AP1000: set to compete

In countries such as Japan and South Korea, new units have been continuously added to the electrical grid, but these additions have only been adequate to sustain their own country's nuclear infrastructure. Elsewhere around the world suppliers of new nuclear plants have had to make dramatic shifts in the product portfolio to focus on fuel and service activities.

However, vital signs within the global electrical industry point to a rekindling utility interest in nuclear power. The reasons are significant, and include the following:

• Nuclear economics are now competitive with the lowest cost energy sources, such as natural gas and combined cycle.

• Environmental benefits derived from nuclear power's elimination of greenhouse gas emissions are yielding high public acceptance scores.

• Step improvements in nuclear plant performance and safety are leading to significantly reduced economic risks associated with nuclear plant ownership and operation.

These points are embodied in the Westinghouse AP1000 plant design currently under review by the US Nuclear Regulatory Commission (NRC) for design certification. The AP1000 features passive safety systems to significantly reduce bulk commodities, and active components requiring stringent performance testing and safety qualification. These reductions not only simplify the plant design, thereby reducing capital costs, but also reduce maintenance and operations testing, and their associated costs. The passive safety systems, in combination with the highly reliable non-safety control systems, provide significant safety improvements as measured by probabilistic risk analysis.

generation costs

The ultimate test of whether or not new nuclear plants will be built comes down to how competitive nuclear power will be compared to other fuel sources. Although the positive circumstances mentioned above have improved the chances that new nuclear will move forward, none are as important as improved capital cost.

A universally accepted premise in the electrical industry is that nuclear plant capital costs are much higher than other fuel sources. Although the truth of this statement is marginal when compared to coal units requiring significant pollution control devices, nuclear still has a relatively high capital cost. Two characteristics help mollify the impact of nuclear plant capital costs.

• Nuclear plant availability factors have improved significantly.

• Nuclear plant operating costs in the USA are now the lowest of all fuels.

The low-cost performance of nuclear plants was confirmed by the Utility Data Institute in the 1999 data for all major generation sources of power. This excellent performance coupled with the rise in oil and natural gas prices has caused utilities to begin re-evaluating the nuclear option for future base load power additions.

Another possible negative impact on fossil fuel costs in the future may be greenhouse gas production. An exact determination of the appropriate generation cost impact that should be assumed for production of these gases is still under debate. However, values range from as little as $25 per ton of carbon to $150. Figure 1 identifies the impact of surcharges for carbon in this range on natural gas and coal generation costs. Although impacts this large clearly provide a distinct advantage to nuclear power, design efforts on next generation nuclear units utilising advanced passive features have not presumed any benefit from environmental penalties associated with fossil fuels.

Design objectives

The design objectives of the advanced passive technology are to provide greatly simplified nuclear plant designs that meet or exceed the latest regulatory requirements and safety goals, whilst being economically competitive with fossil fuel power generation.

Passive safety features

The passive safety systems use natural forces such as gravity, natural circulation, and compressed gas to guarantee plant safety. No pumps, fans, diesel generators, or other rotating machinery are used. A few simple valves align the passive safety systems when they are automatically activated. In most cases, these valves are fail safe - they require power to remain in their normal, closed position. Loss of that power causes them to open their safety alignment. These passive safety systems are significantly simpler than conventional PWR safety systems.

In addition to being simpler, the passive safety systems do not require the large network of safety support systems needed in current generation nuclear plants, such as AC power, HVAC, cooling water systems, and the associated seismic buildings to house these components. These reductions in systems led to the elimination of safety-grade emergency diesel generators and their network of support systems, air start, fuel storage tanks and transfer pumps, and the air intake/exhaust system. Figure 2 illustrates these reductions.

Passive safety systems include: passive safety injection, passive residual heat removal, and passive containment cooling. All of these systems have been designed to meet the NRC single-failure criteria and other recent criteria, including Three Mile Island (TMI) lessons learned, as well as unresolved and generic safety issues. Figure 3 shows the configuration for the safety systems. The use of probabilistic risk assessments (PRAs) has been key in evaluating system design tradeoffs and in quantifying the safety of the design.

AP1000 design

The AP1000 is a logical extension of the AP600 design. Many of the studies coming from earlier AP600 efforts provided a high-confidence level that a two-loop configuration of the passive technology could produce over 1000MWe with minimal changes in the AP600 design. In fact, maintaining as many aspects of the AP600 design as possible became a design objective of the AP1000. The obvious purpose of moving forward with the AP1000 was to optimise the power output, thereby reducing the resulting power generation costs. The objective has been met, and has led to a plant that is competitive with all types of fossil and renewable generation options.

The AP1000 is a two-loop, 1000MWe plant that keeps the same basic design of the AP600 - the nuclear island footprint stays the same as well as the core diameter. The only major change is in the height of the containment (see Figure 4). The key parameters of the AP1000 are shown in Table 1. The component changes for the AP1000 are primarily required to increase the heat transfer of additional power while maintaining safety margins. The most notable changes are the increase of the size of the steam generator heat transfer area, and the larger reactor coolant pump. One of the added benefits of larger reactor coolant pumps is a higher flywheel inertia compared to the AP600, which allows increased margin for departure from nucleate boiling (DNB) for loss-of-flow events, by providing increased coastdown flow to the core. The containment is also larger, but only in height, to accommodate the greater mass and energy of the reactor coolant system.

All of the plant components use proven technology, as seen in Table 2.

Safety assessment

Using the test data developed for the AP600 and similar designs, the AP1000 is capable of meeting higher power output requirements. The PRA shows that the plant simplifications embodied in the Westinghouse advanced passive plant design and providing enhanced safety are not power-level dependent. Results of the PRA studies are shown in Figure 5.

Improved construction schedule

The AP1000 has a site construction schedule of 36 months from first concrete to fuel load. Significant reductions in the construction schedule are absolutely necessary to ensure confidence that projected capital costs are correct. The history of US plants has demonstrated poor performance when measured by gauges that compare final versus original project schedules. One cause of the lengthening of plant schedules in prior years was regulatory delays. The combined operating licence (COL) should address many of the regulatory reasons for these delays, such as design changes and design completion prior to construction. However, reducing the construction period, must be addressed as part of a rigorous construction cost reduction effort.

The AP600 and AP1000 plant construction strategies incorporate changes that directly meet the challenge of shortening construction time. The first benefit of these designs, and probably the most obvious, is the large reductions of bulk quantities and components used in the plant design. Fewer quantities and components mean less installation time with the obvious corollary that if there is less to install, there will be fewer installation mistakes and associated rework.

A second key to shortened construction schedules is the high level of modularisation incorporated in the design. Modularisation on the advanced passive designs was incorporated in both structural and system elements. The Westinghouse advanced passive designs incorporate over 300 modules that would be built in parallel with site activities and incorporated into the construction sequencing. Figure 6 shows one of the structural modules that would be located inside containment.

The Japanese construction company, Obayashi, evaluated the Westinghouse construction schedule and verified that, in Japan, 36 months were more than adequate to complete the necessary tasks at a cost similar to that in the Westinghouse advanced passive plant evaluation. Obayashi used the quantity information and construction drawings supplied by Westinghouse, but assumed Japanese labour rates and productivities to complete the construction sequences. The results of this evaluation were then compared to the Westinghouse results obtained, assuming the plant was built at a typical site in the USA. The similarities in the costs and projected schedule were reassuring from two points of view. First, since construction labour and productivities should be similar in the USA and Japan, Obayashi results indicating a 36-month construction schedule were not only attainable, but could be further shortened, provided the independent verification necessary to validate the reductions in the final construction cost of the plant assumed in the generation costs evaluation. Second, the similarity of the results indicated the cost saving would be realised not only in the USA, but also in Japan.

Competitive generation

Calculating expected economic performance requires establishment of plant operations performance parameters, fuel cycle assumptions, and plant cost data. The plant parameters in Table 3 are consistent with current plant operations in the USA. Using this data yields generation costs below $36/MWh for standard twin AP1000 units constructed at a single site. The bottom line is that the AP1000 will be very competitive with generation using other types of fossil and renewable fuels.

Using the detailed cost buildup of the plant developed by Westinghouse for the Electrical Power Research Institute (EPRI) for the AP600 and adjusting these costs for changes to attain a 1000MWe output for the base design, capital costs for the third in a series of AP1000 plants were determined to be in the range of $1100 to $1200 per kWe, for the overnight capital cost.
Related Articles
Safety issues with AP1000 shielding building, says NRC
Tables

Table 1: Comparison of selected AP1000 parameters
Table 2: Proven technology of new AP1000 components
Table 3: AP1000 costs