He had a leading role in the development of High Explosive Research, Britain's clandestine nuclear programme that started in 1942 during the Second World War which produced the first British atomic bomb in 1952.

As the head of the British delegation working on the Manhattan Project at Los Alamos Laboratory, Penney initially carried out calculations to predict the damage effects generated by the blast wave of an atomic bomb.

[3] He accepted a Commonwealth Fund Fellowship and travelled to the United States, where he became foreign research associate at the University of Wisconsin–Madison, studying under John H. Van Vleck, and was awarded a Master of Arts degree.

He was a guest at Robert Oppenheimer's ranch in New Mexico, saw Babe Ruth play baseball, and was a spectator at the 1932 Los Angeles Olympics.

[1] He changed his mathematical career to physics, and conducted a thorough research and theoretical investigation into the structure of metals and the magnetic properties of crystals with John Lennard-Jones.

[4] In 1935, Penney submitted his final thesis, which contained the fundamental work in the applications of quantum mechanics to the physics of crystals.

[6] With Royal Navy engineer officers Penney designed and supervised development of Bombardon breakwaters, steel structures that formed part of the Mulberry harbours that were placed off the Normandy beaches after the D-Day invasion.

The August 1943 Quebec Agreement provided for British support of the American Manhattan Project, which aimed to develop atomic bombs.

[7] Over the objections of the Admiralty and Imperial College, Penney was sent to join the team of British scientists at the Manhattan Project's Los Alamos Laboratory in New Mexico, where expertise on explosions and their effects was in demand.

Fellow Manhattan Project scientist Rudolf Peierls recalled that:Soon after his arrival he gave a talk about the effect of blast waves on people, including many gruesome details to which his American audience was not accustomed.

[12] Five days later, Penney gave a presentation on the results of the test, during which he predicted that the bomb would level a city of three or four hundred thousand people.

[13] The following month Penney went to Tinian Island as part of Project Alberta, the group of scientists and military personnel that assembled the atomic bombs.

Along with Royal Air Force (RAF) Group Captain Leonard Cheshire from the British Joint Staff Mission in Washington, he represented the United Kingdom.

Big Stink missed its rendezvous with the bomber Bockscar, so they witnessed the flash of the Nagasaki detonation from the air at too great a distance to photograph the fireball and the target was obscured by clouds.

[22][23] In the words of the Foreign Secretary, Ernest Bevin, "We've got to have this thing over here, whatever it costs ... We've got to have the bloody Union Jack flying on top of it.

He gathered 34 senior members of his fledgling team in the library at the Royal Arsenal and gave a two-hour talk on the principles of how to build an atomic bomb.

[32] On 3 October 1952, under the code-name "Operation Hurricane", the first British nuclear device was successfully detonated off the west coast of Australia in the Monte Bello Islands.

The decision to develop a British hydrogen bomb meant that certain scientific information was urgently required and Maralinga was not yet ready, so a second series of tests, Operation Mosaic, was conducted in the Monte Bello Islands in 1956.

[37] Penney was aware of the public relations issues associated with the tests, and made clear-speaking presentations to the Australian press.

Before one series of tests, Lord Carrington, the High Commissioner of the United Kingdom to Australia, described Penney's press presence: "Sir William Penney has established in Australia a reputation which is quite unique: his appearance, his obvious sincerity and honesty, and the general impression he gives that he would rather be digging his garden – and would be, but for the essential nature of his work – have made him a public figure of some magnitude in Australian eyes".

The Prime Minister, Harold Macmillan, hoped to convince the US to change the McMahon Act, which prohibited sharing information even with the British, by demonstrating that the UK had the technology to make a thermonuclear weapon (an H-bomb), and he put Penney in charge of developing this bomb.

[41] The subsequent successful development of British thermonuclear weapons, which coincided with the Sputnik crisis led to the amendment of the McMahon Act, and the re-establishment of the nuclear Special Relationship with the United States.

Penney continued as the government's advisor on arms control and participated in the preliminary discussion leading up to, and the signing of, the Partial Test Ban Treaty in Moscow in July 1963.

[43] Nuclear development was transferred from the Ministry of Supply (MoS) to the newly formed United Kingdom Atomic Energy Authority (UKAEA).

When Sir John Cockcroft left in 1959 to become the master of Churchill College, Cambridge, Penney succeeded him as the Member for Scientific Research.

Penney's management style was to avoid confrontation and seek consensus whenever possible, and the role of the UKAEA in the nuclear power industry was an advisory one.

[56] He was made a life peer, taking the title Baron Penney, of East Hendred in the Royal County of Berkshire on 7 July 1967,[57] and was awarded the Order of Merit in 1969.

[38] Jim McClelland broadly accepted Penney's view but anecdotal evidence to the contrary received wide coverage in the press.

[61] His leadership of the team that exploded the first British hydrogen bomb at Christmas Island was instrumental in restoring the exchange of nuclear technology between Britain and the US in 1958,[40] and he was credited as playing a leading part in the negotiations which led to the Partial Test Ban Treaty in 1963.

[62] His Kronig-Penney model for the behaviour of an electron in a periodic potential is still taught and used today in solid-state physics and is used to explain the origin of band gaps.