Abstract: Chalk River unidentified deposits (CRUD) on pressurized water reactor (PWR) fuel cladding can increase local thermal resistance, promote boric acid enrichment, and aggravate the risks of CRUD-induced power shift and localized cladding degradation. To clarify the coupled heat and mass transfer mechanisms in chimney-structured CRUD, a two-dimensional axisymmetric multiphysics model was developed for a representative unit consisting of one steam chimney and the surrounding porous CRUD. The porous CRUD region was described by steady heat conduction, incompressible flow, Darcy seepage, and advection-diffusion transport of soluble species. Subcooled nucleate boiling (SNB) inside the porous matrix was not explicitly solved as bulk two-phase flow. Instead, evaporative heat removal was represented by a phase-change boundary condition at the chimney-CRUD interface. The local saturation temperature was updated according to boric acid concentration, allowing the temperature, seepage, and solute fields to interact through iterative coupling in COMSOL Multiphysics. The model was compared with Westinghouse Advanced Loop Tester (WALT) data for Rod86 and Rod80. The relative errors in peak cladding absolute temperature are 0.85%-1.81% and 0.75%-1.82%, respectively, while the corresponding absolute temperature deviations are 5.35-11.92 K and 4.68-11.68 K. Under the baseline condition, the high-temperature region is located near the fuel-cladding side and away from the chimney channel, whereas the high-velocity seepage and boric acid enrichment regions are concentrated near the chimney interface on the cladding side. The predicted peak CRUD temperature, maximum chimney-interface superheat, maximum boric acid concentration, and fraction of the active SNB region are 636.25 K, 4.01 K, 71.85 mol/m³, and 57.42%, respectively. These distributions indicate that local overheating drives interfacial evaporation, which further induces liquid replenishment and solute accumulation. Sensitivity analyses were conducted for porosity, chimney radius, chimney density, CRUD thickness, and cladding heat flux. Increasing porosity markedly reduces seepage velocity and boric acid enrichment but has little influence on the overall temperature level. Increasing chimney radius or chimney density generally enhances local heat removal and suppresses interfacial superheat, seepage, boric acid enrichment, and the fraction of the active SNB region. Increasing CRUD thickness has limited effects on peak temperature and maximum superheat, but significantly promotes solute accumulation and increases the absolute length of the active SNB region, although the fraction of the active SNB region varies non-monotonically. Cladding heat flux is identified as the dominant external driver. As it increases from 1.0 to 2.0 MW/m², the peak cladding temperature increases from 630.16 to 642.86 K, the maximum superheat from 2.68 to 5.28 K, the maximum seepage velocity from 0.54 to 2.26 mm/s, the maximum boric acid concentration from 37.04 to 249.17 mol/m³, and the fraction of the active SNB region from 55.18% to 60.59%. The results show that CRUD-related risk cannot be assessed using peak cladding temperature alone because the hottest region and the strongest boric acid enrichment region are not spatially coincident. A combined evaluation of local superheat, seepage behavior, boric acid concentration, and fraction of the active SNB region is therefore more appropriate. The model provides a quantitative basis for identifying high-risk regions, assessing CRUD morphology effects, and supporting PWR fuel design and operational risk evaluation.