MPa Steam Research Articles

98/03134 Distribution of PAH generated in domestic solid fuel boilers

The efficiency of conventional fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide, since the energy crisis in the 1970s. The need to reduce CO2 emission has recently provided an additional incentive to increase efficiency. The main enabling technology in achieving the above goals has been the development of stronger high-temperature materials. Extensive R&D programs have resulted in numerous high strength alloys for heavy section piping, and tubing needed to build boilers. The study reported here is aimed at identifying, evaluating and qualifying the materials needed for the construction of the critical components of coal-fired boilers capable of operating with 760 °C (1400 °F)/35 MPa (5000 psi) steam. The economic viability of such a plant has been explored. Candidate alloys applicable to various ranges of temperature have been identified. Stress rupture tests have been completed on the base metal and on welds to a number of alloys. Steamside oxidation tests in an autoclave at 650 (1200 °F) and 800 °C (1475 °F) have been completed. Fireside corrosion tests have been conducted under conditions simulating those of waterwalls and superheater/reheater tubes. Weldability and fabricability of the alloys have been investigated. The capability of various overlay coatings and diffusion coatings have been examined. This paper provides a status report on the progress achieved to date on this project.

Effect of fluoride contamination on the growth of ZrO 2 films

The mechanism by which fluoride ion degrades the oxide film on Zircaloy-2 has been investigated by deliberately contaminating specimens. Delaying the washing of specimens for 0, 60 and 1800 s after pickling gave sets of, respectively, well-pickled, poorly-pickled and pickle-stained specimens. These were oxidised initially in dry steam (300°C, 3.5 MPa) and were then transferred to water (300°C) for short periods (1, 2 or 7 days). The oxides produced were examined by weight gain, interferometry, impedance spectroscopy and optical, SEM and TEM microscopy. The initial oxidation rates in steam were little different for the three groups of specimens (1 or 2 days), although the interference coloured oxides showed a very different distribution of oxide thicknesses between the well-picked specimens and the other groups. Transfer to water rapidly resulted in thick, friable, porous oxides on the pickle-stained, but not the other specimens, that could not be examined by many techniques because of ready loss of oxide. The techniques that could be applied to these specimens showed that they consisted of apparently large oxide crystallites in multiple layers nearly normal to the oxide metal interface. The original surface topography was still visible in areas where this surface had not spalled, showing that the degradation occurred within the oxide. The severity of this attack was determined by the extent to which the original preparation technique had left oxyfluoride layers on the initial surfaces. It was deduced that these oxyfluoride layers developed porosity in which concentrated fluoride solutions could form during high temperature exposures in water. These solutions attacked the ZrO 2 film by hydrothermal dissolution and recrystallisation to give the large layered platelets in the degraded films. The oxyfluorides appear to be sufficiently hygroscopic that the same degradation process occurred generally in 300°C, 3.5 MPa steam, only locally in 0.1 MPa steam and not in moist air.

The kinetics of uniform oxide formation and its relationship to radiation effects on Zircaloy-2

Specimens of Zircaloy-2 plate were irradiated with 2 MeV proton at 350 °C up to total dose of 1 dpa (displacement per atom). Uniform corrosion tests, at 400 °C in 10.3 MPa steam, were performed in an autoclave. It was found that the radiation enhanced the uniform corrosion rate. The oxide morphology was studied by cross-sectional transmission electron microscopy. The oxide consists of two types of oxide layer, one formed in the early stage which is equiaxial, the other formed later which is columnar. Comparing the as-received and irradiated specimens, it was found that the difference in oxide thickness depends on the development of columnar oxide layer. The effect of radiation on the oxide morphology is considered to be the same in the oxide nucleation stage, but produces a significant difference in the oxide growth stage. It is believed that the stress induced the oxide morphology evolution, and had some effects on the corrosion behavior. A corrosion model is proposed to explain these results.

96/06770 Radioisotope X-ray fluorescence analysis of some elements in fly ash of Afsin-Elbistan power plants

The efficiency of conventional fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide, since the energy crisis in the 1970s. The need to reduce CO2 emission has recently provided an additional incentive to increase efficiency. The main enabling technology in achieving the above goals has been the development of stronger high-temperature materials. Extensive R&D programs have resulted in numerous high strength alloys for heavy section piping, and tubing needed to build boilers. The study reported here is aimed at identifying, evaluating and qualifying the materials needed for the construction of the critical components of coal-fired boilers capable of operating with 760 °C (1400 °F)/35 MPa (5000 psi) steam. The economic viability of such a plant has been explored. Candidate alloys applicable to various ranges of temperature have been identified. Stress rupture tests have been completed on the base metal and on welds to a number of alloys. Steamside oxidation tests in an autoclave at 650 (1200 °F) and 800 °C (1475 °F) have been completed. Fireside corrosion tests have been conducted under conditions simulating those of waterwalls and superheater/reheater tubes. Weldability and fabricability of the alloys have been investigated. The capability of various overlay coatings and diffusion coatings have been examined. This paper provides a status report on the progress achieved to date on this project.

95/03522 Optimization of regimes of intracycle fuel gasification as applied to steam-gas energy units of thermal electric power plants

The effects of pre-oxidation of char from Taiheiyo coal, a non-caking bituminous coal, in the 400–550 °C temperature range on its gasification reactivity with N2-H2O at 0.1 MPa (steam partial pressure of 13.2 kPa) have been investigated. The pre-oxidation of char markedly enhances gasification rates at temperatures between 800 and 900 °C. Reactivity is found to parallel the burn-off level during preoxidation at low temperatures (400–430 °C), whereas at relatively high temperatures (480–550 °C), the burn-off level only affects the reactivity slightly. The amount of CO and CO2 evolved from the preoxidized char by heat treatment is proportional to the burn-off level at low temperatures (400–430 °C), being closely related to the enhancement of the gasification reactivity in steam.

The development of porosity in thick zirconia films

The pore structures of posttransition oxide films on specimens of Zircaloy-4 fuel cladding oxidised in 340–360°C (613–633 K) water or 400°C (673 K), 10 MPa steam, have been examined by impedance spectroscopy, scanning electron microscopy, and replica transmission electron microscopy of the oxide surfaces, the oxide-metal interfaces (after dissolution of the metal) and fractured oxide cross sections. The results were compared with measurements of the total oxide thickness derived from weight gains, FTIR interferometry, impedance measurements with mercury contacts, and metallographic sections. The various measurements of total thickness were in good general agreement. Impedance measurements during soaking with aqueous ammonium nitrate solution showed two characteristically different types of behaviour, while the impedance spectra of the soaked oxides also showed a dichotomy of behaviour, but for different groups of specimens. The impedance spectra showed surprising similarities for specimens from similar cladding batches but with very different oxide thickness and morphologies, suggesting that micrometallurgical differences between batches may influence oxide structure irrespective of formation conditions and final thickness. Electron microscopy showed relatively few cracks and pores in the outer oxide films formed on belt-ground surfaces, except for the thick white oxide patches formed in 400°C steam. Much higher pore densities were seen at the oxide-metal interface than at the surface, in agreement with the higher volume fraction of t-ZrO 2 expected at the oxide-metal interface. Relative pore densities at the oxide-metal interface correlated well with the relative oxidation rates during the final oxidation period.

Effects of proton irradiation on the microstructural evolution and uniform corrosion resistance of Zircaloys

In previous studies of irradiation effects on the nodular corrosion resistance of Zircaloys, the authors proposed a mechanism which involved solute redistribution during irradiation and which enhanced the nodular corrosion resistance of these alloys. Further study of the uniform corrosion resistance of proton-irradiated Zircaloys was performed using a similar technique. Specimens of Zircaloys (with various initial conditions) were irradiated with 1 MeV protons at 350°C up to total dose of 1 dpa at the 2 μm depth. Uniform corrosion tests were performed in an autoclave filled with 400°C and 10.3 MPa steam for up to 130 days. It was found that irradiation increased the uniform corrosion rate. For example, in the as-received (α + β quenched) specimen, the weight gain of the irradiated specimen was three times higher than that of the unirradiated one. The microstructures before and after irradiation were also studied using analytical electron microscopy. Precipitate microchemistry was studied using an attached energy dispersive spectrometer. It was also demonstrated that irradiation-induced precipitate dissolution and solute redistribution in the matrix of Zircaloys occurred. In summary, it is believed that irradiation induced the dissolution of intermetallic precipitates and redistributed solutes more evenly which, in turn, affected the nodular corrosion resistance and the uniform corrosion resistance of Zircaloys in opposite ways. The increased solute concentrations in the Zircaloy matrix enhanced the nodular corrosion resistance of Zircaloys but made the uniform corrosion resistance worse. This result is important in designing high burnup fuel cladding for future advanced light water reactors.

Thermal and hydrothermal properties and microstructure of zirconia-alumina-mullite

Using China clay, zirconia and alumina, zirconia-mullite and zirconia-alumina-mullite have been prepared. Bending strength was found to improve after treatment by annealing at 1140°C and quenching to room temperature, and on autoclaving for 20 h at 0.714 MPa steam pressure. Strength reduced after 16 thermal cycles from 1200°C to room temperature but remained still substantial. Acicular mullite crystals formed in compositions containing larger amount of clay, in addition to irregularly shaped mullite or Al 2O 3 grains.

Hafnium corrosion behavior in high-temperature steam

Corrosion properties for three kinds of hafnium have been examined in 10.5 MPa steam at 773 K. Nuclear grade hafnium formed a shiny black film with a few white nodules, which were found to be monoclinic HfO 2. Hydrogen pick-up fraction during the corrosion amounted to ca. 40% to produce hydrogen dissolved in the hafnium matrix with evolving the remaining 60% hydrogen as H 2 gas. Sponge and crystal bar hafnium were also examined and they showed superior and inferior corrosion resistance, respectively, to nuclear grade hafnium. The corrosion resistance for hafnium increased with increasing the iron content involved in hafnium for these three kinds of hafnium and, correspondingly related to the density of iron-containing second phase particles, which were characterized to be face centered cubic Hf 2Fe. The corrosion mechanism, which was previously proposed for Zircaloy nodular corrosion, was adopted with making minor alterations, to explain the hafnium corrosion properties.

Mechanism of zircaloy nodular corrosion

Zircaloys undergo nodular corrosion in 10.5 MPa steam at 773 K. The corrosion was found to proceed via formations of zirconium dioxide, zirconium hydride and H 2 gas. Clarification of the role played by the H 2 gas has provided a corrosion mechanism wherein the ZrO 2 film is broken to initiate nodular corrosion, when the pressure of the H 2 gas accumulating in the ZrO 2-Zircaloy interface exceeds the pressure the ZrO 2 film can withstand. The mechanism can explain lenticular shape of nodules. Examination of influence of heat treatment and texture on the corrosion behavior supported the presumed mechanism. In connection with the mechanistic studies, second phase particles of Zircaloy have been identified.

Materials Development for HTGR Heat Exchangers

High-temperature, gas-cooled reactors (HTGR’s) are uranium/thorium-fueled, graphite-moderated, helium-cooled systems capable of producing high-temperature primary coolant. Several variants of this system are under active development in the United States and worldwide. In one version, the primary coolant heat is transferred to steam generators producing 538°C/16.5 MPa steam for use in electricity generation or process heat applications. The materials and design technology for steam generators in this system are well developed, relying heavily upon prior experience with fossil-fired steam generators and the steam generators of the commercial HTGR’s. The major work that remains to be done is to complete qualification of the materials and to respond to evolving rules pertinent to elevated-temperature nuclear design and construction. Other versions of the HTGR generate much higher primary coolant gas temperatures (850° to 950°C) and exchange this heat, through intermediate heat exchangers (IHX’s), to a secondary loop for higher temperature process heat applications. Although IHX’s for these systems are typically pressure-balanced (low-stress) units, their design involves several challenges, including the potential interactions between structural materials and impurities present in the HTGR primary coolant. Considerable work is required to qualify materials for IHX applications, including detailed mechanical property characterization, determination of environmental influences on performance, provision of welding materials and procedures for producing joints of adequate strength and integrity, and provisions for wear protection. Some of the work currently under way addressing these issues is described.

Effect of pre-oxidation on reactivity of chars in steam

The effects of pre-oxidation of char from Taiheiyo coal, a non-caking bituminous coal, in the 400–550 °C temperature range on its gasification reactivity with N 2-H 2O at 0.1 MPa (steam partial pressure of 13.2 kPa) have been investigated. The pre-oxidation of char markedly enhances gasification rates at temperatures between 800 and 900 °C. Reactivity is found to parallel the burn-off level during preoxidation at low temperatures (400–430 °C), whereas at relatively high temperatures (480–550 °C), the burn-off level only affects the reactivity slightly. The amount of CO and CO 2 evolved from the preoxidized char by heat treatment is proportional to the burn-off level at low temperatures (400–430 °C), being closely related to the enhancement of the gasification reactivity in steam.