The man who designed Dounreay's famous ''golf-ball'' sphere has died at the age of 86. The 135ft-high steel sphere was built in the late 1950s to house the world's first fast-breeder reactor on the site of a former airfield on the north coast of Scotland.

Richard Brocklesby, a Londoner, got the commission in his position as chief architect to the UK Atomic Energy Authority, which was formed in 1954.

Scientists and engineers working on the fast-reactor programme wanted it housed in a building which could absorb the maximum impact, just in case the unthinkable happened and the novel type of reactor should actually explode.

Mr Brocklesby came up with the concept of a sphere, which did not have the weak points that the corners of a rectangular building would have had.

The main construction work, which involved thousands of steel plates being welded together, was undertaken by Scottish engineering firm Motherwell Bridge.

At peak, around 3000 people were employed on the construction of what was dubbed ''the dome of discovery'' and its associated buildings. It produced its first power in 1959.

Although there were problems, especially when some of the short, squat fuel elements swelled and buckled, the design worked reasonably well and provided valuable lessons for the design of the much larger Prototype Fast-Reactor, which opened in 1974.

By then the risks of a catastrophic explosion had been substantially cut with the use of long, narrow fuel rods, and the reactor was housed in a rock cavern, hanging bat-like from its roof at ground level.

Mr Brocklesby's blueprint was recognised as a masterpiece and has become one of the best-known icons of post-war industrial design.

In 1957, while the dome was still being built, he was nominated as Man of the Year by the Architect's Journal, while his drawings for Dounreay were displayed at the Royal Academy's summer exhibition in London in 1969.

Earlier Eagle comic had shown a dramatic cut away sketch, which made Mr Brocklesby's Dounreay work familiar to a generation of British youth.

Mr Brocklesby was educated at King's College School, Wimbledon. He received his architectural training at Chelsea College of Art and the Bartlett School of Architecture in London. After starting in private practice he joined HM Office of Works in 1937 and helped design military hospitals during the Second World War.

In the immediate post-war he was involved in rebuilding bomb-damaged London, receiving his baptism to the new atomic age in 1949, when commissioned to design Calder Hall at Sellafield, Cumbria.

Although this was the first power station in the world to produce electricity, the British public was not told its priority purpose was to provide enough plutonium, as a by-product of the nuclear fission, for Britain's atomic and hydrogen bomb programmes.

Mr Brocklesby became the chief architect to the authority on its establishment. He retired from the state-owned body in 1969, but continued in private practice for several years after that.

Yesterday, Dounreay's director, Dr Roy Nelson, paid tribute to Mr Brocklesby, describing him as an innovative architect.