Zirconium alloys (Zircaloy-2 and -4) remain the main cladding materials in most light water reactors. The recent
trend is to use niobium-modified zirconium alloys because the Nb addition has been found to improve cladding
performance in high burnup conditions. Hence, new alloys with Nb additions (such as ZirloTM, M5TM etc.) have
recently been developed. Although it is known that creep properties improve, there have been very few data
available to precisely evaluate the creep characteristics of these new commercial alloys. However, the creep
behavior of many Nb-modified zirconium alloys has been studied over a long period of time. In this study, we have
collected creep data on various Nb-containing alloys from the open literature as well as our own study over a wide
range of stresses and temperatures. It is suggested that Nb-modified zirconium alloys behave as a Class-A alloy
(stress exponent of 3) with the Nb atoms forming solute atmospheres around dislocations and thus, impeding
dislocation glide under favorable conditions. On the other hand, Zircaloys exhibit Class-M creep behavior with a
stress exponent of ≥ 4 implying the operation of a dislocation climb-controlled deformation mode.