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Table 1–4. Top 20 U.S. nuclear power plants ranked by lowest non-fuel O&M costs (1999–2001)

Source: B. Schwieger, M. Leonard, S.Taylor, et al.

The top 20 U.S. nuclear power plants ranked by annual power generation are listed in Table 1–5.The average capacity factor for U.S. nuclear power units in 1999 was more than 87%, and according to the EIA, it exceeded 90% in 2002.16 According to the NEI’s president, Joe Colvin, the average capacity factors for the U.S. nuclear power industry for 2001 and 2002 were 90.7% and 91.9%, respectively, and the preliminary data for 2003 gave a figure of 89.4%.17 Explaining 2003’s slight drop-off in performance, Colvin said, “We expect these small year-to-year variations” and expressed the hope that current

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14 Wet-Steam Turbines for Nuclear Power Plants

inspections, replacements, and repairs would further improve industry performances in future years.

Even more important is that the increases in availability are coupled with equally impressive improvements in safety indices.The number of “unusual events” reported to the U.S. Nuclear Regulatory Commission (NRC) dropped from 151 in 1990 to just 18 in 2000. During the same period, the median number of annual unplanned automatic shutdowns per unit dropped from 1.2 to 0, according to the Institute of Nuclear Power Operations (INPO) in Atlanta.

Table 1–5. Top 20 U.S. nuclear power plants ranked by power generation (2001)

Source: P. Boschee,“Industry Report,” Electric Light & Power 80 (11): 21–24.

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Table 1–6 shows similar trends in data of nuclear power generation worldwide over the last few years. Nuclear power plant capacity factor values are higher than that for the United States in seven countries, with Finland having the best three-year average capacity factor of 93.17%.

Table 1–6. Nuclear power generation by countries (2001).

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Source: Data from B. Schwieger, M. Leonard, S.Taylor, et al.

Because of greater capital expenditures and lesser electricity costs as compared to fossil fuel plants, it is more profitable to operate nuclear power plants in a base-load mode, in which case the capacity factor value is primarily determined by the power plant’s reliability and characterizes its availability. For the countries where nuclear power plants account for a significant part of the total installed power generation capacity, some nuclear plants should lower their output during the night and weekend ebbs of power consumption in order to considerably decrease their capacity factor values (Table 1–1).

France is second in the world after the United States in the number of nuclear power units in operation (see Tables 1–1 and 1–6). From 1992 to 1997, the availability of French standard nuclear power units with pressurized water reactors (PWRs) increased from 71.2% to 82.6% (20 units with 900-MW capacity and 34 units with 1,300-MW capacity).18 Simultaneously, their operating expenses decreased from 5.05 to 4.48 centimes/kWh, and the average number of automatic and unplanned shutdowns per 7,000 operating hours fell from 2.9 to 1.4.

The safety level of German nuclear power plants has been raised considerably over the years, enough so that their safety record can

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compare favorably with other countries.19 Operating performances for German nuclear power units in 2002 are shown in Table 1–7. In 2003, 19 German nuclear power plants generated a total of 165.1 TWh of electricity, with an average capacity factor of 87.7%. This is more than 50% of the required power production in the country.20

Likewise, high levels of operating performances are recorded for nuclear power plants in Japan, Russia, South Korea, Finland, Belgium, and other countries.

Table 1–7. Operating performances for German nuclear power units (2002)

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18 Wet-Steam Turbines for Nuclear Power Plants

Main Types of Reactors Used

for Power Production

The vast majority of the nuclear power units in service are equipped with light-water reactors (LWRs), which use light water as both coolant and moderator.21 There are two main types of LWRs: pressurized water reactors (PWRs) and boiling water reactors (BWRs). Pressurized-water reactor versions developed in the former Soviet Union (FSU) are commonly designated as VVER, or WWER. In PWRs, the primary circuit’s water is pumped at relatively high pressure into the reactor vessel where it is heated and then passed to heat exchangers (steam generators), where it boils the secondary circuit’s water, which in turn evaporates and passes to the turbine (Fig. 1–3). In BWRs, the water in the reactor core is allowed to boil, and the produced steam passes directly to the turbine (Fig. 1–4).The nuclear power units with PWRs represent indirect, or two-circuit, cycles, whereas power units with BWRs employ a direct, or single-circuit, cycle. The fuel for both of these reactor types is enriched uranium dioxide, which is clad in zirconium alloy tube assemblies.

Fig. 1–3. Schematic diagram for a nuclear power unit with PWR (1: reactor containment structure; 2: pressure vessel; 3: control rods; 4: steam generator(s); 5: steam turbine; 6: condenser; 7: feed water pump(s); 8: main circulating pump(s))

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Fig. 1–4. Schematic diagram for a nuclear power unit with BWR (1: reactor containment structure; 2: pressure vessel and steam separator; 3: control rods; 4: isolation valves; 5: steam turbine; 6: condenser; 7: feed water pump(s); 8: water pool)

LWRs (both PWRs and BWRs) are the most widespread reactors employed by nuclear power plants throughout the world, with PWRs slightly outnumbering BWRs.The individual electric capacity of power units with LWRs presently reaches 1,400–1,500 MW (Table 1–2). A considerable amount of operating experience with these reactors has been accumulated over the last five decades. They feature high reliability and are economically competitive with fossil fuel power plants in many energy markets.Their greatest advantage is the use of ordinary, inexpensive water as moderator and coolant.

LWRs—both PWRs and BWRs—should be periodically shut down for refueling. Refueling cannot be accomplished under load, while the reactor is working. Commonly, for large-capacity LWRs, refueling takes place at intervals of about 12 to 18 months, and about a third of the core is replaced at each refueling.Thus, the fuel elements must

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20 Wet-Steam Turbines for Nuclear Power Plants

be capable of lasting from three to five years before discharge. For refueling, the reactor has to be shut down and brought to a cold condition. The vessel cover is then removed, allowing the fuel element assemblies to be replaced. In the process of refueling, some of the fuel assemblies in the core are removed and others are moved to different positions so that new fuel assemblies can be loaded. During refueling, it is desirable to undertake all preventive maintenance and repair operations on the unit’s equipment, including any repairs or refurbishment required for the turbine. Therefore, the maintenance schedule should be closely tied with the refueling schedule. In the early 1990s, the reactor refueling process could take up to 90 days. Reducing the amount of refueling time has been one of the main sources of raising availability and capacity factors of nuclear power plants. In 1999, the median refueling time for U.S. nuclear power units with PWRs dropped to 39 days, and some power plant operators have been able to complete refueling procedures in as few as 21 days.22 In 2000, Hope Creek managed to complete the refueling of its 1,000-MW BWR in 32 days and 18 hours—15 days quicker than the plant’s previous best.23

Extending the time interval between refuelings is another way to improve a power unit’s availability and lower the operating costs, along with optimizing maintenance and shortening the duration of the refueling process. Since 1998, the fuel replacement interval has been extended from one year to 18 months for 15 out of 20 standard French 1,300-MW PWR-based nuclear power units, as well as for 16 out of 34 French 900-MW units. 24

Both PWRs and BWRs have massive steel pressure vessels that are only manufactured in a small number of countries with highly developed heavy industry technologies. Because of high power density in the core, a PWR pressure vessel is relatively small, with thick steel walls. The highly pressurized primary circulation system of PWRs is extensive and increases the possibility of a major rupture. The PWR steam generators also often show susceptibility for breaking down. The PWR coolant system becomes highly radioactive during use, and adequate shielding of the operational personnel is an important requirement. BWR-based power units have a single circuit, and the coolant is pumped into the reactor vessel at a lower pressure than in PWR-based units. For a comparable output, a BWR reactor vessel is considerably larger and heavier than one in a PWR,

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The Nuclear Power Industry at the Turn of the 21st Century 21

even though the reactor vessel wall is thinner. The primary coolant system for a BWR, though smaller than that for a PWR, is still very expensive and poses a potential hazard of radioactive leakage. Radioactive steam passing to the turbine also requires special countermeasures for protecting the operational personnel.

In addition to the previously mentioned issues, the main generic disadvantage of all LWRs is the use of enriched uranium, calling for either international supply or domestic industrial enrichment capability.To avoid the need for enriched nuclear fuel, Canada has designed and built a heavy-water-moderated reactor fueled with natural urani- um—the Canada deuterium uranium (CANDU) pressurized heavy-wa- ter reactor (PHWR).As of 2000, 34 PHWRs and CANDU reactors were in operation at nuclear power plants in various countries: Canada (14 units with net individual electric power capacity of up to 750 MW); India (12 units with individual electric power capacity of about 200– 220 MW); South Korea (four units of about 700 MW capacity each); China (two 665-MW units);Argentina (two units of 319 MW and 600 MW capacity); Romania (one unit of 660 MW capacity); and Pakistan (one unit of 125 MW capacity). Similar steam-generating heavy-water reactors (SGHWRs) also used to be developed and installed in the United Kingdom, Japan, and Italy.

In the initial CANDU-based projects, heavy water was used as both moderator and coolant, circulating in separate circuits, with the heavy water in the primary circuit collected in a tubed tank, called a calandria. In the PHWR (advanced CANDU) and SGHWR, heavy water is used only as moderator, with light water used as coolant (Fig. 1–5). The fuel, which is natural uranium dioxide, is arranged in horizontal pressure tubes made of Zircaloy. The use of natural uranium results in much lower fuel fabrication expenditures. With natural uranium, the reactor does not have a high excess of reactivity as it does in reactors using enriched nuclear fuel, and this helps to reduce the likelihood of major power trips.The use of pressure tubes for the fuel in the reactor core allows the coolant to be pressurized without the need for a large steel pressure vessel. In addition, the large core of natural uranium allows for on-load refueling without having to stop operating the unit. Owing to this, the availability and capacity factors of PHWR-based nuclear power units can reach very high values. PHWRs also are efficient producers of plutonium. All of these merits would make the PHWR highly competitive with the LWR, were heavy

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22 Wet-Steam Turbines for Nuclear Power Plants

water not so expensive. The CANDU-type reactors typically exhibit intensive xenon poisoning, accumulating xenon in the reactor core about one hour after a reactor trip.This sometimes makes it difficult to restart the reactor quickly after unplanned, occasional trips. After reactor trips, to enable prompt restarts and shorten the duration of preliminary technological operations, it is desirable to keep the turbine running at the synchronous rotation speed driven by the generator as a motor for about an hour. In addition, the minimal continuous load provided by the CANDU-type reactors is remarkably higher than that for LWRs (about 60% versus 25%).

Fig. 1–5. Schematic diagram for a nuclear power unit with PHWR (1: reactor containment structure; 2: calandria; 3: fuel; 4: steam generator(s); 5: steam turbine; 6: condenser; 7: feed water pump(s); 8: main circulating pump(s); 9: moderator heat exchanger)

The list of the most widespread nuclear power reactors could be supplemented by light-water-cooled graphite-moderated reactors (LWGRs), which are also called pressure-tube graphite reactors (PTGRs).They were designed, constructed, and installed only in the former Soviet Union under the name RBMK—the abbreviation for the Russian term “channel reactor of large capacity.” The ill-fated

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