A new nuclear fuel reprocessing technology, which the United States has allowed South Korea to research into, may work technically, but its viability economically and in terms of security is a "different question," a senior U.S. diplomat said.

The remark by Assistant Secretary of State Thomas Countryman, who represented the U.S. in negotiations on a civil nuclear cooperation pact with Seoul, suggests the U.S. is negative about allowing South Korea to take advantage of the "pyroprocessing" technology.

"The technology has not yet been proven viable. It may be technically viable, but economically and in terms of safeguards and security, it's a different question," the official said during a recent Senate Foreign Relations Committee hearing on the nuclear cooperation pact.

Pyroprocessing refers to a new type of technology for recycling spent nuclear fuel. It poses fewer proliferation risks than the conventional reprocessing technology because it does not involve producing pure plutonium that can be used in nuclear weapons.

It was a key contentious point in negotiations between South Korea and the U.S. to revise their civil nuclear cooperation agreement, with Seoul insisting on using the technology and the U.S. balking at the idea over proliferation concerns.

As a compromise, the U.S. agreed to allow South Korea to conduct research only into the first stage of the technology.

Whether Seoul will be able to fully use the technology is expected to be determined after the sides complete a 10-year joint study into the technology in 2021.

Countryman rejected perceptions that the agreement, which was signed in June and has since been awaiting approval from Congress, has opened the door for South Korea to the technology, insisting that the deal means the decision has been postponed.

"I don't agree that we have opened the door to pyroprocessing. We've just have this discussion. We have agreed to postpone a decision on that until the joint fuel cycle study is completed," he said.

The official also said that South Korea's research into pyroprocessing does not violate the 1992 inter-Korean joint declaration forswearing nuclear reprocessing and uranium enrichment capabilities.

"Our current agreement for joint research with the Republic of Korea on certain aspects of pyroprocessing does not violate that agreement. It is technical research at this point," he said. (Yonhap)