The investigation of three-dimensional transient propagations of dry-out fronts within a nuclear fuel rod bundle is performed, based on experimental and numerical simulations. The spreading of critical heat flux (CHF) fronts across a bundle, caused by gradual decrease of coolant mass flow rate without power increase is predicted, and the locus of dry patches is shown. Obtained results show that a simultaneous occurrence of CHF and rewet multi-fronts in simulated reactor coolant flow transient are less pronounced than in power transients. Due to a possible building of a vapour zone, the CHF front spatial propagation has to be carefully analysed in transient conditions.

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