SEOUL, South Korea — North Korea’s main nuclear complex was discharging hot wastewater in a further sign that the country has restarted a Soviet-era nuclear reactor there that it had used to obtain plutonium fuel for atomic bombs, an American research institute said on Thursday.

Using commercial satellite imagery, the U.S.-Korea Institute at Johns Hopkins University has been monitoring the nuclear complex in Yongbyon, north of Pyongyang, the North Korean capital. Last month, it reported that satellite images from late August showed steam emerging from a generator building adjacent to the five-megawatt reactor, which it said suggested that North Korea was following through on its vow to restart it.

The restarting of the reactor means that the country can produce weapons fuel again. Until the reactor was shut down in 2007 in a short-lived nuclear disarmament deal with Washington, its spent fuel had been the source of plutonium fuel for the North, which conducted three underground nuclear tests between 2006 and last February. North Korea has also said it is running a uranium enrichment program that can provide it with another type of bomb fuel: highly enriched uranium.

In a report posted on its Web site “38 North” on Thursday, the institute said that a Sept. 19 satellite photo showed hot water being released into the Kuryong River, which meanders through the North Korean nuclear complex. The stream of hot water came from the reactor’s generator and turbine building through a recently installed drainpipe, it said. The new buried pipeline, the report said, is part of a new secondary cooling system completed last summer to replace the cooling tower that was demolished in 2008 under the ill-fated deal with Washington.