The Westinghouse AP1000

A Generation III+ PWR Nuclear Power Plant

Overview

The AP1000 was a design initiative from Westinghouse Electric Company LLC to create the next generation of nuclear power plant. The AP1000 started off as the AP600, which was designed to generate 600 MWe of power. The AP600 design was approved by the Nuclear Regulatory Commision in 1998, but Westinghouse realized that it was not a marketable plant due to its low power output. Thus the AP1000 was created, which would output at least 1000 MWe (the actual power output is closer to 1100 MWe) and this plant was seen to be a more marketable choice. The new plant was essentially an upgrade of the AP600 design, with a similar design footprint. The AP1000's output trumps that of today's existing two-loop plants by double the amount of power. The AP1000's design certification was approved in December 2005.

The AP1000 is a Pressurized Water Reactor, which is the type of nuclear power plant Westinghouse is most familiar with, but it makes many improvements to the standard PWR design. The specific improvements that we are going to explore are:

The Simplification of Power Plant Design
Modular Features
Passive Safety Systems

Benefits of Nuclear Power

Though the general public seems to believe that nuclear power plants generate greenhouse gases during production, this is actually not the case. Nuclear power plants emit less than one hundredth of the greenhouse gases that coal or gas powered plants do. The process of running a nuclear plant like the AP1000 produces less than 1% the CO2 produced by natural gas or coal powered plants. Nuclear power plants are responsible for the least amount of CO2 than any other source of energy production.

In addition, the production cost associated with generating nuclear power is 25% fuel and 75% on fixed operation and maintenance, while fossil-fuel-powered plants are actually opposite. Therefore, the cost required to run a nuclear power plant is not largely affected by changes in fuel costs the way that fossil-fuel-powered plants are. They are able to pay back the energy required to build them in less than 2 months of operation.

Though roughly 70% of our country's energy source is from nuclear, countries overseas have widely adopted its use in recent years. Since the disasters at Three Mile Island and Chernobyl, the general public has feared nuclear power. The nuclear industry is aware of this concern and in conjunction with the Nuclear Regulatory Commission, Westinghouse's primary concern is safety and has continuously implemented numerous applications to ensure the safety of the public as well as environment.

More Efficient Power Operations

The key to a more efficient performance from a nuclear power plant is making more electricity for less money. The AP1000 is designed to fulfill this improvement by numerous methods. Its eighteen month fuel cycle allows for regular maintenance and improved fuel costs due to refueling. While the average life cycle of a typical power plant is forty years, AP1000’s extends that by 20 more years. This extended life cycle eliminates the need for uprates, which are typically multi-million dollar procedures performed to increase the energy output of existing plants.

The estimated construction cost of the AP1000 is currently $1400 per KW for the first reactor, and $1000 per KW for additional reactors. This is much less than the standard $5000 per KW of the old second generation reactors. Lowering operating and maintenance requirements save money due to the simpler design and smaller maintenance crew needed. In addition, Westinghouse further developed the AP600 into the AP1000 to fulfill cost effective options. The estimated operating cost for the AP600 was 4.1 to 4.6¢/kWh, which was not a competetive price in the market. Westinghouse has reduced the cost of the AP1000 to about 3.0 to 3.5¢.kWh while simultaneously increasing the power, thus creating a much more lucrative range.

Simplification of Power Plant Design

Simplicity was a key concept behind the design of the AP600, whose blueprint remains almost untouched in the AP1000. Its reactor vessel is essentially the same as those for Westinghouse's traditional three-loop plants, but with nozzle adjustments. The new design however, features 60% fewer valves, 75% less piping, 80% less control cable, 35% fewer pumps, and 50% less seismic building volume than a usual reactor design. Because of this simpler and smaller plant design, it requires less equipment and infrastructure to test and maintain.

The buildings of the AP1000 nuclear power plant have a close nuclear island formation. This is different than the traditional PWR design, which has the buildings more spread out and has a greater footprint. The AP1000’s design is more practical because it houses the primary side components – those that are exposed to radiation – in the same buildings. When constructing a building that is supposed to house radiated components, there is a different level of safety measures that have to be taken. These safety levels lead to great expenses, and the AP1000’s nuclear island design creates a simpler and cheaper alternative to the standard PWR format.

Modular Design

The AP1000 construction costs and scheduling are a direct benefit from the simplistic design. To provide quick and affordable construction methods the AP1000 utitlizes a modular design technique. The major construction and piping and components are designed to be shipped via railcar and able to be easily assembled. These modules help shorten construction schedule, reduce the manpower needed in the field, increased factory-based manufacturing and not on-site, improves quality since major components can be tested while in the factory, and will reduce the construction congestion around assembly time. Due to the Modular Design techniques used the AP1000 should be able to house fuel within 36 months from the first concrete pouring.

The video below shows the modular construction techniques being put into affect.

Passive Safety Systems

The Passive Safety Systems included in the AP1000 design are arguably the greatest improvement. The idea behind the passive safety systems is that when a Reactor Trip occurs – which happens when there is a loss of coolant accident, unsafe rise in pressure, or other safety issue – the passive safety systems will take effect to cool the reactor and prevent meltdown. How the passive safety systems differ from normal safety systems is that the safety measures in the passive systems rely only on gravity and natural convection. In other words, the passive systems require no operator to control them and, more importantly, no on-site or off-site power.

The passive safety systems are considered to be a safer design than the traditional PWR design, and the key word used is Safety. The passive systems require no pumps or mechanical systems, which prevents the risks of mechanical failure. The systems also revolve around the concepts of gravity fed water and natural convection, which are natural laws that will not fail. These systems are also contained within the nuclear island, which means there is no worry about transferring water from an outside building.

The picture below shows the diagram for the AP600 passive containment cooling system. The AP1000 is the same passive safety design as the AP600 has, but it had been upgraded for the new size.

The tanks on the top of the containment vessel hold enough water to cool the reactor for 72 hours straight without the need to add feedwater. The water travels down the side of the steel housing and cools the inside. Circulation at the bottom of the tank is due to natural means, and no pumps or mechanical power is necessary. The circulation is due to the water evaporating, rising to the top, cooling and condensing. This will constantly cool the steel vessel. To aid the cooling, air is brought in and runs across the top of the steel vessel and cools it through natural convection.

To deal with an increase in pressure, the AP1000 has a gravity fed injection system. This is much like a sprinkler system, and the addition of cool water will reduce the pressure and help condense any steam.

There is also a Passive Residual Heat Removal system in place. This is a heat exchanger that, like the majority of the safety system, runs on passive means. In the event of a reactor shut down the heat exchanger removes the excess heat from the steam and feedwater systems.

Conclusion

In Spring of 2007, Westinghouse was selected to build four AP1000 units in China for a contract of $8 billion dollars, the largest international nuclear contract in history. Site construction for these plants began in February 2008, and an additional two units have been contracted to begin building in September 2008. In addition to AP1000 construction in China, contracts for two nuclear reactors were awarded in April 2008 for construction at Vogtle. This is the first agreement for nuclear development since Three Mile Island in 1979. Dan Lipman, senior vice president for nuclear power plants at Westinghouse, said: "Westinghouse and our consortium partner Shaw Group are providing four new plants in China, and we have been identified for no less than 14 plants here in the United States. Other markets are fast emerging."

The AP1000 is an economical solution for the future of nuclear power. Its unrivaled safety systems place it at the frontline of nuclear development and make it a clear choice for an effective future source of energy.