The AP1000: A Pressurized Water Reactor

Introduction

Fig. 1: AP1000 training in China for a reactor currently under construction. (Source: Wikimedia Commons)

The Advanced Passive 1000 (AP1000) is a nuclear pressurized water reactor (PWR) originally designed by Westinghouse Electric Company. With the AP1000, Westinghouse sought to design a reactor that operated using conventional pressurized water reactor principles, while incorporating novel safety measures in order to increase reliability and cost-competiveness relative to competing reactor plants.

History of the AP1000

After decades of research and reactor plant designs, Westinghouse began the preliminary certification process through the U.S. Nuclear Regulatory Commission for its AP1000 in early 2002. [1] After several rounds of design modifications to accommodate the increasingly augmented nuclear safety policies both in the U.S. and globally, construction began on the first AP1000 site in 2008 in China and is nearly completion. [2] As of today, this AP1000 site is scheduled to begin supplying power in late 2017, marking the world's first operational AP1000 plant. [2] As seen in Fig. 1, China has been conducting extensive AP1000 training sessions in preparation for the first operational plant. By the end of 2020, Toshiba, the new majority owner of Westinghouse Electric Company, expects that there will be eight fully operational AP1000 reactor plants (four in China and four in the United States) with plans to expand into several other countries in the next decade.

Pressurized Water Reactors (PWRs)

Fig. 2: The Pressurized Water Reactor process. (Source: Wikimedia Commons)

Pressurized Water Reactors operate using a four-step process, which is illustrated in Fig. 2. Fission in the core of the reactor vessel creates heat, which is carried to the reactor's steam generator. The steam generator contains both a primary and secondary coolant loop. [1] Using heat from the primary coolant loop, water is vaporized in the secondary cooling loop to produce steam. [1] This steam is then directed to the reactor's main turbine. [3] The steam's entrance into the main turbine causes the turbine's generator to turn, which is the source of the reactor's electricity production. [3]

Conventional PWRs also rely on electrically powered pumps to recirculate water which has been condensed from surplus water, as well as safety systems, which require onsite generators in the event of power outages. [3] These power- dependent safety measures were considered a cost-intensive drawback to traditional PWR designs and, as such, Westinghouse saw an opportunity to improve the design aspect with the AP1000 and its innovative passive safety measures, which maintained, if not slightly improved, reliability and cut costs.

AP1000 Design Features

The AP1000 is distinct from other similar pressurized water reactors in the simplicity of its design and operation, which cut down on its lead time to construct and the capital investment required to operate once functional. Relative to a standard reactor plant of a similar size (1117 MWe), the AP1000 has 35% fewer pumps, 80% less safety piping, and 50% fewer safety valves. [1] While this statement might suggest the AP1000 has fewer safety measures, these figures are meant to display the simplified nature of the plant as a whole. Moreover, the lower number of safety pipes and valves actually provides greater safety margins than another current plant model, the Watts Bar, in the event of failures such as a tube rupture or break. [1]

The second major factor that differentiates the AP1000 from several of its competitors is its passive safety mechanisms that handle major reactor accidents. Typical Gen II reactor plants operate off of electrical pump systems that actively replaces water in the event of coolant loss. [4] These pumps often have several redundant "trains" of high and low pressure valves to decrease failure probabilities, which results in significant capital and labor investment for a system that should rarely, if ever, need to be used. To rectify this problem, the AP1000 passive core cooling system sought to simplify the process by utilizing reservoirs that are built to release water into the reactor at specific thresholds that would indicate an emergency. [5] This results in fewer moving parts within the reactor and less initial capital investment into the reactor's safety components.

Conclusion

The AP1000 was originally built by Westinghouse Electric Company to address cost-based issues that were inhibiting the growth of the nuclear energy industry. Nuclear plants required significant capital and labor to both build and maintain once operational. The AP1000 increased nuclear reactors' cost-competitiveness largely through the simplification of its design and safety systems, while also increasing reliability. [4] The simplification of the reactor system significantly decreased the initial capital investment required to fully construct a reactor and reduced the lead time required to build an operational plant. Moreover, the reduction in safety pumps and valves makes the AP1000 a more cost-efficient plant to maintain over its 60-year lifespan, which significantly increases its economic viability.

© JohnHenry Styles III. 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.

References

[1] T.L. Schulz, "Westinghouse AP1000 Advanced Passive Plant," Nucl. Eng. Des. 236, 1547 (2006).

[2] M. L. Wald, "Approval of Reactor Design Clears Path for New Plants," New York Times, 22 Dec 11.

[3] B. Zarubin, "Introduction To Light Water Reactors," Physics 241, Stanford University, Winter 2015.