Through thick glass windows beneath 20 feet of water lurk canisters containing spent nuclear fuel rods, stored after having powered one of the four reactors at this nuclear energy site on South Korea's east coast, 100 miles southeast of Seoul.

"Currently, we have space for spent fuel rods until 2016," says Park Chan-sung, an official at the site, the newest of four nuclear power plant complexes with 20 reactors operating under the aegis of the state-owned Korea Hydro and Nuclear Power Co. "Plans for after 2016 are under discussion."

The issue of what to do with the fuel rods is reaching critical mass. South Korea is a rising manufacturer of nuclear reactors and exporter of nuclear power plants.

Now, it wants to reprocess rather than store its spent fuel rods – despite fears of potential proliferation and questions about a ban on reprocessing imposed by its nuclear cooperation agreement with the United States. The debate is critical not only for South Korea's nuclear energy program but also for efforts to get North Korea to abandon its nuclear weapons. North Korea's leader, Kim Jong-il, talks about "denuclearization" of the Korean Peninsula, suggesting North Korea will persist in its program as long as South Korea and the US also have nuclear capabilities.

South Korean scientists as well as leaders say the South needs the independence to recycle spent fuel rods. But some observers say that could lead to the South producing plutonium for warheads. South Koreans disavow any ambition other than to extract more uranium to fuel reactors. It would then bury the residue.

Secret talks underway

Secret talks are under way, mean­while, on revising the nuclear cooperation agreement.

"Both our countries support the global growth of the peaceful use of nuclear energy," US Ambassador Kathleen Stephens told an influential Korean audience in mid-March. "We will continue our cooperation to guarantee the safety and proliferation-­resistance of nuclear energy."

One US concern is how much faith to place in denials of nuclear ambitions while North Korea refuses to get rid of its weapons program. The US insisted on banning reprocessing in 1972 to frustrate the dream of South Korea's long-ruling Park Chung-hee that his country would become a nuclear power.

That agreement expires in 2014, but it's far from clear if the US and South Korea can resolve their disagreement by then. Pressure could mount in the South for a deterrent while the North produces ever more fissile material, already estimated at enough for six to a dozen warheads, and conducts more underground tests as it did last May and in 2006.

"Does the US want to treat us as a criminal?" asks Kim Tae-woo, vice president of the Korea Institute for Defense Analyses. "Our concern is not to build a nuclear bomb, but how to dispose of spent fuel rods. If the US government continues to oppose us, that will hurt our sentiment."

Reprocessed fuel by any other name...

Koreans try to allay suspicions. "We do not want to use the word 'reprocessing,' " says Choi Jung-bae, director of the nuclear policy division at the Ministry of Science and Technology. "We prefer to say, 'recycling' or 'reused.' "

The difference is more than semantic. "We do not want to produce pure plutonium," says Mr. Choi. "The purpose of recycling is to get only useful elements in spent fuel," including enriched uranium, in a process called pyroprocessing.

At the Korea Atomic Energy Re­search Institute in Daejeon, about 80 miles south of Seoul, scientists call pyroprocessing "a long-term solution" for recycling spent fuel rods without producing weapons-grade plutonium. "The point is, pyroprocessing cannot recover plutonium," says Lee Han-soo, director of nuclear fuel cycle process development. "It cannot compare with normal reprocessing."

Pyroprocessing, say engineers, will make maximum use of the spent fuel rods while vastly reducing the need for waste storage space.

Lee Kwang-seok, director for strategy and international studies, says pyroprocessing, first developed in the US, is "more economic, more problematic-resistant, and has more safeguard ability" than reprocessing systems in use in Japan and France.

But pyroprocessing remains in research and development. "We need more than 10 or 20 years," Mr. Lee says, before it is ready commercially.

In the meantime, Korean engineers ask why South Korea is banned from processing spent fuel rods in the style of the Japanese and French, who do not attempt pyroprocessing. Lee questions if "the Japanese and French argument" that their systems produce too low a percentage of fissile plutonium-239 for warheads is "correct."

Many Koreans see such constraints as an affront that may weaken US-Korean cooperation on military and diplomatic issues. The rift assumes greater importance as South Korea produces more nuclear energy – and competes as a major producer of reactors. Also tangling matters is the prospect of running out of storage space.

A rapid reliance on nuclear energy

Complicating this discussion is the rapid expansion of the South's nuclear energy industry. The government's goal is to produce 100 or so reactors in 20 years, including 80 for export in competition with the US, Japan, France, and China. South Korea's president, Lee Myung-bak, places top priority on the export of reactors.

KEPCO, the state-invested Korea Electric Power Corp., has signed a deal to export four 1,400-megawatt energy reactors to the United Arab Emirates for $20 billion and is looking for more such agreements. Delivery of those reactors is several years off, but South Koreans say they need the US to come to an understanding on recycling for the sake of the export market as well as for burgeoning domestic needs. KEPCO has overall responsibility, while a single company, Doosan Heavy Industries, is building the reactors in the city of Changwon, near the major southeastern port of Pusan. Numerous other companies provide parts and expertise.

South Korea is rapidly becoming reliant on nuclear energy – 20 light-water reactors now produce 40 percent of its energy needs, with 10 more due to go on line in a decade. By 2030, it will derive 60 percent of its energy from nuclear power.

At stake: US relations, and energy independence

Atop a hill overlooking another complex, this one at the historic site at Kori, where South Korea's first reactor began producing power in 1978, Lee Soo-il, a director, points to eight silos near the seafront. Six of them house nuclear reactors – and two more await installation of reactors. "We are trying to make our unit cost-competitive," he says. "Everything is stored here at this site. We are trying to figure out ways to deal with reprocessed spent fuel."

With all the emphasis on building reactors, L. Gordon Flake, director of the Maureen and Mike Mansfield Foundation in Washington, cites the reprocessing tiff as "very dangerous" – possibly "the most important" long-term issue in US-Korea ties.

"The challenge is to put the focus on nuclear responsibility, not nuclear sovereignty," says Mr. Flake, whose organization is dedicated to better US-Asia relations. "Cast as a question of national pride and sovereignty, it could be very damaging."

But at the Korea Atomic Energy Research Institute, physicists were discovered to have enriched tiny amounts of uranium in 2000 without notifying even their own government. The International Atomic Agency in 2004 scolded South Korea for not having reported the experiments, but concluded they had stopped.

Any new US-South Korean nuclear cooperation agreement, says Evans Revere, president of the Korea Society in New York and a former senior US diplomat here, "will have to be in strict compliance" with the nuclear nonproliferation treaty.

Lee Chung-min, ambassador for international security affairs, says South Korea is building "safeguards into our proposal" and "any reprocessing will be under the full purview of the IAEA." He links the issue to the South's rise as an exporter of reactors. At stake, he says, "is a matter of energy independence."