Url changed from http://www.livescience.com/53227-plutonium-238-created-for-s..., which is cribbed from this press release. Like most of the science PR mills, that one does not link to its sources.
https://www.ornl.gov/news/ornl-achieves-milestone-plutonium-...
It's more recent (Dec 22nd, 2015) and discusses a new achievement: first production of Pu-238 in the USA in 30 years.