Published online 27 June 2008 | Nature | doi:10.1038/news.2008.919

News: Briefing

North Korea blows its stack

What does cooling tower's destruction mean for nuclear disarmament?

tower demolitionNorth Korea hopes the demolition is interpreted as a potent symbol of disarmament.Associated Press

North Korea demolished the cooling tower of its only nuclear reactor today in a gesture of its willingness to end its plutonium weapons programme. The demolition is one of a series of recent concessions and disclosures by the republic. But what has been learned about the secretive nation's nuclear programme, and what lies ahead?

What was this reactor used for? The 5 megawatt reactor, located in the formerly secret city of Yongbyon, burned uranium fuel in order to produce plutonium-239 for use in nuclear weapons. The North Koreans had been operating the reactor on and off for decades, but they claim they produced most of their weapons-grade plutonium in a series of campaigns between 2002 and 2005.

How much plutonium did it make? According to documents recently delivered to the United States, the North Koreans say that they have produced a total of 37 kilograms of plutonium. They currently claim to have only 30 kilograms of bomb-grade material, according to Siegfried Hecker, a US nuclear-weapons expert at Stanford University, California, who visited Yongbyon in February. That would be enough for about five nuclear weapons.

So what happened to the other 7 kilograms? It may have been used in tests, including the 2006 underground test of a nuclear device (see Nature 443, 610–611 2006). But Hecker adds that some material was probably also lost during reprocessing — the chemical process by which plutonium is extracted from the reactor's spent fuel rods.

Does the destruction of the cooling tower mean that North Korea can't produce any more plutonium? For the moment, yes. The Yongbyon reactor relied on cooling water to operate safely, and without the tower it would quickly overheat if started up again. However, rebuilding the cooling tower would not be especially difficult and could be completed with months, according to Hecker. "The main effect is symbolic," he says.

So what needs to be done to truly shut down the programme? One significant step would be to remove the reactor's control-rod mechanism, says Hecker. The rods are specialized devices, usually made of cadmium, that allow operators to control the rate of reactions in the core. They would be significantly more difficult to replace than the cooling tower. In addition, steps could be taken to decommission the reprocessing plant, the separate facility where plutonium is extracted from the existing fuel, and the fuel-manufacturing facility.

These and other steps towards nuclear disarmament will be discussed when North Korea holds future talks with South Korea, China, Japan, the United States and Russia.

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How can other countries verify that North Korea isn't keeping some plutonium in reserve? It will be up to highly trained nuclear inspectors to verify the government's claims. They will rely on more than 18,000 pages of newly disclosed documents about the programme, which should include operating records of the reactor and reprocessing plant. In addition, the inspectors will take measurements of key components, such as the graphite blocks used to moderate the reactions and the waste pools where old fuel is stored. From that data they should be able to work backwards and determine how much plutonium North Korea has made. "It's pretty sophisticated stuff, but it's been done," says Hecker.

What is North Korea getting in return for cooperation? On Thursday the United States said it would remove North Korea from its fabled 'axis of evil' and ease trade sanctions against the country. In addition, North Korea has been receiving fuel aid from the negotiating nations and will probably receive further political and economic aid for its continued cooperation. 

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