Cobalt circuit surfaces

Permanent anodes are used in the electrowinning of base metals such as copper, nickel, cobalt, and zinc. The only function of the anode is to transfer electrons from the electrolyte to the external circuit. The usual reactions on the anode surface are oxygen evolution in sulfate electrolytes and chlorine evolution in chloride electrolytes. The permanent anodes have the disadvantages of maintenance costs to clean the... 

Dietzel (29) and Staley (30) have proposed that the chemical reactions at an enamel/metal interface can be considered in terms of electrolytic cells set up between the metals of different electrochemical potential. It has been suggested that cobalt or nickel precipitated in the enamel when in contact with steel surface, forms short-circuited local cells in which iron is the anode. The current flows from iron through the melt to cobalt and back to iron. The result is that iron goes into solution, the surface becomes roughened, and the enamel material anchors itself into the cavities as shown in Figure 6b. 

Michl used double-decker molecules such as the lanthanum and cobalt complesKs 36 and 37 (Figure 28.12), respectively, in which one half is designed to adhere to a fluid mercury surface while the other half points freely into the air, unaffected by the surface [89], yet is still able to rotate around an axis perpendicular to the mercury surface. When the double-decker 36 was adsorbed firmly on mercury under open-circuit conditions, an IR spectroscopic analysis verified that the porphyrin... 

Modification of the electrode started with academic studies on physical and chemical adsorption, i.e., with the appearance of fundamental researches on adsorption of different species on electrode surfaces, both under polarization and at open circuit potential [3]. The properties of similar chemically modified electrodes , in which the modifier consists of a monolayer of a variety of chemical species with different characteristics, possessing (or not) particular properties, were initially studied in a purely electrochemical context, aimed at the collection of fundamental physico-chemical data. A small group of electrochemists were among those involved in these basic studies, envisioning the perspectives opened by the novel systems. In the first, really fascinating, work with similar monomolecular layers, cobalt porphyrin and phthalocyanine, as well as deliberately synthesized dicobalt face-to-face porphyrins were adsorbed on Pt or C surfaces to catalyze molecular oxygen reduction [4]. However, similar systems were not always used or adequately tested in proper amperometric sensing by researchers more interested in electroanalysis dicobalt face-to-face porphirins still constitute a rare example of tailored materials for selective amperometric detection. 

As a very important topic in contamination buildup, the question is still open to what extent the data on corrosion product solubilities in the primary coolant are of importance for the behavior of trace amounts of cobalt. It seems to be still questionable whether cobalt ferrites as a well-defined compound with properties similar to the nickel ferrites can exist under PWR primary coolant conditions, whether cobalt atoms can be incorporated into a nickel ferrite lattice or whether traces of cobalt may be deposited onto the surfaces of the reactor core by adsorption on other, already deposited oxides. Such adsorption processes may occur even on the Zircaloy oxide films in the absence of any net deposition of corrosion products. Experimental investigations of the interaction of dissolved cobalt with heated Zircaloy surfaces (Lister et al., 1983) indicated that at low crud levels in the coolant cobalt deposition on surfaces is dominated by processes involving dissolved species, with adsorption/desorption processes being the responsible mechanisms. The extent of cobalt deposition is controlled by the type of oxide present on the Zircaloy surface thin black films of zirconium oxide will pick up less cobalt from the solution than thick white oxide films, even when the differences in the available surface areas of both types of oxides are taken into account. The deposition process seems to be little affected by the heat flux in the exposed metal. According to Thornton (1992), such adsorption-desorption exchange processes provide a pathway for radioactive species to be transported around the circuit even when the net movement of corrosion products is minimized this means that under such circumstances the processes of activity transport and of corrosion product transport may be decoupled. They may provide a pathway for target nuclides such as Co to be adsorbed onto fuel rod surfaces even in a core which is virtually free of deposited corrosion product particles. 

From the different topics discussed in the preceding sections it can be concluded that there are different possible ways of keeping the buildup of primary circuit contamination low or of reducing already existing contamination levels. Their common aim is to keep the production of radioactive cobalt isotopes low and/or to minimize the transport of the radionuclides produced from the reactor pressure vessel to other regions of the primary circuit and, in addition, to minimize their plate-out on the surfaces there. Because of the importance of low radiation levels for an undisturbed operation and maintenance of the plants, different measures have been attempted with quite varying success. 

Similarly, the consequences of the replacement of the high-cobalt bearing materials on the °Co activity levels in the oxide surface layers of the primary circuit... 

Some authors have claimed that KOH-NH3 primary coolant chemistry as applied in the WER reactors leads to lower radiation dose rates on the primary system. In the course of their work at the DIDO Water Loop, Large and Wood-wark (1989) investigated this type of primary coolant chemistry under conditions which were comparable to those applied in the experiments with LiOH chemistry. Their results showed comparatively low corrosion product concentrations in the coolant, suspended solids as well as dissolved species the radioactivity buildup on the loop surfaces, however, was on the same order as that experienced with coordinated Li/B chemistry. From these findings the authors concluded that the comparatively low radiation dose rates which are reported from VVER plants are not due to the type of coolant chemistry employed, but to the absence of Stellite and Inconel in the primary circuit, i. e. to low cobalt and nickel inventories of the materials in contact with the primary coolant. 

Taking into account that development of a comprehensive model is not feasible given the current state of knowledge, a simple kinetic model Actranf has been elaborated by Harper et al. (1992). In this model, the primary circuit is divided into three main sections the coolant, the in-core surfaces and the out-of-core surfaces. Cobalt is introduced into the system by release from circuit materials, either as Co from any point in the circuit or as Co from activated materials in the neutron field. Both isotopes are removed from the coolant to the purification system by a first-order process, and are also removed from the circuit when they have been deposited on fuel assembUes that are withdrawn from the reactor in the course of refuelling. Co is converted to Co by neutron activation when it is deposited on fuel surfaces or when it is contained in the internals of the reactor pressure vessel. Thus, unlike the models described above, Actranf considers both possible Co sources the calculations showed that activation of Co which is temporarily deposited in the reactor core is only a small contributor when compared with the direct release of Co from in-core sources, in particular in plants equipped with Stellites inside the reactor pressure vessel. Distribution of cobalt isotopes around the surfaces of the primary circuit proceeds by exchange of dissolved species with the coolant deposition and re-release are assumed to be first-order processes. The first applications of the model to operating plants with and without Stellite materials in the primary circuit have yielded encouraging results. 

Observations from operating BWR plants suggest that on the surfaces wetted by high-temperature reactor water, one has to expect a deposition mechanism which is similar to that on the surfaces of a PWR primary circuit. The activation products released from the activated in-core materials, as well as from the fuel rod deposits, as dissolved ions are incorporated into the oxide layers on the austenitic out-of-core surfaces directly from the reactor water. The activated crud which is resuspended from the fuel rod surfaces is also partly deposited on the out-of-core surfaces here, colloid chemistry processes may participate in the deposition process. These corrosion products often show a higher cobalt content than the non-acti-vated corrosion products that are brought in with the feedwater. During the residence time of the particulate corrosion products on the out-of-core surfaces, this excess cobalt content is reduced therefore, the activated crud can be considered as an additional source of ionic cobalt (Alder et al., 1992). 

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