Alan Fraser was a brilliant mathematical physicist who from the 1950s onwards played a leading role in the design of thermonuclear warheads through his distinguished work in the field of radiation hydrodynamics. He was Chief of Mathematical Physics at the United Kingdom Atomic Energy Authority in Aldermaston, Berkshire, from 1976 until 1983.

Alan Fraser was born in Ealing, West London, and won a scholarship to St Paul’s School where he showed an early talent for mathematics. He won a scholarship in mathematics and physics to Peterhouse, Cambridge, where he was awarded a first in part II of the mathematical tripos in 1943, completing the course in only 18 months. He was then summoned to work at the Admiralty Research Establishment during the war. In 1946 he was permitted to return to Cambridge, elected to the John Whitgift Scholarship, and completed his natural sciences tripos part II (physics) eight months later.

In 1950 the Ministry of Supply directed him to present himself at the Armament Research Establishment at Fort Halstead where he was to work on fluid mechanics. At Fort Halstead, the theoretical physics branch was housed in three wartime buildings, known as “the cowsheds”, at the top of Polhill in Kent. One of his colleagues was another talented Cambridge mathematician, Doris Jones, whom he married at Chevening in 1952.

He was swiftly promoted and moved to the new Aldermaston site in 1953 to concentrate on thermonuclear work for William Penney and John Corner, becoming involved in the early theoretical work on the principles of thermonuclear warheads.

By 1957 he was leading the radiation hydrodynamics group in the Theoretical Physics division at Aldermaston. His distinguished work on radiation hydrodynamics led to a Royal Society Paper entitled “Radiation Fronts”, communicated by Sir William Penney in January 1958.

Atomic weapons work by assembling and compressing a ball of metals with a powerful explosive like TNT. The outside blows out and the inside is driven in, so that plutonium in the middle undergoes a very quick fission chain reaction. Ordinary hydrodynamics models the implosion, the metals behaving like fluids in these extreme pressures.

The thermonuclear weapon works by channelling some of the energy of the plutonium explosion to a second assembly of uranium and hydrogen compounds, and must operate before this assembly is affected by the high explosive of the first. The energy transfer is by very high temperature radiation, and it generates pressure in the material surrounding the second assembly compressing it like a super-explosive, and initiating fission and nuclear fusion explosions. Radiation hydrodynamics is the mathematical modelling of the transfer and radiation-driven compression processes.

Fraser also published a number of external reports on this subject in the 1960s which have had a major influence on US research and are referred to in later textbooks by US authors. By this time he was already established with an outstanding reputation in the field of theoretical radiation physics.

Britain had successfully detonated four experimental thermonuclear warheads in the Grapple series of trials based at Christmas Island in the Indian Ocean by November 1957. The success of these tests not only provided a basis for a British nuclear deterrent but also facilitated the signing of the 1958 Bilateral Treaty for Co-operation on Nuclear Weapons with the US. Fraser, who was directly involved in thermonuclear warhead design, played an important role in the collaborative work which took place between the UK and the US following this agreement. He travelled regularly to the Los Alamos, Livermore and Sandia nuclear weapon laboratories and was a member of various joint working groups which were established in 1959 as the method of facilitating the two-way flow of information.

By April 1961 Fraser had been designated a group leader on thermonuclear weapon design. By August 1965 the “magic asterisk” had appeared alongside his name in the AWRE Organisation Chart showing that he was now “senior staff”. In the 1970s he led the work at AWE on laser fusion. This project led to much innovative work, both directly in terms of experiments and indirectly in terms of major advances in computer modelling, particularly in the area of hydrocodes.

In 1976 Corner retired and Fraser took over as Chief of Mathematical Physics at Aldermaston until his retirement in 1983.

He was a chartered mathematician and life Fellow of the Institute of Mathematics and its Applications, and a Fellow of the Institute of Physics.

He was a hugely talented, yet modest, mathematical physicist who looked after his younger staff with patience, encouragement and kindness and was highly respected as a scientist and as a leader.

Fraser was sometimes questioned about his role in nuclear physics and why he chose the life of “skill and science” as a scientific civil servant rather than devoting his formidable intellect to the academic world. He had an unshakeable belief in the importance of the nuclear deterrent as the surest way to peace and committed his working life to that end.

In his private life he was a cultured man and was extremely knowledgeable about classical music. He was a keen amateur botanist and was also actively involved in nature conservancy, with more than 25 years’ service on the area committee of the Hampshire Wildlife Trust, including a long spell as the area chairman of the Hampshire and Isle of Wight Naturalist’s Trust.

His wife, Doris, predeceased him in 2001. He is survived by a daughter.

Dr A. R. Fraser, mathematical physicist, was born on December 9, 1922. He died on November 24, 2008, aged 85