Hot assembly and whole-core thermal-hydraulic analysis of a high power density marine core with neutronic/thermal-hydraulic coupling

In this study, 3D coupled neutronic/thermal-hydraulic (TH) methods are used for hot assembly (HA) and whole-core (WC) analysis of a high power density (HPD) 333 MWth PWR marine core. For HA analysis, the power distribution is found using the WIMS and PANTHER reactor physics codes and COBRA-EN performs the steady-state TH analysis. For WC analysis, PANTHER'S inbuilt TH capability is used, with WIMS supplying temperature-dependent cross-section data. To design HPD cores of between 90 and 250 MW/m³, five cases were chosen by optimizing the fuel pin diameter (D), pin pitch (P) and pitch-to-diameter ratio (P/D). Since steady-state calculations are used in the design process, an overpower condition, typically 18%, was applied in order to accommodate all anticipated transients. The analysis shows that, even in the hot channel at 18% overpower, the minimum departure from nucleate boiling ratio (MDNBR) and fuel temperature remain well within TH margins for all the candidate designs. In the COBRA-EN model, the coolant does not begin to boil unless the core-averaged linear power rating exceeds 27 kW/m for standard geometry, which is 155% higher than the design value. MDNBR, surface heat flux, maximum and average fuel, cladding surface and core outlet temperatures, pressure drop and power peaking factors were determined for steady-state operating conditions to determine whether TH safety limits were satisfied. The results show that it is possible to achieve a core power density of 120 MW/m, a ∼90% improvement on the reference design, while meeting the target core lifetime of 15 effective full-power-years and remaining within TH limits.