Controlled oxide dissolution processes in Pb-Bi and Bi coolants and mass exchange apparatus for monitoring the oxygen regime

Full text of publication follows: The peculiarity of Pb and Pb-Bi melts is their rather high corrosion activity relative to structural materials. In this connection, one of the important problems which arise in using these coolants is the provision of corrosion stability of materials being in contact with them. At the SSC RF IPPE, a method was developed to monitor the oxidation potential of the coolant, which is based on use of the process of dissolution of solid phase lead oxides inserted in the coolant flow (the solid phase method). By this method is meant use as oxidation agent of solid phase lead oxides inserted and retained in a volume restricted section of the circulation loop (reaction vessel of the mass exchange apparatus (MEA)), which communicates via melt with the primary loop. Contacting the circulation lead, the solid phase oxidation agent dissolves, releasing oxygen which is conveyed further with the coolant flow round the whole loop. The report considers the physical-chemical fundamentals, the kinetic parameters and the characteristics of the process of dissolution of solid phase lead oxides in coolants Pb-Bi and Pb. The test procedure is described, and generalization of laboratory and test studies of the dissolution kinetics and mechanism is given. The derived criterion relationship is presented: Sh = 8,7 .10-4.Re1,42.Sc0,83, where Sh, Re and Sc are Sherwood, Reynolds and Schmidt numbers, respectively. The above relationship has a relative calculation error of 25% at the following lead parameters: Re = 1000 - 5000, Sc = 30 - 200, temperature - up to 550 deg. C, velocity - up to 1 m/s and the oxygen activity at mass exchanger inlet, corresponding to the range a[O] n(10-5-10-2). The results are presented of the computational investigation using the 2D and 3D versions of the MASKA-LM computer code being under development on the intensity of oxygen delivery into stagnant zones of the circulation loop. The characteristics of the coolant flow and mass exchangers between the stagnant zones of different configuration and the main flow in the loop are studied. The critical characteristics of the stagnant zones have been obtained, for which the occurrence of corrosion of structural steels is possible due to insufficient income of active oxygen from the circulation loop. It is justified that the process of oxides dissolution can be efficiently employed to control the important process parameter - the thermodynamic activity of oxygen in coolant. In recent years, a number of various mass exchange apparatus have been developed and successfully tested for different test rigs and static facilities with lead-containing coolants. The experience of successful regulation of the preset level of the thermodynamic activity of oxygen dissolved in coolant using the solid phase method was first gained on long time basis (>10000 hours on each test rig). Miniature mass exchange apparatus have been developed and approved for the first time for small scale static facilities. Also, the SACURA automatic system has been developed for the first time to regulate the thermodynamic activity of oxygen dissolved in coolant, which is under operation as part of the TT-2M test rig with lead-bismuth coolant. The mass exchange apparatus developed and tested in the years 2000-2004 enables us to recommend them (to provide corrosion tests) as part of all research test rigs and facilities with lead-containing coolants. (authors)