WHEN the US persuaded North Korea last year to swap its suspected nuclear-weapons program for two modern nuclear-power plants, few arms-control experts and even fewer lawmakers thought the deal would stick. Eleven months later, the ''Agreed Framework'' with the hard-line Communist state is being implemented with greater speed and warmer cooperation than even its strongest backers expected. The accord has not turned North Korea into a democracy nor restored peace to the divided Korean peninsula. Then again, no one claimed it would. What it has done is freeze a dangerous nuclear program that was probably producing weapons-grade plutonium and appeared to be heading toward making nuclear bombs. At the same time, it has opened a largely unpublicized channel of diplomacy between North and South Korea that American officials say, with guarded hope, could contribute to an eventual easing of tensions along the world's most heavily armed border. ''This is an incredible story that's been largely overlooked,'' says Ambassador-at-Large Robert Gallucci, who was the US negotiator of the accord. Since the accord went into effect, North Korea has shut down an existing graphite nuclear reactor and a nuclear-reprocessing facility, stopped construction of two larger nuclear reactors, and begun the process of placing an estimated 8,000 spent nuclear fuel rods, which could be used to make nuclear weapons, into storage. Once stored, the rods cannot be removed without detection and serious radiation hazards. For its part, the US has agreed to help provide two new proliferation-resistant light-water reactors (LWRs) to North Korea, worth an estimated $4 billion. Japan and South Korea will pay the lion's share of the cost. Washington has also agreed to help provide heavy oil to generate heat and electricity for civilian use until the reactors are built, with the US picking up only part of the roughly $50 million a year tab. ''Cal Ripkin has his streak; we've got ours,'' says Mr. Gallucci, referring to the Baltimore Orioles shortstop who broke a consecutive-games-played record this summer. ''Since the Agreed Framework, there hasn't been a day in which we haven't had a freeze on their nuclear program,'' he says. North Korea has also cooperated in the early stages of planning for the two new LWRs, which will be supplied through a seven-nation consortium called the Korean Energy Development Organization (KEDO). In June, the Pyongyang government acquiesced when KEDO specified that South Korea would supply the two LWRs and that a South Korean company would be chosen as the prime contractor. North Korea had previously refused to allow South Korea to supply the reactors. Pyongyang's cooperation North Korea has also granted visas to a KEDO delegation, including South Koreans, to survey the site at Sinpo on the northeastern coast where North Korea wants the reactors built. North and South Koreans have also met face-to-face in recent KEDO meetings in Malaysia, a significant step in light of continuing tensions on the Korean peninsula. And North Korea has also cooperated as American officials prepare spent fuel rods for long-term storage and as KEDO installs a monitoring system to ensure that the American oil shipments to North Korea are not diverted to industrial or military use. ''If everyone had closed their eyes starting the beginning of June and suddenly opened them at the end of September, they would be amazed at the progress that has been achieved,'' says Evan Medeiros, a senior research analyst at the private Arms Control Association in Washington. Diplomatic observers attribute North Korea's newfound agreeability, in part, to the pressure of severe hard times and acute food shortages at home. Optimism is tempered by the fact that the agreement is at the mercy of one of the world's most hawkish and unpredictable governments. ''We're dealing with North Korea,'' Gallucci notes. ''If at any point we forget that, we deserve to be perceived as massively naive without any sense of history. We're not going to make that mistake.'' Deep skepticism remains When the Korea accord was announced last year, critical lawmakers charged that the administration gave more than it got. Despite steady progress toward implementation, many remain skeptical of the agreement and reluctant to grant an administration request for $22 million to pay the American share of the KEDO budget. ''In the past several months there's been fairly positive movement,'' acknowledges Sen. Craig Thomas (R) of Wyoming, a member of the Senate Foreign Relations Committee. ''But there's still overriding anxiety about whether the North Koreans want to comply. I don't think the administration has made its case yet'' for the entire $22 million. KEDO and North Korea are now negotiating a supply contract that will formalize the selection of a site and govern, among other things, training, maintenance, and safety procedures at the two new reactors. Preliminary steps Once the contract is approved, work will begin on the first reactor. But before the main nuclear components are brought in, North Korea will have to open its two nuclear-waste sites to international inspection. That should provide information about how often North Korea reprocessed spent fuel from its existing reactor into plutonium, though not necessarily how much plutonium or whether any of it was used to make nuclear bombs. Once the nuclear components start arriving in North Korea, it will have to begin shipping the spent fuel rods to a third country. By the time the first light-water reactor is completed, according to the agreement, all 8,000 fuel rods must be out of North Korea. And by the time the second LWR is completed, North Korea's existing nuclear plants and reprocessing facility must be dismantled. Administration officials point to the reciprocity that exists at each stage of the estimated 10-year process process, which means that the new reactors will not be built unless North Korea complies strictly with the terms of the ''Agreed Framework.'' More important, says one private source familiar with the accord: ''Every day this process is taking place, North Korea's nuclear-weapons program remains frozen.''