and the neutron that broke it. The mass difference may seem tiny, but it is vital to us in our uses of nuclear fission. The mass difference (in the case of uranium decaying into caesium and rubidium 2.65 × 10 -28 kg ) is the source of energy which powers our nuclear reactors. Our good friend Albert Einstein gave us what is arguably the most quoted equations ever: E = mc 2 [where: E = energy (J) m = mass difference (kg) c = speed of electromagnetic radiation in a vacuum ( 3.0 × 10 8 m.s −1 ) ] So everybody has heard of Einstein’s special theory of relativity, but what are the implications for nuclear fission? Well, it’s as simple as the equation, really. The mass difference is equivalent to the emission of energy. Just multiply the mass (in kg) by 9.0 × 1016 (c2) to find the energy (in J) So for the fission of uranium-235: Mass difference = 2.65 × 10 -28 kg E = mc 2 = 2.65 × 10 -28 × 9.0 × 1016 = 20385 × 10 -11 J That might seem like a pathetic amount of energy, but remember, that’s the amount of energy emitted by one nucleus as it splits. In the kind of chain reaction we use to produce electricity, many moles of nuclei split, and this adds up to a lot of energy! The Fission Reactor When a fissile nucleus absorbs a neutron it suddenly splits into two smaller nuclei, emitting high velocity neutrons which can be absorbed by more fissile nuclei, prompting them to split and set off a chain reaction. An unregulated chain reaction of this type is used in a breeder reactor. The fast neutrons have a high ratio of fission/capture (so more of the neutrons are absorbed by non-fissile nuclides, making them fissile). The rate of fission in a breeder is very fast, so fuel rods need to contain a high percentage of fissile material (meaning that only enriched uranium can be used). Fast reactors breed further fissile material as quickly as they use it, so in theory the fuel rods should last longer. However, fast reactors are difficult to control.

Thermal reactors are much more widely used. A moderator (usually light water but sometimes graphite or heavy water) is used to slow down the neutrons. This increases the likelihood of further fission being triggered, as fewer neutrons are absorbed by non-fissile material e.g. uranium-238. The thermal reactor can use much lower-grade uranium which has not been through the same enrichment as it would have to for a fast reactor. Thermal reactors can even use natural uranium. Useful energy generation is by conversion of the kinetic energy of the fission products into thermal energy. The reactor heats up as these products collide with other atoms, and further heat energy is produced by the decay of radioactive fission products. A coolant (usually water or a liquid metal) circulates past the reactor core, and absorbs the heat generated in it. This heat is carried away from the reactor and used to generate steam which drives turbines. In most reactors, the cooling system is separate from the reactor core but some use the reactor to directly boil water. Control rods are used to limit the number of neutrons which can induce uranium-235 nuclei to split. These control rods are made from a ‘nuclear poison’ and can be inserted into the reactor to absorb neutrons, reducing the number of neutrons available to trigger further fission and thus controlling the power output of the reactor. The extent to which the rods are inserted into the reactor is the key factor in how much they limit the output. In some reactors the coolant acts as a poison, absorbing neutrons and reducing power output. In this case, the power output is controlled by changing the temperature of the coolant. A higher temperature coolant is less dense and therefore less effective at absorbing neutrons, so the power output is greater when the coolant is at a higher temperature.

Why choose fission? • • •

A nuclear fission power plant uses a pitifully small volume of raw materials in comparison to traditional coal or gas burning power stations. Because less fuel is used, the impact of mining radioactive fuels is much smaller than the impact of mining coal or drilling for oil/gas. Because it uses such a small volume of fuel, it doesn’t produce as much waste as traditional power plants.

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Whereas fossil fuel burning plants chuck out their waste gases and fumes into the atmosphere, making it all hot and smelly, waste from fission plants is easier to control and therefore doesn’t pollute the environment.

So why don’t we get all our power from fission? • •

Nuclear power seems to have replaced communism as the British public’s greatest fear. Since the Chernobyl disaster of 1986, people have distrusted fission reactors, so commissioning them has proven difficult. Radioactivity is difficult to control, making the closure of a reactor a lengthy and expensive process. The time-frame for decommissioning a reactor in the UK is 50 years, and it can cost up to £100 million to decommission.

Nuclear Fusion Fusion is the reverse of fission, and is what powers stars. It involves creating high energy collisions between small nuclei resulting in these two nuclei fusing together into a bigger nucleus. Depending on the size of the nuclei, the process can absorb or release energy. Mass Defect Strangely, the mass of a nucleus is less than the sum of the masses of its constituent parts. To use the example of Uranium-235 again: 1 0

n

1.6750 × 10 -27 kg

1 0

p

1.6730 × 10 -27 kg

Predicted mass of a Uranium-235 nucleus = (92 × 1.675×10-27) + (143 × 1.673×10-27) = 3.9344 × 10-25 kg Actual mass of a Uranium-235 nucleus

235 92

U = 3.9014 × 10 -25 kg

Mass defect = (3.9344×10-25) – (3.9014×10-25) = 3.3×10-27 kg As with fission, the missing mass is lost as energy according to Einstein’s mass-energy equivalence: E = mc 2 [where: E = binding energy (J) m = mass lost (kg) c = speed of electromagnetic radiation in a vacuum ( 3.0 × 10 8 m.s −1 ) ] so E = (3.3×10-27) × (3.0×108)2 = 2.97×10-10J Binding Energy This is the energy that would be required to completely dismantle a nucleus into its constituent protons and neutrons. As calculated above, the binding energy of a Uranium-235 nucleus is 2.97×10-10J. This can be converted into mega electron volts (MeV) by dividing by 1.6×10-13: E = 1856.25 MeV. The binding energy per nucleon is obtained by dividing this by the number of nucleons. The binding energy per nucleon for Uranium-235 is 7.90 MeV This figure can be calculated for every element and isotope. Doing so produces this graph: The higher the binding energy per nucleon, the more stable the nucleus (because it requires more energy to be broken apart). Iron is the most stable naturally occurring

element (which is why it is the heaviest element produced by normal nuclear fusion in stars). Fusing two light nuclei together produces one nucleus with a greater binding energy. An increase in binding energy must mean that the product nucleus is lighter than the combined masses of the two lighter nuclei. For example, the fusion of two deuterium nuclei into one helium nucleus: 2 1

2 4 H+ Energy 1 H→ 2 He +

We can find the energy released by finding the change in mass during fusion: (It’s easier to use atomic mass units (u) than kg – 1 u = 1.66×10-27kg) Mass of 21 H = 2.0141 u Mass of 42 He = 4.0026 u Change in mass = 4.0026 – (2×2.0141) = -0.0256 u (a mass change of 1 u releases 930 MeV) So E = 23.81 MeV More common in fusion reactions is the fusion of a one deuterium nucleus ( 21 H ) with one tritium nucleus ( 31 H ) resulting in the creation of a helium nucleus ( 42 He ) and a neutron: Conditions necessary for Fusion to occur In order for fusion to happen, two nuclei have to come into contact. They cannot do this with electrons buzzing around them, so the first step is to completely ionise the nuclei. This can be done by heating a gas to an extremely high temperature so that the electrons separate from the nuclei forming a plasma. The next problem is that within this plasma, individual nuclei will repel one another due to the electrostatic forces of repulsion between like charges. This can be overcome by making the nuclei collide at very high speeds. To simplify, pretend that one nucleus is static and the other is aimed at it. The moving nucleus must have enough initial kinetic energy that it will not come to a stop before reaching the stationary nucleus. The two nuclei must come to within 1 fm of each other in order to fuse (this is the distance at which the Strong Nuclear Interaction operates). The plasma has to be at an extremely high temperature (in the order of 10 8K) in order for this to be possible. Why choose Fusion? • •

The initial fuel for nuclear fusion is deuterium, a naturally occurring isotope of hydrogen which can be obtained from sea water. Tritium can be obtained by bombarding hydrogen or deuterium with neutrons. The products of fusion are not harmful to people or the planet as fission products and greenhouse gases are. In fact, many products could be very useful.

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Because there is no radioactivity involved, fusion is safer than fission and therefore the closure of a fusion plant would not require the same lengthy decommissioning process as a fission plant.

So why don’t we use it? Fusion power is still under development at the moment. So far it has proven very difficult to keep a chain reaction going for longer than around eight minutes (although this is a vast improvement on the record of a few nanoseconds held in the 1950s).

Summary At a first glance, it seems odd that both fusion and fission - seemingly reverse processes - result in a decrease in mass and an increase in binding energy, and therefore both release energy. Surely one of them should require energy? Looking at the binding energy graph helps to explain this problem: It’s all to do with the shape of the graph. Iron is the most stable element because it has the highest binding energy. All other elements and isotopes have a lower binding energy, so any change resulting in products with a higher binding energy means greater stability. For anything smaller to become ‘closer’ to iron, it must undergo fusion and become heavier. In order for a heavier element to achieve greater stability, it must undergo fission or radioactive decay. In simple terms, moving ‘up the curve’ releases in energy, whereas moving ‘down’ it would require energy. The spikes in the curve to the left of iron represent a few rare radioisotopes which are 14 lighter than iron, such as 6 C .

Comparison Cost and energy input:

Energy output : Waste:

Dange rs:

FISSION Requires radioactive isotopes to be found, mined and often enriched. This is costly and has a negative impact on the environment. Building the reactors is very specialised, and decommissioning one takes many years due to the safety risks associated with possible radioactive residue. Waste disposal is expensive as radioactive waste has to be stored for centuries and then buried deep underground. Fission has a very high energy output for the amount of fuel used when compared with traditional combustion-based methods of power generation. Waste products are generally radioactive themselves and usually have very long half-lives. This means that waste cannot be disposed of easily and poses a danger to humans and the natural environment. Meltdowns and explosions are serious safety risks which in some cases can be devastating (such as

FUSION It only works properly at temperatures comparable to those inside the sun, requiring a huge energy input to get started, which is expensive and potentially dangerous. Fusion is still in its infancy so the continued research necessary will cost a lot of money and take a lot of time. Raw materials are (relative to fission) easier to come by: deuterium can be obtained from water and tritium can be made by bombarding hydrogen and deuterium with neutrons. Fusion provides an enormous energy output, far greater than fission or any other power source available to us at this point in time. Waste is not a big problem with fusion as the products are generally harmless and could potentially be useful for other applications.

The high temperatures involved in fusion could be a potential danger, but the real dangers are not currently

in Ukraine). Overheating of moderators leads to fire and explosions, and any breach in the reactor could cause leakage of harmful radioactive substances.

known, as fusion power is not yet a feasible solution to our energy needs.