In the early 1980s, U.S. satellites tracked a growing indigenous nuclear program in North Korea. The North Korean nuclear program began in the late 1950s with cooperation agreements with the Soviet Union on a nuclear research program near Yongbyon. Its first research reactor began operation in 1967. North Korea used indigenous expertise and foreign procurements to build a small nuclear reactor at Yongbyon (5 MWe). It was capable of producing about 6 kilograms (Kg) of plutonium per year and began operating in 1986. Later that year, U.S. satellites detected high explosives testing and a new plant to separate plutonium from the reactor's spent fuel. In addition, construction of two larger reactors (50 MWe at Yongbyon and 200 MWe at Taechon) added evidence of a serious clandestine effort. Although North Korea had joined the Nuclear Nonproliferation Treaty (NPT) in 1985 under Soviet pressure, safeguards inspections began only in 1992, raising questions about how much plutonium North Korea had produced covertly. In 1994, North Korea pledged, under the Agreed Framework with the United States, to freeze its plutonium programs and eventually dismantle them in return for several kinds of assistance. At that time, Western intelligence agencies estimated that North Korea had separated enough plutonium for one or two bombs. North Korea complied with the Agreed Framework, allowing International Atomic Energy Agency (IAEA) seals—including the "canning" of spent fuel rods at the Yongbyon reactor—and permanent remote monitoring and inspectors at its nuclear facilities. When in 2002, U.S. negotiators reportedly presented North Korean officials with evidence of a clandestine uranium enrichment program, the North Korean officials reportedly at first confirmed this, then denied it publicly. The conflict quickly led to the breakdown of the Agreed Framework. The Bush Administration argued that North Korea was in "material breach" of its obligations and, after agreement with South Korea, Japan, and the EU (the other members of the Korean Economic Development Organization, or KEDO), stopped the next shipment of heavy fuel oil. In response, North Korea kicked out international monitors, broke the seals at the Yongbyon nuclear complex, and restarted its reactor and reprocessing plant after an eight-year freeze. Members of the Six-Party Talks—the United States, South Korea, Japan, China, Russia, and North Korea—began meeting in August 2003 to try and resolve the crisis. In September 2005, the Six Parties issued a Joint Statement on how to achieve verifiable denuclearization of the Korean Peninsula, which formed the basis for future agreements. Negotiations broke down, and North Korea tested a nuclear device in October 2006. On February 13, 2007, North Korea reached an agreement with other members of the Six-Party Talks to begin the initial phase (60 days) of implementing the Joint Statement from September 2005 on denuclearization. Phase 1 of this agreement included the shut-down of plutonium production at the Yongbyon nuclear complex in exchange for an initial heavy fuel oil shipment to North Korea. Phase 2 steps include the disablement of facilities at Yongbyon and a "complete and correct" declaration of DPRK nuclear activities, in exchange for delivery of heavy fuel oil and equivalent, and removal of the Trading with the Enemy Act (TWEA) and State Sponsors of Terrorism (SST) designations. The United States provided funding and technical assistance for disablement activities in North Korea until April 2009. Energy assistance was divided evenly between the Six Parties in Phase 2 of the agreement. North Korea submitted a declaration of its past plutonium production activities in June 2008 as agreed in an October 3, 2007, joint statement on "Second-Phase Actions." Thereafter, President Bush removed North Korea from the TWEA list and notified Congress of his intent to lift the SST designation after North Korea agreed to verification provisions. North Korea did not accept initial U.S. verification proposals, and in September 2008, threatened to restart reprocessing plutonium. U.S. officials announced a bilateral agreement on verification in October 2008, and the Bush Administration removed North Korea from the SST List. The agreement was verbal, and North Korea then said that it had not agreed to sampling at nuclear sites, a key element in verifying past plutonium production. The Six Parties met in December 2008, but did not reach agreement on verification measures. Disablement activities at Yongbyon continued through April 2009, when North Korea expelled international monitors. North Korea then announced it would restart its reprocessing plant and boasted progress in uranium enrichment technology development and soon after tested as nuclear device (see detailed discussions below). The February 2007 Denuclearization Action Plan did not address uranium enrichment-related activities or the dismantlement of warheads and instead focused on shutting down and disabling the key plutonium production facilities at Yongbyon. A third phase, to have begun after disablement was complete and a declaration accepted by the Six Parties, was expected to deal with all aspects of North Korea's nuclear program, including weapons, using North Korea's declaration as a basis for future action. Understanding the scope of the program and the weapons capability would require transparency and careful verification for the pledged "complete, verifiable, irreversible" disarmament to be achieved. Six Party Talks have not been held since spring 2009, but the United States and other countries have held bilateral talks with the North since then. North Korea openly acknowledged a uranium enrichment program in 2009, but has said its purpose is the production of fuel for nuclear power. In November 2010, North Korea showed visiting American experts early construction of a 100 MWT light-water reactor and a newly built gas centrifuge uranium enrichment plant, both at the Yongbyon site. The North Koreans claimed the enrichment plant was operational, but this has not been independently confirmed. U.S. officials have said that it is likely other, clandestine enrichment facilities exist. A February 2012 announcement committed North Korea to moratoria on nuclear and long-range missile testing as well as uranium enrichment suspension at Yongbyon under IAEA monitoring. However, an April 2012 satellite launch, which violated UN Security Council resolutions as well as the February missile moratorium, caused a collapse of the February agreement. Following a December 2012 satellite launch and subsequent UN Security Council condemnation, North Korea rejected future denuclearization talks, saying it would only denuclearize when all other nuclear weapons states also did so. A third nuclear test conducted on February 12, 2013, could further complicate diplomatic efforts toward denuclearization. North Korea is widely believed to have mastered the engineering requirements of plutonium production a decade ago and may now be focusing its efforts on mastering uranium enrichment. Acquiring fissile material—plutonium-239 or highly enriched uranium (HEU)—is the key hurdle in nuclear weapons development. Producing these two materials is technically challenging; in comparison, many experts believe weaponization to be relatively easy. North Korea has industrial-scale uranium mining and plants for milling, refining, and converting uranium; it also has a fuel fabrication plant, a nuclear reactor, and a reprocessing plant—in short, everything needed to produce Pu-239. It has recently been built a uranium enrichment facility at Yongbyon that could produce HEU for weapons, or LEU reactor fuel which could be irradiated for plutonium production. In its earlier 5 MWe nuclear reactor, North Korea used magnox fuel—natural uranium (>99%U-238) metal, wrapped in magnesium-alloy cladding to produce plutonium for weapons. About 8,000 fuel rods constitute a fuel core for the reactor. When irradiated in a reactor, natural uranium fuel absorbs a neutron and then decays into plutonium (Pu-239). Fuel that remains in the reactor for a long time becomes contaminated by the isotope Pu-240, which can "poison" the functioning of a nuclear weapon. Spent or irradiated fuel, which poses radiological hazards, must cool after removal from the reactor. The cooling phase, estimated by some at five months, is proportional to the fuel burn-up. Reprocessing to separate plutonium from waste products and uranium is the next step. North Korea used a PUREX separation process, like the United States. After shearing off the fuel cladding, the fuel is dissolved in nitric acid. Components (plutonium, uranium, waste) of the fuel are separated into different streams using organic solvents. In small quantities, separation can be done in hot cells, but larger quantities require significant shielding to prevent deadly exposure to radiation. It operated its 5 MWe nuclear reactor, is believed to have separated Pu from the spent fuel, and has reportedly taken steps toward weaponization. In January 2004, North Korean officials showed an unofficial U.S. delegation alloyed "scrap" from a plutonium (Pu) casting operation. Dr. Siegfried Hecker, a delegation member, assessed that the stated density of the material was consistent with plutonium alloyed with gallium or aluminum. If so, this could indicate a degree of sophistication in North Korea's handling of Pu metal, necessary for weapons production. But without testing the material, Hecker could not confirm that the metal was plutonium or that it was alloyed, or when it was produced. Secretary of State Colin Powell in December 2002 stated, "We now believe [the North Koreans] have a couple of nuclear weapons and have had them for years." In February 2005, North Korea officially announced that it had "manufactured nukes for self-defense." Vice Foreign Minister Kim Gye Gwan has previously said that North Korea possesses multiple bombs and was building more.