Dawn of humanity’s most powerful weapon
August 6 and 9, 2017 marked the 72nd anniversary of the dropping of the nuclear bombs on Hiroshima and Nagasaki. The story behind the development of these bombs is something of an epic, complete with failures, success, joy and sombre reflection.
It began in 1939 when scientists Lise Meitner and Otto Frisch in Germany explained the process Frisch called nuclear fission and were the first to appreciate the potential of the vast amount of energy that could be produced. Alarmingly, as this was happening Adolf Hitler was about to declare war. There was a real risk that Hitler’s Germany would be the first country to use nuclear fission to make a bomb. This galvanised the Americans.
Lieutenant General Leslie Groves was given the responsibility of organising a team of scientists and engineers in an ambitious project to make an atomic bomb before the Germans. It was named The Manhattan Project, after the location of Groves’ office in central Manhattan. Groves had two months to organise the delivery of 1200 tonnes of uranium, appoint scientists to do the work, establish a uranium separation plant at Oak Ridge,Tennessee and find a remote site to test the bomb. He began by choosing physicist Robert Oppenheimer to manage the scientific effort and selected Los Alamos deep in the desert hills of New Mexico for the laboratory.
Technical problems began immediately. The useful form of uranium is called U-235 but it is only 0.7 per cent of the uranium present in natural uranium, the rest is U-238.
Separating U- 235 and U-238 is difficult because they are chemically identical and almost the same mass. The facility at Oak Ridge mainly used an electromagnetic process to separate the two literally, atom by atom. It was extremely slow and would take months before there was enough U-235 available for a single bomb.
The initial bomb design was called a gun-type fission weapon. Inside a metal tube a U-235 bullet was to be fired at a lump of uranium located at the other end. When the bullet hit, the combined mass would become super-critical, meaning that it would generate a runaway nuclear fission reaction – a nuclear explosion.
The bomb, called ‘‘Thin Man’’ because of its length, was constructed, but sufficient quantities of the U-235 from Oak Ridge were not yet available. Fortunately, in spring 1944, chemist Glenn Seaborg suggested that plutonium could be used instead. Plutonium can be made relatively easily using a nuclear reactor, so the reactor at Hanford in Washington was revamped to produce plutonium. But then, calculations showed that the guntype bomb wouldn’t work with plutonium, because plutonium pre-detonates and melts everything before a super-critical mass can be established. The solution was to make a metal sphere of about 2 metres diameter, put a chunk of plutonium in the centre and pack the rest of the sphere with explosives. Detonating the explosives would crush the core and trigger the nuclear explosion. This was called an implosion-type nuclear weapon.
The Americans were unaware at the time that amongst the workforce at Los Alamos there were two scientists spying for the Soviet Union, Klaus Fuchs and Theodore Hall. Both men gave detailed and regular updates to their Soviet masters of how the bomb was being made. (In 1950 Fuchs was imprisoned for 14 years. Hall was never charged).
Meanwhile, supplying the ‘‘Thin Man’’ bomb with U-235 continued at snail’s pace. After three years and $2 billion spent (equivalent to $30 billion today), no bomb had been tested. The test was given the codename ‘‘Trinity’’ and the spherical plutonium implosion bomb, nicknamed ‘‘The Gadget’’, was hoisted to the top of a 35 metre high steel tower – in case they dropped it they resorted to the primitive solution of covering the ground beneath the bomb in mattresses. Finally, on 16-July 1945 the button was pressed and the ‘‘The Gadget’’ bomb dramatically exploded at Alamogordo, south of Los Alamos. On August 6 the uranium gun-type bomb, ‘‘Little Boy’’,was dropped on Hiroshima directly killing 70,000 people, three days later a ‘‘Fat Man’’ plutonium implosion bomb was dropped on Nagasaki killing 40,000. The nuclear weapons age had arrived.