Site Records

Site Name: Chernobyl

Sub Brit site visit 1.6.2006 & 12.4.2007

CHERNOBYL & PRIPYAT
The Chernobyl power station, officially named the V. I. Lenin Power Station, was still an ongoing construction project at the time of the 1986 accident. It was originally intended to have twelve reactors and would have been one of the largest power stations in the then Soviet Union. At the time of the accident in 1986, the plant had four working reactors, another two under construction; with one at 85% completed and another four on the drawing board. All of the reactors, referred to as ‘Units’, were of the RBMK type; a graphite-moderated, light water-cooled design peculiar to the Soviet Union. The design required an electricity supply to drive the motors of the control rods and also to operate the pumps for the cooling water; an aspect which would lead to the disastrous experiment in 1986.

Photo: Unit 5 was still under construction at the time of the accident
Photo by Jane MacGregor

The nearest existing town was Chernobyl, this is the Russian name, the Ukrainian version is Chornobyl. Although this is now preferred, in the context of this article I will use the Russian names as these were in use at the time of the accident. Work began on the power station in the early 1970s and also on a state of the art new town nearby to house the workers. This town, Pripyat, was to be a splendid example of Soviet architecture and was indeed a very pleasant place to live in the 1970s and 80s.

Chernobyl had already suffered one accident in 1982. Unit 1 had been shut down for maintenance and when it was re-started, the valves to some of the water pipes were inadvertently left closed. Some of the fuel melted and there was a leak of radiation, but this was not a true meltdown and other than engineers repairing the damage, there was little exposure of any personnel to radiation. The unit was eventually repaired and put back into service.

Pripyat was a well-planned town with plenty of open spaces. The Cultural Centre, the focal point, had a theatre, cinemas and sports facilities and just outside was the permanent amusement park with a Ferris wheel, dodgems and other attractions. The town was linked to Kiev by rail and also by water and the residents were only a short walk away from the forest and rivers. Everything was new, the shops were well-stocked by Soviet standards and most of the residents were very happy living there.

Photo: Pripyat before the accident. The Olympic sized swimming pool can be seen in the background.
Photo from Prypiat.com

THE MELTDOWN ON 26th APRIL 1986

Unit 4 was to be shut down for routine maintenance, so this was a good opportunity to carry out a test on the turbines driven by the power produced by the reactor. It was not certain whether, should there be a failure, the turbines could generate enough power as they ran down to keep the cooling water pumps working. There was a delay of almost a minute between the power supply failing and the start-up of the diesel generators and this had always been a worry to the engineers. Modifications had been made to the turbines which should ensure that the pumps could be run from them as they ran down. The reactor would need to be run at low power for this test to take place. This of itself was fairly straightforward, but another potential problem was the safety system. The decrease in the water supply or start-up of the back-up generators could trip the safety sensors and give a false reading of an accident, causing the emergency core-cooling system to activate and shut the reactor down automatically.

The reactor seen from a helicopter on 3rd May 1986 with a column of ionized air or perhaps smoke from the graphite fire.

Unit 4 after the explosion (click to enlarge)

The chain of events that were to culminate in the world's worst nuclear accident began at 14.00 on 25 April 1986, when the emergency core-cooling system was switched off for the above reason. The test was scheduled to take place then; however a grid engineer at Kiev telephoned the power station and informed them that the grid required the power from the turbine that was about to be run down. The test was postponed, and the grid engineers gave permission for the turbine to go off-load at 23.00 that evening. A few hours later, at around 01.20 on 26 April 1986, came the disaster. The reactor had been running for hours at only half power; when operating at a reduced capacity the iodine and xenon gases generated as a by-product of the fission process also had a damping action. The operators continued to

reduce the power, but these gases were now interfering with the chain reaction. The reactor was in an ‘iodine well’, attempting to raise the power again from this state is dangerous and the best things to do were either to shut it down, or keep it running for another twenty-four hours to allow the xenon and iodine to decay; this would mean postponing the already delayed test another day. Instead, they attempted to raise its power and withdrew more control rods.

Less than twenty of the two hundred control rods were now in the core. The falling power was causing the pressure to fall in the steam drums, where the water coolant was separated into steam, which went on to drive the turbines, and water to be re-circulated through the core. This fall in pressure in the steam drums would also automatically trip the shutdown mechanism. The operators, under the orders of a senior engineer who was anxious to complete the test, overrode this shutdown mechanism as well. A few minutes later the power in the reactor began rising alarmingly; an operator pushed the

The control room after the accident

emergency shutdown button to drop all of the control rods into the core. At this point an operative entered the control room and reported that he had seen the caps to the fuel channels jumping up and down on the charge face of the reactor.

The control rods jammed, and when the motor was turned off to allow them to fall into the core under their own weight they still did not move-probably because either the channels had become warped by the heat, or the tremendous pressure in the core was holding them out. The water coolant had by now expanded to superheated steam, which then exploded. The blast dislodged the upper shield of the reactor, hurled the heavy charge machine across the hall, and blew off the roof the fourth unit. Valeri Khodemchuk, an engineer whose post was in the reactor hall, was killed in the initial explosion and his body was never recovered.

The damage to the reactor core was horrific. The graphite had been ignited and was now burning, and the initial explosion had caused lumps of the graphite and even pieces of nuclear fuel to be ejected to atmosphere. There was nothing between the intensely radioactive core and the sky, and millions of curies of radioactivity spewed into the air. So deadly was the area that two workers who looked into the reactor from above, for no more than a few minutes, died soon afterwards. They reported seeing not only the light of the fire, but the sinister blue glow of highly radioactive material.

Looking down on the RB MK-1000 reactor head in Unit 4 after the accident.

Firemen fought the fire from the roof of the adjacent turbine hall, which had itself been set alight. They were working unprotected in a highly radioactive area and some began to feel ill-effects after just thirty minutes. By the time they were relieved of duty, most had suffered unsurvivable doses. The heaviest particles fell in the woodland between the power station and Pripyat, killing the trees. New trees have now grown up, but the area remains known as the Red Forest. The fall-out moved around Europe to the north-west, and is still detectable in parts of the UK and elsewhere. Northern Ukraine and Belorussia were particularly badly affected.

Helicopters began over flights of the power station within twenty-four hours, dropping sand and boron into the crater in an attempt to fight the fire, and specialist workers later began tunnelling under the fourth unit to construct a heat exchanger below the ruins. Over the next seven months, a concrete cover was placed over the remains of the fourth unit. Although it is universally known as the ‘sarcophagus’, the official term for it at Chernobyl is the Shelter-Object, the nearest translation of the word used on-site. As with the pilots and diggers, many of those involved in its construction suffered illness and long-term health problems. Although now in urgent need of stabilisation and repair, when the extreme conditions of its construction are taken into account, the Shelter-Object is in its own right a remarkable piece of engineering.

Photo: The sarcophagus or 'Shelter-Object' constructed over the remains of Unit 4
Photo by Nick Catford

Once the Shelter-Object was complete, investigations began inside using remote equipment. The upper shield remained balanced on the rim of the containment vessel with little means of support. It was nicknamed ‘Ylena’ as the pipework dangling down each side of it reminded them of a lady with long hair. When a hole was drilled into the reactor to ascertain the state of the core, the scientists were shocked to find that the containment vessel was empty. Subsequent investigations revealed that the molten nuclear fuel had oozed out of the reactor base, which was forced down by the pressure of the explosion. The fuel poured through pipes and other conduits into the reservoir known as the bubbler pool and the rooms under the reactor, mixing with the packing sand and eventually coalescing into an intensely radioactive lava-like material. Although at the time this was very fortunate, as it encapsulated the nuclear fuel, this now poses one of the worst hazards inside the Shelter-Object. Inside the sarcophagus the temperature is around forty degrees Centigrade and the humidity is high, leading to dripping condensation. This water is now slowly eroding the meltdown lava, and the potential radioactive water and dust should this dry out, is a source of concern.

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