Transient numerical studies on Departure from Nucleate Boiling (DNB) crisis in petal-shape fuel rod bundle assembly

The DNB crisis may cause fuel clad damage or melting which is a reactor safety concern for radiation release. For subcooled boiling, the decisive factor of the DNB boiling crisis is the heat flux. Therefore, it is particularly important to predict critical heat flux. Apply the models of Eulerian two-phase flow and the RPI wall boiling, the DNB in the petal-shaped fuel assembly is simulated. The Critical Heat Flux (CHF), the position of boiling crisis, and temperature field characteristics and void fraction distribution near position of boiling crisis are studied under the transient conditions, such as the sudden drop of the flow rate and the sudden rise of the thermal power. The results show that an increase in the power change ratio induces a proportional rise in CHF, when fuel rod power sudden increases. Wall temperature is higher, when the inlet velocity drops suddenly, and wall temperature increases with inlet velocity drop more quickly, when heating time is the same. The position of the DNB appears at FUEL3, with the two sets of cases; in the upstream and downstream of DNB, wall heat flux undergoes redistribution, peaking at the DNB site before subsequent reduction.