Modeling fuel behavior requires an accurate description of the cladding stress response for both operational and safety considerations. The transient creep response of Zirconium alloys is commonly modeled using a strain hardening rule which is known to hold in cases with monotonously increasing stresses. However, the strain hardening rule is experimentally known to fail in scenarios such as load drop or reversal. In this paper we derive a simple and easily implementable set of rules for primary creep based on experimental results which contradict the strain hardening rule. The primary creep predicted by these rules is compared with data from published thermal creep experiments and Halden in-pile creep experiment IFA-585. The model thus created is shown to perform well in describing both transient stress scenarios with monotonously increasing stress and scenarios involving load drops and reversals.