|Fig. 1: An example of a pressurized water reactor. (Source: Wikimedia Commons)|
In a 1939 paper, Meitner and Frisch first described the phenomenon that we now refer to as fission.  The nucleus absorbed an electron - something rare but by then not unheard of in experiments - but the resulting nucleus destabilized and broke into two. The event released 200 million electron volts of energy, and was the first confirmation of Albert Einstein's 1905 paper proposing that E=mc2.  It is no secret that this discovery precipitated the development of nuclear weapons.
Following World War II, world leaders devoted significant attention and resources to ensuring that nuclear power would not fall into the wrong hands. President Truman insisted: "The hope of civilization lies in international arrangements looking, if possible, to the renunciation of the use and development of the atomic bomb, and directing and encouraging the use of atomic energy and all future scientific information toward peaceful and humanitarian ends."  Subsequently, policy initiatives such as the Atomic Energy Acts and President Eisenhower's Atoms for Peace Program were undertaken with this goal in mind. It is therefore rather ironic that the first design for the world's most commonly employed nuclear reactor - the pressurized water reactor (PWR) - was initially conceived as a generator for a nuclear submarine. 
The first electricity produced by a nuclear rector was produced in December 1951 in Idaho, while Russia soon followed in 1954 with the first nuclear-powered generator.
A nuclear reactor exists for the purpose of containing the reactions that occur during nuclear fission. Nuclear fission is the process through which one atom splits into two, releasing energy. When the atom splits, huge quantities of heat and radiation are released. The two resulting atoms release radiation, as well. The type of radiation (e.g. alpha, beta, gamma) depends on the fuel source used.
It was Niels Bohr who first suggested that the speed of the neutron was an important factor in the fission process, and Enrico Fermi, working in the US, who developed the theory of a 'moderator' - a substance that could slow down high-energy neutrons in a series of collisions until they were moving slowly enough to be captured by another uranium nucleus.  Moderators could include water or graphite. The use of different moderators is often used to distinguish one type of reactor from another.
Uranium and plutonium are the most commonly used elements due to the large size of their nuclei. When a large nucleus collides with a slow-moving neutron, it destabilizes yielding energy and further release of neutrons.
Enriched uranium is typically formed into small pellets, which are then arranged into rods, which are bundled and submerged in a vat of water. These rods are typically 1-2 cm in diameter and can be as long as 7m long. The rods are arranged into a square configuration (with spaces left for coolant water to flow freely through) constituting the core. This core is sheathed in a pressurized, submersible steel case.
Within the bundle, there are a number of control rods composed of material that absorbs neutrons. By raising and lowering these control rods, scientists can control the nuclear reaction rate as desired. The bundle's rods are strategically arranged so that they can be lowered into the bundle completely in order to shut down the reactor in case of emergency.
There are two circuits of a pressurized water reactor. The primary circuit refers to the cooling loops, a series of external pumps that circulate water upwards through the vessel and through the fuel clusters at a pressure of roughly 2,000 psi. Also included in the primary circuit are the pumps that bring water out of the vessel, to the steam generators, and then back to the vessel. The secondary circuit allows water to come to a boil, form steam, and drive the turbines. See Fig. 1 for an illustration of the path of a circuit.
What distinguishes one circuit from another is the pressure at which water is circulated. Whereas the primary circuit's pressure of 2,000 psi does not allow water to boil, the secondary circuit's lower pressure of around 750 psi allows water to boil and steam to form.
Refueling the reactor requires a complete shutdown. Once it has cooled and depressurized, the pressure vessel is removed and its components are changed as needed. A reactor can go up to two years before refueling.
The difference between fission in a reactor and a bomb is the pace of the reactions. For certain isotopes, one fission releases neutrons, which then collide with and split other atoms, and the chain reaction continues. In the case of a bomb, the reaction must happen very fast to yield an explosion. In contrast, a slow chain reaction is desirable in a reactor in order to create heat.
© Alexandra Crerend. 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.
 N. Bohr and J. A. Wheeler, "The Mechanism of Nuclear Fission," Phys. Rev. 56, 426 (1939).
 J. Wood, Nuclear Power (Institution of Energy and Technology, 2007).