|The Maestral sealing laboratory at Pierrelatte
Credit: Technetics Group
The growth forecast in global nuclear generating capacity, estimated by several agencies at the end of 2013 to be at least 17 per cent by 2030, makes it clear that continuous commitment to safety, and the development of systems to eliminate hazardous factors, remain of paramount importance.
Despite their small footprints relative to an entire nuclear power production (NPP) installation, sealing systems play a significant role in ensuring overall safety. Reliable and efficient performance in their specific applications is of the utmost importance.
Maintaining the highest level of safety while ensuring the effective performance of a nuclear power plant typically comes down to one statement: “Keep it tight.” There are many considerations involved in the development and continuous improvement of qualified sealing products, flange assemblies and sealing systems installed in the base of pressurized water reactor nuclear power plants (PWR NPP), as well as important organizational changes and production improvements.
An understanding of the problem, in this case leakage, helps to clarify the solution: the effective sealing of mechanical joints. An outline of the long-term collaboration between the French Atomic Energy Commission (CEA) and Technetics Group operating the Maestral Seal Qualification Laboratory at Pierrlatte illustrates a solution cycle for defining sealing performance.
When used by the media, the term ‘leakage’, as it pertains to nuclear power, is always a cause for alarm. Scientists and technicians in the nuclear energy industry also take the term seriously and view such issues with a results-oriented, systematic approach. The concern expressed by these professionals inherently drives safety, innovation and education, and propels the industry to the highest safety standards. A vital aspect of ensuring safety against leaks and equipment failure is maintained through the use of innovative and highly reliable sealing systems.
Sealing devices and associated technologies are applied wherever joints of components or parts require the prevention of fluid or gas flow. Sealing systems are an integral part of mechanical joints for many essential systems in nuclear installations, as well as between specific systems and components. These systems often include, but are not limited to: reactor pressure vessels; main coolant pumps; steam generators; pressurizers; pipes and pumps; heat exchangers; valves; radioactive waste and fuel element transportation and storage casks; and gates between plant sections and buildings.
In order to design and manufacture a wide range of seals to satisfy a broad array of sealing requirements, including an acceptable leakage rate, it is necessary to establish leakage rate criteria for the selection or design of a suitable seal. A specification that defines a ‘no leak’ or ‘zero leakage’ requirement is, in a technical sense, unrealistic and may prove quite costly.
In characterizing leakage and performing leakage testing, the flow of gas is used. Even at very low pressures, gases behave and flow like fluids. Gas flow is categorized into three different types of flow modes: turbulent flow, laminar flow and molecular flow (see Figure 1).Thus, leakage appears as a result of hardware failure and/or hardware design and is generally measured by pressure per volume over a period of time.
|Figure 1: Modes of gas flow|
Regarding the sealed medium, the molecular size and its relation to the width of the flow path are vital for leak tightness. The flow of media with larger molecules, like oils, is easier to seal than smaller molecules, such as helium or other light gases.
A typical bolted mechanical joint assembly (see Figure 2, p 26), such as those used in nuclear steam supply systems in various applications, relies on each subcomponent to work properly and its successful performance depends on the quality and design of each of the three major subcomponents: flanges (including the flange design, groove dimensions and surface finish); bolts or fasteners; and seal and/or gasket.
|Figure 2: Typical bolted mechanical joint|
These three main components cannot be designed independently of each other. They must be considered together as a system during the design process. If any single part of the bolted joint assembly does not perform properly, the joint as a whole will not achieve expectations and may leak. As a result, the assembly’s leakage, and therefore its leak tightness, is a function of the clamping loads at the bolts, which defines the specific pressure of the mechanical joint. In simpler terms, the smaller the area, the higher the sealing effect.
However, the geometrical dimensions of the assembly work against the performance principle of sealing. Assemblies with large diameter mechanical joints require that the sealing performance increase by magnitudes as the ‘relative leakage’, defined as the leakage per millimetre circumference of the sealing in the assembly, determines the tightness performance of the entire assembled system.
System pressure and the temperature of the medium flowing inside the assembly move the structure and influence its mechanical and geometrical properties. All subcomponents, including the sealing components, age over time, while the electrochemical properties of metal-to-metal connections, as well as chemical ingredients or impurities of the flowing medium, drive corrosion and change surface geometry and condition.
These major influencers introduce several demands that must be fulfilled by the sealing device. First, it must follow the mechanical modifications of the assembly; and second, it must adapt to deformations or the changing conditions of flange mating surfaces.
In this context, we understand that sealing is a ‘controlling system’ which, by its material and mechanical properties, regulates deforming influencers while keeping the assembly below determined leakage rates. ‘Elasticity’ and ‘plasticity’, the main properties of sealing systems, can be exploited to perform controlling functionality.
This controlling system property must be maintained in nuclear steam supply operating systems over a long period of time. Generation III and Generation III+ nuclear reactors are designed for extended fuel reload intervals or extended operating cycles, often between 18 and 24 months. These operating cycles determine the frequency of regular maintenance outages.
A history of sealing approaches
The above principles of leakage led to the following design properties of components:
- For large, heavy flanges with large surfaces and clamping loads/compression loads, sealing was arranged within the mechanical joints as part of the mechanical structure. Gaskets were traditionally bolted to adjust the amount of compression of the seals;
- Mechanical joints required intense maintenance and/or repair, and were leakage prone;
- Costs associated with the construction and manufacturing of flanges were high;
- Increasing the clamping load was the principal method to ensure leakage parameters, which imposed high stresses on bolts, surfaces and sealing materials.
- Graphite and high-nickel alloys were introduced to sealing technology;
- Problems with flanges were tested and settled with improved sealing solutions.
The structural stability and elasticity of a mechanical joint is contrary to the mechanical requirements for a sealing material. Thus, the two functions of mechanical connections for structures (ensuring stability — the metal-to-metal principle) and the sealing function (elasticity and plasticity) were separated. An integrated approach to engineering for flange and sealing designs, based on the exploitation of specific material properties, made it possible to create engineered solutions for sealing tasks while achieving defined sealing performance requirements.
The metal-to-metal concept provides nuclear power plant systems with the ability to seal mechanical joint assemblies together regarding the following criteria: optimal seal compression, low elastic solicitation, rigidity of the assembly, and thermal and pressure transients of the operating conditions.
Seal qualification laboratory
In the early 1970s, France initiated the construction and operation of the European Gaseous Diffusion Uranium Enrichment Consortium (EURODIF) plant for civil uranium production as part of its growing nuclear power industry. From the beginning, it was recognized that a high level of performance and safety was required for the generation of nuclear power. To ensure that these criteria were met, nuclear sealing technologies were implemented through an integrated process of scientific-technical cooperation. This process tested sealing solutions to ensure their precise abilities and technical feasibility.
The French Alternative Energies and Atomic Energy Commission (CEA) partnered with Technetics Group to create a joint sealing laboratory at Pierrelatte and begin the testing and qualification process. This laboratory combines the nuclear expertise and competencies of the CEA with the sealing-specific expertise of a highly specialized, technologically innovative corporation. Work performed at the Maestral Sealing Laboratory is directed toward creating and promoting a solution cycle for defined sealing performance in order to maintain the qualification of that product portfolio and ensure its qualification into the future.
Through integration into the French nuclear sector, as well as within the internationally co-operative nature of nuclear and power technology vendors, NPP operating utilities and main Gen-IV research programmes (ITER, ASTRID, ALLEGRO etc), the laboratory represents a scientific-technical centre of long-term experience and continued feedback from the operation and maintenance of nuclear power plants and facilities worldwide.
This integration also allows for experience, feedback and understanding of almost all types of NPP technology. Clearly defined and measured operating conditions are simulated by respective test stands in the laboratory to monitor seal behaviour and characteristics. These tests are key to providing and maintaining the largest portfolio of engineered qualified sealing solutions in the nuclear industry.
An initial milestone of such R&D includes the qualification and benchmarking of metal-to-metal sealing technologies with spring-energized metal seals. This technology, well known to the industry as Helicoflex, was first applied to the equipment in the EURODIF plant. Consequently, this system and other types of engineered sealing solutions were created and designed on the basis of the metal-to-metal concept in order to fulfil specifically defined sealing performance requirements for a wide range of sealing tasks. These spring-energized metal seals are employed in various components and systems throughout NPPs and other nuclear power-based equipment.
Data and standards
Today, a total of 437 commercial nuclear power reactors are operating in 31 countries. More than half of these plants have been in operation for over 20 years, the majority of which rely on pressurized water reactor systems. This constitutes a tremendous number of operating hours (in the range of several tens of millions), and provides a great deal of respective operating experience and feedback to design and technology providers.
Uranium enrichment and nuclear fuel producers, as well as facilities and container technologies for the transport, treatment and interim storage of radioactive waste and spent fuel, provide large accumulated operational histories of nuclear technology equipment. This creates a tremendous amount of data for nuclear power plants and nuclear component providers with background and experience in the creation of sealing solutions, while providing deep insight into the long-term characteristics of seals and the corresponding NPP component over long operating periods.
As sealing performance is a function of load to the sealing system and the functional area of the seal, Westinghouse Electric Company, the origin of many pressurized water reactor systems, set an initial 5 per cent standard measure for the two main properties. They stated that a seal must maintain “maximal load”, which challenges its elasticity, and “minimal contact seal area in order to maximize sealing stress”, which exploits the plasticity and sealing ability of a resilient seal.
Embedding engineered sealing solutions into a cycle is a promising method for present and future sealing tasks in demanding environments. Specialized laboratories contribute to long-term experience in upgrade processes for existing NPPs, allowing for improved safety, performance and technological support.
The results are more sophisticated seal designs and higher seal compressions, ensuring that elasticity and plasticity remain the most important aspects of sealing performance.
Thomas Ritter is Senior Market Manager, Nuclear at Technetics Group.
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