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Advanced Reactors
Advanced Designs| The IAEA and Advanced Reactor Development | Trends
The world population has nearly doubled over the last three decades and will continue to increase. Current estimates forecast that by 2020 there will be about 8,1 billion people living on this planet, with nearly 90% of the population increase taking place in developing countries. In these countries, current electricity consumption per capita, which may be used as an indicator of standard of living, is very low, one or two orders of magnitude lower than in the industrialised countries. In addition to many urgent needs, such as food, clothes, accommodation and work, availability of energy is an important prerequisite for socio-economic development in all parts of the world.
Depending on the geographic location and the level of industrialisation, final energy is consumed in different forms: as electricity, for transport, or as heat. Among the different energy supply alternatives that countries may choose, nuclear power represents one of the few options that provide a means to produce energy in all forms, i.e. as electricity, low and high temperature process heat, and process steam, both economically and under environmentally acceptable conditions. Environmental concerns about the effects of burning fossil fuels for energy production have stimulated interest in low-polluting energy sources. Further development and deployment of nuclear power may thus appear an attractive option to some nations for contributing to a long term, safe and reliable energy supply.
Over the past three decades, thousands of reactor-years of operating experience have been accumulated with current economic and reliable nuclear energy systems. New generations of nuclear power plants have been, or are being, developed, building upon this background of success and applying lessons learned from the experience of operating plants; hence, the new, advanced designs are anticipated to become even more safe, economic and reliable than their predecessors.
Advanced designs generally incorporate improvements of the safety concepts, including, among others, features that will allow operators more time to perform safety actions, and that will provide even more protection against any possible releases of radioactivity to the environment. Upgraded designs may also include introduction of passive safety features that are based on natural forces, such as convection and gravity, making safety functions less dependent on active systems and components like pumps and valves.
Great attention is paid to making new plants simpler to operate, inspect, maintain and repair, thus increasing their overall cost efficiency. In the event of disturbances and accidents, largely digitalised control, monitoring and protection systems will automatically bring the plant back to normal conditions or to a safe shutdown state, without operator action. Typical design objectives imply that safety actions by the operators shall not be needed within at least 30 minutes, leaving them time to assess the situation carefully before acting. Increased thermal inertia in the reactor system and reduced core power densities are two such design related factors.
With respect to passive safety features, their function will normally also be independent of power supply, (at least following an initiation of the function), by utilisation of thermal hydraulic phenomena, such as density differences due to different temperatures, and elevated water reservoirs, e.g., to permit coolant flow to the reactor system by gravity to top up coolant levels.
Development of a new reactor design is a costly and demanding task that requires a lot of resources. To overcome this problem, co-operation, on a national or international level, has frequently been established, and the advantages of pooling resources in development projects have been demonstrated. As an international forum for exchange of scientific and technical information, the International Atomic Energy Agency (IAEA) is playing a major role in this context, bringing together experts for a world-wide exchange of information about national programmes and co-ordinating research programmes for advanced reactor development projects.
One alternative for meeting future energy needs is to gradually increase the deployment of nuclear energy by introducing advanced designs into the energy supply systems throughout the world. In this context, it may be noted that an advanced design is characterised as a plant design of current interest or merit but which has not yet been constructed or operated; such advanced designs, in turn, can be divided into evolutionary and developmental designs. An evolutionary design is a descendant from an existing plant design featuring design improvements and modifications based on feedback of experience and incorporating new technological achievements; an evolutionary plant design at most requires engineering and confirmatory testing prior to commercial deployment. A developmental design, on the other hand, will incorporate more significant departures from existing plant designs, and may require also construction of a demonstration plant and/or prototype plant prior to large-scale commercial deployment. A developmental design that incorporates radical changes in the design may be called an innovative design, which thus represents a special sub-category of developmental designs.
Advanced designs presently under development comprise three basic types:
- water-cooled reactors;
- fast reactors; and
- gas-cooled reactors.
which are discussed further below.
Water-cooled Reactors
Water-cooled reactors are characterised by the use of water as moderator and coolant; light water reactors (LWRs) utilise ordinary "light" water (H2O) and heavy water reactors (HWRs) use heavy water (D2O), at least as moderator. In heavy water, the hydrogen atoms (H2) have been replaced by deuterium (D2), a heavy isotope of hydrogen.
Some 330 LWR plants are currently operating throughout the world, representing about 75% of all nuclear power plants in operation. This type of reactor has been operating for more than 35 years. Continual upgradings and evolutionary improvements of the LWR plant designs have formed the basis for the development of advanced LWR plant (ALWR) designs, of both boiling water reactor (BWR) and pressurised water reactor (PWR) types. (Some Examples of Advanced LWR Designs).
Both large and medium size reactors are being developed; the former would to a large extent represent extensions of existing designs, whereas the smaller typically would enhance the use of passive safety features to achieve plant simplification and savings in order to become cost competitive in spite of the smaller size. Some developmental designs, in particular innovative designs, like the Swedish PIUS PWR, incorporate safety functions of a different nature compared with the traditional safety features of existing designs, e.g., self-protective functions for reactor shutdown in the event of unacceptable operating conditions and for reliable removal of the decay heat (the heat developed in the fuel after shutdown) from the reactor core.
Fuel improvements have steadily been implemented in operating plants over the years; e.g., enhanced use of burnable absorbers in the fuel and increased fuel burnup have yielded substantial reductions in uranium consumption, some 20% compared with the INFCE (International Fuel Cycle Evaluation) study of 1978/1979.
Recycling of plutonium through mixed oxide (MOX) fuel has become commonplace in a number of countries, such as France, Germany and Switzerland, and several other countries may follow.
Advanced designs will normally try to take full advantage of progress in the fuel field; their core designs will be optimised taking into account advanced burnable absorber strategies and extended fuel burnups. Many of them will have the capability of operation with up to 100% MOX fuel in the core.
HWR plants account for some 7% of all operating nuclear power plants; most of them are CANDUs, the Canadian pressure tube reactor design. Most such plants have operated safely, reliably and competitively from the initial start-up.
Design improvements on the CANDU type reactors focus on enhancing plant safety, increasing plant lifetime to as much as 100 years, simplifying replacement of all components that cannot be designed for such a service lifetime, shortening construction time, and producing electricity at costs that are competitive with other energy sources (e.g. brown coal).
Fast Reactors
Fast reactors use "fast" neutrons for sustaining the fission process, in contrast to water- and gas-cooled reactors that use thermal neutrons. Fast reactors are also commonly known as breeders since they produce fuel, as well as consuming it. Plutonium breeding allows fast reactors to extract sixty times as much energy from uranium as thermal reactors do, which may make them economical and advantageous for countries which lack abundant uranium resources. Increased deployment of nuclear power in the decades to come would likely lead to a depletion of uranium resources, and use of breeder reactors to produce fissile material may become necessary within the next half century.
In the fast neutron spectrum present in such reactions, all transuranic elements become fissionable, and therefore, fast reactors may also contribute to burning of plutonium, arising from operation of other types of reactors and from the dismantling of nuclear weapons, and to decreasing the total inventory of transuranics inside the "macro-system" by transmuting them to energy and fission products; fuel reprocessing and recycling in fast reactors would allow "burning" of the very long-lived transuranic radioisotopes, vastly reducing the required isolation time for high-level waste.
The fast reactors are normally cooled by liquid metal (sodium) and are therefore called liquid metal-cooled fast reactors (LMFRs). Successful LMFR plants have been designed, constructed and operated, e.g. the BN-600 in Russia, the 1200 MWe Superphenix in France, and the 280 MWe Monju in Japan.
The further development of fast reactors is focusing on revised safety and economic requirements for the next generation of nuclear power plants. Work is also continuing on improving fuel burnup and the fuel recycling technology to reduce the amounts of radioactive waste produced at plants.
A number of different types of LMFRs are being studied, as well as wider use of passive systems for further safety; examples are the BN-800M in Russia, the DFBR in Japan, and the PFBR in India. There is also an advanced LMFR design in small and medium power range, developed by General Electric in the USA. This ALMR concept incorporates passive safety features, and includes waste management and reprocessing facilities, as well as fuel manufacturing at the plant site, forming a single nuclear park to minimise the risk of proliferation of fissile materials. Other advanced fast reactors include the European fast reactor (EFR) which may become widely used for recycling of plutonium and electricity production.
Gas-cooled Reactors
High-temperature gas-cooled reactors (HTGR) have been under development for a long period of time, and a number of prototype or demonstration plants have been built, but without complete success in operation. The HTGR is basically a graphite moderated reactor with a gas (helium) as the coolant. The inert He gas and a special fuel design make it possible to operate at temperatures considerably above those in water-cooled reactors, and this, in turn, makes it possible to produce steam at a much higher temperature (and pressure) for supplies to conventional steam turbine generators, yielding a much improved thermal efficiency of the plant, or to produce high-temperature process heat for special applications.
In recent years, development has focused on small, modular units, since experiments and analyses have shown that such units can achieve an exceptional degree of self-protection.
HTGR fuel is not contained in fuel pins clad in metal as in LWR and LMFBR fuel, but in fuel particles. The particles are about 0,2-0,6 mm in size and consist of a mixture of oxide or carbide uranium or thorium or uranium/thorium. In order to retain fission products, each particle is enclosed by several coatings of ceramic material with a high temperature stability. The particles are homogeneously dispersed in a graphite matrix that is subsequently compressed to spherical elements, pebbles, or in the form of rods, filled into fuel channels of a multi-hole graphite block. The particles remain intact and retain virtually all fission products up to a temperature of about 1600 °C. They do not melt at a given threshold temperature and fail only gradually under accident conditions; hence, a sudden release of fission products cannot occur.
Further HTGR development will concentrate on improved plant performance and life extension studies. With respect to the former, much effort is currently devoted to the so-called gas-turbine-cycle in which high temperature gas is conveyed directly to a gas turbine, yielding very high thermal efficiency and promising low energy cost, and to production of very high temperature process heat.
The IAEA and Advanced Reactor Development
Increasing concerns about the environmental impact of burning fossil fuels have led to renewed international interest in nuclear technology. National development programmes have been supplemented by international efforts in advanced reactor development, particularly for those with innovative features. Technical programmes and projects of the IAEA in this field focus on enhanced safety, increased reliability, improved economic efficiency, public understanding, and the possible introduction of advanced reactors.
International Working Groups
Agency activities in the area of development of advanced designs are co-ordinated by three international working groups which meet periodically to discuss and exchange information on national programmes, to identify areas of common interest for co-operation and to advise the Agency on its technical programmes and activities, including the establishment of co-ordinated research programmes. The activities of the groups also include workshops on topics related to the development of advanced designs.
Technical Assistance Programmes
Through its technical assistance programmes, the Agency helps developing countries establish the expertise needed to incorporate advanced reactor technologies into their own research programmes by providing expert advice, training, fellowships and special equipment and research. For countries interested in developing research programmes, the IAEA publishes reports on the status of advanced designs development.
Advanced nuclear reactor designs derive advantage from the extensive operating experience gained from current systems and results of world-wide research and development, the aim being to provide very safe, reliable and economical nuclear power plants, which will also be friendly to the environment.
Improved reactor systems and increasing public awareness and concern about global warming and environmental pollution have led national decision makers to look more closely at the nuclear option for their future electricity generation mix. Successful use of nuclear power in the years to come does not depend only on technical excellence: understanding and acceptance both by the public and by national authorities are an absolute prerequisite.