The past decade, however, has seen stagnation in nuclear power plant construction in the Western industrialised world, slow nuclear power growth in Eastern Europe and expansion only in East Asia. The prospects for nuclear energy have been affected by a number of factors:
- slower economic development and general reductions in the rate of increase in energy demand, coupled with oversupply in some countries;
- the Three Mile Island and Chernobyl accidents with their effect on public confidence in nuclear power;
- slow progress in properly implementing nuclear waste disposal;
- difficulties for utilities in some countries in transforming from a rapidly growing industry to routine operation of ageing facilities;
- electricity supply deregulation;
- increased competition from natural gas.
The turn of the century is potentially a turning point for nuclear power prospects because of:
- increasing world energy consumption, with nuclear power's contribution to reducing greenhouse gas emissions, nuclear fuel resources sustainability, and improvements in operation of current nuclear power plants;
- advanced reactor designs that will improve economics and availability, and further enhance safety;
- continued strengthening of the nuclear power safeguards system.
This paper describes the potential of small and medium sized reactors (SMRs) in addressing current and future nuclear power issues, and gives an overview of SMR development programmes and SMR designs.
Energy Supply and Nuclear Power
Today's global pattern of energy supply is not sustainable (Ref 1). The provision of affordable energy services is a fundamental prerequisite for economic growth and development. The plentiful energy resources in the past and the enormous efforts in research, development and engineering have produced the high living standards enjoyed today by the industrialised countries. For these countries achieving economically, environmentally, and socially sustainable development is a top priority.
Now and in the near future, better living standards and increased employment opportunities for the developing countries are inevitably linked to the provision of substantially more energy services. Taking into account the population growth, the anticipated increase in energy services will require more than twice as much energy production over the next half century. Over the next two decades India plans to triple and China to double the combustion of coal for electricity generation alone. Where transmission and distribution infrastructures are already in place, natural gas will be the preferred fuel for electricity and heat generation and for households. With the increase in the income per capita, and with growing trade volumes in a global market place, the demand for oil product fuelled transportation will expand rapidly.
There is an international consensus that heavy dependency on fossil fuels — which today account for more than 85% of the total energy supply — must be controlled. Their use adversely affects the atmosphere through emissions of greenhouse gases along with other noxious gases and toxic pollutants, thus becoming an obstacle to sustainable development both on a regional and on a global scale. One specific feature of fossil resources is their uneven distribution around the globe. For example, 60% of proven oil reserves are in the Middle East, while 40% of gas resources are in the countries of the former Soviet Union (FSU) and 40% are in the Middle East. Coal is also very unevenly distributed, with more than 80% of the proven reserves being concentrated in three regions, North America, the FSU and China. The uneven distribution of fossil fuel resources and the high cost of transport systems and infrastructures will be additional issues to be taken into account when deciding on future energy supply.
While energy efficiency in generation, transmission and end use, and the new renewable technologies, are an essential element of the sustainable energy policy of the industrialised countries, they may be far from sufficient and in many cases even inadequate to compensate for the expected increase in the demand for energy in the rest of the world. The global challenge is to develop strategies that foster a sustainable energy future, less dependent on fossil fuels.
Nuclear Power for Electricity Supply
From today's point of view of sustainability criteria, the entire energy chain from nuclear fuel production to radioactive waste disposal has very limited emissions of greenhouse gases and other pollutants and does practically no harm to the environment. Furthermore, it is an established technology and commercially available, with 473 nuclear plants currently operating or being built in 32 countries. It can also be reasonably competitive, the resources it uses are plentiful and have no other useful application.
Nuclear power is unevenly used in the world. More than 95% of it is deployed in the industrialised countries and the countries of Central and Eastern Europe and Russia. But the contribution of nuclear energy in the energy mix of the rest of the world, and in particular the developing countries, is very small. While nuclear power has reached a level of saturation in several European countries and North America, it continues to expand in Asia. At the same time countries in Eastern Europe and the FSU, heavily dependent on nuclear power, are experiencing serious difficulties due to a breakdown in the economies and the infrastructure necessary to keep the nuclear power plants operational and to further expand their nuclear power programmes.
The future will see a mix of energy sources. The makeup of this mix cannot be precisely defined — it will depend not only on environmental considerations, but also on technological, political and market factors. The experience to date shows that in most of the countries which have reached a quasi-sustainable level of development, nuclear energy has played an important role in supplying a part of the required energy. Most of these countries will try to preserve their nuclear energy generation and capability and probably will seek to renew it in the future when the life of the current plants is exhausted. Inevitably, the countries whose economies will continue to grow rapidly, will be better placed to include nuclear power in their energy supply system for meeting their energy needs but also for security of supply, environmental awareness and access to high technology.
Non-Electricity Applications of Nuclear
Specific temperature requirements vary greatly for heat applications. They range from about room temperature, for use as hot water and steam for agro-industry, district heating and seawater desalination, to up to 1000°C for process steam and heat for the chemical industry and high pressure injection steam for enhanced oil recovery, oil shale and oil sand processing, oil refinery processes, and refinement of coal and lignite. Water splitting for the production of hydrogen is at the upper end. Up to about 550°C, the heat can be supplied by steam; above that, requirements must be served directly by process heat, since steam pressures become much higher above 550°C. An upper limit of 1000°C for nuclear-supplied process heat is set on the basis of the long term strength capabilities of metallic reactor materials.
Water-cooled reactors offer heat up to 300°C. These types of reactors include pressurised water reactors (PWRs), boiling water reactors (BWRs), pressurised heavy water reactors (PHWRs) and light water cooled, graphite-moderated reactors (LWGRs). Liquid metal fast reactors (LMFRs) produce heat up to 540°C. Gas-cooled reactors reach even higher temperatures, about 650°C for the advanced gas-cooled, graphite-moderated reactor (AGR), and 950°C for the high temperature gas-cooled, graphite-moderated reactor (HTGR).
The primary conversion process in a nuclear reactor is from nuclear energy into heat. This heat can be used in a "dedicated" mode for direct heating purposes, without production of electricity.
Another mode is co-generation of heat and electricity. Parallel co-generation is achieved by the extraction of some of the steam from the secondary side of the steam generator, before it enters the turbine. Series co-generation is achieved by the extraction of steam at some point during its expansion in the turbine, when it is at the right temperature for the intended application. During this cycle, the extracted steam has also been used for electricity production. Series co-generation is ideally suited to industrial processes related to district heating, desalination and agriculture.
More than 80% of the world's energy use is based on fossil energy sources, namely coal, oil and gas. Burning these fuels causes serious environmental problems from emissions of sulphur oxides, nitrogen oxides and carbon dioxide into the atmosphere. One approach to contribute to solving such problems is to use nuclear energy in integrated energy systems. A typical example for the future is the application of nuclear heat to reform natural gas. Using what is known as the HTGR-reforming process, synthesis gas, methanol, hydrogen, heat and electricity are produced from natural gas and uranium. In the process, natural gas is decomposed into mainly hydrogen and carbon monoxide. The main products are methanol, a liquid carbohydron, and hydrogen. Side products are heat and electricity.
Another example of this integrated approach is in the oil industry. Several studies have been performed on the use of nuclear power as a heat source for heavy oil exploitation. They show that under favourable oil market conditions, the nuclear option presents economic and environmental benefits, as compared to conventional methods.
A third example is the integration of coal and nuclear energy in the steel industry. Technologically, this is the most ambitious integration, involving gasification of hard coal heated by hot helium from an HTGR. The intermediate products are synthesis gas and coke, which is used for iron ore reduction. The final products are methanol and pig iron (Ref 1).
Key Nuclear Power Issues
In an increasingly competitive and international global energy market a number of issues will affect not only the choice of nuclear power, but also the extent and manner in which it will be used in a sustainable mix of energy sources:
Enhancing Reactor Safety
Unquestionably, the most convincing demonstration of safety will be through the safe performance of existing plants and the avoidance of any major incident in the future.
Improving Nuclear Power Economics
Nuclear energy also has the potential to provide an economic source of heat for non-electricity applications, including district heating, desalination and high temperature process heat, especially through development and application of small and medium-sized reactors.
Minimising Environmental Impact
Improving Resource Utilisation
While the above issues are for nuclear power in general the following are specific for small and medium reactors.
Nuclear Power in Developing Countries
Non-Electricity Applications
Potential for SMRs to Address Nuclear Issues
Definition of SMRs
For heat-only or co-generation reactors, the range limits are applied to the electrical equivalencies of the thermal power. For very small heat-only reactors, for example, the upper limit adopted is 500 MWth.
It is understood that very small, small, medium or large reactors are relative concepts, related to the power level of the largest reactors in operation. That is, at the time when the largest reactors in operation were of the order of 200 MWe, the corresponding upper limit of the SMR range was 100 MWe, when 600 MWe units came into operation, the SMR range increased to 300 MWe, and so on. As there are no ongoing efforts to further increase the power level of the largest units, the currently accepted SMR range is assumed to prevail for a considerable period.
Applying the current definition of the SMR range, a third of the operating nuclear power reactors would qualify as SMRs. However, it should be noted that at the time when most of these plants were designed and built, they were considered large reactors according to the then-prevailing definition of the term.
The above defined ranges for medium, small and very small reactors expressed in power levels (MWe), are to be interpreted more as orders of magnitude and less as precise numbers. The large variety of reactors with different characteristics which are included in each of these ranges, are intended to respond to different requirements and uses, which need to be taken into account in order to facilitate the assessment of the potential market.
Medium size reactors are eminently power reactors whose objective is electricity generation. They can also be applied as co-generation plants supplying both electricity and heat, but the main product remains electricity. As such, they are intended for introduction into interconnected electricity grid systems of suitable size (at least six to 10 times the unit power) and operated as baseload plants. If operated in the co-generation mode, the heat supply would be up to about 20% of the energy produced. Economic competitiveness with equivalent alternative fossil fuelled plants is expected to be achievable under most conditions.
Small reactors are either power or co-generation reactors which may have a substantial share of heat supply. Due to the size effect, small reactors for electricity generation only, or operated in the co-generation mode, are not expected to be economically competitive with medium or large size nuclear power plants. They are therefore intended for special situations where the interconnected grid size does not admit larger (medium or large size) units and where alternative energy options are relatively expensive.
Very small reactors are not intended for electricity production under commercially competitive conditions as baseload units integrated into interconnected electrical systems. Clearly, very small reactors of current designs are not to be regarded as competitors of large, medium or even small power reactors, of which they are not scaled-down versions. Very small reactors address specific objectives such as the supply of heat and electricity or heat only (at either high or low temperature) for industrial processes, oil extraction, desalination, district heating, etc., propulsion of vessels or for energy supply of concentrated loads in remote locations. They could also serve as focal projects and a very effective stimulus for the development of nuclear infrastructures in countries starting a nuclear power programme.
The consideration of the specific objectives of the reactors corresponding to each power range has major relevance for the assessment of the respective markets (Ref 3).
SMRs and Enhancing Reactor Safety
This approach, which places a maximum emphasis on prevention rather than mitigation, is in fact a basic feature of the INSAG safety principles for future reactors. Some SMRs prevent severe accidents by eliminating the need for forced coolant flow for removing residual heat, while others reduce or even eliminate the necessity for correct operator action to avert or control major occurrences.
SMRs and Developing Countries
Among the developing countries with ongoing nuclear power programmes, China and India represent a substantial market for SMRs. In China, there is an ambitious nuclear power programme firmly supported by the government. In addition to some imported medium size units, a series of domestically designed medium sized reactors, as well as some small and very small units (including heat-only reactors), are expected to be put in place. In India, there is continuing firm governmental support for the nuclear power programme, and a large demand of new capacity. The country is expected to proceed with its programme based on domestically designed SMRs. It is estimated that the market for SMRs in the above two countries is of the order of 20 to 30 units, more than half of which correspond to medium sized reactors.
Argentina, Iran, Korea and Pakistan have ongoing nuclear power programmes, including reactors under construction. In Argentina, follow-up nuclear power plants are expected to be in the medium sized range; the development of a very small domestically designed reactor has been pursued, and there is a plan to build a first unit. In Iran, the construction of two large power reactors has been restarted, and there are plans to acquire some small units. In Pakistan, a further small reactor is expected to be followed by a series of medium sized units. Though large power reactors are the basis for the ongoing nuclear programme in Korea, more units in the medium range are expected. Also, implementation of a domestically designed very small reactor is expected. The estimate for these four countries within the period considered is 10 to 15 units.
Among countries which have not yet initiated nuclear power projects, Turkey and Indonesia are in the acquisition stage of their first units. Both have intended to go nuclear for a long time. Malaysia and Thailand performed studies indicating the convenience of the nuclear option. All four countries are potential markets for medium sized reactors, and in addition Indonesia might implement a very small unit at a remote site. The implementation of 5 to 10 SMRs is expected for this group of countries.
The North African countries (Algeria, Egypt, the Libya, Morocco and Tunisia) show a high degree of interest in initiating nuclear power programmes. All have performed studies and preparations, including, in some cases, attempts to acquire nuclear power reactors. It is expected that further attempts will finally succeed, leading to the implementation of 5 to 10 SMRs, including very small, small and medium sized units.
Several other countries which have not yet initiated nuclear power projects have performed studies and indicated interest in launching nuclear programmes. Belarus has persistent energy supply constraints and might acquire some medium sized units. In Chile, nuclear power could contribute to energy supply diversification in a fast growing economy with corresponding energy and electricity demand growth. In Croatia, a follow-up unit to the 600 MWe plant built in Slovenia was planned; new attempts could lead to a medium sized unit.
Israel has consistently indicated interest in nuclear power; it has a solid nuclear technology infrastructure and could implement a nuclear project, subject to the success of the Middle East peace process. This also applies to Syria, which intends to proceed with medium sized units. Portugal was on the verge of launching a nuclear power programme in the past, but has since desisted; new attempts to implement medium sized units could succeed. Saudi Arabia has very large oil and gas resources, but energy supply diversification seems advisable; a nuclear power programme starting with a very small or small reactor might be launched.
In addition, some other countries have indicated interest in nuclear power and in SMRs in particular, performing studies and building infrastructures: Peru, Uruguay and Bangladesh are examples. There are others, such as Cuba, Romania and the Philippines, where the construction of SMRs was suspended. In these countries, completing these projects would have priority over the initiation of new plants. The estimated market for SMRs in the above group of countries is 5 to 10 units altogether.
These results indicate a market with the rather wide range 50 to 90 units to be implemented up to 2015. The outcome will probably not evolve in all countries according to either the high or the low estimates. It seems reasonable to assume that there will be a certain compensatory effect. Also, it is recognised that forecasts, just like national development plants, tend to err on the optimistic side. Therefore, an overall market estimate of 60 to 70 units seems reasonable.
SMRs and Nuclear Power Competitiveness
SMRs and Non-Electricity Applications
SMR Development Programmes
Several countries in East and South Asia believe strongly that nuclear power will be a principle source of energy in years to come. Small and medium reactors form a major part of this activity. The People's Republic of China has a well developed nuclear capability, having designed, constructed and operated reactors. In many cases, these reactors can be regarded as SMRs and the skills needed to implement them are the same as those needed for terrestrial power plants. China has some 10 000 nuclear engineers in three major centres in different parts of the country, as well as other centres which make a major contribution. There is a particular interest in district heating reactors to help ease the current enormous logistical problems of distributing 11 billion tonnes of coal around the country each year.
In the SMR range, a 300 MWe PWR is in operation in China, and two 600 MWe reactors are under construction. All three reactors are of the evolutionary reactor type. Longer term plans call for development of a 600 MWe passive system. A 5 MWth integrated water cooled reactor has been built and operated for several winter seasons for district heating. A 200 MWth demonstration heating reactor project has been started. A 10 MWth high temperature gas cooled modular reactor for process application is under construction. Technical, safety and economic objectives of the programme have been defined. The test module HTR has been constructed and is expected to go critical by 1999. The system will be used to accumulate experience in plant design, construction, and operation. Several applications, such as electricity generation, steam and district heat generation are planned for the first phase. A process heat application, "methane forming", is planned for the second phase. China is also constructing a 300 MWe LWR in Pakistan.
India has some early reactors of the CANDU type developed by Canada but has adopted a prime policy target of self reliance in nuclear power development, based on heavy water moderated reactors. Four units of the 220 MWe PHWR type are under construction. Additional similar units and two units of a scaled up 500 MWe type are planned. The main objective is to make the most economical use of uranium natural resources in the first phase. In the second phase it is planned to utilise fast breeder reactors fuelled by plutonium generated in phase one. A 500 MWe prototype is in a detailed design stage. India also has large reserves of thorium which exceed its reserves of uranium. The heavy water reactor with its very good neutron economics is well suited to the thorium/U-233 cycle and a programme of R&D work for phase three, aiming at utilisation of the this cycle in an advanced heavy water reactor, has been initiated.
Japan has a high population density and a shortage of suitable sites for nuclear reactors due to the large fraction of the landmass covered by mountainous terrain. This has led to a preference for large reactors on the available sites to maximise the power output from them. In spite of this, there is a very strong and diverse programme of reactor development supported by the big industrial companies, by the national laboratory and by the universities. Three large industrial companies have developed their own LWR designs in the SMR range and the Japan Atomic Energy Research Institute (JAERI) has several more innovative designs.
At the end of 1996 two large reactors were under construction in Japan. The Monju fast breeder reactor (280 MWe), a prototype demonstration plant, is currently undergoing a safety review as a follow up of the incident in 1995. Several different designs are currently being worked on in the SMR range; namely SPWR, MRX, MS-300/600, HSBWR, MDP, 4S and RAPID. SPWR and the marine reactor MRX are integrated PWRs. The MS series are simplified PWRs. HSBWR is a simplified BWR. MDP, 4S and RAPID are small sodium-cooled fast reactors. Preliminary investigations have shown a high level of safety, operability and maintenance. The economics of these systems have been promising. These systems are expected to form part of Japan's next generation of reactors.
Japan has also a development programme for the gas cooled reactor of the small and medium-sized range. A High Temperature Engineering Test Reactor (HTTR) has been under construction since 1991 at O-arai. The 30 MWth reactor will be the first of its kind to be connected to a high temperature process heat utilisation system with an outlet temperature of 850°C. The system will be used as a test and irradiation facility and also utilised to establish the basic technology for advanced HTGRs for nuclear process heat applications. The system is expected to go critical in 1998. However, the main trend in power generation is still taking the line of larger (1000—1300 MWe) evolutionary light water reactors. The guidelines of the programme put user-friendliness, improvement in operability, and flexibility of core design as prime design objectives.
Korea has twelve nuclear power plants (10 PWRs, 2 PHWRs) in operation and has an ambitious programme for the further deployment of nuclear power. The country is not well blessed with indigenous sources of fossil fuel and has to rely on imports. Furthermore, 80% of the countryside consists of mountainous terrain which encourages the installation of large stations to make optimum use of the available sites. Most of the existing plants are of the PWR type, but, since April 1983, PHWRs (679 MWe each) have been added to the grid to give some diversification in supply and operation. Six large PWRs (1000 MWe each) and two medium-sized PHWRs (700 MWe each) are under construction. Large PWRs are expected to form the main component of nuclear power installation in Korea until well into the next century. The optimal combination of PWRs and PHWRs will help to maximise the usage of uranium resources through the utilisation of spent fuel in the future. This choice has been the first phase of a strategy of reactor development in Korea.
The medium-sized PHWR plants form part of the Korean power source, but the standard nuclear power plant, KSNP, with 1000 MWe rating, is expected to form the main stream of the nuclear power generation industry in Korea. On the basis of PWR technology, an advanced integral reactor, the System Integrated Modular Advanced Reactor (SMART) is being conceptually developed. The power output of the reactor will be in the range of 100—600 MWe depending on the purpose of utilisation, such as desalination or power generation. It is expected that the export of nuclear technology to the rest of the world will form part of Korean trade. Streamlining of standardisation, modularisation, prefabrication, and substantial reduction in the construction schedule of small and medium-sized reactors will make Korea a potential nuclear power exporter in the twenty-first century.
In the Russian Federation, there is substantial experience from the development, design, construction and operation of several reactors in the small and medium-sized category. These reactors have been used for electricity generation, heat production and ship propulsion. Reactors that have been used for icebreaker and submarine propulsion are planned to be made available for other applications, not only within the Russia but also to other countries that are interested in their application for electricity generation for remotely located areas or for non-electricity applications.
Currently a project is being implemented that consists of two reactors (KLT-40) mounted on a barge. These reactors have been earlier used for propulsion of icebreakers. The barge is supposed to provide electricity to Pevek in Northern Siberia. Barge mounted reactors may become a near term solution for other countries that need energy, but do not yet have the infrastructure for the introduction of large nuclear power plants. The barge mounted reactors could be operated under the supervision of the vendor and be pulled back to the vendor's location for maintenance and refuelling, thereby avoiding the need for on-site refuelling. Besides KLT-40 (up to about 160 MWth) there are other small reactors under design in Russia for mounting on barges, including the NIKA 75 (75 MWth), UNITHERM (15 MWth) and RUTA-TE (70 MWth).
The CAREM-25 reactor is under development in Argentina by the Atomic Energy Commission (CNEA), which has subcontracted the design and development of the reactor to INVAP SE. The design and development of the fuel elements is carried out by CNEA. The power level of CAREM is 100 MWth, approximately 25 MWe. The intended uses of the reactor are electricity generation, industrial steam production, seawater desalination or district heating. The reactor is also intended to bridge the gap between a research reactor and a larger nuclear power plant, by serving as a focal project for infrastructure development and the transfer of technology, in order to facilitate the launching of a nuclear power programme in a country with no previous nuclear power experience.
The main features of the reactor are light water cooling by natural circulation, low enriched uranium fuel, an integrated and self-pressurised primary system, and a passive heat removal system. The achievement of high levels of safety, simplicity and reliability are the main design criteria. The basic design of CAREM-25 has been completed. The detailed design of the reactor is being performed, and there is a comprehensive research and development effort going on. This consists of various relevant studies and of testing rigs and installations, such as a critical facility, natural convection loop, full scale hydraulic control rod drives, protection system simulator, etc. A preliminary safety analysis report has been completed and presented to the national regulatory authority. It is intended to construct a first project in Argentina.
Examples of SMR Designs
The AP-600
The PXS protects the plant against reactor coolant system breaks, providing the safety functions of core residual heat removal, safety injection, and depressurisation. It uses three passive sources of water for safety injection: the core makeup tanks, the accumulators, and the in-containment refuelling water storage tank (IRWST). These injection sources are directly connected to nozzles on the reactor vessel. Long term injection water is provided by gravity from the IRWST, which is normally isolated from the reactor coolant system by check valves.
The PXS includes a 100% capacity passive residual heat removal heat exchanger, which is connected through inlet and outlet lines to one reactor coolant system loop. The IRWST provides the heat sink for this heat exchanger. Once boiling in the IRWST starts, steam passes to the containment. This steam condenses on the steel containment vessel and, after collection, drains by gravity back into the IRWST. The heat exchanger and the PCS provide indefinite decay heat removal capability.
The PCS provides the ultimate heat sink for the plant. The steel containment vessel provides the heat transfer surface that removes heat from inside the containment and rejects it to the atmosphere. Heat is removed from the outer surface of the containment vessel by natural circulation of air. During an accident, the air cooling is supplemented by evaporation of water, which drains by gravity from a tank located on top of the containment shield building.
The VVER-640
The passive ECCS provides long term residual heat removal in loss of coolant accidents (LOCAs) accompanied by a station blackout. In the first stage, the nitrogen-pressurised hydrotanks will be actuated. When these are empty, the tanks holding cooling water under atmospheric pressure begin to operate. Active elements of the system needed for the function of emergency heat removal are provided with electrical power from storage batteries.
The design basis for the passive residual heat removal system (PHRS) is also a station blackout situation, including loss of emergency power supply. The PHRS consists of four independent trains, each comprising a steam-water heat exchanger, piping for steam supply and condensate return, and battery-operated valves. The heat exchangers are installed in a tank of demineralised water. They are connected to the secondary side of the steam generators in such a way that the steam from the steam generator will flow to the heat exchanger where it condenses, transferring its heat to the water. The condensate will flow back to the steam generator. Coolant motion occurs by natural circulation.
The system for passive heat removal from the containment includes coolers, storage tanks of cooling water and connecting pipelines. Steam released to the containment condenses on the heat exchange surface of the cooler giving heat to the water of a storage tank via natural circulation. Construction of a first pilot plant at the Sosnovy Bor site, near the Leningrad nuclear power station site outside St Petersburg, is under consideration.
The Indian AHWR
The top of the primary containment shell contains the gravity-driven water pool (GDWP). The inventory in the GDWP is sufficient to cool the reactor for three days following an accident. The GDWP inventory is connected to the core through a series of rupture discs and does not involve the use of external power, moving parts or instrumentation.
Isolation condensers (ICs) positioned in the GDWP will transfer decay heat to the GDWP during short, planned reactor shutdowns or following a reactor trip. This is achieved by diversion of the steam flow between the steam drums and the turbine to the ICs. Another set of condensers in the GDWP will cool the primary containment following a LOCA. Simple experiments have demonstrated the feasibility of the passive containment cooling system and more detailed experiments are in progress.
Emergency core cooling is provided from accumulators pressurised with nitrogen, with separation from the PHT system achieved with rupture discs that rupture when post LOCA depressurisation of the PHT system reaches a pre-set level.
South African PBMR
Eskom, the state electricity utility of South Africa, has initiated a detailed economic and technical evaluation of the Pebble Bed Modular Reactor (PBMR) as a potential candidate for future additions to its electricity generation system. The requirements set by Eskom for the installation of new generation capacity include a capital and operation cost which must match (or improve upon) that being achieved by their large coal stations. This currently represents a retail power cost to the customer of approximately two US cents per kWh. Other requirements for the plant include an availability approaching 90%, location and plant size to match the load, public acceptance and environmental cleanliness.
High temperature gas cooled reactors (HTGRs) feature a high degree of safety through reliance on passive safety features. All HTGRs incorporate ceramic coated fuel capable of handling temperatures exceeding 1600°C with core helium outlet temperatures approaching 950°C under normal operating conditions. Consequently, the primary focus for this reactor type is to investigate the generation of electricity via the direct coupling of a gas turbine to the HTGR (resulting in a net plant thermal efficiency approaching 47%), and to evaluate the application of this high temperature primary coolant for industrial applications such as steam and CO2 reforming of methane for the production of hydrogen and subsequent synthesis to other fuels such as methanol.
The conceptual design of the South African PBMR features a helium cooled pebble bed reactor with a power output of 103 Mwe (228 MWth) coupled to a closed cycle gas turbine power conversion system. The three turbo machines are equipped with magnetic bearings. The overall net efficiency of this Brayton cycle system is expected to be ~45%, based on a reactor outlet helium temperature of 900°C and a maximum system pressure of 70 bars.
The PBMR reactor basically builds on German reactor designs utilising the experience from the Thorium High Temperature Reactor and the AVR. These plants utilise a steam cycle in contrast to the Eskom design for a direct cycle helium turbine. The choice of a core design limited to 228 MWth with a diameter of 3.5 m, and the use of graphite constrictions for nuclear control and shutdown outside of the pebble bed, provide conservatism in maintaining the maximum accident fuel temperature to 1600°C. Also, the PBMR is to use a multiple pass regime for on-line constant fuelling of the reactor.
Activities of the IAEA
The IAEA has witnessed a considerable renewal of interest by its member states in the development of SMRs. This is particularly evident in the developing countries where large power plants are not a viable consideration due to the size of the existing electrical grid. This interest was strongly expressed at the 1997 and 1998 IAEA General Conferences, and subsequently reaffirmed at meetings of the Board of Governors.
Many member states have active programmes associated with nuclear power development in the SMR size range. These programmes involve a wide variety of reactor designs, and include plants whose status ranges from being in long term operation to currently undergoing initial conceptual design. The vast majority of these plants are for the production of electricity.
Although nuclear power seems to be focused predominantly on the generation of electricity, the wide range of plant types provides the possibility for nuclear power as an energy source for other applications, such as desalination, district heating, and industrial processes such as hydrogen production through the reforming of methane.
The IAEA has an extensive programme to help support member states in their national SMR efforts. This programme has, as its basic objective, the requirement to provide a venue for international exchange of information on the development of technology and designs of SMRs, in order to enhance their performance, safety and economics.
Though it is not problem free, nuclear power is recognised as having an advantage in contributing to the goals of sustainable development. It has been deployed in the industrialised countries when energy supplies have been insecure and has largely contributed to the stable and predictable energy supply necessary for their economic growth.
About 33% of total primary energy is used to produce electricity. Most of the remaining amount is either used for transportation or converted into hot water, steam and heat. Nuclear plants are now being used to produce about 17% of the world's electricity, from 437 reactors with a total capacity of 352 GWe. Yet only a few of these plants are being used to supply hot water and steam. The total capacity of these few plants is about 5 GW of thermal power, and they are operating in just a few countries, mostly in Canada, China, Kazakhstan, Russia, Slovakia, Switzerland and Ukraine (Ref 2).
With over 8500 reactor year of operation worldwide, nuclear power generally has an excellent safety record. But the Chernobyl accident demonstrated that one very severe nuclear accident has a potential to cause national and regional radioactive contamination. Although safety and environmental impacts are becoming a key issue for all energy sources, many in the general public perceive nuclear power as particularly and intrinsically unsafe. In order to reduce the risk of accidents a number of approaches are used:
Success in meeting this challenge is critical to maintaining a role for nuclear power as a viable energy option. Without getting the economics right, its potential environmental benefits may well become irrelevant. Nuclear power plants will increasingly have to compete directly, in an open energy market, with other suppliers of electricity. This competitive environment has significant implications for plant operations, including among others the need for efficient use of all resources, including personnel; more effective management of plant activities, such as outages and maintenance; and sharing of resources, facilities and services among utilities. The ultimate objective is to provide electricity services at competitive costs without compromising operational safety.
Although nuclear energy has distinct advantages over today's fossil burning systems — in terms of fuel consumed, pollutants emitted and waste produced — a further reduction in environmental concerns can positively influence public attitudes. As the overall health and environmental impact of the reactor and nuclear fuel cycle is small, attention is directed at techniques to deal with spent fuel, accumulated plutonium and radioactive waste. Reprocessing of spent fuel and recycling of most dangerous actinides in future fast reactors is being analysed in some countries as a solution for the fuel cycle back-end issues.
Known and likely resources of uranium should assure a sufficient nuclear fuel supply in the short and medium term even with reactors operating primarily on once-through cycles with disposal of spent fuel. However, as uranium demand increases and reserves are decreased, to meet the requirements of increased nuclear capacity, there will be economic pressure for the optimal use of uranium in a manner that utilises its total potential energy content per unit quantity of ore. Recycling of generated plutonium in thermal reactors and introduction of fast reactors in the longer term is considered in some countries as a solution.
Due to the ever increasing population in developing countries and the need to raise their standard of living, developing countries have a high demand for energy to support socio-economic development. But electricity grids in developing countries are usually smaller than in industrialised countries, and the large NPPs currently being offered (up to about 1400 MWe) on the international market are unsuitable for such countries. New capacity additions should not exceed 10—20% of the grid capacity, therefore many developing countries have to consider plants of about 700 MWe or smaller.
Non-electricity industrial application processes usually require much smaller plant outputs than plants designed for electricity generation or co-generation of heat and electricity. Heat from nuclear power plants cannot be transported over long distances due to potential losses and high costs. Nuclear plants designed for heat production should on the one hand be as close as possible to the point of use, but on the other hand have to be a reasonable distance away due to safety concerns.
The choice of ranges is somewhat arbitrary but it has been the usual practice to take the upper limit of the range of small and medium-sized reactors (SMRs) as approximately half of the power of the largest reactors in operation. Accordingly, reactors up to 700 MWe are currently considered as SMRs. Other limits are defined by continuing to take similar reductions. The ranges adopted therefore are:
SMR designs are not downsized version of larger reactors. In all cases they are taking new design approaches, which result into plants that are simpler, easier to operate and maintain, and that make extensive use of passive and inherent safety components and systems. These safety systems are usually built into the design of SMRs to protect the plant against severe accidents; they can hardly be compromised by malfunctioning equipment or human intervention. They especially exclude mechanisms for reactor core damage and the associated potential radioactivity release.
Due to their lower power output, their simplified designs and their high safety margins, SMRs are prime candidates for deployment in developing countries with small electricity grids or with a need to satisfy demand for non-electricity applications, such a district heating or production of potable water (Ref 4).
The overall trend in nuclear power plant design in most industralised countries has been towards large units, with current power output of about 1400 MWe. The main reason for this has been the economies of scale, which favour large units. Currently there are considerations to go to a power output of 1800 MWe, such as for the European Pressurised Water Reactor, being jointly developed by Siemens and Framatome. SMRs with their low power output are following the opposite strategy. Although the economies of scale do not favour SMRs, there are several reasons why these plant can be competitive with larger units. These include simplified design, shorter construction time, lower overall investment requirement and easier financing.
SMRs have a wide application potential for various industrial heat processes. This includes SMRs that are designed for heat-only production or for co-generation of heat and electricity. Due to their reduced power output, which meets the requirements of developing countries with small electricity grids, SMRs have a broad application potential. Prime candidates for near term industrial process heat applications are mostly in the low temperature range, e.g. district heating, desalination of seawater and process steam and heat supply for industry. In particular, desalination of seawater with nuclear energy is receiving increasing international attention to cope with current and future shortages of potable water.
The Westinghouse Advanced Passive PWR (AP-600) is a 600 MWe design which is conservatively based on proven technology, but with an emphasis on passive safety features. It has been designed by Westinghouse of the United States, under the sponsorship of the US Department of Energy (DOE) and the Electric Power Research Institute (EPRI). The design team includes a number of US and foreign companies and organisations. The AP-600 passive safety-related systems include the passive core cooling system (PXS), the passive containment cooling system (PCS), and the main control room habitability system.
This design of the VVER-640 (V-407) is being developed in Russia by OKB "Gidropress", the Russian National Research Centre "Kurchatov Institute", and LIAEP. The VVER emergency core cooling system (ECCS) includes the following automatically initiated subsystems:
A 220 MWe Advanced Heavy Water Reactor (AHWR) is being developed at the Bhabha Atomic Research Centre in India. The AHWR utilises heavy water moderator and light water coolant with a fuel cycle based on thorium, and a safety approach based on the incorporation of passive safety systems.