The paper presents a model for I-V characteristics of RRAMs based on Ni/HfO2/Si-n+ structures that can be easily implemented in SPICE. It describes the successive formation and rupture of one or several conductive filaments which are responsible for their behavior. For each filament an equivalent circuit is built upon a flux-controlled memristor model whose relationship between the charge and the flux has been particularized to fit the experimental results. The model describes almost perfectly the I-V characteristics during the set and reset operations in quasi-static regime, using a set and a reset fluxes as fitting parameters. However, further work is needed to extend the model to dynamic operation. Experimental work is underway to refine the relationship between the set and reset fluxes and the input rise rate.