Construction methods related to materials of construction

Conventional methods

NPPs utilize large vessels and heat exchangers to produce steam for power generation. They produce steam at much lower temperatures and pressures than fossil-fired power plants. This is especially true when comparing to super-critical and ultra-super-critical coal plant designs. However, NPPs present other challenges to the material performance of piping, components and equipment.

Primarily, nuclear reactors involve significant radiation environments, which are a challenge to long term integrity of all materials, including metals. Safety concerns also produce rigorous requirements. Finally, repair and replacement within NPPs can be relatively more expensive than at other types of plants, driving the need for extended life and improved performance.

Pressurized water reactor (PWR) steam generator material performance is a prime example of the evolution of materials used in NPPs. Steam generators are large tube and shell heat exchangers that pass pressurized reactor coolant through the tubes and produce steam on the shell side. Two of the most important factors affecting a tube’s vulnerability to degradation are its material and heat treatment. The two types of tube material conventionally used for PWR steam generator tubes are alloy 600 and alloy 690. The two types of heat treatment applied to these materials to improve their mechanical and corrosion resistance properties are mill annealing and thermal treatment.

Early operations of NPPs did not pay close attention to the impact of secondary cycle water chemistry on the material performance in the steam generators. The dominant cause of tube degradation was thinning of the mill annealed alloy 600 steam generator tube walls due to the chemistry of the water flowing around them. Changes to feed-water chemistry control programmes have virtually eliminated this problem with tube thinning. After tube thinning, tube denting became a primary concern. Denting results from the corrosion of the carbon steel support plates and the buildup of corrosion product in the crevices between tubes and the tube support plates. Measures have been taken to control denting, including further changes in the chemistry of the secondary side water chemistry.

Even after corrections to water chemistry controls were made across the industry, the initial alloy materials used for steam generators proved ineffective in long term resistance to stress corrosion cracking and pitting. As mill annealed alloy 600 steam generator tubes began exhibiting degradation in the early 1970s, the design of future steam generators pursued improvements to reduce the likelihood of corrosion. This resulted in the alloy 600 tubes being subjected to a high temperature thermal treatment to improve their resistance to corrosion. Although no significant degradation problems have been observed in plants with thermally treated alloy 600 steam generator

tubes, plants which replaced their steam generators since 1989 have primarily used tubes fabricated from thermally treated alloy 690, which is more corrosion resistant than thermally treated alloy 600. Most of the newer steam generators, including the aforementioned replacement steam generators, have features that make the tubes less susceptible to corrosion-related damage. These include using stainless steel tube support plates to minimize the likelihood of denting, and new fabrication techniques to minimize mechanical stress on tubes.

For reactor pressure vessels (RPVs), long term exposure to high neutron flux rates has caused embrittlement of the vessel walls. RPVs, which contain the nuclear fuel in NPPs, are made of thick steel plates that are welded together. Neutrons from the fuel in the reactor irradiate the vessel as the reactor operates. This can embrittle the steel or lower its strength, subsequently making it less capable of withstanding flaws that may be present.

Embrittlement usually occurs at a vessel’s ‘beltline’, that section of the vessel wall closest to the nuclear fuel.

Many PWR operating plants use core designs that reduce the number of neutrons that reach the vessel wall.

These design features therefore reduce the rate of embrittlement in the reactor vessels. Other factors also contribute to the degree to which a particular vessel material becomes embrittled. Steels with a higher proportion of copper and nickel will tend to be more susceptible to embrittlement than steels with lower proportions of these two elements.

Advanced methods

Materials management matrixes

Decades of worldwide NPP operational history have generated a materials performance knowledge base. The knowledge and experience have resulted in improved materials being available in current markets. The new generation of NPP projects has an opportunity to take advantage of these advances, provided the vast collective experience can be distilled and applied effectively. International research and standards organizations continue to gather and distil this type of information for the industry as a whole. One such effort is the Electrical Power Research Institute (EPRI) Materials Management activity under their Advanced Nuclear Technology Programme.

Reference [7] is an example of such work.

The abstract of Ref. [7] is quoted as follows:

“For advanced light water reactor (ALWR) designs currently being considered for new construction, an opportunity exits to leverage operating plant experience to proactively identify and manage materials performance issues. In many cases, evaluation of operating plant issues and implementation of mitigation or management technologies hold the potential to significantly reduce operating costs over the life of these new plants. Potential benefits include prevention of degraded conditions, more efficient and accurate inspections, and reductions in repair and replacement costs. To fill this need, EPRI’s Advanced Nuclear Technology (ANT) has initiated a materials management matrix (MMM) initiative. The focus of the project is on identification of gaps and opportunities that, if addressed in at appropriate times in the life cycle of the plant, could significantly improve plant materials performance.”

Materials for sensors and electronics

Emerging technologies in I&C focus on advanced sensors that can affect safety and operating margins. These include the following areas:

— Sensors and measurement systems;

— Communications media and networking;

— Microprocessors and other integrated circuits (ICs);

— Computational platforms (computers, programmable logic controllers, application specific ICs, etc.);

— Diagnostics and prognostics;

— Control and decision;

— Human–system interactions;

— High-integrity software.

Sensor and measurement systems technology that has progressed to becoming potentially applicable includes silicon carbide neutron-flux monitoring. This sensor has the potential to combine the functions of current three-range flux monitors into a single system. Data on important parameters such as drift, accuracy, repeatability and degradation over time needs to be accomplished with review and acceptance by regulatory bodies. It is important that a combined neutron monitor based on this technology should exhibit the full dynamic range from startup to 100% power, and should also be shown to not degrade over the long term when at 100% power.

Another sensor with potential applicability is the fuel mimic power monitor (or constant temperature detector). The instrument represents a unique sensing technology in that it provides a direct measurement of the nuclear energy deposited into a fuel mimic mass. This type of sensor will have the advantage of providing an accurate and direct measurement of reactor power. Data on signal drift needs to be accomplished with review and acceptance by the regulatory body.

The Johnson noise thermometer (JNT) also has potential for NPP applications. The JNT is inherently drift-free and insensitive to the material condition of the sensor. As a result of the latter characteristic, a sensor based on Johnson noise is immune to the contamination and thermo-mechanical response shifts that plague thermocouples and resistance thermometers. This advantage should improve operating as well as safety margins in NPPs. JNTs include a relatively long response time because of the long integration time required to make a temperature measurement. Addressing this has been accomplished in a dual-mode thermometer by making the temperature measurement as a simple resistance measurement whose resistance to temperature conversion is quasi-continuously updated using Johnson noise. Thus application of the JNT in NPPs could be as an on-line, self-calibrating resistance temperature detector. The regulatory implication of self-calibration is that required calibration intervals could be relaxed.

Advances in IC technologies include radiation-hardened ICs and system-on-a-chip (SoC) devices. The potential impact toward facilitating smart sensors and sensor networks in containment applications continues to be valuable reason for radiation-hard ICs. Likewise, the potential for sensing applications using SoC circuitry that can be located in harsh environments and then changed out (perhaps robotically) on a periodic basis provides motivation for development. Specialized applications for SoC devices are challenged by the high degree of on-chip integration for complex systems, power consumption and manufacturability, including testability and reliability.

The special application of the SoC technique to nuclear instrument development and control performance improvement is appropriate. Considerable progress has been made in radiation-hardened IC development using materials such as silicon-on-insulator. Another innovation in radiation-hardened chip technology is the integration of chalcogenide material with the radiation-hardened complementary metal oxide semiconductor (CMOS) process to produce memory arrays. Chalcogenide-based memory is highly radiation resistant. As stated earlier, the regulatory implication of these advances in radiation-hardened IC technology is that equipment such as smart transmitters could find their way into containment environments in the near future.

In the area of nanoelectronics, advances in the development and reinvention of the vacuum tube show promise for high-radiation and high-temperature environments.

Advances have reached a density of 109/cm², comparable to the density of metal oxide semiconductor (MOS) transistors. Nanotriodes are advantageous over MOS technology in radiation resistance and temperature tolerance.

This technology could enable smart sensors to migrate into the most inhospitable areas within the reactor containment.

4.4.3.2. Materials of construction for rigs and special tools Conventional methods

Conventional construction methods have generally used heavy steel rigs, stays and cables to ensure sufficient strength for purpose.

Advanced methods

Lightweight materials such as fibre-reinforced composites are reaching adequate strength to perform well in construction environments. Opportunity exists to reduce weight, manoeuvrability and flexibility (where desired) of construction tools and rigs so that construction efficiency can be increased.

Lightweight materials are also being used by military and logistics organizations for movable bridges and similar structures. Such applications may be useful at NPP construction sites for haul roads, etc.

Similar materials are questionable for structural building work within NPPs because of the current cost and issues with qualification under radiological and fire loading conditions. However, composite materials have been used in selective permanent building applications in NPPs such as fuel pool personnel bridges and other movable, non-safety appurtenances.

4.5. ECO-FRIENDLY (GREEN BUILDING) DESIGN