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| Fig. 1: Nuclear Fission Reaction (Source: Wikimedia Commons). |
It has been argued that nuclear fission is a necessary part of the worlds portfolio in the fight for cleaner sources of energy. In fact, some argue that it is the only developed form of energy capable of replacing fossil fuels in meeting our energy demands safely and economically. [1] Indeed, the energy density of Uranium-235 fission is upwards of 70 Terajoules/kg (7.0 × 1013 J kg-1). This is about a million times more energy dense than the combustion of hydrocarbons or fossil fuels which is about 45 Megajoules/kg (4.5 × 107 J kg-1). [2] Furthermore, fission power plants have been shown to be able to run reliably at higher capacity factors the ratio of actual energy output to theoretical energy output over a given time period than other forms of energy production. This also places it ahead of other forms of energy generation which may depend on weather conditions such as solar and wind.
With such high energy density and capacity factor, nuclear energy should seemingly be at the forefront of the fight to move from fossil fuels and hydrocarbons as energy sources especially since it has minimal contribution to greenhouse gas emission. That is, however, not the case as nuclear energy suffers from not-in-my-backyard sentiments, economic concerns, and a waste problem that must be addressed. The not-in-my-backyard sentiments come from fear of nuclear accidents such as the infamous Fukushima event in 2011 and subsequent difficulties trying to find places to build nuclear plants as very few residents are willing to let such plants be built in their community. The economic concerns primarily stem from initial costs of building nuclear plants which tend to be higher than those of fossil fuel plants. [3] Lastly, the waste problem relates to determining what to do with the by-products of nuclear fission which can be radioactive for 100s or 1000s of years. This report aims to review this waste problem by considering just how much waste would be generated in a world fully running on nuclear energy and what sort of resources would be needed to safely store this waste.
During nuclear fission, unstable isotopes of uranium and plutonium are hit by neutrons prompting them to break down into smaller atoms as illustrated in Fig. 1 and, in the process, release vast amounts of energy in the form of heat and light. This energy is then captured by water surrounding the fuel and is transferred to a turbine by steam. The spinning turbine then powers a generator producing electricity.
The smaller atoms produced by the fission processes, however, are often unstable and take some time to reach stable states. In the process, they release atomic and subatomic particles, and light collectively referred to as radiation. This radiation can be dangerous to humans so this radioactive waste must be stored till it is stable. This process may take place over the course of seconds or minutes but may also take 1000s of years depending on the atoms in question. Furthermore, what atoms are present is random but it is fairly certain that at least some of the long-lived isotopes are present after nuclear fission so the waste must be stored for as long as the longer lived radioactive waste would take to become stable on the order of at least 1000 years (length of time for decay of Cs-137 and Sr-90). [4]
To understand what this storage question could mean for a world fully run on nuclear fission, the current rate of nuclear waste generated can be examined then extrapolations may be made to understand its implications. According to the Congressional Research Service, the United States generates about 2,000 metric tons of spent nuclear fuel per year - representing the most radioactive by-products of the nuclear fission reactions. [5] This is the waste from generating about 8 exajoules per year via nuclear fission. [6]
This can be used to infer how much spent nuclear fuel is generated each year worldwide. According to the BP Statistical Review, about 24 exajoules of energy was produced by nuclear power plants towards world energy demands in 2020. This gives a nuclear world waste mass of
| 2,000 tonnes y-1 × | 2.4 × 1019 J 8.0 × 1018 J |
= | 6,000 tonnes y-1 |
Considering that the energy the world uses annually is about 556 exajoules, the mass of waste that would be produced by a completely nuclear-powered world would be [7]
| 2,000 tonnes y-1 × | 5.56 × 1020 J 8.0 × 1018 J |
= | 1.39 × 105 tonnes y-1 |
This is just the mass of waste generated. To understand how much space is needed, it needs to be converted to volume. Getting the exact density is unclear but it is estimated the United States has about 88,000 metric tons of spent nuclear fuel which should fit in a volume the size of a football field at a depth of 8 yards. [8] The corresponding density it
| 88,000 tonnes ×
(1.0936 yd m-1)3 5333 yd2 × 8 yd |
= | 2.697 tonnes m-3 |
The volume of waste that would be produced each year is then
| 1.39 × 105 tonnes 2.697 tonnes m-3 |
= | 5.15 × 104 m3 |
This is the volume that would be needed to store the spent nuclear fuel produced in one year. As mentioned, this may need to be stored for up to 1,000 years for safety reasons. This implies that about 51 million m3 of storage would be needed for storage. For comparison, a standard Olympic size swimming pool is about 2500 m3 suggesting the world would need over 20,000 Olympic size swimming pools for storing this waste. Furthermore, it is important to note that the spent nuclear fuel is a fraction of the waste actually produced in nuclear reactors as the equipment (e.g filters) used for running the reactors, components of the reactors, and even clothing and tools also become radioactive to some degree and would need to be treated or stored (though not necessarily for the same length of time).
That said, this space requirement is actually not as intractable as it seems. For comparison, 82 million barrels of oil was produced in the world each day in 2010. [6] The corresponding volume of oil produced each year is
| 8.2 × 107 bbl day-1
× 365 days y-1 6.29 bbl m-3 |
= | 4.79 × 109 m3 y-1 |
That means in a year, the world produces, in oil, 93 times the volume required to store the nuclear waste accumulated over 1000 years for a world run on nuclear energy.
Indeed, nuclear fission is a viable option for offsetting the current use of fossil fuels. Its energy density and capacity factor should make it one of the top choices for this. It suffers from a bit of a storage problem that seems prohibitive but is not intractable. Additionally, proponents have suggested irradiating the waste to quicken breakdown while others are looking into ways to reuse the waste products. Breeder plants have been proposed which may be able to increase nuclear energy efficiency up to 100 times. [9] These technologies are fairly new so storage is still the most employed method of dealing with this waste. Even as posed, however, storage may still be acceptable just considering the volumes needed. That said, it inherently poses a security concern as radioactive waste, in the wrong hands, could prove dangerous. This would need to be properly studied and accounted for when building storage units.
© Adura Jibodu. 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.
[1] B. W. Brook et al., "Why Nuclear Energy Is Sustainable and Has to Be Part of the Energy Mix," Sustain. Mater. Technol. 1-2, 8 (2014).
[2] O. Eriksson, "Nuclear Power and Resource Efficiency - a Proposal for a Revised Primary Energy Factor," Sustainability 9, 1063 (2017).
[3] "The Future of the Nuclear Fuel Cycle," Massachusetts Institute of Technology, 2011.
[4] A. G. Croff, M. S. Liberman and G. W. Morrison, "Graphical and Tabular Summaries of Decay Characteristics for Once-Through PWR, LMFBR and FFTF Fuel Cycle Materials," Oak Ridge National Laboratyry, ORNL/TM-8061, January 1982.
[5] "U.S. Spent Nuclear Fuel Storage," Congressional Research Service, R42513, May 2012, p. 11.
[6] "BP Statistical Review 2011," British Petroleum, June 2011.
[7] "BP Statistical Review 2021, British Petroleum, June 2021.
[8] J. Thompson, "Nuclear Power Is Clean If You Ignore All the Waste," High Country News, 1 Jan 20.
[9] B. L. Cohen, "Breeder Reactors: A Renewable Energy Source," Am. J. Phys. 51, 75 (1983).
