How much iron does a star fuse before going supernova?
Wikipedia says the iron-nickel core is inert, and doesn't mention any endothermic iron fusion reactions, just the core collapse, which converts the core to neutrons (and a not insignificant proportion of protons &amp; electrons), and releases a lot of neutrinos. The nickel is radioactive, decaying to cobalt &amp; iron. 

The silicon burning phase leading to nickel only lasts about 5 days. The inert core collapses when it exceeds the Chandrasekhar limit of about 1.2 M$\odot$.