to all major South Korean cities. Two rail lines, Gyeongui and Donghae Bukbu Line, to North Korea are now being reconnected. The Korean high-speed rail system, KTX, provides high-speed service along Gyeongbu and Honam Line. Major cities including Seoul, Busan, Incheon, Daegu, Daejeon and Gwangju have urban rapid transit systems. Express bus terminals are available in most cities. South Korea's main gateway and largest airport is Incheon International Airport, serving passengers in 2016. Other international airports include Gimpo, Busan and Jeju. There are also many airports that were built as part of the infrastructure boom but are barely used. There are also many heliports. The national carrier, Korean Air served over 26,800,000 passengers, including almost 19,000,000 international passengers in 2016. A second carrier, Asiana Airlines also serves domestic and international traffic. Combined, South Korean airlines serve 297 international routes. Smaller airlines, such as Jeju Air, provide domestic service with lower fares. South Korea is the world's fifth-largest nuclear power producer and the second-largest in Asia . Nuclear power in South Korea supplies 45% of electricity production, and research is very active with investigation into a variety of advanced reactors, including a small modular reactor, a liquid-metal fast/transmutation reactor and a high-temperature hydrogen generation design. Fuel production and waste handling technologies have also been developed locally. It is also a member of the ITER project. South Korea is an emerging exporter of nuclear reactors, having concluded agreements with the UAE to build and maintain four advanced nuclear reactors, with Jordan for a research nuclear reactor, and with Argentina for construction and repair of heavy-water nuclear reactors. , South Korea and Turkey are in negotiations regarding construction of two nuclear reactors. South Korea is also preparing to bid on construction of a light-water nuclear reactor for Argentina. South Korea is not allowed to enrich uranium or develop traditional uranium enrichment technology on its own, because of US political pressure, unlike most major nuclear powers such as Japan, Germany, and France, competitors of South Korea in the international nuclear market. This impediment to South Korea's indigenous nuclear industrial undertaking has sparked occasional diplomatic rows between the two allies. While South Korea is successful in exporting its electricity-generating nuclear technology and nuclear reactors, it cannot capitalize on the market for nuclear enrichment facilities and refineries, preventing it from further expanding its export niche. South Korea has sought unique technologies such as pyroprocessing to circumvent