There is a shared interest in the nuclear community for nuclear power to continue as a viable participant in future electricity markets. This implies an industry that is forward looking and change-oriented at all levels, including investors, managers, designers and regulators, and a focus on plants that generate power efficiently, safely, profitably and at competitive costs. It also implies the need for all members of the nuclear community to re-examine their assumptions, goals and practices, to find new approaches, by working together, that are consistent with today’s and tomorrow’s commercial realities.
Design organizations are challenged to develop advanced reactors with
considerably lower capital costs and shorter construction times;
simplified designs which achieve high safety levels in the most cost effective manner;
sizes (including small and medium sizes with load following capability) appropriate to grid capacity and owner investment capability;
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high levels of standardization and modularization incorporating the latest technological advances.
Regulatory agencies are challenged to
move to more risk-informed safety requirements for new plants;
establish design certification procedures which don’t take too long, and which are sufficiently flexible to accommodate
incorporation of advances in technology,
design changes which maintain adequate safety level at reduced cost, and
a variety of users needs (e.g. needs for a variety of plant sizes); and
establish international consensus regarding commonly acceptable safety requirements that would facilitate development of standardized designs, so that such designs could be used in several countries.
Power generating organizations are challenged to set user requirements for new designs that result in the most cost effective solutions while meeting safety requirements and which are profitable under changing market conditions.
41 Appendix
TECHNOLOGIES FOR EXTENDING PLANT LIFETIME, IMPROVING AVAILABILITY AND REDUCING OPERATION AND MAINTENANCE COSTS With deregulation and privatization of electricity markets, nuclear plant operators are experiencing an operating environment with increased competition from other suppliers of electricity. This competitive environment has significant implications for plant operations, including efficient use of all resources; more effective management of plant activities, such as outages and maintenance; and sharing of resources, facilities and services among power generators. The overall result is a significant reduction in operation and maintenance costs.
Important means of improving economics involve technologies for plant life extension, and for increasing plant availability. Both require attention to both the nuclear island and the balance of plant. Nuclear power plants worldwide are improving their energy availability factors. The world average has increased from below 70 percent in 1983 to 82 percent in 200020, with some power generators achieving significantly higher values. This is being achieved through integrated programmes including personnel training, quality assurance, improved maintenance planning, longer fuel cycles, as well as technological advances in plant components and systems, and in component inspection, maintenance, repair and replacement techniques.
International co-operation is playing a key role in this success. The various programmes of the World Association of Nuclear Operators (WANO) to exchange information and encourage communication of experience, programmes of the OECD/NEA and the European Commission to improve component inspection techniques, and the activities of the IAEA including projects in nuclear power plant performance assessment and feedback, effective quality management, and information exchange meetings on technology advances [30] [31] [32], are important examples of international co-operation to improve plant performance.
Technologies for extending the lifetime of current plants
The economic life of a plant is defined by the market, and the cost of continued operation of the plant may not coincide with the period of the plant license. Different countries take different approaches in setting the duration of the operating license for a nuclear plant. In some countries, there is a “licensed life” with possible consideration of life extension, while in others there is a “periodic safety assessment” to approve the plant for a further fixed period of operation. Political decisions to end the operation of a plant before it reaches its technical or economic lifetime also occur. Provided the economics of the plant are favourable, there is a considerable incentive to seek life extension.
Life extension up to a total of 60 years is being pursued in several countries. In the USA, the Nuclear Regulatory Commission has already issued twenty-year extensions of the operating licenses for several nuclear power plants. It is anticipated that most nuclear plants in the USA will seek similar extensions of their operating licenses over the next several years. The
20 Based on IAEA Power Reactor Information System (PRIS) data. In PRIS, the energy availability factor is defined as 100 [1-EL/Em] with Em being the net electrical energy which would have been produced at maximum capacity under continuous operation during the reference period, and EL is the electrical energy which could have been produced during the reference period by the unavailable capacity. (The numbers reported here are for plants with capacity greater than 100 MW(e) and with more than one year of commercial operation).
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incentive for seeking license extensions well in advance of the expiration of the initial 40-year license, is quite simple to understand. If the plant owner knows he will be allowed to continue operating for a longer period of time, there is less risk in making capital investments needed for future operations.
Life extension requires qualification of components and structures related to safety performance supported by fatigue analysis and ageing effects analysis. Components addressed include the reactor pressure vessels of LWRs or the pressure tubes of HWRs, reactor internals, steam generators, pumps and valves. Life extension typically involves at least particular component replacement. Because the reactor pressure vessel cannot easily be replaced, vessel annealing has been an important approach for alleviating vessel embrittlement with age. HWR pressure tubes have been replaced in 5 units, and experience now predicts that this can be achieved with a 9 to 12 month shutdown after about 30 years of operation.
The ability to qualify systems, components and structures for life extension is facilitated by life management programmes that monitor and establish their conditions. Key elements of these programmes include:
periodic inspections of major components such as the LWR reactor vessel, HWR pressure tubes, primary system piping and steam generator tubing;
effective maintenance;
development of inspection and repair technologies, and the acquisition of material data and operational data related to ageing.
Materials technology programmes have been established by nuclear plant operators and government organizations focused on understanding and managing materials condition and performance issues with the goals of determining and increasing component residual service life. Specific focus has been on components such as pressure vessels and internals, reactor pressure tubes, primary system piping and steam generator tubing.
Each of the principal light water and heavy water reactor types has experienced material-related problems during the service life of the initial versions. These have been overcome in various ways including replacement with more resistant materials and changes in the chemistry of the water environment or by design modifications based on full scale model testing.
BWR reactors, for example, have experienced cracking of reactor internals (e.g. the core shroud) made of type 304 stainless steel due to inter-granular stress corrosion cracking.
Inspection, repair and replacement techniques have been developed using extensive laboratory, and in-plant data as well as full-scale mock-up test facilities to detect and correct the damage.
PWRs have suffered from cracking of vessel head penetrations, core barrel and bottom mounted instrumentation adapters. Reactor vessel head replacement has been needed at several PWRs because the nickel-based Alloy 600 sleeve material was susceptible to stress corrosion cracking. The replacement heads include improved materials for penetrations (e.g.
Alloy 690) and integrated forged head designs.
HWRs have experienced problems related to pressure tube and boiler tubes. The condition of the initial pressure tube alloy (Zircaloy-2) deteriorated from the failure of tube supports to
43 prevent thermal gradients from developing in the tube and causing hydride formation and cracking. This problem necessitated pressure tube replacement in 5 units with improved designs of spacer tube supports. The replacement alloy was Zr-2.5Nb, which picks up significantly less corrosion hydrogen than Zircaloy-2 during service.
Although significant progress has been made in understanding irradiation and thermal degradation of LWR reactor vessel steels, some aspects are still not fully understood. In particular, further work is essential on the qualification of remedial measures such as annealing and repairs. The international efforts of the IAEA Working Group on Nuclear Plant Life Management and the OECD Nuclear Energy Agency (NEA) Principal Working Group 3 (PWG3) provide national contacts between institutions working in this field. The European Network AMES programme (Ageing Materials Evaluation and Studies) was initiated in 1993 including material studies to improve the understanding of the effects of irradiation damage, ageing and annealing, micro-structural model development and studies on irradiation and thermal degradation of materials for new reactors.
The pressure tubes of HWRs have and continue to receive considerable study to determine life limits before fullscale replacement. Deformation by irradiation enhanced creep and growth of the zirconium alloy tubes limits the life to about 30 years although development results predict a steady increase.
The dominant cause of damage to pressurized water reactor steam generators has been corrosion of the tubing and support structures. The alloy, Inconel 600, has suffered extensive corrosion failures in many plants, sometimes requiring steam generator replacement. Large efforts in several countries have been and are being carried out to control and improve the service environment to extend the service life of steam generator tubes. New alloys (e.g. alloy I800 and Inconel 690) have been shown to have superior corrosion resistance compared to Inconel 600 and are now favoured for new and replacement PWR steam generators.
With regard to in-service inspection of primary circuit components (especially the reactor vessel, primary piping and steam generator tubes), the EC and OECD/NEA have conducted major international efforts for more than 20 years to improve the capability and reliability of non-destructive evaluation methods. Improvements have been introduced through procedures specifically adapted to the defects to be detected.
A major international effort to improve the assessment of the capability and reliability of inspection techniques and procedures for non-destructive evaluation of structural components has been carried out since 1974 in the Programme for the Inspection of Steel Components (PISC) by the European Commission in co-operation with OECD/Nuclear Energy Agency.
Inspection methods for pressure vessels, dissimilar metal welds, primary piping welds, and steam generator tubes have been evaluated. This work culminated in the publication of the first edition of the European Methodology for Inspection Qualification in 1995, and the second in 1997.
For HWRs, specific inspection techniques have been developed to detect flaws in the relatively thin-walled zirconium alloy pressure tube tubing and primary circuit tubing most of which is also small diameter (<100mm dia). For zirconium tubing the inspections can use both ultrasonic and eddy current techniques to increase overall the detection sensitivity to small flaws.
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Technologies for improving performance of current plants
Considerable improvement in outage time and generation costs can still be achieved through technical and administrative measures. Examples of technologies for improved performance include high burnup fuel (which supports longer cycle length), power up-rating to achieve higher output, computer-aided systems to provide early indication of sensor or component degradation, and simpler systems for PWRs and HWRs for control of hydrogen during accident conditions (systems that require considerably less testing and maintenance and thereby reduce outage duration).
Power up-rating often involves replacement of some heat transfer and power conversion equipment, as well as analyses required to support the license to operate at higher power levels. Steam turbine efficiency has increased over the past 20 years, and remodelling of turbines can result in an increase of power output without any upgrading of the reactor. Many operators have made technical improvements to take advantage of power up-rating. In Sweden, for example, the nuclear industry has added approximately 600 MW(e) of capacity by improving its existing stations.
Improved performance at current plants is also supported by implementation of activities to analyse information from operation of components and systems to understand the causes of unavailability, and to improve work processes during maintenance.
Procedures for planning and carrying out maintenance influence reliability and availability.
Good planning and organization of work during planned outages can strongly contribute to shortening outage duration and thereby contribute to availability improvements. For example, formation of special outage management teams with a focus on close communication influences the efficiency of the execution of work during an outage. Efficient outage manage-ment also deals with logistics support including requiremanage-ments for special tools and availability of spare parts as these factors influence outage duration.
New I&C and control room technologies which can contribute to improved plant performance are being backfitted into operating plants. These include:
digital instrumentation and control, including self-diagnostic systems; and
control room and man-machine interface improvements with due consideration of human factors engineering.
Changes in regulatory policy can also facilitate improvements in plant reliability and availability. As an example, The U.S. Nuclear Regulatory Commission's new Risk-Informed Performance-Based regulation policy is expected to contribute in this area because it is a goal-oriented, rather that a means-oriented system focusing on what licensees must achieve rather than on what they must do. Simplification of technical specifications is an application that should reduce the number and length of plant outages, as well as the number of plant personnel required to implement the ‘tech spec’ requirements. For example, simplifications in surveillance and maintenance activities for emergency diesel generators are expected to result from the goal oriented practices of this new policy. Another benefit from this new policy may be the ability to reclassify many of the systems, structures, and components in the nuclear plants, so that they are not required to fully satisfy all of the special conditions normally imposed upon safety-grade equipment. Instead, many of them could be purchased and maintained to commercial quality standards, in the future.
45 REFERENCES
[1] Nucleonics Week, Jan. 11, 2001, p. 3.
[2] Nucleonics Week, April 5, 2001, p. 6.
[3] INTERNATIONAL ATOMIC ENERGY AGENCY, Status of Advanced Light Water Cooled Reactor Designs: 1996, IAEA-TECDOC-968, IAEA, Vienna (1997).
[4] INTERNATIONAL ATOMIC ENERGY AGENCY, Advances in Heavy Water Reactor Technology IAEA-TECDOC-984, Vienna (1997).
[5] INTERNATIONAL ATOMIC ENERGY AGENCY, Evolutionary Water Cooled Reactors: Strategic Issues, Technologies and Economic Viability, Proceedings of a symposium held in Seoul, 30 November–4 December 1998, IAEA-TECDOC-1117, Vienna (1999).
[6] DAVIS, George, “Assuring the Competitiveness of New Nuclear Plants in a Deregulated U.S. Market” paper presented at the Global Foundation Conference (1998).
[7] OECD/NEA-IEA, Projected Costs of Generating Electricity — Update 1998.
[8] OECD/NEA, Reduction of Capital Costs of Nuclear Power Plants, OECD 2000.
[9] ELECTRIC POWER RESEARCH INSTITUTE, Advanced Light Water Reactor Utility Requirements Document (Volume I).
[10] INTERNATIONAL ATOMIC ENERGY AGENCY, Objectives for the Development of Advanced Nuclear Plants, IAEA-TECDOC-682, Vienna (1993).
[11] LEE, J.S., et al., “Design features in Korea next generation reactor focused on performance and economic viability”, Performance of Operating and Advanced Light Water Reactor Designs, IAEA-TECDOC-1245, IAEA, Vienna (2001).
[12] INTERNATIONAL ATOMIC ENERGY AGENCY, HWRs: Status and Projected Development, Technical Reports Series No. 407, IAEA, Vienna (2002).
[13] UJIHARA, N., “Development, operating experience and future plan of ABWR in Japan”, Performance of Operating and Advanced Light Water Reactor Designs, IAEA-TECDOC-1245, IAEA, Vienna (2001).
[14] INTERNATIONAL ATOMIC ENERGY AGENCY, Strategies for Competitive Nuclear Power Plants, IAEA-TECDOC-1123, Vienna (1999).
[15] INTERNATIONAL NUCLEAR SAFETY ADVISORY GROUP, Basic Safety Principles for Nuclear Power Plants, Safety Series No. 75-INSAG-3, IAEA, Vienna (1988).
[16] INTERNATIONAL NUCLEAR SAFETY ADVISORY GROUP, The Safety of Nuclear Power, Safety Series No. 75-INSAG-5, IAEA, Vienna (1992).
[17] INTERNATIONAL NUCLEAR SAFETY ADVISORY GROUP, Defence in Depth in Nuclear Safety, INSAG-10, IAEA, Vienna (1996).
[18] INTERNATIONAL NUCLEAR SAFETY ADVISORY GROUP, Basic Safety Principles for Nuclear Power Plants, Safety Series No. 75-3 Rev. 1, INSAG-12, IAEA, Vienna (1999).
[19] The Safety of Nuclear Power: Strategy for the Future (Proc. Conf. Vienna, 1991) IAEA, Vienna (1992).
[20] INTERNATIONAL ATOMIC ENERGY AGENCY, Development of Safety Principles for the Design of Future Nuclear Power Plants, IAEA-TECDOC-801, Vienna (1995).
[21] INTERNATIONAL ATOMIC ENERGY AGENCY, Applications of Probabilistic Safety Assessment (PSA) for Nuclear Power Plants, IAEA-TECDOC-1200, Vienna (2001).
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[22] INTERNATIONAL ATOMIC ENERGY AGENCY, The Role of Probabilistic Safety Assessment and Probabilistic Safety Criteria in Nuclear Power Plant Safety: A Safety Report, Safety Series No. 106, IAEA, Vienna (1992).
[23] UK HEALTH AND SAFETY EXECUTIVE, Safety Assessment Principles for Nuclear Power Plants, HMSO, London (1992).
[24] U.S. Department of Energy, Report to Congress on Small Modular Nuclear Reactors, May 2001.
[25] GASPARINI, M., “The IAEA Safety Standards for the Design: Application to Small and Medium Sized Reactors” Paper presented at IAEA Seminar on Status and Prospects for Small and Medium Sized Reactors.
[26] INTERNATIONAL ATOMIC ENERGY AGENCY, Safety Related Terms for Advanced Nuclear Power Plants, IAEA-TECDOC-626, Vienna (1991).
[27] INTERNATIONAL ATOMIC ENERGY AGENCY, Progress in Design, Research and Development and Testing of Safety Systems for Advanced Water Cooled Reactors, IAEA-TECDOC-872, Vienna (1996).
[28] INTERNATIONAL ATOMIC ENERGY AGENCY, Technical Feasibility and Reliability of Passive Safety Systems for Nuclear Power Plants, IAEA-TECDOC-920, Vienna (1996).
[29] INTERNATIONAL ATOMIC ENERGY AGENCY, Experimental Tests and Qualification of Analytical Methods to Address Thermohydraulic Phenomena in Advanced Water Cooled Reactors, IAEA-TECDOC-1149, Vienna (2000).
[30] INTERNATIONAL ATOMIC ENERGY AGENCY, Technologies for Improving Availability and Reliability of Current and Future Water Cooled Nuclear Power Plants, IAEA-TECDOC-1054, Vienna (1998).
[31] INTERNATIONAL ATOMIC ENERGY AGENCY, Technologies for Improving Current and Future Light Water Reactor Operation and Maintenance: Development on the Basis of Experience, IAEA-TECDOC-1175, Vienna (2000).
[32] INTERNATIONAL ATOMIC ENERGY AGENCY, Performance of Operating and Advanced Light Water Reactor Designs, IAEA-TECDOC-1245, Vienna (2001).
[33] A. Voß, The Ability of the Various Types of Power Generation to Compete on the Liberalized Market; VGB Power Tech 4/2001.
ANNEXES
Annex 1