Rainer Karlsch said that new research in Soviet and also Western
archives, along with measurements carried out at one of the test
sites, provided evidence for the existence of the weapon.
"The important thing in my book is the finding that the Germans had an
atomic reactor near Berlin which was running for a short while,
perhaps some days or weeks," he told the BBC.
"The second important finding was the atomic tests carried out in
Thuringia and on the Baltic Sea."