[en] Knowledge of the thermodynamic stabilities of metals and their oxides is important for understanding material degradation and corrosion product transport in a supercritical water-cooled reactor (SCWR). The thermodynamic stability under various conditions can be illustrated using graphical representations such as Pourbaix and Ellingham diagrams. This paper will show how a combination of Pourbaix and Ellingham diagrams is necessary to characterize the thermodynamic stability of alloys and oxide films under proposed SCWR coolant conditions. Results predicted using both Pourbaix and Ellingham diagrams will be compared with literature data. The advantages and disadvantages of these methods under SCWR conditions will also be discussed. (author)

[en] Elucidating the relationship between coolant parameters and corrosion of materials proposed for Generation IV supercritical water-cooled reactors (SCWRs) remains a challenge. The properties of the supercritical water (SCW) coolant change significantly from core inlet (∼350 ^oC, 25 MPa) to outlet (as high as 625 ^oC, 25 MPa). A mechanistic framework is required to interpret materials degradation data and predict long-term behaviour. Different corrosion mechanisms exist in high-density sub-critical/near-critical water and in low-density SCW. Understanding surface, rather than bulk, coolant chemistry is required; for example, the structure of SCW near the surface and the local oxide solubility. Oxide solubility also determines the transport of radioactive species out of the core. This paper describes the link between the physical chemistry of SCW and the corrosion of SCWR materials, including corrosion product release. (author)

[en] This report describes the effort to develop in-situ characterisation of steam generator deposits using remote raman spectroscopy to determine the chemical composition and semi-quantitative measurement of their concentrations. Information on the composition of the deposits is necessary in order to establish the optimal cleaning conditions and procedures. Furthermore, the composition of the deposits also provides information on the conditions that exist within the steam generator and the feedtrain. The raman spectra of the three most common iron oxide phases found in the CANDU deposits (hematite, magnetite and nickel ferrite) are shown

[en] To minimize corrosion and corrosion product transport in a reactor, it is necessary to implement a chemistry regime, and monitor and control the chemistry to within specifications. This paper highlights three areas of concern for chemistry control in a supercritical water-cooled reactor (SCWR), reviews water chemistries used in fossil-fired and nuclear power plants, discusses modern techniques for high-temperature chemistry control and monitoring, and discusses how chemistry control might affect design. The paper concludes by summarizing the challenges for chemistry control and monitoring in a SCWR, and identifies the need for chemistry control and system design to work together. (author)

[en] The adsorption of ammonia, morpholine, ethanolamine, and dimethylamine onto the surfaces of colloidal magnetite and hematite was measured at 25^oC. The effect of the adsorption on the surface potential was quantified by measuring the resulting shift in the isoelectric point of the corrosion products and by the direct measurement of the surface interaction force between the corrosion products and Inconel 600. These measurements have served to support the hypothesis that adsorption of amine affects the magnetite deposition rate by lowering the force of repulsion between magnetite and the surface of Inconel 600. The deposition rate of hematite increased as the oxygen concentration increased. A mechanism to account for enhanced deposition rates at high mixture qualities (> 0.35) has been identified and shown to predict behaviour that is consistent with both experimental and plant data. As a result of this investigation, several criteria are proposed to reduce the extent of corrosion product deposition on the tube bundle. Low hematite deposition is favoured by a low concentration of dissolved oxygen, and low magnetite deposition is favoured by choosing an amine for pH control that has little tendency to adsorb onto the surface of magnetite. To minimize adsorption the amine should have a high base strength and a large 'footprint' on the surface of magnetite. To prevent enhanced deposition at high mixture qualities, it is proposed that a modified amine be used that will reduce the surface tension or the elasticity of the steam-water interface or both

[en] Chemical precipitates formed in the recovery water following a Loss of Coolant Accident (LOCA) have the potential to increase head loss across the Emergency Core Cooling System (ECCS) strainer, and could lead to cavitation of the ECCS pumps, pump failure and loss of core cooling. Atomic Energy of Canada Limited (AECL) has been involved in the investigation of chemical effects on head loss for its CANDU® and PWR (Pressurized Water Reactor) customers. The chemical constituents of the recovery sump water depend on the combination of chemistry control additives, fission products, radiolysis products (e.g., nitric acid), and the corrosion and dissolution products from metals, concrete, and insulation materials. Some of these dissolution and corrosion products (e.g., aluminum and calcium) may form significant quantities of precipitates. The presence of chemistry control additives such as lithium and sodium hydroxide, trisodium phosphate (TSP) and boric acid can significantly influence the precipitates formed. While a number of compounds may be shown to be thermodynamically possible under the conditions assumed for precipitation, kinetic factors play a large role in the type and morphology of precipitates observed. Precipitation is also influenced by insulation debris, which can trap precipitates and act as nucleation sites for heterogeneous precipitation. This paper outlines the AECL approach to resolving the issue of chemical effects on ECCS strainer head loss, which includes modeling, bench top testing and reduced-scale testing; the latter conducted using a temperature-controlled variable-flow closed-loop test rig that includes an AECL Finned Strainer® test section equipped with a differential pressure transmitter. Models of corrosion product release and the types of precipitates expected in post-LOCA sumps are discussed. Finally, this paper discusses reduced-scale test results and presents a possible method for chemical effects head loss modeling. (author)

[en] This is the third in a series of reports from an investigation co-funded by the Electric Power Research Institute (EPRI) and by Atomic Energy of Canada Limited (AECL) into the effectiveness of alternative amines for controlling the rate of tube-bundle fouling under steam generator (SG) operating conditions. The objectives of this investigation are to determine whether the fouling rate depends on the amine used for pH control, to identify those factors that influence the effectiveness, and use this information to optimize the selection of an amine for chemistry control and deposit control in the steam cycle and steam generator, respectively. Work to date has demonstrated that the rate of particle deposition under steam generator operating conditions is strongly influenced by surface chemistry (Turner et al., 1997; Turner et al., 1999). This dependence upon surface chemistry is illustrated by the difference between the deposition rates measured for hematite and magnetite, and by the dependence of the particle deposition rate on the amine used for pH control. Deposition rates of hematite were found to be more than 10 times greater than those for magnetite under similar test conditions (Turner et al., 1997). At 270^oC and pH_T 6.2, the surfaces of hematite and magnetite are predicted to be positively charged and negatively charged, respectively (Shoonen, 1994). Measurements of the point of zero charge (PZC) of magnetite at temperatures from 25^oC to 290^oC by Wesolowski et al. (1999) have confirmed that magnetite is negatively charged at the stated conditions. A PZC of 4.2 was measured for Alloy 600 at 25^oC (Balakrishnan and Turner, un-published results), and its surface is expected to remain negatively charged for alkaline chemistry over the temperature range of interest. Therefore, there will be a repulsive force between the surfaces of magnetite particles and Alloy 600 at 270^oC and pH_T 6.2 that is absent for hematite particles depositing under the same conditions. This difference is consistent with the higher deposition rates found for hematite particles on Alloy 600. The deposition rate of hematite was also found to be sensitive to the redox conditions in the test loop (Turner et al., 1999). Thus, the highest deposition rates were measured when there was a residual of dissolved oxygen in the loop water and no hydrazine, whereas the deposition rates tended to decrease towards those of magnetite in tests with residual hydrazine and little or no dissolved oxygen. This result, again, points to the importance of surface charge in governing the rate of particle deposition. The dependence of the particle deposition rate on the amine used for pH control was postulated to be associated with differences in the degree of adsorption of amine onto the surface of the magnetite particles and the associated effect this would have on surface charge. Amine molecules exist in solution as both the neutral, A, and hydrolysed, HA⁺, species. Adsorption of the latter was postulated to make the surface of magnetite less negative and, consequently, to reduce the force of repulsion between the magnetite particles and Alloy 600. Subsequent measurements at 25^oC showed that the amine molecules did adsorb onto magnetite to varying degrees, and that this adsorption was associated with a decrease in the force of repulsion between the surfaces of magnetite and Alloy 600 (Turner et al., 1999). There also appeared to be a correlation between the amount of amine adsorbed at 25^oC and the deposition rate measured at 270^oC. The reduction in surface charge deduced from the force measurements was small, however, compared to the amount of amine adsorbed. Also, both the adsorption and the base strength of the amines decrease with increasing temperature which makes the connection between adsorption measurements at 25^oC and deposition behaviour at 270^oC less certain. Doubt as to the validity of the hypothesis was also cast by the results of Wesolowski et al. (1999) and Benezeth et al. (2000) who found no effect of the adsorption of amine on the high-temperature surface charge properties of magnetite. Results from further experiments to determine the mechanism by which amines affect the rate of tube-bundle fouling and, thus, facilitate optimization of the amine used for pH control in the steam cycle and deposit control in the steam generator are reported here. The hypothesis that the amine affects the deposition rate through a surface-charge effect was tested by measuring the deposition rate of magnetite as a function of amine concentration in buffered solutions at constant pH_T. Measurements of the kinetics of adsorption and desorption of amine as a function of temperature were also conducted to help relate the surface chemistry at 25^oC with deposition behaviour measured at 270^oC. Finally, the relationship between hydrophobicity and the particle deposition rate was investigated further by measuring the effect of amine on the wetting angle at temperatures up to 220^oC (the practical limit of the apparatus). (author)

[en] Fouling of secondary-side of nuclear steam generators by corrosion products transported by the feedwater is a serious problem. The build-up of deposits on the tubes can lower the heat transfer and provide sites for localised corrosion by the impurities. The deposits are removed by chemical and/or mechanical cleaning techniques. For the optimal conditions and procedures, the composition of the deposits is required. This composition also gives information on the chemistry conditions within the steam generator and feedtrain. At the present time, samples of deposits are removed from the steam generator and characterised ex-situ. In this procedure the sample may undergo chemical and physical surface changes. Hence an in-situ technique for determining the chemical composition of the compounds present would be valuable. This paper describes the development of a remote characterisation technique based on vibrational spectroscopy. The remote Raman spectroscopy uses fiber optics to transmit laser energy to the inspection region and scattered light back to the spectrometer. Thus Raman spectroscopy is applied to make in-situ deposit analysis. Raman spectra of some of the more common chemical compound in the deposits are presented

[en] The dissolution kinetics of Si₃N₄ in alkaline solutions were studied at temperatures ranging from 333 to 573 K. The results show that there is little dissolution of Si₃N₄ below 373 K. Above 373 K, there is release of the major constituents of silicon nitride into solution. The release rate of silicon follows zero order reaction kinetics d[Si]/dt = k and that of nitrogen (as [NH₄⁺]) follows the power law [NH₄⁺] = k t^a, where a has a value between 1.1 and 1.2. These release rates increase with increasing temperature. At 573 K, release of aluminum, a minor component of the Si₃N₄ studied, is also observed and follows first order reaction kinetics. (author)

[en] The identification of suitable materials, and an appropriate water chemistry regime to minimize their degradation, are key challenges in the development of a Supercritical Water-cooled Reactor. In recent years, a significant body of research has been carried out to characterize the behavior of materials under conditions expected in an SCWR. With the increased amount of data available, it is now possible to evaluate the experimental uncertainties and to assess the role of key variables. It is also becoming possible to more accurately identify the mechanisms through which the SCWR environment (e.g., water chemistry) will interact with materials leading to degradation. In this paper, the interface between materials and chemistry is examined in the context of the role of dissolved oxygen in the corrosion of candidate alloys. Some suggestions for future work are given. (author)