Applying thermal-hydraulic design limits for the highly moderated MOX-fueled APWRs

A feasibility study on using 100% mixed-oxide (MOX) fuel in an advanced pressurized water reactor with increased moderating ratio RMA (Reacteurs a Moderation Accrue) was initiated by the Commissariat a` l`Energie Atomique. In the proposed core all the design parameters will be as close as possible to the French 1450-MW (electric) pressurized water reactor (PWR) except the fuel pin diameter, which will be reduced to achieve higher moderating ratios. The purpose of this study was to identify the conditions for which the same minimum departure from nucleate boiling ratio (MDNBR) can be obtained in a 100% MOX core as in the reference PWR during normal and upset operating conditions, while trying to achieve the highest possible moderating ratio. The thermal-hydraulic feasibility of using fuel assemblies with smaller diameter fuel pins was investigated. An assembly capable of burning 100% MOX fuel with reduced fuel pin diameter and increased number of mixing grids has been designed.