The graphite in Reactor No 4 had been burning for almost 24 hours when the Chernobyl Commission decided the only way to extinguish the fire was to smother it. The scientists suggested sand, boron and lead, to absorb radiation and cool the melting core - 4,000 tons would do it, dropped into the blazing reactor from the air. On the afternoon of the 27th, two Mi-8 helicopters from Kiev began the first of hundreds of firefighting sorties. The pilots navigated through a forest of pylons surrounding the power station to hover 100 metres above the burning building, and, aiming by eye, dropped individual bags of sand from the helicopters' open doors. The radiation directly over the reactor was such that the pilots soon began being sick in the air; eventually they started flying in respirators, and sliding lead panels under their seats. By 1 May, they had dropped 4,450 tons of sand into the reactor.
But on 2 May, the engineers and physicists at Chernobyl made a horrifying discovery: the temperature of the core and the volume of radionuclides rising from it were both increasing. They suspected that the whole helicopter operation had been a terrible mistake: the sheer weight of everything they had dropped on the reactor from the air - including 2,400 tons of lead - had not only caused structural damage but was pressing the hot reactor core against its concrete base. And if the uranium reached meltdown temperature - 2,900C -a single sphere of molten fuel would burn through the concrete foundations of the reactor building, and keep going until it reached the water table. At that moment, there would be another explosion, exponentially more devastating than the first; the three remaining reactors would be destroyed in a nuclear blast that would render Ukraine, Belarus and Russia uninhabitable for decades to come.
'That was the most terrifying thing,' says Veniamin Prianichnikov. 'We were petrified of meltdown, walking around like zombies.'
A plan was devised: to freeze the Earth around the reactor with liquid nitrogen, and then build a heat exchanger in the ground beneath it to cool the core and prevent meltdown. Prianichnikov himself was sent in with temperature and radiation probes to discover how long they had before the core burned through the two metres of concrete foundations; meanwhile, miners were summoned from the coalfaces of Donetsk and the subway projects in Kiev to dig tunnels beneath the reactor. The scientists feared that pneumatic drills could disturb the foundations of the reactor, so they worked with hand tools, in conditions where wearing protective clothing was practically impossible, amid extraordinary fields of radioactivity. To freeze the ground, all the liquid nitrogen in the western Soviet Union was sent to Chernobyl: when it didn't arrive quickly enough, director Brukhanov received a late-night telephone call from the minister in charge of the operation. 'Find the nitrogen,' he was told, 'or you'll be shot.'
On 10 May, the fire finally subsided; it now seems possible that the graphite simply burnt itself out. The nitrogen was found, and the subterranean heat exchanger built, but by mid-May the temperature of the core had dropped to 270C; the exchanger was never even turned on. 'The miners died for nothing,' says Prianichnikov. 'Everything we did was a waste of time.'
