The U.S. Energy Department said in a statement the higher radiation level was discovered during a routine check of a leak detection pit, the Los Angeles Times reported Friday.
The source of contamination could be a leak in a secondary containment shell since a slow leak had been found in the primary tank, spokeswoman Lori Gamache said.
State officials have pressed federal energy officials to deal with the leaking tanks, which are beyond their projected life spans.
Gov. Jay Inslee said in a statement he continued to have "serious concerns regarding the pace of addressing the leaking tanks." While the leak was not an immediate danger, Inslee said he would insist on an "acceleration of remediation of all the tanks."
A watchdog group, the Hanford Challenge, said Energy Secretary Ernest Moniz had visited the site two days before the announcement and said the leak was contained and had not grown.
"Today's news highlights the need for further transparency and urgent action," the group said in a statement.
Inslee said the Energy Department had assigned engineers to determine the source of the contamination, but that the process could take several days, CNN reported.
The Hanford facility is considered the most contaminated nuclear site in the nation. It is also the oldest in the world, with plutonium from the site used to make the atom bomb detonated over Nagasaki, Japan, in World War II. It borders the Columbia River.