|Fig. 1: Nuclear Power Plant (Source: Wikimedia Commons)
Renewable energy can be naturally replenished indefinitely on a human timescale and comes from renewable sources. It is often also referred to as clean energy. Some examples of sources of renewable energy include rain, sunlight, wind, geothermal, waves, and tides. There are many benefits of renewable energy. It can strategically help to diversify the energy supply so that there is less of a dependence on imported fuels. It is environmentally beneficial because using renewable energy reduces many types of air pollution and generates no greenhouse gasses. It is economically beneficial because the manufacturing, installation, and maintenance of renewable energy plants create jobs and development. 
Nuclear energy is the energy that comes from the core of an atom (the nucleus). In order to capture nuclear energy, it must be released from the atom. Nuclear fission is a process in which an atom splits into two or more smaller nuclei. A nuclear reactor is a device that can start and control a chain of nuclear fission reactions to produce electricity. The core of the reactor contains uranium fuel. Fission takes place inside nuclear power plants. Specifically, nuclear power plants generate electricity by using steam to spin turbines (see Fig. 1). This steam is generated by the heat produced during nuclear fission. 
One of the main components of classifying an energy source as renewable is whether or not it can be considered a clean energy source. Here we define a clean energy source as one that does not release greenhouse gasses during production or release air pollution. This is what makes clean energy sources much more environmentally friendly and sustainable than fossil fuels. The main byproduct of nuclear energy production is excess steam. This excess steam is recycled into the atmosphere as clean water vapor. However, there is also radioactive material that can be generated as a byproduct of nuclear fission. This material is highly toxic, and can negatively affect the environment if not properly disposed of, especially as it can last for thousands of years. Although it does not release greenhouse gasses, the radioactive pollutants technically go against the definition of a renewable energy resource.
Another key component of classifying an energy source as renewable is that it can be naturally replenished indefinitely on a human timescale. As discussed in the introduction, conventionally nuclear power plants use uranium as their primary form of fuel. These conventional reactors use U-235 specifically which is a non-renewable resource. However, in 1983, Dr. Bernard Cohen, a physicist, and professor at the University of Pittsburgh proposed that natural uranium extracted from seawater (U-238) is effectively inexhaustible. Specifically, he was able to show that breeder reactors using nuclear fission can supply energy needs as long as the remainder of the sun's current lifetime which is about five billion years.  This makes the resource naturally replenishable on a human timescale. The reason for this is that breeder reactors are nuclear reactors that consume less fissile material than they generate.  Breeders do use uranium fuel much more completely than light water reactors, but after more uranium reserves were found in the 1960s, interest in them declined. 
Cohen's argument holds only as he believes that the amount of uranium available is much higher than what is considered currently to be extractable (by accounting for the uranium in the earth's crust and in seawater). Specifically, he uses the facts that seawater contains 3.3 × 10-9 (3.3 parts per billion) of uranium and that 3.2 × 102 tonne/yr of uranium enters the oceans from rivers, to show that all the world's energy requirements for five billion years existence of life on Earth could be provided by breeder reactors.  However, as the uranium deposit on earth is finite at the end of the day, it does not technically make the process sustainable for an indefinite period.
Overall, as nuclear power plants currently depend on a finite supply of uranium and release radioactive waste, nuclear energy cannot generally be considered a renewable energy source. However, as it does not release greenhouse gasses, it can still be considered a low-carbon fuel that can help fight against climate change.
© Sowmya Patapati. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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