South Korea and the United States have "effectively" concluded their nuclear cooperation accord that would allow Seoul to reprocess spent nuclear fuel for the purpose of research and development, albeit in a limited way, government sources said Sunday.

Seoul and Washington have been negotiating for more than four years to revise the 1974 accord over Seoul's civilian nuclear energy use, also known as the "123 agreement."

Under a revised accord, South Korea is likely to secure some autonomy in dealing with spent nuclear fuel for R&D purposes if doing so does not pose any risks of nuclear proliferation, they said.

Currently, Seoul should seek consent from the United States case by case when tinkering with spent nuke fuel.

"The two countries are in the last stage of fine-tuning some details after reaching agreement on major points of the nuke accord," a source said, asking not to be named.

South Korea has been in talks with the aim of meeting its three goals for the negotiations -- winning the right to deal with spent nuke fuel, securing stable supply of nuclear fuels and promotion of exports of nuclear power plants.

The main sticking point of the accord talks was whether Seoul would be able to win the right to enrich uranium and reprocess spent nuclear fuel to some extent to address the headache of growing nuclear waste.

Despite Seoul's call, the U.S. has been reluctant to do so, apparently due to possible negative impact on its global nonproliferation campaign amid concerns over North Korea's nuclear weapons program.

The new accord seems to allow Seoul leeway to deal with spent nuclear fuel in a limited way while still backing up the U.S. nonproliferation policy, the sources said.

The Seoul-Washington accord is not likely to contain set of clauses called the "Gold Standard" that explicitly prohibit uranium enrichment and reprocessing, they said. In 2008, the U.S. clinched a nuke pact with the United Arab Emirates that includes such clauses.

South Korea and the U.S. have agreed to do their best to hold final talks and conclude the nuclear deal "within several weeks," saying that there has been much progress over the accord, according to a statement following a recent meeting of their foreign ministers in Germany.

The two sides are expected to hold their final negotiation soon to initial the agreement.

A draft of a new nuclear pact between Seoul and Washington would contain the wording of promoting "strategic cooperation" in the nuclear energy issues among the two sides, according to officials.

South Korea has been seeking to upgrade its strategic cooperation with the U.S. by taking into account its enhanced status in the nuclear power industry.

The nuke accord was supposed to expire in March last year, but the two countries agreed to extend it by two years to March 2016 in order to buy time for further negotiations. (Yonhap)