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Text A

    The pressurized-water reactor (PWR) evolved from the reactor design for the propulsion system of the world's first nuclear submarine the U.S.S. Nautilus, launched in January 1954, A 60-MW prototype PWR installed at Shipping port, Pa., for electric power generation began delivering for commercial use in 1957. This was the first nuclear power plant in the United States.

    In the PWR, ordinary pure water under pressure is used both as the moderator and the coolant. A moderator is used in a reactor core to slow down the neutrons from their initial fission energies because lower-energy neutrons have a greater probability of being absorbed in the fuel to produce more fissions. The heat from the nuclear chain reaction is removed from the reactor by a coolant, which may include ordinary water, heavy water, liquid metal, molten salt, or gas.

    The water passes through the reactor core at a pressure of about 2.250 pounds per square inch and a temperature of 600°F (315°C) enters a heat exchanger in which steam is produced to drive a turbine, which in turn drives an electric generator. There are two distinct fluid loops in the design so that the coolant for the reactor core does not mix with the fluid for driving the turbine. Because of the high pressure in the coolant loop, the water there cannot boil, even at a temperature of 600°F. A PWR constructed for commercial power generation is very large because a large reactor provides power at a lower unit cost than a smaller reactor does.

Text B

    The boiling water reactor (BWR) was developed from PWR. In the BWR, cooling water enters the reactor core and is heated when it travels between the nuclear fuel elements. The water boils under the influence of the heat generated in the reactor core. The steam is collected at the top of the reactor vessel and is subsequently delivered to a turbine that drives an electric generator. The pressure in the reactor is kept at about 1.000 psi (2 kgs/sq em), and the steam temperature is about 545UF (285°C). Under these conditions, boiling can occur.

    A single-cycle BWR  has only one fluid loop and thus has fewer main components than does a PWR. In a -dual cycle BWR, a secondary fluid is used for the production of steam. The components of a BWR tend to be very large. For an 800-MW electrical power plant, a reactor vessel about 70 feet (21 meters) high and 20 feet (6 meters) in diameter is required.

    The Fukushima I Nuclear Power Plant consists of six BWRs. On March 11, 2011, reactors 4, 5 and 6 had been shut down prior to the earthquake for planned maintenance. The remaining reactors were shut down automatically after the earthquake, but the subsequent tsunami flooded the plant, knocking out emergency generators needed to run pumps which cool and control the reactors. Over the following days there was evidence of partial nuclear meltdowns in reactors 1, 2 and 3; hydrogen explosions destroyed the upper cladding of the building housing reactors 1 and 3; an explosion damaged reactor 2's containment; and severe fires broke out at reactor 4.

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