Overview of Chernobyl Zone

The Chornobyl NPP

The nuclear power plant was build for shortage of electricity in Central energetical area of Soviet Union. Chornobyl region was chosen from 16 sites, as as one with best conditions for building such facility. Construction works started in 1970, together with city for emploees Pripyat. 
In 1983 four units were operational and 2 more were planned to come online in next years. All units were equipped with RBMK 1000 reactors, an early Generation II reactors and the oldest comertial reactor design, using minimalistic design, regular (light) water for cooling, graphite for moderation and natural uranium for fuel (instead of more expensive enriched uranium) cheap enough, large and powerful. One of advantages of RBMK type of reactor allows to perform reloading nuclear fule without reducing power output.

The city of Pripyat

In the same time with start of construction of Chornobyl NPP, the satellite town was founded. In the very beginning it was just a village with dorms of first builders, that quickly expanded to developed city, that featured high level of life, perfect infrastructure and wide availability of consumer goods. As for 1986, Pripyat consisted of 5 and a half districts, grouped in an aproximate triangle shape around central square, and became a home for nearly 50,000 people of more than 25 nationalities.

The city had more than 10 stores, huge trade center with nearly the first supermarket in USSR, 14(!) kindergartens, professional swimming pool, 5 schools, cinema, music school and more. The local Palace of Culture, which was the community center, featured nearly professional-level theater, library, sport center and famous Pripyat's disco Edison-2.

However, shortly after 16th birthday of the city, everything changed...

Disaster at Chornobyl NPP Unit IV

On 26.4.1986 a series of events were leading to an accident at Vladimir Ilič Lenin nuclear power plant in Ukraine (at that time part of Soviet union). This accident is one of 2 nuclear disasters categorized as Level 7 – Major accident on International Nuclear Event Scale, created after 1986. 
An explosion took place on unit 4 of power plant located 120 km north from Kyiv, causing the release of radioactivity to the environment and evacuation of 135 000 people from region.

The accident occurred during an experiment scheduled to test a potential safety emergency core cooling feature, which took place during a normal shutdown procedure. Cooling pumps needs electricity, in case of grid failure, reactors had three backup diesel generators. To start them up it required 15 seconds, but took 60-75 seconds to reach full speed. This gap, could be solved by using rotational energy from the steam turbine for generating required electrical power. The actual accident happened due to a combination of many factors.

Test was postponded by Kiev electrical grid controller, because another power station went offline and allowed further reduction of power level only after midnight. 
At this time reactor was already poisoned by fission byproduct xenon-135, which is reaction-inhibiting neutron absorber. Power continued to decrease without any operator action. Reactor power output dropped further, as control rods were mistakenly inserted too far. Combination of these factors put the reactor into an unintended near-shutdown state. To restore power, control room stuff decided to disabe the automatic system governing the control rods and manually extracting the majority of the reactor control rods to their upper limits.

High poisoning level and operation of the reactor on low power level, unstable core temperature, the levels in the steam/water separator drums and coolant flow, and possible instability of neutron flux triggered alarms. As part of the test plan, extra water pumps were activated, increasing the water flow. The increased coolant flow produced an increase in the inlet coolant temperature of the reactor core.

The flow exceeded the allowed limit, triggering an alarm of low steam pressure in the steam separators. At the same time, the extra water flow lowered the overall core temperature and reduced the existing steam voids in the core and the steam separators. Since water weakly absorbs neutrons (and the higher density of liquid water makes it a better absorber than steam), turning on additional pumps decreased the reactor power further still. Turning off two of the circulation pumps to reduce feed water flow, in an effort to increase steam pressure, and also to remove more manual control rods to maintain power led to an extremely unstable reactor configuration. Automated system that could insert all control rods and shut down reactor was disabled to maintain the power level. At 1:23:04a.m. 26. 4 1986 the experiment began. The steam to the turbines was shut off, and a run down of the turbine generator began and diesel generator started and sequentially picked up loads. 
As the momentum of the turbine generator decreased, the water flow rate decreased, leading to increased formation of steam voids (bubbles) in the core. Because of the positive void coefficient of the RBMK reactor at low reactor power levels, it was now primed to cast a positive feedback loop, in which the formation of steam voids reduced the ability of the liquid water coolant to absorb neutrons, which in turn increased the reactor's power output.

At 1:23:40, as recorded by the SKALA centralized control system, an emergency shutdown of the reactor, which inadvertently triggered the explosion, was initiated.
After the AZ-5 button was pressed, the insertion of control rods into the reactor core began. A big problem was a flawed graphite-tip control rod design (kept secret, as sensitive material known only to some physicists), which initially displaced coolant before inserting neutron-absorbing material to slow the reaction, this increased the reaction rate in the lower half of the core. A massive power spike occurred, the core overheated, and resulted in the initial explosion. Some of the fuel rods fractured, control rod columns were deformed by heat and causing the control rods to become stuck at one-third insertion. The subsequent course of events was not registered by instruments, it is known only as a result of mathematical simulation. A great rise in power first caused an increase in fuel temperature and massive steam buildup, leading to a rapid increase in steam pressure. Then the reactor jumped to around 30GW thermal, ten times the normal operational output. It was not possible to reconstruct the precise sequence of the processes that led to the destruction of the reactor and the power unit building.

There is a general understanding that it was steam from the wrecked channels entering the reactor's inner structure that caused the destruction of the reactor casing, tearing off and lifting the 1200ton upper plate. This explosion ruptured further fuel channels, and as a result the remaining coolant flashed to steam and escaped the reactor core. Graphite fire was burning by now, greatly contributing to the spread of radioactive material and fallout of outlying areas. A second, more powerful explosion occurred about two or three seconds after the first, dispersed the damaged core and effectively terminated the nuclear chain reaction. Burning pieces of material and sparks shot into the air above the reactor. Some of them fell on to the roof of the machine hall and started a fire. 

Most important was now to extinguish fire on building of unit 4 and protect unit 3. Reactor Nr. 3 was shut down at 5am, leavging there only operators responsible for emergency cooling systems.

Initial stage of liquidation

First on the scene was a Chornobyl Power Station firefighter brigade, together with firemen from nearby city Pripyat and town Chornobyl. 
Director of NPP asked for first radiation readings. Geiger counter capable of measuring up to 1000R/s was burried under collapsed part of building, another one failed when turned on. Scale was 3.6R/h, needle was off scale. Because this incorrect reading, operatores believed, that reactor can be saved, so they were putting more and more water in.

Most of firemen and last shift later died on acute radiation sicnes. The fires were extinguished by 5:00am, but the fire inside reactor 4 continued to burn until 10 May 1986 and was extinguished by combined effort of helicopters dropping over 5000 tons of sand, lead, clay, and neutron absorbing boron onto the burning reactor and injection of liquid nitrogen. It is now known that virtually none of the neutron absorbers reached the core.

To clean debris from roof, robots and remote-controlled bulldozers were assembled, but their electronics and bateries were not shielded enough, so they faild quickly. 
The nearby city of Pripyat was evacuated on 27 April. To expedite the evacuation, residents were told to bring only what was necessary, and that it would be only temporery. As a result, most personal belongings were left behind, and remain there untill decontamination, when most was burried in special places. In next days continued evacuation of 10km Zone and 30km Zone. Several buildings in Pripyat were officially kept open after the disaster to be used by workers still involved with the plant. These included the Jupiter Factory which closed in 1996, Azure Swimming Pool which closed in 1998 and few more facilities. 

At the time there was still fear that the reactor could re-enter a self-sustaining nuclear chain-reaction and explode again, and a new containment structure was planned to prevent rain entering and triggering such an explosion. 
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. Some workers could only spend a maximum of 40 seconds at a time working on the rooftops of the surrounding buildings because of the extremely high doses of radiation given off by the blocks of graphite and other debris.
 By December 1986, a large concrete Shelter object had been erected to seal off the reactor and its contents.

Chornobyl Zone today

In 1986, the Exclusion Zone was created. Today, it covers an area of approximately 2,600 km2, although its shape has changed and its size has been expanded few times. From its beginning it was not a barren land — but a territory, where lots of facilities involved in management of this territory are in operation. Currently, it is a restricted area under the management of State Agency of Exclusion Zone with the center at Chornobyl town, that was converted to main administrative location of the Zone. In this town, there is all needed infrastructure for nearly 3,000 workers that involved in radiological monitoring, science, forest management, fire safety, and so on.

1987 some of eldery people returned to the 30km Zone, as many were not satisfied with conditions of new places for living. Approximately 150 of them live today in their houses, all in outer, 30-km Zone. 
Zone became unique place — same time an open air laboratory, a territory with restrict access to hazardous areas, natural barrier to reduce the spread of radiological contamination, site of radiological and ecological monitoring activities, and a Europe's biggest reservation of forest fauna. 

Radioactive contamination

The territory of the zone is polluted unevenly. Spots of hyperintensive pollution were created first by wind and rain spreading radioactive dust at the time of the accident, and subsequently by numerous burial sites for various material and equipment used in decontamination. Zone authorities pay attention to protecting such spots from tourists, scrap hunters and wildfires. The major difference from effects of nuclear blast is that in case of Chornobyl disaster, the area was polluted with long-living substances, therefore, it won't be reused for at least few next thousands years.

All the visits to the zone being performed based on safe routes, that passed detailed check-up on levels of abmient radioactivity and contamination concentrations, thus, considered possible for third-party visiotrs stay.