We refer in text to a variety of nuclear reactors. To clarify our terms, we offer this elementary description of the differing types of reactors that concern historians. These are all commercial power reactors, designed to produce electricity. Other sorts of reactors include: weapons or production reactors, designed to produce plutonium; research reactors, designed principally to produce neutrons, which are used in various kinds of physical, chemical, agricultural, industrial, and even criminal and archaeological research; and propulsion reactors, whose purpose is providing the motive source for naval surface and submarine vessels.
American power reactors are predominantly of two kinds: boiling water reactors (BWR) and pressurized water reactors (PWR). Because both use ordinary water as the coolant and transfer medium, water just like that coming out of a household tap, they are generically called light water reactors (LWRs), to distinguish them from reactors that use deuterium, so-called heavy water.
The PWR is the more common type of light water reactor. It is manufactured by Westinghouse. As its name indicates, the water that serves as a neutron moderator, coolant, and transfer medium circulates through the core around the fuel rods under high pressure-- approximately 150 atmospheres. Being under pressure, this water does not boil, a design feature that avoids the problems of having the coolant boiling around the core. It circulates through a route called the primary loop, into a steam generator (which is a heat