https://en.wikipedia.org/wiki/Lise_Meitner

Sir Marcus Laurence Elwin “Mark” Oliphant AC KBE FRS FAA FTSE (8 October 1901 – 14 July 2000) was an Australian physicist and humanitarian who played an important role in the first experimental demonstration of nuclear fusion and also the development of nuclear weapons.
Born and raised in Adelaide, South Australia, Oliphant graduated from the University of Adelaide in 1922. He was awarded an 1851 Exhibition Scholarship in 1927 on the strength of the research he had done on mercury, and went to England, where he studied under Sir Ernest Rutherford at the University of Cambridge’s Cavendish Laboratory. There, he used a particle accelerator to fire heavy hydrogen nuclei (deuterons) at various targets. He discovered the nuclei of helium-3 (helions) and tritium (tritons). He also discovered that when they reacted with each other, the particles that were released had far more energy than they started with. Energy had been liberated from inside the nucleus, and he realised that this was a result of nuclear fusion.
Oliphant left the Cavendish Laboratory in 1937 to become the Poynting Professor of Physics at the University of Birmingham. He attempted to build a 60-inch (150 cm) cyclotron at the university, but its completion was postponed by the outbreak of the Second World War in Europe in 1939. He became involved with the development of radar, heading a group at the University of Birmingham that included John Randall and Harry Boot. They created a radical new design, the cavity magnetron, that made microwave radar possible. Oliphant also formed part of the MAUD Committee, which reported in July 1941, that an atomic bomb was not only feasible, but might be produced as early as 1943. Oliphant was instrumental in spreading the word of this finding in the United States, thereby starting what became the Manhattan Project. Later in the war, he worked on it with his friend Ernest Lawrence at the Radiation Laboratory in Berkeley, California, developing electromagnetic isotope separation.
After the war, Oliphant returned to Australia as the first Director of the Research School of Physical Sciences and Engineering at the new Australian National University (ANU), where he initiated the design and construction of the world’s largest (500 megajoule) homopolar generator. He retired in 1967, but was appointed Governor of South Australia on the advice of Premier, Don Dunstan. He assisted in the founding of the Australian Democrats political party, and he was the chairman of the meeting in Melbourne in 1977 at which the party was launched. Late in life he watched his wife, Rosa, suffer before her death in 1987, and he became an advocate for voluntary euthanasia. He died in Canberra in 2000.
https://en.wikipedia.org/wiki/Mark_Oliphant

The Frisch–Peierls memorandum was the first technical exposition of a practical nuclear weapon. It was written by expatriate German physicists Otto Frisch and Rudolf Peierls in March 1940 while they were both working for Mark Oliphant at the University of Birmingham in Britain during World War II.
The memorandum contained the first calculations about the size of the critical mass of fissile material needed for an atomic bomb. It revealed for the first time that the amount required might be small enough to incorporate into a bomb that could be delivered by air. It also anticipated the strategic and moral implications of nuclear weapons.
It helped send both Britain and America down a path which led to the MAUD Committee, the Tube Alloys project, the Manhattan Project, and ultimately the atomic bombings of Hiroshima and Nagasaki.
https://en.wikipedia.org/wiki/Frisch%E2%80%93Peierls_memorandum

Sir Rudolf Ernst Peierls, CBE (/ˈpaɪ.ərlz/; German: [ˈpaɪɐls]; 5 June 1907 – 19 September 1995) was a German-born British physicist who played a major role in the Manhattan Project and Tube Alloys, Britain’s nuclear programme. His obituary in Physics Today described him as “a major player in the drama of the eruption of nuclear physics into world affairs”.
Peierls studied physics at the University of Berlin, at the University of Munich under Arnold Sommerfeld, the University of Leipzig under Werner Heisenberg, and ETH Zurich under Wolfgang Pauli. After receiving his DPhil from Leipzig in 1929, he became an assistant to Pauli in Zurich. In 1932, he was awarded a Rockefeller Fellowship, which he used to study in Rome under Enrico Fermi, and then at the Cavendish Laboratory at the University of Cambridge under Ralph H. Fowler. Due to Adolf Hitler’s rise to power in Germany, he elected to not return home in 1933, but to remain in Britain, where he worked with Hans Bethe at the University of Manchester, then at the Mond Laboratory at Cambridge. In 1937, Mark Oliphant, the newly-appointed Australian professor of physics at the University of Birmingham recruited him for a new chair there in applied mathematics.
In March 1940, Peierls co-authored the Frisch–Peierls memorandum with Otto Robert Frisch. This short paper was the first to set out that one could construct an atomic bomb from a small amount of fissile uranium-235. Until then it had been assumed that such a bomb would require many tons of uranium, and consequently was impractical to build and use. The paper was pivotal in igniting the interest of first the British and later the American authorities in nuclear weapons. He was also responsible for the recruitment of his compatriot Klaus Fuchs to work on Tube Alloys, as the British nuclear weapons project was called, which resulted in Peierls falling under suspicion when Fuchs was exposed as a spy for the Soviet Union in 1950.https://en.wikipedia.org/wiki/Rudolf_Peierls

Otto Robert Frisch FRS (1 October 1904 – 22 September 1979) was an Austrian physicist. With Lise Meitner he advanced the first theoretical explanation of nuclear fission (coining the term) and first experimentally detected the fission by-products. Later, with his collaborator Rudolf Peierls[1] he designed the first theoretical mechanism for the detonation of an atomic bomb in 1940.https://en.wikipedia.org/wiki/Otto_Robert_Frisch