First of all let's take a moment of silence in memory of those who have lived there and the team that made the concrete sarcophagus over the reactor. I wanted for a long time to make this post about Chernobyl and Pripyat,and now I've made it.Today there are still high readiations near the power plant.22 years have past from the disaster that started at 01:23:44 a.m .Further explosion and the resulting fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area. Nearly thirty to forty times more fallout was released than had been by the atomic bombings of Hiroshima and Nagasaki.Two more reactors, no. 5 and 6, capable of producing 1 GW each, were under construction at the time of the accident.Chernobyl was the worst nuclear power plant accident in history!



Fatal experiment - How it all happened...

At 1:23:04 AM the experiment began. The unstable state of the reactor was not reflected in any way on the control panel, and it did not appear that anyone in the reactor crew was aware of any danger. The steam to the turbines was shut off and, as the momentum of the turbine generator drove the water pumps, the water flow rate decreased, decreasing the absorption of neutrons by the coolant and causing the coolant to heat up. As the coolant boiled, pockets of steam formed voids in the coolant lines. Due to the RBMK reactor-type's large positive void coefficient, the steam bubbles increased the power of the reactor. As soon as the reactor power increased, the positive feedback that had acted to drive reactor power down now acted to increase it further. As power increased, the Xe-135 poison began to be burned faster than it was being produced by I-135 decay, which increased power, resulting in more steam generation, a faster Xe-135 burn, and so on. With the manual and automatic control rods removed, nothing prevented a runaway reaction.

With reactor output rapidy increasing, the operators pressed the AZ-5 ("Rapid Emergency Defense 5") button at 1:23:40, that ordered a "SCRAM" – a shutdown of the reactor, fully inserting all control rods, including the manual control rods that had been incautiously withdrawn earlier. It is unclear whether it was done as an emergency measure, or simply as a routine method of shutting down the reactor upon the completion of an experiment (the reactor was scheduled to be shut down for routine maintenance). It is usually suggested that the SCRAM was ordered as a response to the unexpected rapid power increase.

The slow speed of the control rod insertion mechanism (18–20 seconds to complete), and the flawed graphite-tip rod design which initially reduces the amount of coolant present, meant that the SCRAM actually increased the reaction rate. At this point an energy spike occurred and some of the fuel rods began to fracture, placing fragments of the fuel rods in line with the control rod columns. The rods became stuck after being inserted only one-third of the way, and were therefore unable to stop the reaction. At this point nothing could be done to stop the disaster. By 1:23:47 the reactor jumped to around 30 GW thermal, ten times the normal operational output. The fuel rods began to melt and the steam pressure rapidly increased, causing a large steam explosion. Generated steam travelled vertically along the rod channels in the reactor, rupturing the coolant tubes and then blowing the 2,000 tonne lid off the reactor.After part of the roof blew off, the inrush of oxygen, combined with the extremely high temperature of the reactor fuel and graphite moderator, started a graphite fire, worsened by flammable materials used in the original construction of the roof for reactor 4. This fire greatly contributed to the spread of radioactive material and the contamination of outlying areas.

Radiation levels

At the time of the disaster, the plant's staff were not aware of the true radiation levels, which led to severe misassessments of the situation. The radiation levels in the worst-hit areas of the reactor building have been estimated to be 5.6 röntgen per second (R/s), which is equivalent to 20,000 röntgen per hour (R/h). A lethal dose is around 500 röntgen over 5 hours, so in some areas, unprotected workers received fatal doses within several minutes. However, a dosimeter capable of measuring up to 1,000 R/s was inaccessible due to the explosion, and another one failed when turned on. All remaining dosimeters had limits of 0.001 R/s and therefore read "off scale". Thus, the reactor crew could ascertain only that the radiation levels were somewhere above 0.001 R/s (3.6 R/h), while the true levels were 5,600 times higher in some areas.

Because of the fallacious low readings, the reactor crew chief Alexander Akimov assumed that the reactor was intact. The evidence of pieces of graphite and reactor fuel lying around the building was ignored, and the readings of another dosimeter brought in by 4:30 a.m. were dismissed under the assumption that the new dosimeter must have been defective. Akimov stayed with his crew in the reactor building until morning, trying to pump water into the reactor. None of them wore any protective gear. Most of them, including Akimov, died from radiation exposure within three weeks.

Debris removal

The worst of the radioactive debris was collected inside what was left of the reactor, much of it shoveled in by liquidators wearing heavy protective gear (dubbed "bio-robots" by the military); these workers could only spend a maximum of 40 minutes at a time working on the rooftops of the surrounding buildings due to the extremely high doses of radiation given off by the blocks of graphite and other debris. The reactor itself was covered with bags containing sand, lead and boric acid thrown off helicopters (some 5,000 metric tonnes during the week following the accident). By December 1986 a large concrete sarcophagus had been erected, to seal off the reactor and its contents.

Many of the vehicles used by the "liquidators" remain parked in a field in the Chernobyl area to this day, most giving off doses of 10-30 röntgen/hr. over 20 years after the disaster.

I hope you will all enjoy reading the article and watchind the pictures collected.Special thanks to Elena Filatova and Pierpaolo Mittica for sharing some of their pictures taken in the "zone".For those interested in trip's please contact me.