The pace at which Richard Moore and others of scientific or technical bent were sent out — often after only rudimentary training — to defuse unexploded bombs and mines indicates the severity of the problem facing the country in the spring of 1940.

Even before the fall of France in June, the Luftwaffe began a programme of bombing British ports and mining their approaches. Many bombs and mines remained a threat because they did not explode, either by fault or design.

German non-contact mines triggered by a ship’s magnetic field posed a sinister challenge — they could not be swept by conventional methods — so an Admiralty department was formed to design counter-measures. To prevent their secret mechanism falling into British hands, the mines were fitted with self-destruction devices in case they accidentally fell on land.

Confident that this mechanism was infallible, on the night of September 16-17, 1940, the Luftwaffe dropped a number of the mines by parachute on London — no doubt seeking to inflict massive damage. In the event, however, several of the self-destruction devices did not work and the mines failed to explode.

Moore, then a sub-lieutenant RNVR, had joined the Admiralty mine counter-measures department from the cruiser Effingham, where he had been serving as assistant torpedo officer. Immediate action was required on the morning of September 17, and he volunteered to join the head of department, Lieutenant-Commander Dick Ryan, in dealing with unexploded mines in Edmonton and Walthamstow. The two men had only a theoretical understanding of the self-destruction mechanism and none at all of the magnetic triggering device.

The first mine examined lay at an angle of 30 degrees with its nose buried in the ground, close to an Army headquarters. They were informed that ticking had been heard from the mine but it was silent when they arrived. This matched their theory that the self-destruction mechanism had a 22-second delay clock. However, it was impossible to know at what point this particular clock had stopped until it was extracted from the mine.

Their method of making safe the mine was simple — provided it could be completed without restarting the clock. The three-inch ring closing the fuse aperture had to be unscrewed and a length of cord attached to the fuse to enable it to be hauled out from a safe distance behind cover and exploded harmlessly on the ground. The operation was successfully completed on the first three mines.

With this experience to hand, Ryan decided to form two teams, each of an officer with a chief petty officer assistant, to deal with remainder. Four mines fell on Dagenham on the night of September 20-21, but only one had exploded. While Ryan and his assistant dealt with one on an airfield, Moore and Chief Petty Officer George Wheeler accompanied police officers to the site of the remaining two.

The first had fallen on a road outside a factory, preventing work. Moore saw that impact on striking the road had distorted the fuse ring so that it could not be unscrewed. He borrowed a drill from the factory and drilled out two gaps on opposite sides of the ring so that it broke into halves, allowing extraction of the fuse.

He was in the process of removing the mine’s magnetic trigger when Ryan arrived. Having been assured that Moore’s was “safe” he went on to examine the fourth mine. Following on shortly afterwards, Moore was within 200 yards of the site when there was a massive explosion ahead. Lieutenant-Commander Ryan and his assistant, Chief Petty Officer Reg Ellingworth, were both killed.

For his gallantry and “undaunted devotion to duty” in disarming five of the magnetic mines, with which he and Ryan had only a nodding acqaintance, Moore was among the first recipients of the George Cross, instituted by King George VI and announced by him in a broadcast to the nation on September 23, 1940, two days after the deaths of Lieutenant- Commander Ryan and CPO Ellingworth. The decoration recognises “heroism in circumstances of extreme danger” by civilians or members of the armed services for actions when military decorations for gallantry would not normally be awarded. It ranks second only to the Victoria Cross.

Lieutenant-Commander Ryan and CPO Ellingworth were both awarded the George Cross posthumously and Moore’s assistant, CPO George Wheeler, received the British Empire Medal.

Richard Valentine Moore was born in 1916, the only son of Randall and Ellen Moore. He was educated at the Strand School and London University, where he took his BSc in engineering. After gradating in 1936 he joined the County of London Electricity Supply Company, in which he worked until the outbreak of war in 1939. When war was declared he joined the RNVR.

Moore served with the special mines counter-measures section of the Admiralty until he was appointed torpedo officer of the light cruiser Dido in the Mediterranean in February 1942. He saw action in support of the Eighth Army along the coast of the Western Desert and during the disastrous Operation Vigorous — when Rear-Admiral Sir Philip Vian tried to fight a relief convoy through to Malta from the Eastern Mediterranean in June 1942. He then served in support of the landing forces in Sicily, Salerno and Anzio in 1943. During the final year of the war he served as deputy director of torpedoes and mining on the Admiralty delegation in Washington.

After leaving the Royal Navy with the rank of lieutenant-commander in 1946, he joined the Atomic Energy Research Establishment at Harwell, where he worked until 1953, when he went to the Department of Atomic Energy, Risley.

Later that year he became involved in the design and construction of Britain’s first nuclear power station, Calder Hall, which opened in 1956. He was appointed chief design engineer in 1955.

From 1958 to 1961 he was director of reactor design for the United Kingdom Atomic Energy Authority, 1958-61, and he was managing director of its reactor group, 1961-76. He was also a member of the UKAEA from 1971 to 1971 to 1976.

Moore was appointed CBE in 1963 and was the Faraday Lecturer in 1966.

He married Ruby Edith Fair in 1944. She predeceased him. He is survived by two of their three sons.

Richard Moore, GC, CBE, wartime bomb disposal officer and nuclear industry scientist, was born on February 14, 1916. He died on April 25, 2003, aged 87.