The OSCAR code: modelling and simulation of the corrosion product behaviour under nucleate boiling conditions in PWRs

摘要

The PWR primary circuit materials are subject to general corrosion leading to metallic elements (mainly Fe, Ni, Cr, Mn, Co) transfer and subsequent ion precipitation and particle deposition processes on the primary circuit surfaces. When deposited on fuel rods, these species are activated by neutron flux. Thus, crud erosion and dissolution processes result in primary coolant activities. During a normal operating cycle in an EDF PWR, the volume activities in the coolant are relatively stable (usually about 10-30 Bq.g-1 in 58Co). During some cycles (depending on fuel management), a significant increase in 58Co and 51Cr volume activities can be observed (10 to 100 times the ordinary volume activities). This increase in volume activities is due to local sub-cooled nucleate boiling on the hottest parts of some fuel assemblies. Indeed, it is well known that nucleate boiling enhances the deposition and precipitation processes. Crud growth in boiling conditions is related to different phenomena: 1. Enrichment: concentration increases at crud-coolant interface, 2. Boiling deposition by vaporisation: fluid vaporization at wall results in particle deposition and ion precipitation. As the crud growth, boiling occurs in the crud itself, so do the ion precipitation and deposition of the small particles. 3. Boiling deposition by trapping: some of the small particles trapped at the interface bubble/fluid make deposit when a bubble leaves the wall. 4. Enhanced erosion: turbulences generated by bubbles collapsing close to the wall and spalling above a certain deposit thickness enhance erosion; it results in the release of particle agglomerates. These phenomena have been modelled in the OSCAR V1.3 code. In this article, we present the modelling of these mass transfer mechanisms in boiling conditions and we show that the crud amount on fuel rods and the volume activities in the primary coolant in case of boiling calculated by OSCAR are in accordance with the experimental feedback on French PWRs.