[1] Shortly after the Leipzig L-IV atomic pile—worked on by Werner Heisenberg and Robert Döpel—demonstrated Germany's first signs of neutron propagation, the device was checked for a possible heavy water leak.
The burning uranium boiled the water jacket, generating enough steam pressure to blow the reactor apart.
[2][3] This happened after 20 days of operation when Werner Paschen opened the machine at the request of Döpel after blisters formed at the gasket.
[1] As glowing uranium powder shot to the 6-meter-high ceiling and the apparatus heated up to 1000 degrees, Heisenberg was asked for help but could not provide it.
They had achieved the first net neutron production of the German program, three years after the first such pile in history by Hans von Halban and colleagues in Paris.