NEA Issue Brief: An analysis of principal nuclear issues
No. 5, June 1989

Advanced water reactor technology

I. What are water reactors?

Water Reactors are nuclear power plants that use water to control and remove the heat from the nuclear fuel in order to convert heat to electricity. The first nuclear.power plant operated by a utility company to produce electricity on a commercial grid was a water- cooled reactor started up in 1957 at Shippingport, Pennsylvania, in the United States. Since that time, over 330 water-cooled reactors have been built and have produced electricity throughout the world. Water-cooled reactors comprise over 80 per cent of all the world's nuclear power plants; the remainder are gas-cooled reactors, liquid metal-cooled reactors or graphite-moderated water-cooled reactors.

There are two basic types of water reactors: those cooled by ordinary water, called the light water type, and the heavy water type. The majority of the world's water reactors are Light Water Reactors (LWRs), including both Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs). Heavy water reactors are discussed in Section X.

II. Why are improved water reactors desirable?

It is typical of any technology that, as experience is gained, opportunities arise for improving the performance and economics of the technology. Such is the case for water reactors, which have a very large experience base in most industrialised countries. Most of these countries reason that, if they are to continue to utilise nuclear power to maintain national energy security in the future, they should take advantage of the experience already gained to further improve future water reactors. Hence, many of the industrial countries have extensive programmes under way to advance the technology of water reactors.

III. What are the objectives of advanced water reactor development?

Most advanced water reactor development programmes have the dual aim of improving safety and reducing environmental impacts while at the same time providing further reductions In the cost of generating electricity. Environmental and public health benefits are to be achieved through improvements in reactor design and operation to further reduce the already low probability and potential consequences of accidents -the so-called "residual risk" of reactor operations.

While it may appear to some that these goals conflict with each other, in most cases, they do not. The objectives of improved safety, reliability and economics being sought in advanced water reactors can be, by and large, complementary. The same enhancements that lead to improved reliability and economics--for example, simplified system design and improved ruggedness of structures and components--may also lead to improved safety.

IV. Can we improve existing operating water reactors?

Many of the lessons learned from the past 30 years of water reactor operation have been and continue to be applied to the design and operation of existing water reactors. Indeed many of the advances in technology being considered for future plants came about because of this direct experience. Examples are the lessons learned from the 1979 accident at Three Mile Island (TMI). Major improvements, such as in the design of control rooms and instrument systems, have been made in existing plants as a result of lessons learned at TMI and further improvements are being made in designs for future plants. In addition, several systems have recently been established among all countries which operate nuclear power plants to share operational experience in an effort to enhance the safety and performance of existing reactors. These include the Incident Reporting Systems of the NEA and the International Atomic Energy Agency (IAEA), as well as the recently organised World Association of Nuclear Operators (WANO).

V. What changes are being made reactors?

Many different approaches are being taken to improve future water reactors. In some cases, incremental changes in design are being incorporated in plants now being built. These changes include improvements to make plants easier and more economical to operate and maintain, changes to plant safety systems to reduce the already low residual risk of reactor operation, and changes to the fuel design to,increase the efficiency of fuel utilisation. In other cases, designs are being changed to reduce the complexity of the plant and to improve its safety. An example is the new design for a containment structure which can be cooled by natural circulation only. Such programmes will take a number of years to develop and, hence, are not yet ready for construction commitments. In still other cases, more revolutionary designs are being developed which may require the operation of a prototype plant to test the design prior to commercial commitment.

These approaches are exemplified in the following projects now being carried out in some of the OECD countries:

A. The French advanced PWR, the N4, is under construction in Chooz, France. This 1400 MWe project aims at reducing the cost of nuclear electric power through use of advanced components such as pumps, heat exchangers and turbines which lower the capital cost and increase the efficiency of operation. The safety of plant operation is enhanced through improvements in the man-machine interface and other features.

The N4 Advanced PWR at Chooz, France

Other countries have similar near-term advanced water reactor projects. For example, three large advanced PWRs called Convoy were recently completed in Germany. The United Kingdom's first commercial water reactor, the Sizewell B PWR, is now under construction with startup scheduled in 1994. A large ad- vanced BWR, jointly designed by Japanese and U.S. firms, is under construction In Japan.

B. Two mid-sized (about 600 MWe) advanced LWRs are under development in the United States, with the major focus on plant simplification. For example, the advanced PWR has 32 per cent fewer valves, 35 per cent fewer pumps, and 45 per cent less pipe than a traditional PWR of comparable rating. These simplifications are expected to greatly enhance the safety and reliability of plant operation. Significant reductions in the cost and schedule of plant construction are expected both from plant simplification and from the application of modular construction techniques and a greater scope for factory assembly. Major emphasis is also placed on passive safety features which put less reliance on human intervention for accident management. For example, emergency core cooling systems will not rely on pumping systems requiring diesel generated electric power, and containments can be cooled using natural rather than forced circulation. Such a passive cooled containment system (PCCS) is shown below:

A mid-sized BWR being developed in the U.S. is expected to include similar improvements in simplicity, safety and economics. Several OECD Member countries, including Italy, Japan, France and the Netherlands, are participating with the U.S. Electric Power Research Institute in various aspects of the mid-sized advanced LWR development programme.

C. The PIUS (Process Inherent Ultimate Safety) reactor is under development in Sweden. Because this reactor system has marked departures from existing water reactor systems in areas such as reactivity control and primary coolant system configuration, a largescale prototype system (or demonstration plant) should probably be constructed to confirm the reliability of the system. A primary design goal of this system (and a similar system in Japan, the Intrinsically Safe Economical Reactor (ISER), is enhanced protection of the core during postulated accidents. The goal is for core degradation accidents to be prevented by passive means without reliance on the function of active components and/or operator action following conceivable accidents.

VI. How long will it take to secure the benefits of advanced water reactors?

In addition to the state of technical development, the schedule for deployment of these advanced water reactor systems depends in many cases on evolving government policies. In many countries, ample resources and capabilities are now available for investment in advanced water reactor deployment, provided national governments take a leadership role in creating a stable regulatory climate to provide the conditions needed for private investment. Various forms of international co-operation such as exchanges of information and joint projects could also make a valuable contribution to the development of advanced reactors.

VII. What about inherently safe reactors?

Improvements already made to existing reactors and planned for future water reactors have and will continue to achieve significant reductions in the residual risk of reactor operations. Many of these features rely on passive safety features, or inherently safe choricteristics. That is, they require less reliance on human participation and intervention to ensure the reactor is always in a safe condition.

However, it is not technically sound to claim that any reactor, be it a water reactor or some other type, is an "inherently safe reactor". The use of this term is a misnomer and is inappropriate for any power generating technology. All energy technologies pose some risk, and it is necessary to evaluate the risks and benefits of each technology carefully and objectively before reaching decisions on new applications.

VIII. Are small reactors safer?

Two smaller reactors producing 500 megawatts of electricity each are not necessarily safer than one large reactor producing 1 000 megawatts. Although some features which can be provided in smaller reactors place greater reliance on more passive safety features compared to existing larger reactors, many other factors which affect overall safety are either not a function of size or favour large reactors over smaller ones. therefore, electric supply organisations and their national governments will continue to select the unit size of new water reactors based on utility-specific factors such as grid size, economics, and energy demand growth.

IX. Will advanced water reactors be less costly?

The cost of nuclear power compared to alternatives varies considerably with the age of the plant and from country to country. In many cases, nuclear power is more economic than electricity generated by coal or oil. In other cases, where lengthy construction schedules and resulting high costs were incurred, or where coal, oil or gas are currently available at low cost, nuclear power is more expensive.

One of the major objectives of the advanced water reactor programmes in most countries is to further improve the economics of nuclear power. Features being included in all advanced water reactors focus on simplifying the design, using modular construction, and other steps to reduce the initial capital cost. In some countries, a coherent energy policy and the introduction of a streamlined and stable licensing system is a critical prerequisite to such cost reduction efforts. These factors, together with the anticipated increasing costs of available fossil fuel alternatives, such as the cost of new pollution control systems, are expected to enhance the economics of nuclear power in future years.

X. Heavy water reactors

The heavy water reactor uses a molecular variation of ordinary water comprised of two atoms of deuterium for every atom of oxygen, instead of the usual two atoms of hydrogen for every atom of oxygen. A deuterium atom is twice as heavy as an ordinary hydrogen atom. Heavy water has different nuclear properties than ordinary light water, although its appearance and chemical behaviour are the same.

Commercial heavy water reactors were pioneered by the Canadians and are often called CANDUs (for Canadian Deuterium reactors). Among the advanced heavy water reactors being studied are two which have been developed by Canada: the CANDU 3 with a rating of 450 MWe and the CANDU 6 Mark II with a 600 MWe rating.

Canada is also developing a larger advanced heavy water reactor (the CANDU 6 Mark III) which will have a rating of 700 MWe to 1150 MWe and will feature major advances in safety, reliability and economics. The design programme began in mid-1987 and is expected to produce a design for project commitment in the early 1990s.

An Advanced Thermal Reactor, a 606 MWe heavy water moderated BWR, is under development in Japan, with plans for commercial operation in the late 1990s. This project is intended to demonstrate major improvements in fuel utilisation and is viewed by Japan as a possible transition to the ultimate Introduction of fast breeder reactors.

XI. Role of the Nuclear Energy Agency

Development and deployment of advanced water reactors should be the responsibility of the individual Member countries. To share the costs and risks of such development, a number of countries are already co-operating in the development of specific advanced water reactor systems. In view of wider interest in the potential benefits of these programmes, the NEA will examine the various positions on development and deployment of advanced water reactors in Member countries.


  1. A Study on Advanced Water-Cooled Reactor Technologies, a Report by an Expert Group, NEA (to be published in 1989).

  2. Status of Advanced Technology and Design for Water Cooled Reactors, Light Water Reactors, IAEA TECDOC-479, 1988 (a companion document on heavy water reactors is under preparation).

  3. World Association Gets Ready to Bring Utilities together in a Global Network, J. Varley, Nuclear Engineering International, Oct. 1988.

  4. Advanced CANDU Reactors, J.T. Dunn et al., presented to the IAEA Technical Committee, Dec. 1988 (AECL Report 9817).

  5. State of the Advanced Model N4-- French Design PWR Related to the Nuclear Power Plants of France, Dennielou- et al., Proceedings of the American Power Conference, May 1987.

  6. Progress of the U.S. Advanced Light Water Reactor Program, McGoff, et al., IAEA Conference on Nuclear Power Performance and Safety, Oct. 1987.

  7. PIUS--The Next Generation Water Reactor, Pederson, et al., ANS Topical Meeting on Safety of Next Generation Power Reactors, May 1988.

  8. AP 600 Offers a Simpler Way to Greater Safety, Operability and Maintainability, Vijuk et al., Nuclear Engineering International, Nov. 1988.

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