, there are 177 tanks at Hanford, 149 of which have a single shell. Historically single shell tanks were used for storing radioactive liquid waste and designed to last 20 years. By 2005, some liquid waste was transferred from single shell tanks to (safer) double shell tanks. A substantial amount of residue remains in the older single shell tanks with one containing an estimated 447,000 gallons (1,700 m3) of radioactive sludge, for example. It is believed that up to six of these "empty" tanks are leaking. Two tanks are reportedly leaking at a rate of 300 gallons (1,136 liters) per year each, while the remaining four tanks are leaking at a rate of 15 gallons (57 liters) per year each.