Can plutonium be used to fuel nuclear reactors?

Yes, but it would not be used in a pure form, because it would be too concentrated for a power reactor. In the UK and France plutonium has been used in what is called MOX (Mixed Oxide) Fuel, where plutonium and uranium oxides are mixed to make fuel with roughly the same fissile content as enriched uranium fuel. I don't believe this technique has been used yet in the US,where spent fuel processing is not in operation so the plutonium is not being separated to make it available. However there may be plutonium available from ex-military stocks, and this could be used if required to supplement the amount of U-235 available.