By Paul Rincon
Science reporter, BBC News website
The glass bottle was discovered during a clean-up operation
A bottle discarded at a waste site in the US contains the oldest sample of bomb-grade plutonium made in a nuclear reactor, scientists say.
The sample dates to 1944 and is a relic from the infancy of the US nuclear weapons programme.
A team from the Pacific Northwest National Laboratory used nuclear forensic techniques to date the sample and track down its origins.
Details appear in the latest edition of the journal Analytical Chemistry.
The researchers have described their study as "nuclear archaeology".
The type of plutonium in the bottle - known as Pu-239 - is a so-called alpha emitter. These alpha particles are too bulky to penetrate skin or paper, but they can cause poisoning if swallowed or inhaled.
It has a half-life (the time it takes for half the radioactive nuclei in a sample to decay) of 24,110 years.
The bottle in question was discovered in a burial trench at the Hanford nuclear site in Washington state, north-western US.
Established as part of the Manhattan Project in 1943, Hanford was home to the world's first full-scale plutonium production facility.
The Manhattan Project was the US' bid to build the world's first nuclear weapon during World War II. The project's roots lay in fears that Nazi Germany was investigating similar technology.
It culminated in the nuclear bombs dropped on Hiroshima and Nagasaki in Japan at the end of the war.
Plutonium produced at the Hanford site was used in Trinity - the world's first nuclear weapon test - on 16 July 1945 and in the plutonium bomb dropped on Nagasaki, Japan, on 10 August 1945.
To recover bomb-grade plutonium, spent nuclear fuel was transported from a reactor to a chemical re-processing plant.
Here, the small amount of plutonium produced in the reactor was separated from the remaining uranium and waste fission products.
The Hanford site is now the focus of a massive environmental cleanup effort due to high levels of radioactive waste that remain at the site.
While excavating a burial trench in December 2004, clean-up personnel discovered a safe which contained a jug filled with whitish liquid slurry.
Further tests revealed the bottle contained a type of plutonium made by re-processing spent fuel in a manner consistent with early operations at Hanford.
Realising the historic potential of the find, Jon Schwantes and colleagues from the Pacific Northwest National Laboratory carried out further tests on the sample.
In order to determine its age, the researchers analysed the different forms, or isotopes, of plutonium and uranium in the sample. They found it had been separated from the spent fuel in 1944.
The bottle was held in a safe buried at a disposal site
In order to determine which reactor had produced the sample, they compared plutonium isotope ratios from the contents of the bottle against technical data from nuclear research reactors that were operating at the time the sample was made.
Their results strongly suggested the plutonium was manufactured at the prototype X-10 reactor at Oak Ridge in Tennessee, which began operating in 1943, a year after the Manhattan Project was authorised.
The types of forensic techniques used in the study are also vital for determining the sources, origins and routes of smuggled radioactive materials.
"The frequency of smuggling events involving radioactive materials is supply driven and is on the rise worldwide," the researchers write in Analytical Chemistry.
They added: "It is likely that (given) the current nuclear renaissance and greater access to these materials by the public, smuggling events involving fissionable materials may rise in the near future."