For the past four decades, he went by the name Sergei Alekseevich Gegechkori, a fake identity crafted by the secret policemen his father once commanded. 'They said I needed a new name to protect me from the rage of the people,' he recalls. 'My friends, though, all call me Sergo.'
And this is the name that links this jolly, roly-poly, 69-year-old with the most chilling figure of Soviet history.
He was named Sergo at birth in the Georgian capital of Tbilisi in honour of Sergo Ordzhonokidze, a Bolshevik who conquered the Caucasus for Communism but whose brutality shocked even Lenin into denouncing him as a thug.
But it is Sergo's full name - the one he hopes to get put in a new passport issued by the new independent state of Ukraine - that recalls the bloodiest episodes of all: Sergo Lavrentievich Beria.
His father was Lavrenti Beria, head of the NKVD from 1938 until 1946 and overall chief of perhaps the most ruthless security apparatus ever assembled until Stalin's and then his own death in 1953. After he died, owners of the Great Soviet Encyclopaedia were invited to paste over a glowing entry on Beria with a substitute text on the Bering Sea.
'I am not trying to rehabilitate my father. No one in that regime can be rehabilitated but things should be known,' says Sergo, who has just published his memoirs in Russian: My Father: Lavrenti Beria.
He describes life at home with a man usually remembered as a prolific murderer and insatiable womaniser: 'He liked history and loved books. He had a very good library. All of us were educated people.' A rocket scientist by training, Sergo remembers the family mansion on Vspolny Lane, where they lived after moving from Tbilisi to Moscow in 1938, as a sanctuary of civilised conversation. Vistors, he said, included the Cambridge spy, Kim Philby, the American nuclear scientist, Robert Oppenheimer, and Golda Meir, Israel's ambassador to Moscow. A frequent caller was Stalin's daugher, Svetlana, whom he remembers fondly as a lost little girl but whom he also criticises for turning against her father, whom Sergo never refers to as Stalin but always by the more cosily respectful Josef Vissarionovich.
He says his father always slept at home and rejects tales of epic lechery, although he does concede at least one child from an illicit liaison. 'Lots of old maids now claim they were his mistress. I know only one thing. In the early 1950s father told me: Life is very strange; you have a little sister.'
The Great Terror did sometimes impinge: 'It was very unpleasant to know they are listening when you are alone with your wife. I asked my father to do something. He said: 'You are a scientist, why don't you make something so they cannot hear what you do in your bedroom?' '
Sometimes, though, Sergo did the eavesdropping. He says he was flown to Tehran at the end of 1943 to help translate tape-recordings of bugged conversations between Roosevelt and Churchill at the first Allied meeting to which Stalin had been invited.
'I am fed up of reading rubbish about my father,' he says. 'He was not a monster. He was smart. He was soft.'
One thing he wants put straight is how his father died. The official version is that he was executed on 24 December 1953 for treason, counter-revolutionary conspiracy and other crimes. The case was heard in camera by a kangaroo court. Scholars are still mystified about what really happened.
Sergo insists his father died six months earlier, on the day Khrushchev moved to fill a power vacuum left by Stalin's death in March. 'The outer wall where my father had his study and bedroom had big holes from bullets. There had obviously been shooting. The window was broken and so was the door. We were standing outside for about 20 minutes when some people came out with a stretcher covered by a tarpaulin. They put it in an armoured car. Then one of our bodyguards shouted: 'Sergo, that is your father'.'
Arrested that night along with his mother, Nino, he was held first at a dacha outside Moscow and then at Lefortovo Prison. Freed a year later, they moved to Sverdlovsk, a city where a young building foreman called Boris Yeltsin was beginning his climb of the Communist Party hierarchy. Sergo resumed work on missile systems and later moved to Kiev.
Lavrenti Beria's widow died two years ago. Sergo went back to Georgia with his own son, also a scientist, to bury her. They had a talk with Eduard Shevardnadze about designing an anti-aircraft system.
Sergo then travelled alone to his father's birthplace in Merkheuli, a village in the war-ravaged region of Abhazia on the Black Sea. Four decades after Beria's death, though, the whereabouts of his grave is still secret. Along with Stalin, another Georgian but one buried in Red Square, Lavrenti Beria is Russia's nightmare. Also its excuse. 'Right up to Gorbachev they all kept saying the same thing: everything was well with the Party until those two Georgians made us all so unhappy.'