RICHLAND, Wash. (AP) — The federal Department of Energy says there is a slow leak in the oldest double-shell waste tank at the Hanford nuclear reservation that's allowing highly radioactive and hazardous waste to leak into the space between the inner and outer shells.

The agency said Monday that no waste is believed to have leaked from the outer shell into the soil beneath the underground tank on the south-central Washington reservation.

The Tri-City Herald reports that earlier testing showed that some of the material seen between the two walls of the tank was radioactive waste. Now DOE has confirmed a leak. While the exact amount isn't known, the agency says perhaps a couple tablespoons of additional waste were seen between the two tank walls between Thursday and Sunday.

The tank is one of 28 double-shell containers used to hold waste from older, leak-prone single-shell tanks.

They hold 56 million gallons of radioactive waste from the past production of weapons plutonium until it can be treated for disposal.