The environmental concerns associated with nuclear reactors are very different than with fossil fuel–burning power plants. On the positive side, reactors do not emit greenhouse gases or other atmospheric pollutants. They also use significantly less fuel in terms of weight than coal-fired plants. Reactors for electricity production have had an excellent safety record for the most part, and they have been used in many countries generally without incident.
Three major accidents mar that record; they occurred at Three Mile Island, Pennsylvania, in 1979; Chernobyl, Ukraine, in 1986; and Fukushima, Japan, in 2011. In addition, several experimental reactors have experienced partial meltdowns and other accidents in the early years of reactor development. As a result of these accidents, approval for new plants, particularly in the United States, has been slowed and the public perception of nuclear power tends to be negative.
In spite of these setbacks, new reactors are now being constructed and a number of reactors in the United States have been upgraded in order to increase power output, and more upgrades are planned for the future.
The first of the three incidents occurred at Three Mile Island in Pennsylvania in March 1979. The reactor was a PWR type that had a partial meltdown when feedwater pumps lost power, causing the steam generator to shut down. A valve that should have closed failed, and water began to drain from the reactor. There were no sensors to warn operators of an impending disaster, and they reacted incorrectly, shutting off the emergency cooling system. The core became exposed and suffered a partial meltdown before the situation was brought under control. Although no lives were lost, public perception of the safety of nuclear reactors was seriously compromised as a result. In the years since the disaster, the Institute of Nuclear Power Operations was formed to monitor the industry’s best practices, and the safety record of the US nuclear industry was improved and has been excellent since then. In addition, federal regulation of nuclear plants was strengthened and a new Nuclear Regulatory Commission was assigned to oversee reactor operations.
A much more serious accident occurred in the Ukraine at the Chernobyl nuclear plant in April 1986. The reactor was a graphite-moderated reactor with two major design flaws as cited by the Nuclear Safety Advisory Group of the International Atomic Energy Agency (IAEA). The flawed design problem was compounded by having inexperienced operators perform a questionable experiment that included disabling the emergency cooling systems and not following specific guidelines. All of this combined to create a catastrophic accident that started as a steam explosion. This explosion was followed three to four seconds later by a second massive explosion that killed two engineers immediately and caused thirty-one deaths within three months due to radiation sickness. Hundreds of others, mostly emergency responders, were diagnosed with radiation sickness, and an entire nearby town was permanently abandoned due to high levels of radiation.
Other environmental effects included radiation in groundwater that affected fish, and radiation sickness and cancers in cattle, horses, and wildlife. The effects continue, with latent cancers and sickness still showing up. At Chernobyl, the melted fuel rods have been entombed in tons of concrete and a huge confinement sarcophagus was begun in 2010. Other reactors on the site are still operating but an exclusion zone that is 31 km (19 mi) in radius around the site is still largely uninhabited.
Another major disaster involving nuclear reactors took place in Japan in March 2011. At this time, an earthquake—the largest on record—was centered off the coast of Japan and triggered a devastating tsunami that swept over cities and farmland in northern Japan. It struck the Fukushima Daiichi Nuclear Power Station, a complex of six reactors. The effects of the tsunami were devastating, but they were compounded by the nuclear reactors losing cooling water as a result of backup power failures; explosions and leaks of radioactive gas occurred in three of the Fukushima reactors, and a partial meltdown was recorded in at least one of the plants. Large quantities of radioactive material were released into the atmosphere, and radioactive water was released into the ocean. Nearby farmland was contaminated with radiation, resulting in restrictions on the distribution and consumption of foods from the area. Some 80,000 people in nearby towns have been evacuated, and many will never be allowed back in the area. A few weeks after the tsunami, the disaster was placed at the same level of severity (level 7) as the Chernobyl meltdown. Public confidence in nuclear energy was severely shaken as a result, and the accident has forced numerous countries to rethink their nuclear ambitions. At least twenty-five reactor projects have been shut down or cancelled in Europe in the aftermath, although China and other countries have continued to pursue nuclear power with a number of new projects. The industry has implemented safety enhancements and upgrades from lessons learned from Fukushima. In the United States, a complete review of safety and emergency preparedness was implemented for various possible disasters, including earthquakes, fires, explosions, and terrorist acts. Interest in nuclear power continues worldwide, and the United States is building new reactors in South Carolina and Georgia.
Disposal of radioactive waste from reactors is a huge issue that has not been resolved. Radioactive waste includes spent fuel rods and all other waste, and it is classified by the level of radioactivity. Products with short half-lives tend to be more radioactive because they disintegrate rapidly over time. High-level waste includes spent fuel rods that are highly radioactive and thermally hot. Spent fuel rods are stored in water for approximately ten years to cool them to safe temperatures for handling and to reduce the radiation.
After about ten years, radiation levels subside enough for the fuel rods to be reprocessed, extracting leftover fuel and plutonium for reuse, but the remaining high-level waste is dangerous for thousands of years (plutonium, for example, has a half-life of 24,000 years.) Because of concerns with proliferation of nuclear weapons, reprocessing of 238U is banned in many countries, including the United States.
Much of the high-level waste from reactors has been allowed to accumulate onsite and in reprocessing plants, postponing the eventual requirement to have a disposal plan in place. The problem is that it must be contained safely for thousands of years. Geological burial of waste is the only viable solution, but a site needs to be identified that is geologically stable, and safe transportation of the waste needs to be addressed.
There is no experience base for predicting geological events for tens of thousands of years. In the United States, Yucca Mountain in Nevada (100 miles northwest of Las Vegas) was the leading candidate for years, but there is considerable opposition to this site in part because of groundwater, transportation, and potential ground movement (earthquakes). Groundwater could be contaminated if it mixes with waste and can contribute to corrosion problems for the containers. After more than $10 billion was spent on this project, there is still no determination on what the United States should do for long-term storage of waste.
The Yucca Mountain project has an uncertain future because of litigation; meanwhile, the amount of radioactive waste continues to accumulate at scattered locations and the problem gets worse.
Nuclear power plants use steam-driven turbines and massive amounts of cooling water. The water issue is the same as that discussed for fossil fuel–burning plants. The temperature of outgoing water from one plant (Alabama’s Brown’s Ferry plant) has been too warm in several recent summers, requiring the plant to cut power output (in one case, for five consecutive weeks). Incoming water can also be too hot to provide the required cooling, which can force cutbacks.
Fusion reactors offer the promise of nearly limitless clean energy. Fusion, as already defined, is the process of putting together light nuclei to form heavier nuclei and release energy. While fusion reactions are common in particle accelerators, the goal for energy production is to produce more energy than is consumed in a controlled reaction, an elusive goal that has not been achieved. Fusion reactors require two charged particles that repel each other to come together and react, a very difficult process requiring extreme temperatures like those in the interior of the sun.
Fusion-Triggered Fission Fusion-triggered fission is being investigated at the National Ignition Facility (NIF). The idea is to use lasers to trigger fusion reactions in tiny pellets of D-T that react in a chamber. The new twist is to line the walls of the chamber with a blanket of uranium or other fuel. The fast neutrons from the fusion would cause fission to occur in the blanket, thus increasing the power over fusion energy alone.
One of the significant benefits from this method is that it can help eliminate reactor waste. The fuel for fusion reactors is almost endless. One fuel is a mixture of deuterium (written as 2H or D) and tritium (written as 3H or T). Deuterium and tritium are isotopes of hydrogen (one proton), with one neutron for deuterium and two neutrons for tritium. Deuterium is readily available in seawater and tritium can be produced from lithium, which is available in quantities that would last literally millions of years. The basic reaction between deuterium and tritium is written as follows:
n is a high-energy (14.1 MeV) neutron
This reaction, called the D-T reaction, produces a high-energy neutron, but it is not a chain reaction (the neutron does not induce further fusion events).
There are two approaches to making a fusion reactor. One is a pellet method, in which small pellets of D-T fuel are compressed by high-powered lasers, creating helium and releasing energy; this method is referred to as an inertial fusion project. The major effort for this approach is at the National Ignition Facility (NIF), which is part of the Lawrence Livermore National Laboratory. The NIF brings 192 high-power lasers to bear on a tiny target of D-T located at the center of a reaction chamber, replicating conditions at the center of stars. A major milestone was reached in the summer of 2012, when a laser pulse with a peak power of 522.6 terawatts (TW) was fired; however, the goal of achieving ignition continues to be elusive and very expensive. The NIF continues to be a major program element in the United States for developing fusion energy by advancing the science of fusion.
The second approach to making a fusion reactor is to confine super-hot plasma (108 K) of D-T fuel long enough for an appreciable fraction of the D-T to react and produce energy. There are various efforts at confinement. One project is called Alcator C Mod, which is a Tokamak design used by researchers at the Massachusetts Institute of Technology (MIT) to investigate the stability, heating, and transport properties of plasmas. The Tokamak has a doughnut-shaped reaction area; it was originally a Russian idea for confining the plasma.
The most ambitious Tokamak to date is a huge cooperative project called the international thermonuclear experimental reactor (ITER), which is under construction in Cadarache, France, and is funded by various countries. The goal of ITER is to produce more power than it consumes and to show that fusion energy is an achievable goal. The ITER Tokamak heats the plasma using neutral beam injection and two sources of high-frequency electromagnetic waves. Energy is extracted by slowing down high-energy neutrons from the fusion reaction in a blanket that surrounds the reaction and using the resulting heat to convert water to steam.
Figure 1 shows a cutaway view of the planned reactor. The original scheduled date for deuterium-tritium operation is March 2027, with the goal of ultimately taking ITER to 500 MW of fusion power.
Figure 1: The ITER Tokamak Fusion Reactor. The reaction takes place in the hollow, doughnut-shaped area in the center. (Source: Courtesy of ITER Organization.)
Fusion-based reactors have many advantages, including no greenhouse gas emission. There are no radioactive fission fragments or fuel rods to recycle, but some radioactive waste is produced due to the activation of materials by high-energy neutrons. The biggest advantage is safety because there is no chain reaction; the fuel is consumed much like the gas in a kitchen stove. A failure in a fusion reactor will simply shut it down, so there is no possibility of a Chernobyl-like accident.