Routing neptunium to a single product in spent nuclear fuel reprocessing is a significant challenge. In this work, we have further improved the simulation of neptunium extraction in an advanced PUREX flowsheet by applying a revised model of the Np(V)-Np(VI) redox reaction kinetics, a new nitric acid radiolysis model and by evaluating various models for the nitrous acid distribution coefficient. The Np disproportionation reaction is shown to have a negligible effect. The models are validated against published ‘cold test’ experimental results; the ‘hot test’ simulation suggests high neptunium radiolysis could help to achieve high recoveries using this flowsheet.