Day by Day

14 March 2011

Japan, anatomy of their nuclear reactors...

The reactors at risk in Japan are BWR, boiling water reactors. Reactors in use in the United States, specifically those operated at Plant Hatch in Georgia, are also BWR units. These units are simplified designs that provide dry steam to power electricity generating turbines.

Plant Technical Description

Plant Hatch is a two-unit General Electric Boiling Water Reactor (BWR). Its basic design is known in the industry as a BWR-4. Unit 1 began commercial operation on December 31, 1975 and has a gross electrical output of 924 MWe. Unit 2 began commercial operation on September 5, 1979 and also has a gross electrical output of 924 MWe. These gross electrical outputs represent the new higher outputs after the implementation of the Power Uprate Program. The Power Uprate Program resulted in an Operating License amendment allowing both Hatch units to increase reactor thermal power by about 10%.

Construction began at the plant in 1968. The Architect/Engineer duty was a partnership effort between Bechtel Power Corporation and Southern Company Services. Georgia Power Company was the constructor. Unit 1 was completed at a cost of $414 million, while Unit 2 cost $520 million to finish. The Operating License for Hatch Unit 1 was granted on August 6, 1974. Hatch Unit 2 received its Operating License on June 13, 1978.

The reactors at Plant Hatch boil water using heat produced from the controlled splitting of Uranium atoms in a nuclear chain reaction. The steam produced is used to drive a turbine connected to a generator (there is a separate turbine-generator system for each reactor). The generator converts the rotating energy of the turbine into electrical power for our customers. Once the steam's energy has been extracted in the turbine, it is exhausted to the condenser where it cools and returns to a liquid state. Pumps then return the liquid water to the reactor through a series of feedwater heaters. The feedwater heaters increase the efficiency of the plant by transferring waste heat from certain stages of the turbine back into the system. Raising the temperature of the incoming feedwater improves the overall thermodynamic efficiency of the plant.

Simplified BWR Flowpath

Boiling Water Reactors operate by using the heat generated from fissioning (splitting) Uranium atoms to directly boil water. The steam produced is then dryed to reduce its moisture content and routed directly to a turbine. In contrast, Pressurized Water Reactors operate by using the energy from Uranium atom fissioning to first heat water under high pressure. The high pressure prevents the water from boiling inside the reactor vessel itself. This hot water is then ducted over to a Steam Generator where it is used to boil water in a secondary loop. The steam from this secondary loop is routed to a turbine. Both designs have their advantages. BWRs are simpler designs, but the steam produced does carry small amounts of radioactive gases into the turbine systems. This requires extra shielding around turbine-generator components and also restricts access to these areas during power operation. PWRs are more complex designs, but because of the two-loop system, the steam produced normally contains no radioactive gases and thus the turbine systems don't require additional shielding. PWR turbine-generator systems are also more accessible during power operation. The figure below shows the reactor vessel design for a GE BWR 4.

General Electric BWR 4 Reactor Vessel

Both Hatch units utilize the General Electric Mark I Primary Containment System. In the Mark I design, the reactor is surrounded by a steel drywell encircled by a steel wetwell (torus). This drywell/torus assembly is then encased in a steel-reinforced concrete secondary containment structure known as the Reactor Building. The torus is used as a large heat sink to rapidly condense any steam that may escape into the drywell during large pipe-break accidents.

General Electric Mark I Primary Containment System

The generator, a General Electric 1000 MVA unit, is powered by a General Electric 1800 RPM steam turbine with one High Pressure and two Low Pressure rotors. The reactor steam dome pressure is typically 1031 PSIG and the steam flowrate is approximately 10.9 million lbs/hour at full power. Cooling water for each unit's two condensers is provided by a closed circulating water system with four forced-draft cooling towers. Make-up water is provided from the Altamaha River to replace water volume lost to evaporation and "drift".