Controlled Fission

A nuclear reactor operates by the controlled fission of 235U. Fission occurs at a slow steady rate, rather than suddenly in a fraction of a second, as in a bomb. Fission produces heat, and this heat is used to generate electricity, in the same way that the heat of burning oil or coal generates electricity in a conventional power plant.

Continuous operation depends on each fission producing neutrons for the next fissions. But we do not look for the multiplying effect. Rather, after each fission we want to produce, on the average, one additional neutron to initiate another fission. In that way the process doesn't grow rapidly, but continues at a constant rate.


It is not economically practical to generate electricity using highly enriched uranium. Some reactors operate with natural uranium (0.7% 235U), some with slightly enriched uranium (3% 235U). Since weapons require about 90% 235U, the uranium used in reactors cannot be diverted to weapons use.

Slow Neutrons

The presence of large amounts of 238U in the reactor imposes important constraints on the reactor's design. As neutrons collide with the 238U nuclei there is a growing chance of them being absorbed (producing 239U). There will eventually be so much absorption, that not enough neutrons will remain to initiate further fissions, and to maintain the reactor in operation. On the other hand, when neutrons strike a 235U nucleus, there is a high probability that they will be absorbed, and produce the unstable 236U, which then fissions. This probability is greater if the neutron is moving relatively slowly.

[To be more precise, the neutrons emitted in fission are moving, typically, with speeds around 1/10 of the speed of light. The probability of absorption in 235U is much greater if the speed is brought down to the region of 1/100,000 of the speed of light.]

The Moderator

Thus, reactors are designed to operate with a moderator. This is a substance, spread throughout the reaction space, containing light nuclei that do not absorb neutrons. In collision with a light nucleus (such as hydrogen or carbon), the neutron scatters, and as result loses a considerable fraction of its energy. After a few such scatterings, its energy gets down to the level where it has a high probability of absorption by 235U. The moderator in most American reactors is water.

Fuel Rods; Water

The reactor consists of a large pool of water. Immersed in this pool are a large number of fuel rods. These are narrow cylinders that contain small pellets (about the size of a pea) of uranium. Neutrons released in one fission will usually travel out of one of the fuel rods, and have to pass through some water before they encounter uranium in another fuel rod. The effect of the water, primarily its hydrogen nuclei, is to slow the neutrons, and allow them to initiate further fissions. Without the moderator fission probability is too low and the reactor stops.

Control Rods

The third major element in the reactor (beyond the fuel and the moderator) is the control rods. To keep the reactor running at a constant rate there are cylinders in the pool made of material that readily absorbs neutrons. One example is boron, which has two stable isotopes, 105B and 115B. The 105B readily undergoes a neutron absorption reaction, to form 115B. If the reactor begins to increase its rate of fission, the control rods are moved a little deeper into the pool. They absorb more neutrons, there are fewer fissions, and the reactor slows down. If the reactor runs too slowly, the control rods are moved a little out of the pool.

Energy Production

The effect of continuous fission is to raise the temperature of the whole system. The energy of fission appears in the form of the motion of fission fragments flying apart from each other. This is happening in nuclei all over the reactor. Temperature is really nothing more than a measure of the average speeds (or, to be more precise, the energies) of the atomic particles. So the fuel, with lots of fissioning nuclei, is at high temperature.

Pressurized Water

The water in the reactor is not stationary. Cool water is constantly flowing into the reactor and getting heated by contact with the fuel rods. Then, at high temperature, it flows out. Here is another important part of reactor technology: The temperature reached in a nuclear reactor is in the range of 300 degrees Celsius. This is higher than the usual boiling point of water, 100 degrees. But the boiling point of water is not always 100 degrees. It can be increased if the water is kept under high pressure (higher than the usual atmospheric pressure of about 76 cm or 30 inches as measured by a mercury barometer).

[This is a general property of the phases of matter: solid, liquid and gas. The temperature at which a phase change occurs can be quite different at different pressures. This point is further discussed in a link, which is not a required part of this site on reactors.]

This pressurized hot water is what produces electricity. Outside of the nuclear reactor swimming pool, the water is allowed to vaporize, forming steam under high pressure. The steam causes the rotation of coils of wire that turn in the space between the poles of a magnet. That "electrodynamic" effect causes the flow of electric current.

Functions of Water
Water in the reactor is thus a heat transfer device, a cooling system (to keep the fuel from getting too hot), and the moderator.

In some reactors the moderator is heavy water, water in which the hydrogen isotopes are 2H instead of 1H. In other reactors the moderator is carbon in the form of graphite. The very first reactor, built by Enrico Fermi in 1943, used a graphite moderator.

Nuclear Fear


Nuclear science has been part of public knowledge for a full century now, and nuclear reactors have been known for more than half a century. Nonetheless, the public still retains a certain fear of nuclear technology which far exceeds its fear of other technologies (e.g. electricity, or the internal combustion engine). To some this fear is irrational, and holds back important progress; to others this fear is justified. One focus of nuclear fear is the possibility of a nuclear accidents in a reactor: runaway production of energy (as in a bomb), or a release of radioactivity to the environment. We have seen that reactor design precludes a runaway nuclear explosion, but a chemical explosion did occur in the accident at Chernobyl in Ukraine, as well as a serious release of radioactivity. This event, however, was by far the worst nuclear accident, and the Chernobyl design was faulty.

Nuclear Waste

A second focus of nuclear fear is the consequences of long-term storage of nuclear waste.

New Technologies

The design of today's nuclear reactors had its beginning in the years immediately after World War II. The reactor was originally planned as a small unit to be used to power nuclear submarines. It was then modified, and enlarged, for large-scale generation of civilian electricity. Thus, the questions of long-term safety and environmental protection that concern people today were never considered in the early years when nuclear power was being introduced. In anticipation of a continuing and expanding need for nuclear power, both in the U.S. and elsewhere, many scientists and engineers today -- with the support of the federal government -- are working on new designs for reactors, still using the fission of heavy elements as the basic source of energy. One of these is the heavy-metal reactor.

Why Nuclear Energy?

Since the Three-Mile Island accident in 1979, no new nuclear power plants have been built. Although safety concerns have played a role, the major reason is that nuclear plants have become much less competitive than was anticipated. To a large extent this is because the price of oil has stayed low, and because coal has come into wider use for generating electricity. Still, nuclear power is an important part of the energy scene. There are more than 70 nuclear plants operating in this country. In France, about 80% of the electricity is generated by nuclear plants, and nuclear energy continues to be important in Russia and Japan. In the world, 17% of all electricity is generated by nuclear power.

Supporters of nuclear energy argue that it is unwise to rely as heavily as we do on fossil fuel sources (oil, coal, natural gas), for four reasons: (1) supplies of these are limited (although estimating how much there is in the earth is not as reliable as we would like); (2) oil comes from politically unstable parts of the globe; (3) burning fossil fuels produces air pollution (toxic gases such as carbon monoxide and sulfur oxides); (4) global warming due to increased levels of carbon dioxide, is a consequence of burning fossil fuels, and nuclear fission is a source that does not produce CO2.

Supporters also claim that new designs will make nuclear plants safer and cheaper. One of the reasons for the increased cost of these plants is the need to add safety devices to a design that goes back to the 1950s, a time when safety was not a paramount consideration.

Alternative Fuels

Opponents of nuclear energy look to the development of renewable energy sources (direct use of sunlight, wind, biomass fuels). A recent study, sponsored by a group that strongly favors renewables, concludes that renewables could supply half of America's energy needs within about 40 years. This leaves the problem of what to do during the next few decades. Fusion energy is also not likely to be practicable until 40 or 50 years from now. Thus, some see nuclear energy as a stopgap measure, to supply energy for the next few decades, until something else is ready.


Critics also point to the increased risk of the proliferation of nuclear weapons if reactors are built more widely around the world, and nuclear materials are generally more available. The Fukushima accident heightened fear of nuclear reactors in many places around the world.

Bottom lines

In the U.S. there is a resurgence of support for nuclear energy at present, supported also by the Obama administration. Applications for licensing of new nuclear plants have increased considerably in recent years. In Germany the government has embarked on plan to radically alter the country's energy supply, eliminating many reactors right now, and all nuclear energy by 2022.


  • Use natural or low-enriched uranium.
  • Slow neutrons: The moderator
  • "Swimming pool" layout
  • Control rods; absorb neutrons
  • Water as moderator, coolant, heat transfer device
  • Pressurized water
  • Arguments against nuclear energy:
    • Accidents
    • Waste
    • Expense
  • Arguments for nuclear energy:
    • Pollution from fossil fuels
    • Political instability in oil-rich areas
    • Does not contribute to global warming.
  • Renewables: Cannot supply all energy needs for several decades.
  • Current trends