On December 20th, 1951, four glowing light bulbs in Arco, Idaho heralded the first use of nuclear power for electricity generation . Today, twenty-nine countries operate nuclear power plants, and these produce about 14% of the world’s electricity . This electricity comes from a seemingly ideal source: nuclear power is cost-effective, does not rely on fossil fuels, and emits the same carbon equivalent per kilowatt-hour as wind and hydropower . However, the 25th anniversary of the Chernobyl disaster and the recent and evolving Fukushima Daiichi disaster serve as reminders that the benefits of nuclear power do not come without risks.
Basics of nuclear energy
All matter is composed of atoms. Atoms contain a dense nucleus composed of protons and, in all cases but hydrogen, neutrons (Figure 1). The nucleus (plural, nuclei) is surrounded by a cloud of electrons, which contribute very little to the overall mass of an atom. The number of protons defines the atomic number, which is distinct for each element. The number of neutrons defines the isotope number. Many elements have multiple isotopes, and these vary in their stability.
Figure 1. Diagram of an atom. (Illustration by Leila Ross)
There are two main types of nuclear reactions: fusion and fission. In a fusion reaction, the nuclei of two atoms are joined together to create a heavier atom. Nuclear fission, which powers nuclear reactors, is the splitting of nuclei.
The nuclei of some large atoms like uranium, thorium, and plutonium are unstable and can be split when bombarded with neutrons. The splitting of a large nucleus releases multiple lighter nuclei, a large amount of energy, and free neutrons. If these neutrons collide with other unstable nuclei, they can create a chain reaction that continues the nuclear fission process . If a “critical mass” of nuclear fuel is used, the chain reaction can become self-sustaining. Chain reactions have the possibility of releasing enormous amounts of energy, but can also run out of control.
How to control a chain reaction
Controlled chain reactions, which are used in nuclear power plants and many medical applications, employ several methods to slow down or absorb the neutrons produced by nuclear fission, which in turn slows the fission rate until the reaction eventually stops. In nuclear power plants, nuclear fuel is typically shaped as rods, which are arranged in the nuclear reactor core. These fuel rods contain small pellets of fuel clad in a corrosion-resistant zirconium alloy that allows neutrons to pass through. The energy released in the fission reaction is used to boil water into steam that spins turbines to produce electricity. Fuel rods are submerged in a circulating water bath to prevent overheating. Control rods made of neutron-absorbing materials (e.g., cadmium, hafnium, boron) can be inserted or withdrawn from the reactor core to adjust the rate of the reaction . The reactor core is surrounded by thick walls of concrete and steel to help shield the reaction from the outside world and the world from the reaction, especially in the event of a meltdown.
Meltdowns and Fukushima Daiichi
The large amounts of heat released by fission reactions must be tightly controlled. The term “meltdown” refers to any situation in which the reactor core is not covered by water, leaving the nuclear fuel to overheat and melt. In the Fukushima Daiichi plant disaster, the reactor core survived the magnitude 8.9 earthquake, but not the tsunami and power outages that followed. Although many of the details are still unclear, most reports hold that all six of the plant’s reactors were shut down after the earthquake, and that control rods were inserted into their cores to absorb neutrons [2,5]. Control rod insertion greatly slows the rate of the fission reaction, but cannot stop it right away. If all else is maintained, the control rods will gradually slow and eventually stop the fission reaction.
The Fukushima Daiichi plant appears to have been able to slow the fission reaction, but not to maintain its fuel-cooling systems. The water used for cooling the reactor core leaked, leading to an increase in temperature. The electricity-dependent emergency cooling systems failed when the tsunami cut off the connection to grid power and overwhelmed the backup diesel generators. From currently available information, it is known that after the cooling failure, there were explosions in four of the six reactors, and a partial meltdown occurred .
Hydrogen explosions and radioactivity?
The failure of the Fukushima Daiichi cooling systems led to an increase in temperature, which is believed to have increased the pressure inside the core as the remaining water became steam. High temperatures and pressures can cause the zirconium alloy used to suspend the nuclear fuel to become reactive or even melt . This is problematic on two levels – first, a reaction between the alloy and steam that occurs at high temperatures releases explosive hydrogen gas. Venting the reactor core helps prevent a dangerous build-up of hydrogen gas, but also releases radioactive material to the outside world. Some of this material is directly linked to human cancers, and people can become sick or even die from radiation exposure . However, allowing hydrogen gas to accumulate in the reactor core could lead to an explosion, which could lead to disastrously large releases of radiation.
Second, if the zirconium alloy were to melt, the fuel pellets would no longer be separated. This could allow neutrons released from a fission reaction to bombard previously inaccessible nuclear fuel. If enough fuel is present, a “critical mass” could restart the chain reaction — but this time, without many or possibly any of the measures previously in place to control the reaction. This does not turn a power plant into a nuclear weapon, as power plants should not have the large amount of fuel required for a weapon on-site, but it is still a dangerous situation.
Figure 2. Diagram of the General Electric boiling water reactor (BWR) 3 Mark 1 containment design at Fukushima Daiichi. (Image courtesy of the Nuclear Energy Institute )
New reactor design and retrofitting
The Fukushima Daiichi plant is a generation II reactor, similar to many of the nuclear power plants built in the United States in the 1970s  (Figure 2). It is the same model as six (out of 104) US plants . Generation II reactors lack several of the safety features present in generation III and beyond. As nuclear power plants are quite expensive to build, retrofitting the existing generation II plants in addition to building improved generation III+ reactors may be a useful compromise.
In the event of a nuclear power disaster, the first priority is to stop the fission reaction, then to cool the fuel. The fission reaction is slowed and then stopped by insertion of neutron-absorbing control rods. This process requires electrical power in generation II plants. Some modern plants suspend the control rods above their slots with electromagnets. If power is lost, the control rods are released into place . To cool the fuel, older reactors rely on electrically-powered pumps to circulate water in the core, while generation III+ reactors use a passive cooling system that does not rely on electricity. Cooling water is also stored in towers above the core to replenish the water in the core by gravity in the event of a leak. To prevent the build-up of explosive hydrogen gas and the release of radioactive material, generation III+ reactors have automatic pressure-release valves that vent gases in the core into a large container . Overall, generation III+ reactors seek to have “defense in depth”, a term referring to having multiple, independent solutions for potential problems. For example, for power supply, these defenses could include having multiple connections to the power grid, backup diesel generators in several places, and sufficient battery power to ride out a disaster .
Future improvements will involve the partnering of chemistry, materials science, engineering, and politics. One largely unexplored option is to develop an alternative to the zirconium alloy used to encase the nuclear fuel. While the zirconium alloy is largely unreactive and allows neutrons to pass through, the risk of hydrogen release is a liability. Additionally, although some newer reactors have large tanks to capture materials when the core is vented, there are no good methods for isolating these harmful materials. Finally, there must be a plan for safely transporting and storing spent nuclear fuel. Fukushima Daiichi had four times the recommended amount of spent fuel stored on-site, increasing the risk of reaching a critical mass of fuel and restarting the chain reaction . Spent fuel can currently be stored in special cooling pools or dry casks, but as this fuel will remain dangerous to humans for millennia, there need to be more robust solutions. Although several countries have proposed long-term storage of spent fuel entombed underground or in mountains [2,5] there is no consensus.
Leila Ross is a PhD student in the department of Biological Chemistry and Molecular Pharmacology at Harvard Medical School.
 The U.S. Department of Energy (DOE), Office of Nuclear Energy
Includes a concise history of nuclear energy development for power generation, medicine, and weapons. http://www.ne.doe.gov/
 The International Atomic Energy Agency (IAEA)
Includes a reflection on the Chernobyl disaster 25 years ago, and a useful booklet called “International Status and Prospects of Nuclear Power, 2010 Edition”. http://www.iaea.org/
 Bohr, N, & Wheeler, J.A. (1939) The mechanism of nuclear fission. Phys. Rev. 56(5):426-450.
Classic scientific paper first describing nuclear fission. Requires personal or institutional access. http://link.aps.org/doi/10.1103/PhysRev.56.426, DOI: 10.1103/PhysRev.56.426
 World Nuclear Association (WNA)
Includes a public information section geared towards the general public and schoolchildren. http://www.world-nuclear.org/
 The United States Nuclear Regulatory Commission (NRC) http://www.nrc.gov/
 Clery, D. (2011) Current designs address safety problems in Fukushima reactors. Science 331:1506.
 Nuclear Energy Institute (NEI)
Includes a “Resources and Stats” section with useful data, graphs, timelines, and summaries. http://www.nei.org/