SMALL AND MEDIUM NUCLEAR REACTORS

Transcription

1 Small and Medium Nuclear Reactors SMALL AND MEDIUM NUCLEAR REACTORS 1. INTRODUCTION In recent years throughout the world, and in member states of IAEA International Atomic Energy Agency, renewed interest has been observed in the development and application of small and medium power nuclear reactors [1]. This is a direction opposite to that so far preferred by suppliers of commercialised power reactors, the installed capacities of which have greatly exceeded the barrier of 1,000 MWe. For example, the output of EPR Evolutionary Power Reactor, European Pressurized Reactor offered by French company Areva is 1,650 MWe. IAEA defines small power reactors (small reactors) as those with an installed electric capacity up to 300 MWe, and medium power reactors (medium reactors) as those with an installed electric capacity from 300 MWe to 700 MWe [2]. This author points out that the English word reactor means a power unit with a nuclear reactor as well as a nuclear reactor itself. The term small and medium power reactors is commonly shortened to small and medium reactors (SMR), which is not accidental, since as a rule, besides smaller installed capacity, they also have smaller sizes. 2. SMALL REACTOR FEATURES According to the IAEA definition, 139 of 442 power reactors (as of 2008) are regarded as small and medium reactors (SMR) [2]. This follows from the fact that at the beginning of the nuclear power technology era reactors featured smaller power outputs. For example, Shippingport, the first reactor in the U.S., had a power output of 60 MWe. They accounted for a stage of the development towards large power reactors, the power of which now exceeds 1,000 MWe. In recent years the world s interest has grown in nuclear reactors, for which the small power is designed intentionally. They are often referred to as deliberately small reactors (DSR). Typically, this category includes the following types of reactors: a) research b) test c) prototype & demonstration d) propulsion [2]. They are not of interest to electrical power engineering, which focuses on those that output heat to a coolant to produce electricity and / or district / process heating. Abstract Recent years have brought about increased interest in small and medium reactors with 700 MW or less output power, as an alternative to large commercialized nuclear units. Currently developed small and medium reactors can compete with large reactors due to the following advantages: 1) smaller sizes allowing manufacture of reactor components in supervised factories 2) less heat output from the secondary circuit, which facilitates location selection 3) less investment and financial risk 4) improved power system stability. The most advanced small nuclear reactor designs appear to be the light water reactors with integrated primary systems, such as Westinghouse IRIS and NuScale, and Toshiba 4S fast-neutron sodium-cooled reactor. The latter is expected to be installed in Galena, Alaska. The main barriers to small reactor technology development are: too many competing projects, fear of new reactor technologies, and perception of small units through the prism of the economy of scale. 39

2 40 In many countries DSR technology is currently being researched and developed. They include: Russia, Japan, the United States, India, China, Argentina, and South Korea. There are many aspects to which attention should be drawn when comparing small nuclear reactors with large reactors LR. These aspects have been discussed in [1, 2 6], and the discussion is synthesised below: Manufacturing of reactor structural components There are more opportunities to manufacture components for physically smaller reactors, because only a few manufacturers in the world are able to make large steel components for modern large reactors. This may change and the number of suppliers will increase, but it is a highly capital and time intensive process. Besides, forged elements of small reactors may be supplied domestically. Transportation of reactor structural components The use of large tanks in LWR light water reactors restricts the choice of location primarily to sites on a coast or along major rivers. Small reactors may be transported by rail, road, or river (on barges), because they are much lighter. Reactor construction process A significant portion of the work associated with construction of a power unit with a nuclear reactor is executed on the site of its later operation. The option of manufacturing many components of small reactors in factories, which are strictly controlled, and of on-site assembling (not manufacturing) them not only reduces the uncertainty associated with the construction cost and schedule, but also increases the reactor reliability and safety. Amount of radionuclides produced by fission in reactor The amount of radionuclides is proportional to the reactor capacity, hence small reactors produce fewer of them than large reactors. This is manifested in the option to reduce the sizes of reactor shields, plant sites, and EPZ emergency planning zones (in the US: EPZ a zone within 10 miles radius around the plant). Susceptibility to accidents Elimination of the systems of water injection to a reactor in emergencies (e.g. broken pipeline connecting a reactor vessel with a steam generator) reduces the cost, but requires an integrated reactor vessel design that includes a steam generator and pressuriser. This approach is applied to deliberately small nuclear reactors. It has the advantage of eliminating large diameter pipes through which primary circuit cooling water flows. Decay heat removal Compared to large reactors, small reactors are capable of more efficient removal of the decay heat in the event of reactor outage. This is so for the following reasons: a) reactors with less output power feature less decay power b) smaller reactor core volume allows for more efficient heat conduction c) heat removal from the outer surface of the vessel is more efficient (although the heat dissipation surface area is smaller) the reactor core volume has a greater impact on the amount of heat given off. Reactor location selection The reduced amount of radionuclides in small and medium reactors is manifested in the size reduction of plant sites and emergency planning zones. This creates the possibility to apply the small reactor technology to electricity and heat cogeneration, reducing the losses of heat transmission over long distances, while the EPZ reduction allows locating a reactor closer to populated areas. Lighter and smaller nuclear islands of small reactors can be founded on seismic isolators, resulting in greater reactor design standardization and reduced susceptibility to the effects of earthquakes. Heat demand characteristics Smaller reactors are more flexible to consumer requirements, especially with regard to the use of process heat from a reactor. For economic viability of a plant, the excess thermal power produced by its reactor has to find a consumer. Use of water for secondary system cooling Due to the need to give off large amounts of heat to the environment from power plants, including dispersal in an open circuit to water reservoirs, the problem of plant location arises. As regards nuclear power plants, this problem mounts, because, due to the lower thermal cycle efficiency, such a plant gives

3 Small and Medium Nuclear Reactors off more heat to the environment (water, air) than a conventional coal-fired power plant, assuming the same energy production. Where available location options are scarce in terms of water cooling (too little water body surface or too little water flow), an alternative may be a small reactor which needs much less cooling per power unit. Growing power demand in local power grids Smaller nuclear plants allow for easier adjustment to gradual increases in power demand characterized by low dynamics, which is in a way reflected in the reactor economics and flexible operating characteristics. Plant construction overall capital expenditures Usually the main economic indicator is specific investment cost, relative to the plant s installed electric capacity. However, no less important, if not more important, criterion is the sum of overall capital expenditures. This is particularly important for clients with reduced capacity to finance nuclear power units that today cost even as much as ca. 5 billion EUR. Small units are easier to finance by smaller customers, such as less wealthy states or smaller power utilities. Economies of scale The prevailing belief is that larger nuclear reactors are cheaper per unit of installed capacity due to the economies of scale. The relevance of economies of scale, however, may be reduced by: modular construction, standardized components manufacturing in factories, learning by doing process, simplified reactor structure, compact design, etc. In addition, economies of scale could be applied to compare reactors of the same structure, and all indications are that large reactors and small and medium reactors will be significantly different in terms of design. Investment risk As regards capital expenditure projects, besides economic indicators, cash flows are also very important. From this perspective, it may be advantageous to build four smaller nuclear power units than a single large unit (with the same installed power as the four small blocks have), while maintaining such a development sequence that the following block is built after the completion of the previous one. Then such another unit s construction is partially funded by the revenues from the previous unit s commissioning. This approach can significantly mitigate the capex project s financial risk. In addition, a smaller unit s construction is less susceptible to delays in implementation that add to the project s risk. It is anticipated that the construction of a unit with a small reactor will take three years, and with a large reactor five years [5]. Another important feature of this approach is small reactors better adaptability to changing market conditions. Power system constraints Small and medium reactors may be used in a power grid with limited capacity of installed generation sources, in which a deviation of the active power balance in excess of 10% of the sources installed capacity may jeopardize the power system s operation and stability. They may also be located far away from civilization in order to avoid construction of a long power transmission lines. Many power systems are not suited to the connection of power units in excess of 1,000 MW. Smaller reactors can be advantageous in systems with large renewable sources based generation, wind farms in particular, where load follow in the power system is necessary. Small reactors offer better flexibility in this respect, also in the perspective of the electric power sector s development towards smart grids. These characteristics show that under certain conditions, the construction of small nuclear reactors would be more advantageous than the construction of large reactors. There remain, however, many barriers to overcome of a technological, social, and economic nature. Overcoming technical barriers is the subject of research conducted within numerous demonstration and development projects. 3. SELECTED SMALL AND MEDIUM REACTOR PROJECTS According to an IAEA report, there are more than 60 projects of small and medium reactors, listed in [1]. This creates certain obstacles and threats to the technology development because of dispersion of the involved resources and lack of standardization. This chapter presents selected small and medium reactor projects, especially those for which the commercialization moment seems to be the closest. The projects are listed in Tab. 1. Below some selected reactor projects are discussed in more detail. 41

4 42 Tab. 1. Selected small and medium reactor projects. Based on [2, 3] # Specification IRIS NuScale 4S 1 Designer Westinghouse NuScale Toshiba 2 Primary circuit coolant light water light water sodium 3 Coolant circulation forced natural forced 4 Primary circuit configuration integrated integrated pool 5 Electric power [MW] (to 50) 6 Reactor output temperature [ C] Secondary circuit configuration intermediate intermediate intermediate 8 Heat cycle Rankine Rankine Rankine 9 Reactor vessel diameter [m] Reactor vessel height [m] Fuel UO2 UO2 U-Zr 12 Fuel enrichment rate [%] <5 < Fuel cycle duration [a] Scheduled launch International Reactor Innovative and Secure (IRIS) as well NuScale are deliberately small Integral Primary System Reactors IPSR. The IRIS project is developed by Westinghouse and based on the known light water reactor technology, which will soon allow for the launch of the FOAK first-of-a-kind unit. The solution s main features [2] include: Availability of installed capacity in the range of 100 to 350 MWe. Typical unit installed capacity is 335 MWe (reactor thermal power output 1000 Wt). The reactor s structure is modular All primary system components (core, control rods, drive mechanisms, steam generators, primary coolant pumps, pressuriser) are integrated in a single reactor vessel The reactor core consists of 89 fuel assemblies (289 rods each) known from pressurized water reactors (PWR) containing uranium oxide UO2 fuel enriched at the rate of ca. 5%. The expected in-core fuel cycle duration is 3.5 years, and fuel burn-up is 50,000 MWd / t The reactivity is controlled by solid burnable absorbers and control rods, as well as using soluble boron. NuScale, on the other hand, is situated at the opposite pole in terms of IPSR reactors power range. Its design installed capacity is 45 MWe. The technology provider s idea is to develop an installation with a large number of units, e.g. 10, of a total power of 450 MWe. Its features are similar to those of IRIS. Instead of 89 cassettes (IRIS) there are 24 fuel assemblies designed for NuScale, although an in-core fuel cycle extension is considered from 30 to 60 months, using a fuel with an enrichment rate of 8%. An important difference is the use of natural circulation of coolant in the reactor vessel [2]. A relatively interesting project, also due to the anticipated short time of its completion, seems to be the 4S reactor (Super Safe Small and Simple) developed by Toshiba Corp. and Central Research Institute of Electric Power Industry (CRIEPI) in Japan. The 4S reactor is a small power, fast neutron, sodium cooled reactor. It employs passive safety systems, which allow improving their economics [3]. The first planned location is Galena, Alaska. The reactor planned for installation in Alaska will have an installed electrical power of 10 MW (thermal power 30 MJ / s). However, an option to increase the reactor s power in subsequent installations up to 50 MW is considered. The fuel will be an alloy of zirconium and uranium (U-Zr). The intermediate system that uses sodium as coolant takes heat from the reactor and delivers to the steam generator in the secondary circuit a Rankine steam heat cycle [2]. The 4S reactor s basic design assumptions include [3]: No need to refuel for over ten years (ultimately throughout the useful life of thirty years, if possible). Simple control of fuel burn-up with no control rod drive mechanism CRDM) Minimization of reactor system controls Load follow operation with no need to activate the reactor control system

5 Small and Medium Nuclear Reactors Minimization of reactor components repairs and inspections. Negative temperature coefficient of reactivity Security system independent of the emergency power supply systems and the decay heat removal system (this system is passive and does not require power supply from the plant s auxiliary system). A very interesting feature, and also the 4S reactor s key technological innovation, is its core, in which fuel is made of metal, and its burning is controlled by a neutron reflector, allowing the economical balance of neutrons in the fuel. 4. SUMMARY Once appropriately technologically matured, small and medium reactors (SMR) will compete with large reactors. The modern fleet of large reactors features high standards of safety, and in this way it will define the technology offerings market. At first glance, it seems that economies of scale may be an issue. Due to their integrated modular design, small reactors will have to prove their competitiveness. Also the associated equipment will have to be developed, as well as the control and measurement instruments and automation (for example inside the tank containing the reactor s integrated primary circuit) for small and medium reactors [2]. Non-technical challenges will include [2]: Too many competing SMR projects Widespread perception of large, centralized nuclear power plants as a better solution, since they are considered high risk objects and their centralization in locations remote from major cities is desirable High capital expenditures and anxieties related to disasters at nuclear power plants as well as concerns from the early era of nuclear technology have created a kind of fear of projects referred to as the first of a kind FOAK) Longer in-core fuel cycle will require ongoing monitoring, diagnostics, and forecasting of the reactor s technical condition The use of reactors for cogeneration or polygeneration will require development of automation systems designed to balance demand for two or more products offered by a unit with a reactor. Yet another issue is management of the waste generated during current production or remaining at the end of a reactor life cycle, like for the 4S reactor, in which fuel is not supplemented throughout its technological life cycle. Currently available large commercial reactors are designed for a sixty year useful life cycle, though certainly reactor operators will seek to extend it. Small and medium reactors at the moment are designed for thirty years of use. After its technical lifetime completion a nuclear power plant must be decommissioned and dismantled, which requires substantial outlays. In light of the high demand for new powergeneration sources, and even the need to build distributed generation sources based on renewable energy resources or natural gas, it seems unlikely that this technology, despite its advantages, will be adapted to the Polish national powers system over the next two decades. Certainly, it can be used in places such as Galena, Alaska, which are far away from civilization. Time will tell whether it will be possible to significantly simplify the reactor design in such a way as not to endanger the safety of the population and whether the public will accept dispersed nuclear reactors in view of an alternative in the form of a smaller number of such sources, but with higher installed capacity. Overcoming the barriers of economies of scale would encourage an alternative approach to the construction of power plants with large units the construction of plants of the same capacity, but with a respectively larger number of units. 43

6 44 REFERENCES 1. Kuznetsov V., IAEA activities for innovative Small and Medium sized Reactors (SMRs), Progress in Nuclear Energy 4 7, no. 1 4, 2005, pp Ingersoll D. T., Deliberately small reactors and the second nuclear era, Progress in Nuclear Energy 51, 2009, pp Ueda N., Kinoshita I., Minato A., Shigeo K., Yokoyama T., Maruyama S., Sodium Cooled Small Fast Long-Life Reactor 4S, Progress in Nuclear Energy 4 7, no. 1 4, 2005, pp Carelli M., Garrone P., Locatelli G., Mancini M., Mycoff C., Trucco P., Ricotta M. E., Economic features of integral, modular, small-to-medium size reactors, Progress in Nuclear Energy 52, 2010, pp Shropshire D., Economic viability of small to medium-sized reactors deployed in future European energy markets, Progress in Nuclear Energy 53, 2011, pp Locatelli G., Mancini M., The role of the reactor size for an investment in the nuclear sector: An evaluation of non-financial parameters, Progress in Nuclear Energy 53, 2011, pp