The arrest must be a devastating blow to Professor Lumu, who was overseeing Cren-K, one of two reactors at the Centre Regional d'Etudes Nucleaire de Kinshasa on the southeastern outskirts of the city. The reactors have operated in defiance of the US, which since the overthrow of dictator Mobutu Sese Seko in 1997 has been trying to have the facility shut down.

A few years ago Professor Lumu had scoffed at suggestions his reactors posed a risk to global security.

'Would I be sitting here if I did not think it was safe,' he asked, referring to the reactors just down the corridor from his office.

The Democratic Republic of Congo may have lurched from one crisis to another for the past 47 years, enduring looting, mutinies, insurrection and invasion but, Professor Lumu said, it was a safe place to run a nuclear reactor or two.

The Congo acquired its first reactor in the late 1950s, thanks to colonialist idealism and the US Atoms for Peace programme. The deal was something of a quid pro quo for the uranium mined at Shinkolobwe, about 1,000km southeast of Kinshasa, which had produced the uranium for the bombs dropped on Nagasaki and Hiroshima.

All Shinkolobwe's uranium was now controlled by the CIA, Professor Lumu said.

The idea of conducting nuclear research in the Congo was first dreamed up by a Belgian missionary, Luc Gillon, one of the founders of the University of Leopoldville. Gillon was instrumental in the Belgian colony acquiring a Triga reactor from an American manufacturer shortly before independence in 1960. The Mark I Triga was one of the first to be used anywhere. The Mark II was installed several years later and became a favourite plaything of Mobutu.

'Mobutu came here many times,' said Professor Lumu, showing giant photographs of Mobutu at the control panel of the Mark II pulse reactor.

Congo commentator Michela Wrong wrote in her book In the Footsteps of Mr Kurtz: 'The question of whether this potentially hazardous invention would be an appropriate inheritance for an unstable government terrified of its own mutinous army does not appear to have troubled the well-intentioned Monsignor Gillon.'

Professor Lumu, however, said Congo's nuclear research programme was a far more serious and credible matter than some neocolonial pursuit. It was also properly managed, he insisted.

The fact that in recent years the Cren-K building had been hit by a bomb and that two fuel rods had gone missing was no indication to the contrary, said Professor Lumu.

Yes, he admitted, a bomb had hit the building in 1997, but it was not so mysterious as the scant reports about the incident had suggested. The bomb had been among ordnance jettisoned by a military aircraft whose crew lost control as the plane flew over the university. It had occurred at the height of the insurrection against Mobutu, but it was an accident, he said.

Cren-K's track record in safeguarding its fuel rods was a problem, the professor admitted. News reports the following year said one fuel rod had gone missing, but in fact two disappeared. Italian police recovered one from the Sicilian mafia but Professor Lumu had no idea about the whereabouts of the other, one of 138 on the campus.

The issue of the 138 fuel rods has become more sensitive in recent years and the US Department of Energy wants the Commissariat General a l'Energie Atomique to hand them all over to prevent them being used to make 'dirty bombs'.

'We accepted that they could get the 58 spent fuel rods from the Mark I reactor, but not the rest,' said Professor Lumu. Giving up the 80 in the Mark II would mean shutting down the reactor. The commissariat would do so only 'if they give us a new one so that we can continue working on our research', Professor Lumu said.

The Americans have refused, Professor Lumu has held on to his fuel rods, and the matter has been referred back to the International Atomic Energy Agency in Vienna. Whether Professor Lumu's arrest last week is only the latest round in this negotiation remains to be seen.