Differences between Korea and the U.S. over a decades-old nuclear energy pact are casting a shadow over the first talks between Presidents Park Geun-hye and Barack Obama scheduled for early May.

Park plans to visit Washington in her first trip as president which is expected to focus on their joint response to a North Korea-fueled security crisis.

The two leaders will also discuss how to share costs for 28,500 U.S. troops stationed here, the transfer of wartime operational control slated for 2015 and other bilateral and global issues.

Her choice of the U.S. as her first overseas destination since taking office on Feb. 25 apparently reflects spiraling tension on the peninsula in the wake of North Korea’s third nuclear test, ensuing new U.N. sanctions and the two allies’ joint military drills.

For Park, working with Washington is pivotal in dealing with Pyongyang’s newfound lust for brinkmanship, with its “nuclear umbrella” also covering Japan, another staunch U.S. ally.

For Obama, the two countries’ alliance is a linchpin of peace and security in the region on which his government is laying greater store via its “pivot” to Asia.

But emerging as a potential fault line is their nuclear cooperation agreement sealed in 1965 for the commercial use of atomic energy. Seoul and Washington have been discussing an amendment of the pact, last revised in 1974 and due to expire next March.

Under the so-called 123 deal, South Korea is banned from reprocessing spent fuel generated in its 23 civilian reactors to prevent possible enrichment of fissile materials.

The country is also a member of the Non-Proliferation Treaty. It supplies about 40 percent of its electricity from atomic power.

But it resulted in some 10,000 tons of radioactive waste, fears of radiation exposure and related public protests. Storages in Gyeongju, North Gyeongsang Province are forecast to reach capacity in 2016.

Seoul is pushing for a new technology called pyroprocessing and fast reactors as a solution to curb its spent fuel inventory without extracting plutonium or uranium used to make nuclear weapons, a view that remains as a source of controversy.

The allies are in separate talks over joint research on pyroprocessing, which some officials and scientists believe will be a step closer to revising the decades-old accord.

During a meeting with senior U.S. officials in mid-January, Park called for greater efforts to break through the impasse.

Foreign Minister Yun Byung-se said the country’s goal was “to lay the groundwork to expand the use of our peaceful nuclear energy” during his confirmation hearing late last month.

But the outlook appears still murky as Washington’s nonproliferation campaign is already under stress in the face of a regional arms race and continuing nuclear development by rogue states like North Korea and Iran.

“Both sides are not budging an inch. The U.S. thinks that it had already offered a concession by agreeing to longer missile ranges last year,” a government official said on condition of anonymity because of the sensitivity of the issue.

Given time constraints, the most likely outcome for now may be an extension of the existing agreement, observers say.

News reports have suggested that the two countries are considering one or two-year extensions.

“Debate over whether the ROK should have the freedom to pyroprocess is ultimately not about whether a pyroprocessing system is safeguardable or whether the ROK will divert nuclear material for a clandestine nuclear program,” Mark Hibbs, a senior associate at the Carnegie Endowment for International Peace’s nuclear policy program, wrote recently.

“It is about whether the U.S. has confidence that the ROK will stay in the NPT and not obtain a capability to produce nuclear weapons in a short period of time,” he said.

By Shin Hyon-hee (heeshin@heraldcorp.com)