Ten years ago, helicopters played a major part in preventing greater catastrophe at a nuclear reactor in Chernobyl.

Tony Booth/MINSK

ON 26 APRIL, 1986, the number four reactor at the Chernobyl nuclear power station in Ukraine, then part of the Soviet Union, began to melt down after a turbogenerator experiment went wrong. A sudden power surge caused the graphite core to burn fiercely - threatening to ignite the neighbouring number three reactor. A similar graphite fire had occurred at the Windscale nuclear-power station in the UK in 1957, but that fire was quenched by a massive wave of water. This option was not possible at Chernobyl because not enough water could be pumped into the reactor core - and too little would cause an even greater catastrophe.

The decision was taken, within 36h of the accident, to employ helicopters to drop a mixture of sand, lead, clay and boron directly on to the exposed reactor core.

The boron was to absorb the neutrons in the core and prevent the fire from re-igniting, and the lead was to stem the flow of radiation which was shooting 3,000ft (900m) into the sky. The sand, dolomite and clay were used to bind the mixture together.

Soviet air force and civilian helicopters from as far afield as Siberia, more than 3,000km (1,600nm) away, were scrambled in an effort to seal the cracked reactor. Five Mil helicopter types were used for the clean-up operation: the Mi-2 (Hoplite), Mi-6 (Hook), Mi-8 (Hip), the combat Mi-24R (Hind) and the heavylift Mi-26 (Halo). They were also used to provide liaison capabilities, for humanitarian missions and to drop supplies to the thousands of "liquidators" on the ground, assisting in the clean-up operation and preventing the fire and radiation from spreading

The versatile Polish-built Mi-2 light-utility helicopter fulfilled one of its military roles as a reconnaissance aircraft when used to measure radiation levels some distance from the main reactor. It was also used for messenger liaison, but was not flown in the reactor area until radiation levels had significantly diminished.

The 42,500kg Mi-6 heavylift helicopter, with its payload capability of 12,000kg was used to drop sand, lead and boron on to the cracked reactor. Debris, smoke and the reactor chimney all posed hazards for the pilots trying to reach the drop site. They were assisted by ground controllers who timed their approach paths and instructed the crews on exactly when to drop their loads.

Military Mi-8s were used to lower supplies to ground workers. Later, fitted with external spray systems, they helped drop a bonding mixture over the reactor area to prevent contaminated dirt from spreading. Aeroflot-supplied versions executed precise drops of the chemical in bulk form, using their own pilots who were trained for Arctic oil-pipe laying and fire-fighting control in the former Soviet Union.

The Mi-8's four-axis autopilot gives it added yaw, pitch and roll stabilisation under any flight conditions. This made it ideal for precision flying close to the exploded reactor.

The Mi-24R had been designed for nuclear, biological and chemical warfare, and was first identified by the West through its missions at Chernobyl. Col Etuev Vassilivich says: "The Mi-24R is specially designed for this kind of deployment. The nose-mounted machine gun can be replaced by a highly accurate dosimeter [radiation monitor], which is a standard variant on this type of combat helicopter. These Mi-24 variants were then used to assess radiation levels, take ground samples and find the safe paths needed to lead civilians out of the area."

The bulk of work at Chernobyl was carried out by Mi-26s, as only they could withstand the tremendous heat and radiation above the exposed reactor core for any length of time. All drop missions were monitored by closed-circuit cameras, standard equipment in this type, allowing the pilot to observe the target and slung payloads on a television screen.

This equipment gave the Mi-26 the edge over the Mi-6, as the former could be hovered over the crater for longer, executing drop loads with a 100% success rate. The Mi-6s could not hover and loads had to be dropped at speeds of about 110kt [60km/h] over the reactor. The Mi-6 crews had only a couple of seconds to complete their drop missions - achieving a success rate of about 50%.

Radiation dose

Vassilivich flew from 27 April until 1 May, by which time he had received his permitted radiation dose. This, for all helicopter crews, was 25 roentgens in three days. Once at this level, crews were never allowed to fly over Chernobyl again. "There were 30 Mi-26s altogether, seven from my unit," says Vassilivich, "and between sunrise and sunset we flew missions from a sector 15km from the reactor. We could not fly at night as it was too dangerous going so close to the rector without adequate light."

On the first day, crews dropped sand through the open door of the helicopter, one sack at a time, but then technicians developed a way of using containers attached to small brake parachutes to make the job more effective. The new containers were dropped from a specially mounted quick-release mechanism from a height of 1,650-2,000ft. Up to eight sacks at a time were then able to be dropped from a net slung beneath the helicopter.

On the first day of operations, 96 missions were flown. On day two, this number had almost doubled, to 186. The Mi-26 can carry an internal or external maximum payload of 20,000kg - but only about 2,000-3,000kg could be dropped at a time. A full payload dropped from a safe hovering position would have further damaged the crippled reactor and run the risk of more leakage. The full crew on the first missions of two pilots, flight engineer and navigator was soon cut to two pilots and the flight engineer. The other crew members were quickly considered unnecessary for these types of missions.

From 28 April, Mi-26 crews were instructed to also drop lead on the open reactor. This was first delivered as pellets and shrapnel and later as heavy sheets. The crews first used some of the lead to cover the flooring in their helicopters - thus cutting down their own exposure levels by two and a half times.

Col Oleg Chichcov (now retired) was serving as an Mi-26 instructor in Siberia when the disaster happened. By 3 May, 1986, he had trained ten new crews for Mi-26 operations over Chernobyl, hand-picked from existing Mi-6 personnel. Chichcov says: "The problem was not training the Mi-6 crews to fly the Mi-26, as it is a relatively easy conversion, only taking 4h flying time, but it was harder training crews to deal with its external payload capabilities." He arrived near Chernobyl on 4 May.

Chichcov adds: "Helicopters were flying from four or five sites at different safe distances around the reactor. At least ten Aeroflot Mi-8s flew from my base which was further back than the others." The Mi-8s were loaded with many litres of the bonding agent to douse the whole Chernobyl site after the military helicopters had covered the reactor with their payloads. So much was needed that a specially constructed railway was laid from the main track to the improvised base. This enabled the substance to be piped straight into tankers and then directly into the Aeroflot Mi-8s.

In all the dangerous missions flown in those critical two weeks, only one helicopter was lost. Chichcov recalls: "There were very large cranes around the reactor, and when the blast occurred some of them were swinging free. I was assigned a mission to fly over this area, but I refused to do so until the cranes were fixed."

The crew that relieved Chichcov agreed to take the mission. Unfortunately, it took the helicopter too close. The wind carried the crane and - coming out of the Sun - the crew did not see its arm swing into them. The arm clipped the machine's main rotor. It shattered, killing all the crew instantly when the helicopter hit the ground.

contaminated helicopters

Helicopters were washed down at the end of each day's operations, but large amounts of radioactive debris had been sucked into the engines. In addition, the undersides of each machine became highly contaminated. Vassilivich says: "Some of the first helicopters used were grounded on 1-2 May and in just those two days the grass around them had turned yellow from the radiation from the aircraft."

By the time the main clean-up operations ended, on 13 May, 1986, more than 5,000t of mixture had been dropped on to the reactor. A fire-fighting tactic which had never been used before was pronounced successful to the world's press in Vienna on 13 May , but helicopters continued to be flown on missions over Chernobyl until the end of 1986.

All machinery and vehicles used in those crucial two weeks were dumped in purpose-built machine graveyards to await destruction and burial, as they were considered to be too contaminated for continued use.

Helicopters were no exception. The Air Force Ministry in Belarus claims that, since 1986, every machine used has now been destroyed and buried as radiation damage caused to the engines and lower fuselages deemed them unfit to ever be flown again.

Source: Flight International