Passive Safety-Related Systems
The AP1000’s passive safety systems include the passive core cooling system, containment isolation, passive containment cooling system, and the main control room emergency habitability system.
A major safety advantage of passive plants is that long-term accident mitigation is maintained without operator action or reliance on off-site or on-site AC power. Instead of relying on active components, the AP1000 relies on natural circulation to keep the core and containment from overheating. For example, in the event of a design-basis accident, such as a coolant pipe break, the plant is designed to achieve and maintain safe shutdown conditions. To provide high reliability, these systems are designed to move to their safeguard positions upon loss of power or upon receipt of a safeguards actuation signal — that is, a single move powered by multiple, reliable Class 1E DC batteries.
The passive safety system design does not require the large network of active safety-grade support systems (such as AC power, diesel generators, and HVAC) that are needed in a typical nuclear plant. Therefore, less Seismic Category I building volume is required to house the safety equipment, resulting in an approximately 45% smaller footprint compared to an existing nuclear power plant with the same generating capability. This provides a large capital cost savings, as seismic structures cost roughly three times as much as nonseismic structures.
The AP1000 uses extensively analyzed and tested passive systems to improve the defense-in-depth safety of the plant. The ACRS and the NRC have scrutinized these systems and ruled that they meet all the required criteria.
These defense-in-depth capabilities for accident mitigation result in extremely low core-damage probabilities while minimizing occurrences of containment flooding, pressurization, and heat-up. For example, the AP1000’s probabilistic risk assessment (PRA) core damage frequency (CDF) is 1/100 of the CDF of currently operating plants and 1/20 of the maximum CDF deemed acceptable for new, advanced reactor designs.
The AP1000 is designed to mitigate a postulated severe accident such as core melt. Additional features and improvements include the absence of bottom-mounted in-core instrumentation and a lack of vessel penetrations below the top of the core. Having the core lower in the reactor vessel minimizes core temperature excursions during loss-of-coolant accidents. The AP1000 operator can flood the reactor cavity space immediately, thereby surrounding the reactor vessel with water. The cooling is sufficient to prevent molten core debris in the lower head from melting the steel vessel wall and spilling into the containment.
Improved Operations and Maintenance Efficiencies
Operating U.S. nuclear plants are already competitive producers of electricity compared with coal-fired plants. They also have the advantage that fuel accounts for about 25% of production costs for nuclear power, while the remaining 75% is for fixed costs of operation and maintenance. Therefore, nuclear power production is much less sensitive to changes in fuel costs than fossil-fueled plants, where fuel can account for 75% or more of the production costs.
As an added benefit, the AP1000 reactor has several design features that improve plant production, enhance worker safety, and reduce costs:
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The modular plant design and component standardization ensures a high degree of reliability, requiring significantly reduced maintenance, staging, and testing and inspection requirements.
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An 18-month fuel cycle results in improved availability and reduced overall fuel costs.
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Radiation exposure and the volume of generated plant radwaste are reduced.
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A 60-year design life.
Competitors in the race to build the next generation of U.S. nuclear plants may be slow out of the blocks, but we expect the level of activity will accelerate in 2010 as companies that are serious about constructing Generation III+ reactors ramp up staff and on-site construction presence in preparation for a full construction release in 2011 – 2013. We expect Plant Vogtle to be the first of the next generation of nuclear plants to enter commercial service during 2016.
—James M. Hylko (jhylko1@msn.com) is a POWER contributing editor.
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