I am currently working on thermal-hydraulics of a small PWR. The configurations are: Thermal power (MWth) 333 Core height is 1.8 m Pressure (MPa) 15.5 Total rod diameter (mm) 9.50 pin pitch (mm) 12.65 Pitch/diameter ratio 1.33 Flow Area (m2) 8.92E-05 Intlet mass flow/area (kg/m2/s) 3384.46 Radial peaking factor 1.64 Axial peaking factor 1.57 Composite peaking factor 2.57 Total fuel mass (tonne) 18.05 Power density (MW/m3) 61.6 Average linear rating (kW/m) 9.90 Peak linear rating (kW/m) 27.00 * I have performed hot channel analysis on COBRA-EN code and got Pressure Drop of 30 KPa. * But for high power density, I have changed fuel rod diameter to 11 mm with (constant pin pitch) and I have got Pressure Drop of 70 KPa a. What is the highest and acceptable limit of "Pressure Drop" ? b. In "constant mass flow", MDNBR is improving with the increase in fuel rod diameter. Why is it increasing? c. In "constant mass flux", MDNBR is decreasing with the increase in fuel rod diameter. Why is it decreasing? d. Why fuel centerline temperature is decreasing with increase in fuel rod diameter? e. It seems that tighter fuel lattices (constant pin pitch) have improved effect in DNBR (same mass flow) and fuel and cladding surface temperature. The only problem is that "Pressure Drop" is increasing. Except the increase in "Pressure Drop" , are there any thermal-hydraulic problems that we can expect? Thank you very much!!!