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Thermal Nuclear Power Plants

By: Adam Eldaly

Thermal Nuclear power stations power around 11% of the world’s electricity. It’s done through the composition of various structures and sections which all work together to produce electrical energy from heat generated by nuclear reactions. However, they only utilize fission.

Principle's Behind Nuclear Power Plant

Fission

Fissile atoms are those that can undergo Fission because they can absorb a neutron. Fission is the process of where the atomic nucleus [Protons and neutrons which are held by the strong nuclear force] are bombarded with Neutrons, therefore making it unstable. It then splits into 2 or more isotopes with almost equal mass and additional neutrons. The rest of the Mass according to follows the law of conservation of energy which is the energy released in the fission is equal to the total mass of the parent's atoms minus the total mass of the daughter atoms. The energy can thereby be calculated using Einstien’s E=mc2. Henceforth the energy is released in the form of radiation kinetic energy or heat.

Fusion occurs when two small nuclei such as (hydrogen and Helium) combine to form a larger nuclei. Similarly, to Fission, the mass of the product is less to the mass of the reactants. This is because it also follows the law of conversation of energy and hence releases energy in the reaction. However, unlike Fission, they are much harder to create and don’t release neutrons.

Its difficult to create it because most nuclei are positively charged, hence exert an electrostatic force of repulsion to the other atoms. Hence to overcome it, the nuclei must have a huge amount of energy which translates to temperatures of 100 million degrees Celsius and to be traveling at almost the speed of light in order to overcome the electrostatic force.

Fussion

Therefore, fusion power doesn’t exist because of two reasons. Firstly, it’s too difficult to heat hydrogen gas to 100 million degrees which creates plasma. It’s even harder to sustain this temperature to control the plasma with powerful magnets so the atoms can fuse and release energy. Secondly, the amount of energy required for to produce that amount of heat on Earth, is much higher than the output of usable energy. Henceforth, Fission is the reaction used in Nuclear power plants

What is utilised in the Power Plants?

Uranium is the most sought out fuel to be used in the Thermal Nuclear power plant for Fission. The reason being is that Uranium 235 [U-235], is the only naturally occurring isotope capable of undergoing fission. However only 0.7% of Natural Uranium is U-235 compared to the rest U-238. Therefore, there is an extraneous extraction process in order to attain the U-235 from U-238

How Fission is used in the Nuclear Power Plants

Nuclear power plants utilise the method of a chain reaction with U-235. In fission [as shown below]

The product of the fission consists of Barium, krypton energy in the form of heat and 3 neutrons. These neutrons therefore create a continuous cycle by instigating and starting other fission reactions with other U-235. Hence, massive amounts of heat can be generated

The Chain Reaction

Parts and How the Reactor Works

Control and Fuel Rods

The Reactor Core

Firstly, The reactor core is where the nuclear reaction and release of thermal energy occurs. It composes of 3 main components; Fuel rods, Control rods and a Moderator. It is covered by a containment building which is reinforced concrete as a safety measure in case on an accident

There are many types of reactor systems; however, the Fuel and control rods are controlled variables and same throughout.

A single fuel rod contains the Uranium-235 pallets that was earlier discussed so it can undergo Fission. There are many Fuel rods that fit into the reactor [depending on the vessel size; from 100s to 1000s].

Control rods that are composed of materials such as cadmium and boron that are neutron absorbing material that are raised or lowered into the reactor in order to speed or slow the reaction chain and have control over it. They do this so they can allow an average of one neutron released from fission to create another fission reaction; and remove then remove the excess neutrons.

Moderator

The rods explained can then be used in various systems with different moderators. However there will be only two described; The pressurised water systems (78% of total Nuclear Power Plants) and Boiling water reactor (15%). The rest include:

- Advanced Gas-cooled reactor (3%)

- Light Water graphite-moderated reactor (3%)

- Fast Neutron reactor (Only 2 made, and in the development)

Both of these systems utilises water as their moderator. The water slows down the high energy neutrons that are emitted during fission that have energy of about 1Mev to a thermal neutron [have thermal equilibrium with the moderator] with energy of about 0.04eV in order for the Neutron to travel at a speed that enables the U-235 to ‘absorb’ the neutron, thus creating the chain. The water slows it down by having the high-energy neutrons collide with the water molecules, thus losing the energy and slowing down. The water also absorbs all the energy produced from the fission thus, heating up.

However, there are two different types of water that can be used; heavy and light. And are used in the corresponding pressurised heavy water reactor (10% of total Nuclear Power Plants) and pressurised water reactor (68% of Nuclear Power Plants)

Unriched fuel is when the Uranium has less composition of (U-235) and riched is the opposite. U-235 has more of a chance compared to U-238 to undergo fission merely because of its natural composition and configurations of neutrons, protons and radioactivity. Light water (normal water), absorbs more neutrons compared to heavy water (has more has higher percentage of deuterium oxide). The (D20) is composed of a nucleus with 1 neutron and proton, therefore, absorbs less neutrons compared to the light water. Hence by, heavy water is utilised when the fuel is has less percentage of U-235 and light water is used when the fuel has high percentage of U-235.

The water is heated beyond the temperature of boiling point (100 degrees Celsius). However, the way the systems utilise the heated water differs:

In the pressurised water reactor, there is a pressuriser attached to the nuclear core reactor which maintains the waters pressure at a level that doesn’t enable it to boil and release steam. This water is then pumped through a heat exchanger (steam generator) which is in contact with the water in the heat exchanger. The first law of thermodynamics is involved as it states that the change in internal energy of a closed system, will be equal to the energy added minus the work done by the system on its surroundings. Therefore, the heated water does work on the water in the heat exchanger and energy is added due to heating; hence there is a increase change in temperature to eventually change form and release steam. This steam is then piped to a turbine which is also connected to the generator. The steam makes the turbine spin which makes the generator spine therefore it converts the mechanical energy into electrical energy which is supplied to the users through the cables [under/over ground]. The steam then continues down into the condenser which converts it back to water, so it can then be pumped through the cooling towers. The hot water runs through the cooling tower which is then cooled down so it can be pumped back into the reactor core

In the Boiling water reactor, the only difference is there is no pressuriser. That’s why the heated water in the reactor, is left to convert to steam and continue the same process as the pressurised water system

Coolant/Turbine/Generator/Condenser/Cooling tower

Pro's and Con of Nuclear Powerplant's

Pro's

  • Low greenhouse gas emissions. The greenhouse gases are emitted only by the extraction of Uranium-235 and the transportation of it. The science behind it is because the products of the fission that occurs in the reactor, doesn't release carbon or any greenhouse gases like fossil fuels. But rather just heats up water which is contained in the power plant to generate electricity, and then cooled down in the 'cooling towers' which only release water vapour. This heated water is then reused

  • Highest power output from all energy sources. One power plant produces on average 1 Gigawatt which equates to around 300,000 homes. This is due to the trillions of atoms of Uranium 235 that can undergo fission

  • Economic benefit. One power plant provides 400-700 permanent jobs, and thousands during its construction phase. Compared to that of Coal with 70 and Hydro with 50.
  • Not renewable. This is for the mere fact that Uranium is a limited supply

  • Potential Nuclear accidents [Extreme safety measures]. The Chernobyl power plant explosion happened on Saturday 26/1986 in Ukraine. This was due to lack of safety design and precaution, which resulted in an explosion killing 30 operators

  • High starting costs/building time. With about 10 years building time and requiring 14-23 bilion USD.

  • Nuclear waste and Storing it safely. The nuclear waste is the unused by-products of all the fissile reactions that occur in the core. Its a risk to humans because it's radioactive, hence release radiation which can harm Human DNA. Therefore, they are stored in very robust containers, for thousands of years left to decay because of its long half-life.

Con's

Because of the potential of a Nuclear Power Plant accident, there are many safety precautions that are done: (below are some)

1. Stress tests. These are a series of tests to measure the safety of the power reactor in the case of natural disaster (earthquake, flooding and so on)

2. Continuous improvement. This is because they are constructed with the plan of operating for decades. Hence to avoid any complications with ageing systems, instruments; they must regularly update it

3. Measuring exposure to radiation. This is to ensure that it meets safety standard for the public and workers

4. Emergency contingency plans/drills. So, in the case of the emergency, they understand what is to be done to minimise any further consequences

Safety Measures

As shown in the diagram, Australia generates its 84% of its electricity form unsustainable sources; which harm the environment, possess to people’s health risk and will eventually run out. Therefore, Australia should adopt the Nuclear power plant. The reasons being include that, it generates at least 40% more electricity than the coal and natural from same quantity of inputs. Hence, will further benefit the community as it can be provided at cheaper rates as seen in the US. It is much safer to the environment compared to the other methods used. Because we have 33% of the world’s uranium; it would be very easy to sustain the fuel used; hence making it cheaper by removing all transportation costs. Finally, it will increase the economy by requiring more jobs to build and operate it.

Use of Nuclear Plant in Australia

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