Boiling flows are an extremely efficient mechanism for the transfer of ultrahigh heat fluxes and used in numerous industrial applications. In this paper, the accuracy of computational fluid dynamics in predicting the temperature distributions and heat transfer performance is examined within a nuclear fusion reactor divertor. The aim is to establish the role of computational fluid dynamics (CFD) within the design of complicated high heat flux components using a semi-mechanistic approach to flow boiling that is independent of geometry and flow conditions. An Eulerian–Eulerian two-fluid method is developed and a conjugate heat transfer model is validated against the existing experimental data where available. Overall, a satisfactory accuracy is achieved in the prediction of several important quantities. Temperature distribution throughout the divertor is found to be highly accurate and aligns with the physical testing across two expected operating regimes. Additionally, the system heat transfer coefficients and coolant temperatures are close to the assumptions already established within the literature. Heat transfer enhancement is a critical component of the divertor design, and a twisted-tape insert appears to be necessary for the system to withstand ultrahigh heat fluxes encountered within the fusion reactor. The results show that the inclusion of a twisted tape improved the heat transfer coefficient of the system by almost 45% allowing the divertor to withstand the required heat fluxes of 10 MW/m2 and 20 MW/m2.