SUKHUMI, Georgia (Reuters) - Behind a thicket of weeds and broken window panes, one of the former Soviet Union's dark secrets is the laboratory where captured German scientists worked to build an atomic bomb for Josef Stalin.
The Sukhumi Institute still exists, in a state of limbo. Limping along under semi-siege in Abkhazia, a breakaway region of Georgia whose existence the rest of the world does not recognize, its Cold War past has been all but forgotten.
Once, around 250 German specialists lived here with their families and built centrifuges to separate uranium isotopes. Now a money-making sideline for the few scientists who keep the institute's research going is designing household heaters.
Deputy director Vladimir Kunitsky does have ambitious hopes for the institute, which was nearly wrecked by the separatist war that engulfed this region on the shores of the Black Sea after the Soviet Union collapsed.
He would like to turn part of the former bomb laboratory into a sanatorium, combining cutting-edge treatments using radioactive sources and a beautiful location a short walk from the Black Sea.
"We are preserving some kind of potential," he said in his bare office, where the paint is peeling off the walls.
Abkhazia's unrecognized status, and the suspicion with which many countries regard it, will make realizing his plan difficult. In the late 1990s, the institute was a focus of international concern with reports that radioactive fuel had gone missing, although a United Nations inspection concluded all the nuclear material was in order.
Kunitsky, who first came to Sukhumi as a small boy when his serviceman father was assigned to keep an eye on the Germans, thinks the institute has a duty to keep going: "We are not letting it die out."
Set in the lush grounds of an estate built by the brother of Russia's last tsar, the institute bears witness to Abkhazia's past as a chic holiday destination for the Russian elite.
Its neo-classical concert hall is a gutted shell after an incendiary round landed on it. Nearby, two huge lion statues stand guard outside a laboratory building with ceilings that have caved in.
Six decades ago, the institute's residents arrived under very different circumstances. After World War Two, German scientists were needed to help Moscow compete in the Cold War.
As the war drew to an end, Soviet intelligence homed in on two individuals: Manfred von Ardenne, the son of a Prussian army officer and head of an electron physics laboratory in Berlin, and Gustav Hertz, who had won Nobel Prize in 1925 for his work on experimental physics and was director of a research laboratory at the Siemens Company.
"They were invited," said Kunitsky. "But they were invited in such a way that they could not refuse."
The story told by institute staff is that the Germans were taken to Moscow and brought before Lavrenty Beria, the feared head of Stalin's secret police who was also in charge of the nuclear program.
He gave them a choice of working in Siberia, the Crimean Peninsula or Abkhazia, a region of Soviet Georgia where Beria and Stalin both had holiday homes. They chose Abkhazia.
"They were set the task of building a nuclear bomb," said Alexander Chachakov, another deputy director.
"We cannot say that without us they (the Soviet Union) would not have made the bomb ... We were not the only people working on this," he said. "But we achieved our objectives."
Once installed in Sukhumi with their families the Germans were set to work on the centrifuges that are a vital step in creating the fissile material needed for a bomb.
For nearly a decade, the institute became their home. Hertz had a house built to resemble his home in Germany, and they were assigned servants, who doubled as secret police informers.
They grew vegetables in their gardens, laid on dances and concerts and took part in sports competitions.
By the early 1950s, their Soviet masters decided the Germans had served their purpose, and started to let them go home. By 1955 the last of the Germans was gone, though one Austrian who had married a local woman stayed on.
Today, the only evidence of their presence is a cluster of eight graves in Sukhumi's municipal cemetery, where Germans who died during their stay were buried.
The names have been worn away from the rough concrete gravestones except on one, which reads, in German: "Gustav Treff, born 22.5.11, died 14.12.51."
The few remaining scientists are too preoccupied with surviving to dwell on the past.
After the Germans left, their Soviet understudies thought up civilian uses for nuclear power. Their speciality was mini-reactors to generate electricity in remote places. One was used on a space satellite.
Then the Soviet Union began to disintegrate, and Abkhazia was tipped into ethnic conflict. The Abkhaz decided they did not want to be part of newly independent Georgia.
At one point, when Georgian forces fought their way back into Sukhumi, institute staff formed patrols to stop marauders stealing their equipment.
When large-scale fighting ended in 1994, Abkhazia's separatists had driven out the Georgians but they were isolated, under blockade and not recognized by any state. The separatists still shoot down Georgian spy planes from time to time.
Many of the institute's scientists left for Israel, the United States and Germany. Those who stayed are matter-of-fact about living under siege.
"Nothing terrible about that," said Kunitsky. "The whole republic lives in difficult conditions, and not just us."
Editing by Sara Ledwith