In 2004, clean-up work uncovered a battered, rusted, and broken old safe containing a glass jug inside which was 400 milliliters of plutonium (see photo, top right).

Recent tests by Jon Schwartz' team at the Pacific Northwest National Laboratory in Richland, Washington, has shown this plutonium was the first ever processed at the site, and the first made on a usable scale anywhere in the world.

Schwantes and colleagues used the fact that plutonium naturally decays to uranium to date the sample to 1946, give or take 4.5 years, by comparing the amounts of the two metals present inside the jug. Its age allowed the team to establish that the plutonium must have come from one of four reactors - out of 11 in the US at the time - from which fuel was reprocessed into plutonium.

Three of those reactors were on the Hanford site, with the fourth at Oak Ridge in Tennessee. Comparing the minor plutonium isotopes in the sample to signatures for each of the four reactors showed that the sample came from the X-10 reactor at Oak Ridge.