The Yongbyon nuclear complex has been the beating heart of North Korea's nuclear programme for more than 50 years -- a source of great domestic pride and even greater global anxiety.

The mere suggestion -- based on satellite imagery -- that its main plutonium reactor may be back up and running has been greeted with grave concern and Russian warnings of a Chernobyl-like disaster.

The founder of North Korea's ruling dynasty, Kim Il-Sung, launched the country's nuclear programme in the 1950s, sending scientists and engineers to the Soviet Union for training.

With Soviet help, the complex at Yongbyon was established around 90 kilometres (60 miles) north of Pyongyang in the early 1960s and a Soviet-made two-megawatt reactor came on line in 1965.

For the next 15 years, North Korea slowly weaned itself off its initial dependence on Soviet technology and fuel and, in 1979, construction began on the five-megawatt reactor that is now back in the headlines.

The gas-cooled facility began operating in 1986. At the time, the North told the UN's International Atomic Energy Agency (IAEA) that its sole purpose was electricity generation.

But suspicions of a dual military purpose were immediately aroused, given that the reactor's design meant it could easily produce weapons-grade plutonium.

North Korea signed the nuclear Non-Proliferation Treaty (NPT) in 1985 but didn't allow any inspection of the Yongbyon facilities until 1992.

When IAEA inspectors eventually got access, their findings fuelled the dual-use suspicions, triggering a cat-and-mouse game that would be reprised many times in the decades to come, as Yongbyon took centre stage in the drama of North Korea's nuclear ambitions.

Moves in 1994 to remove spent fuel rods from Yongbyon triggered the first nuclear crisis with the United States.

The Pentagon drew up plans to bomb the facility but diplomacy involving former US president Jimmy Carter and others averted a clash and led to an eight-year shutdown.

Under a 1994 "Agreed Framework" deal with the United States, an international consortium started work on two proliferation-resistant light-water reactors.

The United States also provided an interim 500,000 tonnes a year of heavy fuel oil, although shipments were often delayed.

The deal collapsed in 2002 -- the year president George W. Bush labelled North Korea part of an "axis of evil" -- when Washington accused Pyongyang of running a secret programme to produce highly enriched uranium.

North Korea denied the charge but restarted Yongbyon, expelled IAEA inspectors and announced it was leaving the NPT.

In 2006, suspicions were turned into reality when North Korea conducted its first nuclear test.

But just four months later it reached a six-nation deal which promised energy aid and major diplomatic and security benefits in return for full denuclearisation.

Yongbyon was shut down in July 2007 and Pyongyang began disabling key plants there, publicly demolishing the plutonium reactor's cooling tower in 2008.

Six-party negotiations -- involving the two Koreas, China, Japan, Russia and the United States -- later stalled because of disputes about ways to verify the North's declared nuclear activities.

In April 2009, Pyongyang said it was leaving the talks, again expelled IAEA inspectors and announced it was resuming its nuclear weapons programme.

A month later it exploded its second nuclear device.

North Korea then announced its intention to build an indigenous experimental light-water reactor and, in 2010, revealed it was enriching uranium when it allowed foreign experts to visit a centrifuge facility at the Yongbyon complex.

The development fuelled international concern that the North was developing a two-track nuclear weapons programme of both plutonium and uranium bombs.

Pyongyang's see-saw strategy shifted again in February 2012 when, after talks with the United States -- it offered a moratorium on further nuclear and missile tests and on its uranium enrichment programme.

The deal was short-lived and the North carried out a successful long-range rocket launch followed by its third -- and most powerful -- nuclear test in February this year.

As military tensions soared on the Korean peninsula, Pyongyang announced in April a complete overhaul and upgrade of its Yongbyon facilities -- including the plutonium reactor.