Nuclear Fuel

Regarding nuclear fuel, there has been huge controversy in public whether it is a solution to our increasing demand for energy. However, before discussing about the advantages and disadvantages of nuclear fuel, we first have to learn how it works.

Nuclear fuel works very like a thermal power station, but the way it heats the water is different from the other sources. First of all, enriched uranium is required to make up the fuel. Enriched fuel contains 3% U-235 and 97% U-238 which has higher percentage of U-235 than the natural uranium. Higher percentage of U-235 is needed for the fuel rod since U-235 is a good absorber of neutrons which is significant for the nuclear fission reaction in vessel.

Nuclear fission is described as:

Using mass deficit and E=mc^2, it is found out that about 200MeV or 3×10^-11J is released in the form of kinetic energy per fission. These fast neutrons with high kinetic energy now turn into thermal neutrons which heats the water. After that, it follows the same process as thermal power station except that reactor vessel has to be in closed-system. This is because turbine steam cannot directly flow into the reactor vessel since there is a probability of leakage of radioactive material to outside of the reactor vessel.

Now, we will learn the functions of some major instruments in nuclear reactor.

Moderator: The function of moderator is to moderate the speeds of the neutrons. Some of the most effective moderators are water and carbon in the form of graphite. When high speed neutrons inelastically collide with the moderator, there is transfer of energy to the atoms of moderator. Moderator is important because it lets neutrons to lose enough kinetic energy so that they can move at the thermal speeds and also have high chance cause further fission.

Control rod: The function of control rod is to control the rate of nuclear fission by absorbing the neutrons. These rods are often made of boron or other elements that absorb neutron very well. Absorbing neutron controls the rate of nuclear fission because it can slow down the rate of chain reaction. Chain reaction is caused by neutrons in the product of nuclear fission. Control rod is important because if it doesn’t control the rate of reaction, then the fuel rods can be overheated and melt. This is very dangerous since it leads to nuclear radiation disaster.

Chain reaction is shown below:

Sankey Diagram

As we learned previously, energy sources go through several processes before it becomes the useful energy we use and we can provide a visual representation of this through Sankey diagram. According to the Oxford IB Physics textbook, Sankey diagram is a visual representation of the flow of the energy in a device or in a process.

When drawing a Sankey diagram, there are some rules to remember:

• Each energy source and loss in the process is represented by an arrow.
• The diagram is drawn to scale with the width of an arrow being proportional to the amount of energy it represents.
• The energy flow is drawn from left to right.
• When energy is lost from the system it moves to the top or bottom of the diagram.
• Power transfers as well as energy flows can be represented.

If we flow these rules, then we can come up with a Sankey diagram for a lamp.

From the Sankey diagram, we can notice that large portion of the total energy is leaked through power generation and internal energy of light bulb. Any energy that is leaked and no longer useful is called degraded energy. Hence through the Sankey diagram, we can also know the efficiency of energies.

Thermal Power Station

According to the definition in Oxford IB Physics textbook, thermal power station is one in which a primary source of energy is converted first into internal energy and then to electrical energy.

So how does thermal power station work? In order to enhance the understanding, we will use a diagram.

To begin with, energy from the primary fuel converts into internal energy and this heats the water in vessel (boiler). Heated water then turns into steam with high temperature and pressure. After that, steam goes through the turbine and turns the blade in the turbine. The kinetic energy from the rotation then activates the electricity generator which produces the electrical energy that we need. While this is happening, the steam that passed through the turbine goes back to condenser and becomes liquid state. Then this liquid water goes back to the vessel in order to become steam again.

Specific Energy and Energy Density

When we have multiple options of energy to choose from, how do we know which energy contains more energy than the other energy? Some ways we can use are specific energy and energy density.

So what is the difference between specific energy and energy density? Specific energy is the number of joules that can be released by each kilogram of the fuel. Meanwhile, energy density is the number of joules that can be released from 1m^3 of a fuel. Hence the main difference would be while specific energy deals with the amount of energy per mass, energy density deals with the amount of energy per volume.

Table above shows specific energy and energy density of some common fuels we use in the world today except for nuclear fusion which can be a great energy source if we somehow find a way to operate it with affordable cost.

Renewable and Non-Renewable Energy Sources

In media, we see a lot of articles or news that refer to renewable and non-renewable energy sources. But many people don’t specifically know what they really are and the difference between those categories.

Best way to differentiate renewable and non-renewable energy is looking at the rates at which they are being consumed and replaced. For example, coal and oil are non-renewable resources because they can only be replaced over very long geological times. On the other hand, biomass, renewable source, can be replaced in relatively short time since it uses biological materials such as trees which replenish much faster compared to fossil fuels. Moreover, hydropower, solar energy and wind energy are also renewable energy since their sources are constantly generated from the nature.

Table above shows how many primary sources that we use in the world today.

Primary and Secondary Energy

We use many different kinds of energy in our daily lives. However, we can divide those energies into two categories: primary source and secondary source.

Primary source is one that has not been transformed or converted before use by the consumer. Such examples can be coal, petroleum, oil, and wind energy. They are primary source because coal, for example, is burned in furnace in order to convert the chemical potential energy into the internal energy of the water and surroundings. Moreover, kinetic energy in the wind is also converted to electricity which we will later know as secondary energy.

Secondary source is one that results from the transformation of a primary source. For example, as mentioned above, electricity is a secondary energy since it is the result from the transformation of kinetic energy from the wind. However, there is another secondary source that we are not familiar with: hydrogen. By obtaining hydrogen from hydrocarbon or water, we can react it with oxygen which releases relatively large amounts of energy.

What Do We Learn in Unit 8?

According to the IB Syllabus, Topic 8 (Energy Production)  is divided into two units:

• 8.1-Energy resources
• 8.2-Thermal energy transfer

Even though this unit only requires 8 hours, which is relatively small compared to other units, it is one of the most units to learn since various kinds of energy production bring tremendous amount of impact to our Earth’s environment. Moreover, this not only matters to scientists, but also to everyone since if we want to make appropriate decisions about the future of our energy provision, we need to have a good understanding of this topic.

Throughout the unit, we will learn:

• Describing the basic features of fossil fuel power stations, nuclear power stations, wind generators, pumped storage hydroelectric systems, and solar power cells
• Describing the differences between photovoltaic cells and solar heating panels
• Solving problems relevant to energy transformations in the context of these generating systems
• Discussing safety issues and risks associated with the production of nuclear power
• Sketching and interpreting Sankey diagrams
• Solving specific energy and energy density problems
• Sketching and interpreting graphs showing the variation of intensity with wavelength for bodies emitting thermal radiation at different temperatures
• Solving problems involving Stefan-Boltzmann and Wien’s laws
• Describing the effects of the Earth’s atmosphere on the mean surface temperature
• Solving albedo, emissivity, solar constant, and the average temperature of the Earth problems