The United States and South Korea have been partners in nuclear technology since 1972. The agreement governing that partnership, signed in its current form in 1974, was originally set to expire this month after forty years in effect. Because US and South Korean negotiators could not finalize the language of the new agreement before the deadline, last spring both sides agreed to a two-year extension of the existing agreement, which was passed by congress in January and signed by President Obama in February. This essentially gives negotiators until early 2015 to nail down the new agreement, allowing time for both the American and Korean legislatures to approve the deal and make it law before the extension expires on March 19, 2016.
The US has several such pacts, called 123 agreements, with other partners in nuclear technology around the world, such as Japan and the United Arab Emirates. The purpose of these deals is to ensure the peaceful use of atomic power and to govern the transfer of technologies and fuels between the signatories. American companies built South Korea’s first nuclear reactors and patented American designs and technologies are used under license agreements in many of Korea’s other reactors built since then. The US is also a major source of nuclear fuel for Korea, which derives around 30% of domestic electricity from its 23 nuclear reactors, and has plans to expand both the number of reactors and the size of their role in the domestic electricity supply.
The bill extending the current US-Korea 123 agreement stipulates that congress must receive updates every 180 days regarding the status of the negotiations until such time as a deal is finalized. The bill can be viewed here.
Update (3/20/2014): On March 18th, the State Department announced that the extension agreement officially entered into force with immediate effect.