[en] The high operating pressure and temperature of the Canadian SCWR raise the material-selection requirements for various in-core and out-of-core components. Material selection for the feedtrain is based on boiling water reactor and SCW fossil-power plant best practices, while selection of components downstream of the core is based on the latest advances in material development for ultra-supercritical fossil-power plants. The most challenging requirements are for in-core components. While high nickel stainless steels or nickel-based alloys are considered appropriate for components outside the irradiated region (e.g., inlet and outlet plenums, tubesheet, etc.), these materials may not be appropriate for components in regions (e.g., fuel-assembly) subjected to a high neutron flux due to the unfavorable neutron absorption characteristics of nickel. A number of stainless steels appear suitable for in-core components that encounter a maximum coolant temperature of 625 ^oC. However, these materials may not be suitable for use as the fuel cladding, as the current maximum design limit for the fuel-cladding temperature is 800 ^oC. A significant effort has been devoted to assessing potential candidate fuel cladding alloys. A collapsible cladding has been selected for the Canadian SCWR fuel, reducing the material strength requirements. Corrosion, cracking, creep and irradiation damage then become the limiting phenomena. Corrosion leads to thinning of the cladding wall, challenging its integrity at high burnup when the internal pressure of the fuel element due to fission gas build-up becomes high. Furthermore, the build-up of corrosion products on the cladding surface will reduce the effectiveness of heat transfer from the cladding to the coolant, leading to high cladding and fuel temperatures. Cladding cracking on both the coolant and fuel sides can lead to fuel failure and must be assessed. This paper outlines the key materials requirements, major knowledge gaps, and the program in place to address these gaps. (author)

[en] There is a strong interplay between coolant chemistry and materials selection in any nuclear power plant system. To achieve the design life of reactor components it is necessary to monitor and control relevant chemistry parameters, such as ionic conductivity, pH, concentrations of dissolved ions and redox species (e.g., hydrogen, hydrazine, oxygen) and the concentrations of suspended corrosion products. Chemistry specifications are set to achieve a balance between the sometimes conflicting requirements to minimize corrosion and radiological dose and to minimize operating and maintenance costs over the lifetime of the plant. For the past decade, Atomic Energy of Canada Limited (AECL) has taken a rigorous and disciplined approach to reviewing and updating all aspects of chemistry control in the CANDU® nuclear power plant (NPP). This approach has included proactively reviewing chemistry operating experience from existing CANDU and other water-cooled NPPs worldwide to identify and address emerging issues, updating all of our chemistry control documentation to ensure that each chemistry parameter is linked to a specific requirement (e.g., reduce activity transport, monitor for condenser leak) and incorporating the latest results from our Research & Development (R&D) programs to ensure that all chemistry specifications are supported by a sound rationale. The results of this review and update have been incorporated into updated chemistry specifications and, in some cases, modified operating procedures for new and existing plants. In addition, recommendations have been made for design modifications to improve chemistry control in new build plants, especially during periods of shutdown and startup when chemistry control has traditionally been more challenging. Chemistry control in new-build CANDU plants will rely increasingly on the use of on-line instrumentation interfaced directly to AECL's state-of-the-art chemistry monitoring, diagnostics and analysis system to facilitate improved chemistry control and to help staff to proactively identify and address emerging issues before they result in a loss of performance. This paper will outline AECL's chemistry control philosophy, and provide specific examples to illustrate how changes to plant design, materials, operational procedures, and chemistry specifications are being implemented to support improved chemistry performance in existing and new-build CANDU plants. (author)

[en] In a supercritical water reactor (SCWR), the decrease in water density from subcritical to supercritical regions in the reactor core results in a decrease of solubility and deposition of most corrosion products. Such deposition in the reactor core can seriously affect fuel performance, thermal hydraulics and activity transport in an SCWR. In addition, the dissolution of the oxides formed by deposition or corrosion can release radioactive corrosion products into the supercritical water (SCW) coolant. In a boiling water reactor, the phase change that occurs upon boiling and the very low solubility of metal salts in steam prevents most radioactive species from being transported by the steam to the turbines. In an SCWR core, there is no phase change in the coolant, only a density change. While the solubilities of relevant corrosion products are low, they are not negligible, and ion pairs, the dominant solution species in low density SCW, can be transported by the SCW coolant to the turbines. Thus it is likely that radioactive corrosion products will be transported to the SCWR high pressure turbine where they will deposit due to the changes in temperature and pressure. This paper reviews the solubilities of potential corrosion products in SCW and presents a semi-quantitative prediction of the amount of activity transport expected in an SCWR. Potential mitigating strategies will be briefly discussed. (author)

[en] The three key materials performance metrics of the Canadian Supercritical Water-cooled Reactor (SCWR) core internals, and particularly those of the fuel cladding, are the lifetime wall loss due to corrosion, the oxide thickness developed on the cladding between fuel cycles, and the likelihood of cracking. Other materials performance requirements are mitigated by design. The present paper establishes acceptance criteria for corrosion wall loss and oxide film thickness of the fuel cladding and evaluates the corrosion performance of Alloy 800, Alloy 214, and Alloy 625 with respect to these criteria. (author)

[en] One of the key challenges in the development of a CANDU pressure-tube supercritical water-cooled reactor (SCWR) is the selection of materials appropriate for in-core use. Such materials must be able to withstand the high-temperature, corrosive environment, and effects of irradiation encountered in the core, while at the same time minimizing parasitic neutron absorption. Achieving the appropriate balance between reactor physics and materials requirements necessitates knowledge of both materials properties of candidate alloys and their impact on lattice physics. In this paper, lattice physics calculations have been performed for the CANDU-SCWR for several categories of candidate in-core materials. In addition, a simple relation is derived that can be used to estimate the relative influence of in-core materials on lattice reactivity and fuel discharge burnup, based on material chemical composition and density. (author)

[en] The GIF (Generation 4 International Forum) SCWR (Super-Critical Water-cooled Reactor) materials and chemistry provisional project management board (PPMB) has identified two major challenges that must be overcome to ensure the safe and reliable performance of an SCWR. First,insufficient data are available for any single alloy to unequivocally ensure its performance in an SCWR, especially for alloys to be used for in-core components. Secondly, current understanding of super-critical water chemistry is inadequate to specify a chemistry control strategy, as the result of the large changes in physical and chemical properties of water through the critical point, coupled with the as yet poorly understood effects of water radiolysis. This paper broadly outlines these work packages, describes some of the key challenges, and presents some of the progress being made to overcome these challenges

[en] There is a strong interplay between coolant chemistry and materials selection in any nuclear power plant system. To achieve the design life of reactor components it is necessary to monitor and control relevant chemistry parameters, such as ionic conductivity, pH, concentrations of dissolved ions and redox species (e.g., hydrogen, hydrazine, oxygen) and the concentrations of suspended corrosion products. Chemistry specifications are set to achieve a balance between the sometimes conflicting requirements to minimize corrosion and radiological dose and to minimize operating and maintenance costs over the lifetime of the plant. For the past decade, Atomic Energy of Canada Limited (AECL) has taken a rigorous and disciplined approach to reviewing and updating all aspects of chemistry control in the CANDU® nuclear power plant (NPP). This approach has included proactively reviewing chemistry operating experience from existing CANDU® and other water-cooled NPPs worldwide to identify and address emerging issues, updating all of our chemistry control documentation to ensure that each chemistry parameter is linked to a specific requirement (e.g., reduce activity transport, monitor for condenser leak) and incorporating the latest results from our Research and Development (R and D) programs to ensure that all chemistry specifications are supported by a sound rationale. The results of this review and update have been incorporated into updated chemistry specifications and, in some cases, modified operating procedures for new and existing plants. In addition, recommendations have been made for design modifications to improve chemistry control in new build plants, especially during periods of shutdown and startup when chemistry control has traditionally been more challenging. Chemistry control in new-build CANDU® plants will rely increasingly on the use of on-line instrumentation interfaced directly to AECL's state-of-the-art chemistry monitoring, diagnostics and analysis system to facilitate improved chemistry control and to help staff to proactively identify and address emerging issues before they result in a loss of performance. This paper will outline AECL's chemistry control philosophy, and provide specific examples to illustrate how changes to plant design, materials, operational procedures, and chemistry specifications are being implemented to support improved chemistry performance in existing and new-build CANDU® plants. (author)

[en] The CANDU supercritical water-cooled reactor (CANDU-SCWR) is a pressure tube reactor intended to operate with a coolant pressure of 25 MPa and temperatures ranging between 350°C (core inlet) and 625°C (core outlet). Along the length of a fuel channel, there is a drastic decrease in the coolant density and dielectric constant, which is expected to result in a rapid decrease in the solubility of corrosion products. Therefore, it is anticipated that corrosion product deposition onto the cladding and liner in an SCWR fuel channel will be much greater than in conventional water-cooled reactors operating below the critical point of water. While optimized materials selection and chemistry control strategies may mitigate corrosion and corrosion product deposition to some degree, it may not be possible to completely eliminate corrosion product deposition within SCWR fuel channels. Corrosion product deposition on fuel cladding will have a negative impact on the neutron economy of the CANDU-SCWR because of parasitic absorption of neutrons within the deposited material. In this paper, lattice physics calculations are used to assess the impact of corrosion product deposition on fuel exit burnup, based on corrosion product deposition rates estimated for prototypical SCWR conditions. (author)

[en] Most corrosion testing in support of Supercritical Water-cooled Reactor (SCWR) development has been performed at constant pressure, the fluid density changing as the temperature changes. While experimentally simple and analogous to the SCWR (which will operate at essentially constant in-core pressure) data interpretation is not straightforward because supercritical water properties change significantly in the vicinity of the critical point. In addition, it is difficult to separate the pressure (density) effects, which affect the number of water molecules available for reaction at the surface, from changes in corrosion rate due to temperature. The pressure effect is of practical importance because testing at lower pressures is experimentally simpler than testing at 25 MPa, facilitating materials screening at temperatures > 500 ℃. In addition, a significant body of corrosion data in superheated steam exists, including in-reactor testing, and understanding pressure effects on corrosion would facilitate the use of these data for SCWR development. This paper summarizes available data on the effects of pressure on corrosion in SCW, synergisms with other key variables, and outlines a mechanistic framework for data interpretation. (author)

[en] Operating water-cooled nuclear reactors at temperatures above the thermodynamic critical temperature is a natural evolution, conferring much higher thermodynamic efficiency. The water chemistry conditions in the core of such a supercritical water-cooled reactor will be extreme, a combination of high temperatures, high pressures, and irradiation. However, even at 300 ^oC water is already a very different fluid than at 25 ^oC, with a much lower dielectric constant and extent of hydrogen bonding. These changes affect solute-solvent interactions, which in turn affect properties such as the stability of ions versus ion pairs and the rates of chemical reactions that affect practical phenomena such as corrosion reactions, deposition of radioactive species and water radiolysis. This paper highlights the impacts of on-going R&D work on Supercritical Water-cooled Reactor water chemistry on our understanding of the water chemistry of Generation II and III reactors. Applications and implications of these insights will be drawn from areas such as water radiolysis, corrosion, and corrosion product transport. (author)