Oxide layer with coolants

The depth of etched and oxide layers with different coolants. (From Zhang, C., Ohmori, H., Kato, T., and Morita, N., Prec Eng J Int Soc Free Eng Nanotechnol, 25, 56, 2001a. With permission.)... 

Fortunately, it forms a relatively thick oxide layer on the surface of the metal which prevents further reaction. This gives us the use of a light, strong metal for items such as door handles and cooking foil. There are many reactive metals you would not expect to find many uses for. For example, sodium is so reactive it is stored under oil to prevent it coming into contact with water and the air. However, because it has a low melting point and is a good conductor of heat, it can be used as a coolant in nuclear reactors. 

Furthermore impurities in the helium coolant, mainly the air constituents, can cause corrosion effects on the outside reformer tube walls which eventually change its properties. Measurements of impurity contents in Dragon and AVR revealed a large scattering of the data. Experimental results obtained within the Dragon project indicate a strong corrosion of aluminum and titanium, i.e., the formation of Cr-, Mn-, Si-, and Ti-oxide layers, and an increased corrosion rate in moist helium compared with a dry atmosphere [26]. 

Besides the chemical and radiochemical composition, other properties of the collected materials are also often of interest, such as the natine of the chemical compounds present in these substances. For example, the structure of oxide compounds after isolation from the base material or from the coolant is analyzed by X-ray diffractometry or by Mdssbauer spectrometry. Other microanalytical techniques can be directly applied to oxide layers deposited on surfaces, e. g. of steam generator tube sections. Examples in this field are Auger electron spectroscopy for the determination of element concentrations in micrometer areas and X-ray induced photoelectron spectroscopy for the determination of the chemical states of the individual elements. In order to obtain depth profiles over the thickness of the oxide layer, these techniques often are combined with an argon sputtering process (e. g. Schuster et al., 1988), which removes nanometer fractions from the swface prior to the next analysis step. By y spectrometry of the specimen after each sputtering step, the profile of the radionuclides in the oxide layer can also be determined. 

The radionuclides incorporated into the oxide layers, which lead to a radiation field in the surrounding area, are mainly the activated corrosion product nuclides, above all Co and Co. Out of the fission products present in the primary coolant during plant operation with failed fuel rods in the reactor core, iodine and cesium isotopes are not deposited into the surface oxide layers this reactor experience is consistent with the general chemical properties of these elements which do not allow the formation of insoluble compounds under the prevailing conditions (with the sole exception of Agl, see Section 4.3.3.1.2.). On the other hand, fission product elements that are able to form insoluble compounds (such as oxides, hydroxides or ferrites) in the primary coolant are incorporated almost quantitatively into the contamination layers (see Section 4.3.3.1.4.). However, because of the usually low concentrations of polyvalent fission products in the primary coolant, only in very rare cases will these radionculides make a measurable contribution to the total contamination level for this reason, they will not be treated in this context. 

The postulated equilibrium between the radionuclide concentration in the coolant and the area-related radioactivity on the surfaces of the wall materials requires a clear correlation between the concentrations of relevant radionuclides (in particular of Co) in the coolant and the radionuclide inventory in the oxide layers, i. e. that higher numbers in the coolant will result, with a certain time delay, in higher activities in the oxide layers and vice versa. As can be seen from Fig. 4.36. (according to Marchl and Riess, 1993 a), a clear relationship exists between the average Co activity concentration in the primary coolant over a fuel cycle and the radiation dose rate at the main coolant pipework, even when different plants with different ages and different Stellite surface areas inside the reactor pressure vessel are compared. 

In the Cora code, the corrosion product layers outside the reactor core are rather arbitrarily subdivided into two layers, a transient one and a permanently deposited one. Supply to the transient layer occurs via deposition of suspended particles from the coolant, release from it includes erosion of particles back to the coolant as well as transport into the permanently deposited layer and partial conversion into dissolved species. In a comparable manner, the supply of nuclides to the permanent layer is assumed to result from transfer from the transient layer and the exchange equilibrium with the dissolved species present in the coolant. The deposition coefficients of suspended solids can be calculated on the basis of particle size and flow characteristics. The coefficients of relevance for the permanently deposited layer, including ionic transfer mechanisms between liquid and solid phases, can be derived from theoretical considerations as well as from laboratory studies of corrosion product solubilities. Finally, diffusion rates of nuclides at the interphase layers are needed, from the oxide layer to the coolant as well as in the reverse direction. These data can be obtained in part by theoretical considerations and by measurements at the plants. 

Similar to PWR conditions, cobalt shows a preferential release to the coolant as compared to iron, indicating a comparatively weak cobalt retention in the oxide layers formed on the surfaces of the materials. Furthermore, the results show a chromium release which initially increases more than linearily with time and a somewhat irregular Co release behavior which was possibly influenced by errors in the measurement of the very low Co activity concentrations present in the experimental solutions. Since the °Co release rate is almost constant over time, the equivalent penetration under BWR operating conditions is lower than that under PWR conditions for the time period of the experiments however, when integrated over the 30-year operational life of the plant, the Co release under BWR conditions is significantly higher than that under PWR operating conditions. 

IV-1. The corrosion of steels and alloys that are in contact with the primary coolant leads to the in situ growth of an oxide layer and the release of ions into the coolant. The driving force for this mechanism is the concentration gradient between the bulk of the coolant and pores in the oxide layer. 

In the grinding of many metals, reaction of the freshly cut metal surface with air forms an oxide layer which can effectively prevent adhesion of the metal surface to the abrasive, to the bond, or to itself. This facilitates the abrasive process. Substances which can react more rapidly with fresh metal surfaces than does air, or which can melt and cover the surface with a protective layer, are called active grinding aids. The most common ways such substances are used is in the oils or water-based coolants for wet grinding and as impregnants or fillers in the bond of the abrasive article for dry grinding. 

For optical pyrometry, the well would serve as a blackbody hole 1 cm (0,39 in.) in diameter by 5 cm (1.97 in.) deep, the bottom of which would be in close thermal contact with the coolant stream. The minimum wall thickness at the bottom of the blackbody hole would have been set by pressure containment considerations The exterior surfaces of the well would be refractory metal with a very low vapor pressure that does not form stable oxide layers, such as Mo, Re, or W, The surface could be textured or grovm via chemical vapor deposition for increased emissivity. However, a simple polished metal surface likely would suffice. Some recalibration schemes, which require one to reflect a laser pulse off the measurement surface, would prefer a lower emissivity and likely would not benefit from a blackbody hole. 

Ever) year our planet is bombarded with enough energy from the Sun to destroy all life. Only the ozone in the stratosphere protects us from that onslaught. The ozone, though, is threatened by modern life styles. Chemicals used as coolants and propellants, such as chlorofluorocarbons (CFCs), and the nitrogen oxides in jet exhausts, have been found to create holes in Earth s protective ozone layer. Because they act as catalysts, even small amounts of these chemicals can cause large changes in the vast reaches of the stratosphere. 

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