MODELING OF PLATINUM DEPOSITION ON BWR SURFACES DURING ON-LINE NOBLECHEM

摘要

The On-line NobleChem (OLNC) process is the second generation noble metal process that was developed to protect boiling water reactor (BWR) internal and recirculation system piping from intergranular stress corrosion cracking (IGSCC). OLNC applications are performed by injecting the platinum chemical solution, containing Na_2Pt(OH)_6, into the reactor feedwater during power operation, with reactor coolant temperatures near 540 °F (282 °C). The platinum chemical decomposes to sodium hydroxide as the metal deposits on piping and internals surfaces. Surface Pt catalyzes the recombination reaction of hydrogen with oxidants to lower the electrochemical corrosion potential (ECP) to levels at which IGSCC is mitigated. Platinum mass inputs for individual OLNC applications are roughly 250 - 900 g. The particle sizes of platinum deposits are significantly smaller than those of classic NobleChem (NMCA) applications, which are performed during hot shutdown. Laboratory studies indicate with NMCA applications, noble metal particle size distributions on stainless steel surfaces are in the range of 50 - 150 nm, whereas with OLNC typical platinum particle sizes range between 2 and 12 nm. The smaller particle size of the noble metal deposits provides a more effective surface coverage. Based on industry experience with OLNC, the need for methodologies to quantify the distribution of the injected platinum deposited on surfaces within the reactor system has been identified. Mass deposition amounts and particle size can be measured on available monitoring specimens and retrievable artifacts; however, it is not practical to obtain and analyze samples from all plant surfaces. As a result the EPRI chaired BWR Vessel and Internals Protection (BWRVIP) committee has undertaken a project to model the platinum deposition around the primary circuit. A multi-box model is under development which takes into account deposition and release of Pt particles from surfaces in each section of the BWR, including structural components, piping and the fuel channel. Deposition in sample lines is also modelled, as platinum deposits can alter sample line chemical concentrations from those in the reactor coolant. Plant and laboratory results are used to adjust model parameters and benchmark model output. This paper presents the basis for the model and output results, which are compared to available measured deposition results.