Hungarian-American physicist Leo Szilard conceived of the idea of the nuclear chain reaction while waiting for a traffic light in Bloomsbury, London.

Leo Szilard was a Hungarian-born physicist, biologist and inventor who made numerous important discoveries in nuclear physics and the biological sciences. He conceived the nuclear chain reaction in 1933, and patented the idea in 1936. In late 1939 he wrote the letter for Albert Einstein's signature that resulted in the Manhattan Project that built the atomic bomb, and then in 1945 wrote the Szilard petition asking president Harry S. Truman to demonstrate the bomb without dropping it on civilians. According to György Marx, he was one of the Hungarian scientists known as The Martians.

In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series or "positive feedback loop" of these reactions. The specific nuclear reaction may be the fission of heavy isotopes. A nuclear chain reaction releases several million times more energy per reaction than any chemical reaction.

Bloomsbury is a district in the West End of London, part of the London Borough of Camden in England. It is considered a fashionable residential area, and is the location of numerous cultural, intellectual, and educational institutions. Bloomsbury is home of the British Museum, the largest museum in the United Kingdom, and several educational institutions, including University College London and a number of other colleges and institutes of the University of London as well as its central headquarters, the New College of the Humanities, the University of Law, the Royal Academy of Dramatic Art, the British Medical Association and many others. Bloomsbury is an intellectual and literary hub for London, as home of world-known Bloomsbury Publishing, publishers of the Harry Potter series, and namesake of the Bloomsbury Group, a group of British intellectuals which included author Virginia Woolf, biographer Lytton Strachey, and economist John Maynard Keynes.