The city suffered heavy bomb damage during World War II's "Birmingham Blitz". The city was also the scene of two scientific discoveries that were to prove critical to the outcome of the war.[82] Otto Frisch and Rudolf Peierls first described how a practical nuclear weapon could be constructed in the Frisch–Peierls memorandum of 1940,[83] the same year that the cavity magnetron, the key component of radar and later of microwave ovens, was invented by John Randall and Henry Boot.[84] Details of these two discoveries, together with an outline of the first jet engine invented by Frank Whittle in nearby Rugby, were taken to the United States by the Tizard Mission in September 1940, in a single black box later described by an official American historian as "the most valuable cargo ever brought to our shores".[85]
The city was extensively redeveloped during the 1950s and 1960s.[86][better source needed] This included the construction of large tower block estates, such as Castle Vale. The Bull Ring was reconstructed and New Street station was redeveloped. In the decades following World War II, the ethnic makeup of Birmingham changed significantly, as it received waves of immigration from the Commonwealth of Nations and beyond.[87] The city's population peaked in 1951 at 1,113,000 residents.[79]