On the 26 April 1986, the worst-man-made nuclear disaster had occured at the confluence of Rivers Pripyat and Dnieper, near the historic city Kiev, the capital of the Republic of Ukraine, situated about 130 km. away from the epicentre of the nuclear disaster. The capital was saved by the winds that carried radiation plume over it but failed to shower. But more than 3 million population of surrounding towns-Pripyat, Dnieper and the satellite town of Chernobyl within 30 km. of nuclear power stations, housing engineers, chemists, workers and their wives and children­had to be evacuated on war footing.

As the entire reactor structure was turned into inferno it could only be surmised by experts as what might have happened inside the critical mass of fission in the uranium fuel core. Under orders of President Gorbachev that the scientists must speak the truth, an official scientific chronology of the critical moments of the accident were made public. That disaster started within a few seconds between 0100 hours and 0124 hours of the 26th.

Power inside the reactor of 1,000 MW capacity RBMK (boiling-water, graphite-uranium reactor) was to be reduced to 200 MW thermal power for annual maintenance. As this low level of power could only be achieved by removing control rods from the core this led to xenon poisoning of the reactor. Because the 200 MW energy level was well below the designed required power. The operators were still hopeful that the system of safety would work somehow. But due to the low power the coolant flow rate through the core could not be raised to the designed level. This caused sudden reduction in steam formation leading to undue fall in pressure in the steam drums. To control falling rate of steam, the operators closed valve of a turbine generator bypass. Under the written rules this was dangerous situation demanding immediate shut down of reactor.

According to safety rules a minimum of fifteen control rods must always be present in the core. But the operators could not follow the computer modeling command that indicated that the number of control rods inside the reactor were only six, seven or eight— far less than half the designed safety requirement. Due to lack of sufficient flow of coolant in the fuel, core and not having sufficient number of control rods, the power suddenly began to rise faster than the automatic control rods could be lowered in. Since lowering of the control rods took upto 20 seconds the fatal few seconds delay led to steep rise in temperature in the core that produced a heat transfer crisis. Within three seconds (seconds) the reactor power surged to 530 MW and continued to rise exponentially. This led to intensive generation of steam inside the reactor splitting fuel channels and within 2-3 seconds at 0123.48 two explosions occured one after the other. Burning debris and flames shot into the air, and falling on the machine room started secondary fires all around the nuclear power complex.

‘China Syndrome’ of meltdown had taken place inside the reactor core. Thermal explosion and outbreak of fires in over thirty places were due to high-temperature and falling uranium core fragments on to the roofs of adjacent buildings. Hydrogen stores and disesl stocks had aided the volcano. In this burning-hell, the fire fighting took precedence over radiation protection. Different measures were required for different kinds of fire: water for cable rooms, gas for control rooms, and foam where the oil was ignited. The flames leaping out from the melting core of reactor No 4 were approaching towards the sister reactors nearby. Twenty-eight firemen valiently fought the catastrophic advances. But they did not return to receive the official awards.

The Russian scientists were faced with the problem of controlling the graphite fire in the reactor core. Its source had to be contained with heat-absorbent materials. Hundreds of army helicopters were deployed to drop the sacks of about 5000 tonnes of compounds of boron, dolomite, sand, clay and lead on the epicentre of the burning reactor. But the sky was covered with hot gases and radiation dust. Many pilots could fly only one sorty, some attempted 2 or 3 times, and almost all of them were contaminated and suffered untimely death. The Russian Air Force veterans of the Second World War heroically went in the task of air-dropping into the mouth of volcanic opening. But the posterity would remember their sacrifice at this high-tech scientific alter.

Inspite of all these giagantic efforts at controlling the fire the temperature into the core was 2000 degree Celsius and it was still a burning volcano. The most serious problem was how to stop burning lava from sinking underneath into the womb of the earth? and coming into contact with underground water? But now the foundation structure was collapsing under the weight 5000 tonnes. The Russians constructed a "cooling slab" platform beneath the sinking reactor. This consisted of a reinforced concrete slab incorporating a flat heat exchanger, a monolithic reinforced huge concrete slab sufficiently strong enough to support the dangerously sinking structure from the underneath.

The entire core and its burning surroundings were, bit by bit encased inside the 5000 tonnes of heavy material. This well-secured reactor was now provided with one metre thick concrete walls all around it. Isolated but burning inside, the entomed reactor had been named "sarcophagus" meaning ‘the flesh-eating stone-cofftn’ or tomb. It had been provided with a ventilation system with heavy duty filters and radiation monitoring devices. There is an 18 metre steel pipe, 10 centimeters in diameter, encasing measuring instruments for constantly monitoring temperature and radiation levels. Sarcophagus is meant to last not more that 50 years but the scientists are busy in working out more advanced scientific monitoring system that would keep this sarcophagus intact and safe for thousands of years. The estimated cost is $ 200 billions! But when would this currently holding up sarcophagus give way to another Chernobyl explosion? No one can tell. The fear is equally serious and the consequences would be catastrophic.

High geared machines involved in the fire fighting and radiation control operation had to be destroyed along with millions of tonnes of vegetables, fruits, milk and dairy products. Millions of milch and meat animals had to be destroyed. From Kiev, 250,00 children were evacuated. But to provide clean water to the region, more than 400 deep water wells had to be bored. Hundreds of helicopters and thousands of trucks had to be abandoned that could not be destroyed or cleaned up.

Resources : 1. Chernobyl : The Real Story, by Richard F. Mould (Pergamon Press, 1988)