Introduction To Light Water Reactors

Bobby Zarubin
March 7, 2016

Submitted as coursework for PH241, Stanford University, Winter 2015

Typical Design

Fig. 1: Pressure Water Reactor. (Source: Wikimedia Commons)

The light water reactor is a type of thermal- neutron reactor that utilizes normal water as opposed to heavy water, a form of water that contains a larger amount of the hydrogen isotope deuterium. Normal water that contains the hydrogen-1 isotope called protium. Light water reactors use water as both and a coolant method and a neutron moderator that reduces the speed of fast moving neutrons. Light water reactors produce heat by controlled nuclear fission. There are several different vital parts of light water reactors that make the generation of nuclear energy possible. The nuclear reactions take place in the nuclear reactor core, where the nuclear fuel components are contained. The core is made of nuclear fuel rods that are pencil thin and are about 3.7 m long. These rods are grouped by the hundreds in bundles called fuel assemblies and are filled with pellets of uranium or uranium oxide. Control rods are lowered into the core and are filled with pellets of substances such as hafnium or cadmium that are responsible for capturing neutrons. The neutrons that are absorbed by the control rods cannot take part in the chain reaction, however, when the control rods are raised out of the core, more neutrons strike the fissile U-235 (or Pu-239) nuclei in nearby fuel rods, and the chain reaction intensifies as the control rods are out of the way. [1] All of these components are enclosed in a water-filled steel pressure vessel, called the reactor vessel. The heat generated by controlled nuclear fission turns the water into steam, which drives the power- generating turbines. After the steam flows through the turbines, the steam turns back into water in the condenser. In the two types of Light Water reactors addressed below, this process takes place slightly differently.

Pressurized Water Reactors

Pressurized water reactors (PWRs) make up the majority of all Western nuclear power plants. In a PWR, as depicted in Fig. 1, water is pumped under high pressure to the reactor core where it is heated by the energy generated by the fission of atoms. There are two main systems used to convert the heat generated by the reactions in the core into electrical power for residential and industrial use. The primary system transfers the heat to the steam generator, where the secondary system takes over. The secondary system transfers the steam formed in the steam generator to the main turbine generator, where the conversion to electricity takes place. After passing through the low-pressure turbine, the steam is directed to the main condenser. The cool water that is flowing through the tubes in the condenser is responsible for removing excess heat from the steam, which allows the steam to condense. After this process, the water is pumped back to the steam generator to be recycled through over and over again. In contrast to a boiling water reactor, pressure in the primary coolant loop prevents the water from boiling within the reactor.

Fig. 2: Boiling Water Reactor. (Source: Wikimedia Commons)

Boiling Water Reactors

The boiling water (BWR) reactor, depicted in Fig. 2, is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor (PWR). The major differences in the operation of a BWR from other nuclear systems is the steam void formation in the core and that in a BWR, the reactor core heats water, which turns to steam and powers a steam turbine. In a PWR, the reactor core heats water, but does not boil it. Inside the boiling water reactor (BWR) vessel, a steam water mixture is produced when pure water moves upward through the core absorbing heat. The steam-water mixture leaves the top of the core and enters the two stages of moisture separation, where water droplets are removed before the steam is allowed to enter the steam line. The steam line directs the steam to power the main turbine and the attached electrical generator. The unused steam is exhausted to the condenser where it is condensed back into water. The resulting water is pumped out of the condenser through a series of pumps and back to the reactor vessel to be reused and recycled through again. The operator can vary coolant flow through the core and change reactor power by adjusting the recirculation pumps and jet pumps.


Nuclear power plants using light water reactors have contributed 20% of the of the electrical power in the United States over the past two decades and are responsible for over 70% of non greenhouse gas emitting electrical power generation in the United States. [1] As the electrical demand continues to increase, most presently operating nuclear power plants will be nearing their 60-year operating licenses. The Light Water Reactor Sustainability (LWRS) Program is working on developing the currently operating facilities to operate beyond the 60-year license while maintaining long-term reliability, safety, security and productivity.

© Bobby Zaraubin. 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] "2012-2013 Information Digest," U.S. Nuclear Regulatory Commission, "NUREG-1350, Vol. 24," August 2012, p. 28.