53 3 Technical project description
3.1 Operating principles of nuclear power plants
Nuclear power plants produce electricity in the same manner as large condensing power plants using fossil fuels – by heat-ing water into steam and lettheat-ing the steam rotate a turbogen-erator. The main difference between nuclear power plants and conventional condensing power plants is the method of production of the energy required for heating the water: in nuclear power plants, the heat is produced in a reactor using the energy released by splitting atom nuclei, whereas in con-densing power plants, the water is heated by burning suitable fuel, such as coal, in a boiler.
The plant type considered for this project is the pres-surized water reactor. The operation of a prespres-surized water reactor is discussed in more detail in section 3.2. In a pres-surized water reactor (Figure 3-1), the high-pressure water exiting the reactor is led into steam generators. In the steam generators, the water flowing in a separate secondary circuit turns into steam, which is then used to rotate a turbine and an electric generator.
In Finland, the fuel used in nuclear power plants is isotope U-235-enriched uranium dioxide (UO2). Enriched uranium dioxide contains 3–5 percent of isotope U-235, whereas natu-ral uranium only contains less than one percent of the same isotope. The fuel is introduced into the reactor in the form of ceramic pellets placed in hermetically sealed tubes called fuel rods, which are bundled into fuel bundles (Figure 3-2).
Figure 3-1. The operating principle of a pressurized water reactor.
Control rods
Feed water pump
Core
Primary circuit pump
Feed water tank
Condenser Fuel bundles
Primary circuit Secondary circuit
Reactor
Pressurizer
Cooling water Turbine Steam
generator
Containment building shell
Generator Electricity Figure 3-2. Fuel pellets, fuel rods, fuel bundles.
The use of uranium as fuel is based on the heat gen-erated in the splitting, or fission, of atomic nuclei. When neutrons collide with a fissionable atomic nucleus, the latter splits into two lighter nuclei. At the same time, new neutrons, neutrinos, and energy are released. The neutrons released following the splitting of the nucleus can cause new fissions, which enables a chain reaction to start. The fission of U-235 nuclei forms a self-maintaining, controlled chain reaction that enables controlled heat production.
The heat produced in nuclear power plants or other ther-mal power plants (such as coal, oil, or gas plants) cannot be fully converted into electricity. Due to this, part of the heat produced is removed from the power plant using condensers.
In the condensers, the low-pressure steam exiting the steam turbines releases energy and turns back into water.
54 3 Technical project description In Finland, condensers are cooled using cooling water taken
directly from a natural water system. The cooling water, the temperature of which rises by 10–12 °C in the process, is then returned back to the water system. In nuclear power plants, more than one-third of the thermal energy generated in the reactor can be converted into electric energy.
Nuclear power plants are best suited as base load plants, which means that they are used continuously at constant power except for a few weeks’ maintenance outages at 12–24-month intervals. Plants are designed for an opera-tional lifetime of at least 60 years.
3.2 Description of the plant type
The most widely used reactor type is the light water reac-tor. The reactors of the nuclear power plants currently in operation in Finland are light water reactors. Light water reactors use regular water to maintain the chain reaction, to cool the reactor, and to transfer heat from the reactor core to the power plant’s process systems. The alternative light water reactor subtypes are the boiling water reactor and the pressurized water reactor. The subtype considered for this project is the pressurized water reactor.
In a pressurized water reactor, the fuel heats the water, but the high pressure (approximately 160 bar) prevents the water from boiling. The temperature of the water inside the reactor reaches a maximum temperature of approximately 330 °C. High-pressure water is led from the reactor to sep-arate steam generators. In the steam generators, the water is distributed into small-diameter heat transfer tubes. Heat transfers from the hot water led from the reactor through the walls of the heat transfer tubes to water flowing in a sep-arate circuit (secondary circuit), which is maintained under lower pressure (60–70 bar). The water in the secondary cir-cuit turns into steam which is then led to the turbine rotat-ing the electric generator (Figure 3-1).
As the reactor system and the secondary circuit are com-pletely separated from each other, the water circulating in the secondary circuit is not radioactive.
In Finland, the currently operating reactors at the Lo -viisa power plant and the reactor of the new power plant unit currently under construction in Olkiluoto are pressur-ized water reactors.
3.2.1 The Rosatom pressurized water reactor plant
Rosatom’s AES-2006 pressurized water reactor plant (Figure 3-3) is a modern third-generation nuclear power plant, which comes in two different versions. The plant ver-sion chosen by Fennovoima is AES-2006 / V491. Table 1-1 in Chapter 1 contains basic data on the plant.
The AES-2006 plants are based on VVER technology, which has been developed and used for more than 40 years and consequently offers the benefit of long-term opera-tional experience. The plant version considered for Fenno-voima’s project is the latest development step in the VVER plant series. VVER plants have a history of safe operation spanning over 30 years in locations such as Loviisa.
Contracts have been made to build AES-2006 plants in several countries. Additionally, plants of this type are cur-rently under construction in Russia and Belarus. In total, 13 AES-2006 plants are currently under construction or in contract phase. In Russia, the first plant unit of phase II of the Leningrad plant site in Sosnovy Bor is currently under construction. The construction began in 2008. Additionally, two plant units are under construction in both Kaliningrad and Novovoronez.
The target of the safety design of the plant has from the start been to comply with the requirements of IAEA’s safety guidelines and standards, European Utility Require-ments (EUR), and Russia’s own national regulations and requirements. The designing of Fennovoima’s plant will be
Figure 3-3. Rosatom’s AES-2006 pressurized water reactor plant.
55 3 Technical project description
continued so that it will also fulfill the requirements of the Finnish authorities. Chapter 4 presents safety requirements and principles pertaining to the designing and construction of nuclear power plants.
In terms of safety solutions, AES-2006/V491 equals the Western third-generation nuclear power plants. The safety of the plant is based on both active and passive systems.
Active systems are systems that require a separate motive power (for example, electric power) to operate. Among the important safety features of the AES-2006 are addi-tional passive safety systems, driven by natural circulation and gravity. Being independent from the supply of elec-tric power, their operation can be maintained even in the unlikely event of total loss of power supply and unavailabil-ity of the back-up power generators.
Four redundant active systems are provided for cooling the reactor after shutdown. One of them is sufficient to per-form the safety function. The active safety systems are plied with motive power from a diesel-backed power sup-ply. The safety systems used to cool the reactor are installed in four separate divisions within the safeguard building.
Alternatively, the reactor can be cooled down using passive systems. In this case, heat is extracted from the steam gener-ators to pools located outside the containment building.
In this reactor type, the reactor power is controlled using 121 control rods arranged in 12 rod banks. The high number of control rods (in comparison to other pressurized water plants) improves safety. The control rod system is designed so that in the case of loss of power, gravity causes the con-trol rods to fall into the reactor core. The reactor can be shut down with or without operating the control rods, and the reactor power can also be controlled by injecting boric acid into the reactor.
The plant type features a double-shell containment building. The inner containment shell is made of pre-stressed reinforced concrete that is capable of withstanding the tensile stresses caused by overpressure under accident conditions. The outer containment shell is a thicker struc-ture made of reinforced concrete that is capable of with-standing external collision loads, including a passenger airplane crash.
The possibility of a severe reactor accident – a partial meltdown of the reactor core – is also considered in the design of the plant. To cope with a severe accident, the con-tainment building is equipped with a core catcher. The core catcher is located below the reactor. It receives the reactor core in the case that the core melts through the reactor pres-sure vessel. The core catcher cools down the core melt and prevents any adverse effects to the containment structures.
The core melt is cooled down by spraying it with water from above. Spraying the core melt with water reduces the dispersion of radioactive substances inside the containment building. The water vapor generated in the core catcher is cooled down using the reactor building’s passive cooling system. This allows for maintaining the integrity of the containment building even during severe accidents and, consequently, limiting the dispersion of radioactive releases outside the containment building.
3.3 Safety of the plant site
The Hanhikivi headland in Pyhäjoki was selected as the plant site in 2011. At that time, the remaining alternative site locations were Karsikko in Simo and the Hanhikivi headland in Pyhäjoki. Several different factors were taken into account in selecting the site location. Special emphasis was placed on safety, technical feasibility, environmental and nature-related matters, building costs, and the regional community’s willingness and capability to accept the pro-ject. Dozens of specific issues falling under these general themes were looked into.
The most important issues in terms of safety were the population of, and activities taking place in, the immediately surrounding area, effective implementation of safety systems and emergency response arrangements, arrangement of the intake and outlet of cooling water in a reliable manner under various conditions, and the soil and bedrock properties.
The assessment revealed no significant differences between the Hanhikivi headland and Karsikko. In the end, the selection of the Hanhikivi headland was supported by, among other things, higher integrity of the bedrock and lower seismic design values, which affect the dimensioning of the nuclear power plant building and the equipment installed inside it.
3.3.1 Conditions at the plant site
Various extreme natural phenomena, accidents, and human activity in the vicinity of the plant site will be comprehen-sively taken into account in the design of the nuclear power plant. The conditions occurring at the plant site have been examined in numerous different studies and surveys con-ducted to ensure sufficient consideration of all factors in the design of the nuclear power plant.
A study on the level of sea water, conducted by the Finnish Meteorological Institute, assesses the change in the average sea water level and the probability of occurrence of exceptionally low or high sea water levels in the sea area of Pyhäjoki. The study is based on a comprehensive interna-tional literature review on changes in the water level of the oceans during the next hundred years. According to the most likely scenario, the average water level will drop slightly in the course of the current century, because land uplift has a larger effect on the water level of the Bothnian Bay than the global rise in the level of sea water. To ensure that even a sig-nificant rise in the level of sea water during the current cen-tury will not affect the safety of the plant, sufficient margins are applied in determining the elevation at which the plant will be constructed. (Johansson et al. 2008, 2010)
In addition to the level of sea water, other sea-related phe-nomena that may have an effect on the plant’s cooling water intake have been investigated as well. These include pack ice, frazil ice dam (a dam effect caused by formation of ice crys-tals in subcooled water), migration of sediments, and a severe oil spill accident at sea. The design solutions relating to the plant’s sea water intake will be selected on the basis of the results of the investigations such that the possible effects of
56 3 Technical project description various phenomena are taken comprehensively into account.
Another study conducted by The Finnish Meteorolog-ical Institute assesses the probability of extreme phenom-ena relating to temperature, rainfall, snow load, and wind velocity in Pyhäjoki, as well as the effect of climate change on the occurrence of these phenomena. The study is based on predictions on global climate development, which are used to model the occurrence of local weather conditions.
(Ilmatieteen laitos 2008a)
Table 3-1 presents the extreme values of selected natural phenomena. The estimated recurrence level of these extreme values in Pyhäjoki is 1,000 years. This means that on an aver-age, each value given in the table occurs once in a thousand years. The weather phenomena considered in the design of the nuclear power plant are significantly more intense than those indicated by the values given in the table.
Soil and bedrock surveys have been conducted to assess the occurrence and probability of earthquakes at the plant site and in its vicinity, to examine the seismic properties of the plant site, and to map any faults occurring at the plant site with the help of various soundings and bedrock analyses.
The results of the surveys will be utilized in the design solu-tions of the plant structures, systems, and equipment.
Several other surveys have been conducted at the plant site as well. These surveys have concerned matters such as the geological and geophysical properties, bedrock, groundwater, and water quality at the plant site.
The effects of human activity in the vicinity of the plant site on the safety of the plant have been assessed. There are no heavy industry sites, gas pipelines, railroads, airports, or harbors in the immediate vicinity of the plant site. Conse-quently, it has been assessed that the risks relating to the transport, handling, and storage of hazardous substances are very small. The probability of an airplane colliding with the plant has been assessed to be extremely small. Furthermore, the plant will be designed to withstand a large commercial airplane crash.
Fennovoima presented the Radiation and Nuclear Safety Authority (STUK) with the studies and surveys relating to the plant site in conjunction with the submittal of the original application for Decision-in-Principle in 2009. The reports on plant site investigations conducted after this were submitted
to STUK in October 2013 together with other reports. These reports will form the basis for STUK’s statement concerning the safety of the AES-2006 plant alternative. STUK is cur-rently preparing the statement, which will be given to the Ministry of Employment and the Economy in spring 2014.
3.4 Best available technique and energy efficiency of the plant
3.4.1 Best available technique
The primary criterion applied in the designing and building of the nuclear power plant is safety. The releases of radioac-tive substances due to the operation of the nuclear power plant and the environmental radiation levels will be kept as low as reasonably achievable (ALARA principle). The limi-tation of releases and radiation levels will be implemented by applying best available techniques and procedures. This chapter (Chapter 3) discusses treatment methods for liquids and gases containing radioactive substances as well as spent fuel and operating waste. The basic principles for the plant’s safety design are discussed in Chapter 4.
As regards the treatment of other discharges and emis-sions and ordinary waste, the best technically and eco-nomically methods will be applied. This section describes techniques possibly applicable to, for example, waste water treatment, waste management, and energy efficiency.
3.4.2 Energy efficiency
The nuclear power plant will be designed for the highest possible energy efficiency. The goal is to maximize the pro-duction of electricity and minimize the energy consumption of the power plant operations and the amount of waste heat discharged into the sea with the cooling water. As safety is the starting point of the design, the implementation of solu-tions improving energy efficiency will be considered on a case-by-case basis.
The dimensioning and technical solutions of the turbine plant will have a significant impact on the energy efficiency of the nuclear power plant. If correctly dimensioned, the Extreme values of natural phenomena
Sea level (cm) in the N2000 system in 2075
(and in 2008) min -179 (-152)
max +201 (+228)
Temperature, momentary (°C) min -42.8
max 33.9
Temperature, 24-hour average (°C) min -35.3
max 22.0
Rainfall (mm) 24 h 84.6
7 days 126.7
Snow load (kg/m²) 190.5
Wind velocity (m/sec.) gust, 3 sec. 34.7
average, 10 min. 31.2 Table 3-1. Extreme
values of natural phenomena in Pyhäjoki, with average occurence rate of once in a thousand years.
57 3 Technical project description
efficiency a turbine may be up to a few percentage points higher compared to a plant with a turbine that has not been optimized for the operating conditions. The low tempera-ture of the sea water of the Bothnian Bay will be taken into account in the optimization of the turbine by equipping it with suitably long turbine blades and a sufficient outlet area.
This maximizes the expansion ratio of the steam produced at the reactor plant in the turbine and, consequently, the yield of kinetic energy that rotates the turbine and transforms into electricity in the generator.
In the winter, when the cooling water is cold, the effi-ciency of electricity production is estimated to be approxi-mately 39 %. Part of the produced electricity will be used for the nuclear power plant’s processes. These include, in par-ticular, the pumping of cooling and process waters into their points of use and ventilation/air-conditioning. After deduct-ing the plant’s auxiliary power requirement from the amount of electricity produced by the plant, the resulting overall net efficiency will be approximately 36 %. The overall net effi-ciency of Nordic nuclear power plants is typically 30–34 %.
The heat produced in the nuclear power plant cannot be fully converted into electricity. Due to this, part of the heat produced will be removed from the power plant by way of condensation. The condenser will be cooled using water taken directly from the sea. The cooling water, the temper-ature of which rises by 10–12 °C in the process, will then be returned back to the sea. Another alternative for implement-ing the condensation process would be to transfer the extra heat directly to the atmosphere via cooling towers. However, direct cooling of the condenser with constantly cold sea
The heat produced in the nuclear power plant cannot be fully converted into electricity. Due to this, part of the heat produced will be removed from the power plant by way of condensation. The condenser will be cooled using water taken directly from the sea. The cooling water, the temper-ature of which rises by 10–12 °C in the process, will then be returned back to the sea. Another alternative for implement-ing the condensation process would be to transfer the extra heat directly to the atmosphere via cooling towers. However, direct cooling of the condenser with constantly cold sea