BEIJING (Reuters) - After securing the shutdown of a North Korean nuclear reactor, negotiators flying into Beijing on Tuesday face the much harder task of coaxing Pyongyang to give up a trove of atomic secrets long guarded as vital for its survival.

Envoys from six countries are scheduled to hold two days of talks starting on Wednesday after U.N. nuclear inspectors verified the shutdown of its Yongbyon reactor.

The reactor produces material that can be turned into weapons-grade plutonium and in February North Korea agreed to close it in return for 50,000 tonnes of heavy fuel oil, which began moving there from South Korea last week.

North and South Korea, the United States, China, Japan and Russia will now start to explore how to permanently scrap the Yongbyon complex and terminate North Korea's nuclear weapons potential.

China welcomed the closure of the Soviet-era reactor and top North Korean negotiator Kim Kye-gwan was quoted by Xinhua news agency as saying the first phase of the deal had been accomplished.

"So the talks will focus on the sequence of the obligation and actions to be taken by the concerned parties at the second phase under February 13 agreement," he was quoted as saying in Pyongyang.

Envoys and experts said the next stage promises to be contentious and volatile even by the bruising standards of the talks that began in 2003.

"We've got to the first mountain in a pretty big mountain range. Yongbyon was the first step in a very tough process," said Peter Beck, who analyses North Korea from Seoul for the International Crisis Group, a non-government think tank.

"I think we're still closer to a breakdown than a breakthrough."

Pyongyang refused to close Yongbyon until Washington cleared away an international snarl-up over some $25 million of its funds unfrozen from a Macau bank as part of the February deal.

North Korea officials had called the bank demand a test of U.S. "sincerity" and such testing of wills is likely to shape the forthcoming phase of talks.

"I think you have to look at each stage as more difficult than the previous stage. It is a little like one of those video games -- every level becomes more difficult than the previous level," U.S. negotiator Christopher Hill said in Seoul on Monday.

He urged the North to permanently disable Yongbyon and give a full account of its nuclear programs, which Washington has said included efforts to enrich uranium -- another route to making the explosive core of nuclear weapons.

The impoverished North will get 950,000 more tonnes of oil, security assurances and improved access to international trade if it completely scraps its nuclear arms programs.

But verifying a full deal would entail sweeping inspections of the isolated regime that has long warded off international intrusion.

North Korea is likely to resist such scrutiny and could easily back away from a deal, said Zhang Liangui, a Chinese expert on the North.

"It's still an unanswered question whether North Korea has the will to abandon nuclear weapons. We can't assume further concessions are a matter of course," said Zhang, a researcher at the Central Party School, a top think tank in Beijing.

After throwing out International Atomic Energy Agency inspectors in late 2002, North Korea quit the nuclear Non-Proliferation Treaty, which the IAEA enforces.

In 2005, Pyongyang declared it had nuclear arms, and last October it alarmed the world with its first test detonation.

China could push for a tighter schedule of disarmament moves and concessions after the months of delays, said Zhang.

"Without a timetable to keep discipline this could become a Marathon that never ends."

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