This two-part paper presents an updated version of the long term MIT-NTHU modeling effort on the hydraulics of hexagonal arrays of wire wrapped fuel pins. It has been stimulated by the emergence of technical licensing considerations from the current development efforts for MYRRHA, the accelerator-driven system cooled by lead–bismuth eutectic being conducted at SCK CEN in Belgium. This added licensing perspective has been introduced into this hexagonal array modeling effort through the recent collaboration of this paper’s lead author with the MIT-NTHU team resulting in the creation of the new model called the Pacio-Chen-Todreas Detailed model (PCTD). Models used for licensing calculations must be able to predict pin bundle hydraulic and thermal behavior within a prescribed upper bound of uncertainty. This must be done for all postulated scenarios with sufficient accuracy to maintain required safety margins. This capability requires a predictive model of sufficient detail to represent the bundle friction and flow distribution behavior. The Upgraded Cheng and Todreas (UCTD) model from 2018 is unique in this ability and was selected as the basis for this effort. The major improvement needed was correction of the consistent over-prediction of flow velocity in the edge subchannels. This was addressed by recognizing and adding an additional physical mechanism of momentum exchange due to the flow mixing effect resulting from the wire rod spacers. While this added momentum exchange has also slightly improved the UCTD bulk friction factor prediction, most importantly it has provided for corrected prediction of subchannel axial velocity. Since the velocities in the corner, edge and interior subchannels differ, this velocity distribution has been characterized as the bundle flow split. Its accurate prediction is essential for prediction of maximum rod cladding temperature the key bundle safety parameter. Part I, here following, summarizes the existing UCTD and then presents the new PCTD model including the governing equations, the constitutive models, the empirical parameters, an example calculation and an associated statistical evaluation. Part II, published separately in this issue, presents the experimental data base from which the empirical parameters in the new model are calibrated. Based on the analysis of these data, it is suggested that future experimental efforts focus on the laminar flow regime (relevant in accidental conditions) and flow-split data. Moreover, numerical simulations are expected to play an increasing role in the safety assessment, and the database collected in this work can be used for a benchmarking exercise to evaluate further model developments.