Chernobyl disaster


On 26 April 1986, reactor no.4 of the Chernobyl Nuclear Power Plant, located near Pripyat, a city in the Ukrainian SSR, Soviet Union, exploded, causing what has become known as the Chernobyl disaster or the Chernobyl catastrophe. With dozens of direct casualties, it is one of only two nuclear energy accidents rated at the maximum severity on the International Nuclear Event Scale, the other being the 2011 Fukushima nuclear accident. The response involved more than 500,000 personnel and cost an estimated 18billion rubles. It remains the worst nuclear disaster and the most expensive disaster in history, with an estimated cost of
US$700 billion.
The disaster occurred while running a test to simulate cooling the reactor during an accident in blackout conditions. The operators carried out the test despite an accidental drop in reactor power, and due to a design issue attempting to shut down the reactor in those conditions resulted in a dramatic power surge. The reactor components ruptured and lost coolant, and the resulting steam explosions and meltdown destroyed the reactor building. This was followed by a reactor core fire that spread radioactive contaminants across the Soviet Union and Europe. A 10 km exclusion zone was established 36 hours after the accident, initially evacuating around 49,000 people. This was later expanded to 30 km, resulting in the evacuation of approximately 68,000 more people.
Following the explosion, which killed two engineers and severely burned two others, an emergency operation began to put out the fires and stabilize the reactor. Of the 237 workers hospitalized, 134 showed symptoms of acute radiation syndrome ; 28 of them died within three months. Over the next decade, 14 more workers died of various causes mostly unrelated to radiation exposure. It is the only instance in commercial nuclear power history where radiation-related fatalities occurred. As of 2005, 6000 cases of childhood thyroid cancer occurred within the affected populations, "a large fraction" being attributed to the disaster. The United Nations Scientific Committee on the Effects of Atomic Radiation estimates fewer than 100 deaths have resulted from the fallout. Predictions of the eventual total death toll vary; a 2006 World Health Organization study projected 9,000 cancer-related fatalities in Ukraine, Belarus, and Russia.
Pripyat was abandoned and replaced by the purpose-built city of Slavutych. The Chernobyl Nuclear Power Plant sarcophagus, completed in December 1986, reduced the spread of radioactive contamination and provided radiological protection for the crews of the undamaged reactors. In 2016–2018, the Chernobyl New Safe Confinement was constructed around the old sarcophagus to enable the removal of the reactor debris, with clean-up scheduled for completion by 2065.

Accident sequence

Background

Reactor cooling after shutdown

In nuclear-reactor operation, most heat is generated by nuclear fission, but over 6% comes from radioactive decay, which continues after the reactor shuts down. Continued coolant circulation is essential to prevent core overheating or a core meltdown. RBMK reactors, like those at Chernobyl, use water as a coolant, circulated by electrically driven pumps. Reactor no.4 had 1,661 individual fuel channels, requiring over 45 million litres of coolant per hour for the entire reactor.
In case of a total power loss, each of Chernobyl's reactors had three backup diesel generators, but they took 60–75 seconds to reach full load and generate the 5.5 MW needed to run one main pump. Special counterweights on each pump provided coolant via inertia to bridge the gap to generator startup. However, a potential safety risk existed in the event that a station blackout occurred simultaneously with the rupture of a coolant pipe. In this scenario the emergency core cooling system is needed to pump additional water into the core.
It had been theorized that the rotational momentum of the reactor's steam turbine could be used to generate the required electrical power to operate the ECCS via the feedwater pumps. The turbine's speed would run down as energy was taken from it, but analysis indicated that there might be sufficient energy to provide electrical power to run the coolant pumps for 45 seconds. This would not quite bridge the gap between an external power failure and the full availability of the emergency generators, but would alleviate the situation.

Safety test

The turbine run-down energy capability still needed to be confirmed experimentally, and previous tests had ended unsuccessfully. An initial test carried out in 1982 indicated that the excitation voltage of the turbine-generator was insufficient. The electrical system was modified and the test was repeated in 1984, but again proved unsuccessful. In 1985, the test was conducted a third time, but also yielded no results due to a problem with the recording equipment. The test procedure was to be run again in 1986 and was scheduled to take place during a controlled power-down of reactor no.4, which was preparatory to a planned maintenance outage.
A test procedure had been written, but the authors were not aware of the unusual RBMK-1000 reactor behaviour under the planned operating conditions. It was regarded as purely an electrical test of the generator, even though it involved critical unit systems. According to the existing regulations, such a test did not require approval by either the chief design authority for the reactor or the nuclear safety regulator. The test program called for disabling the emergency core cooling system, a passive/active system of core cooling intended to provide water to the core in a loss-of-coolant accident. Approval from the site chief engineer had been obtained according to regulations.
The test procedure was intended to run as follows:
  1. The reactor thermal power was to be reduced to between 700 MW and 1,000 MW
  2. The steam-turbine generator was to be run at normal operating speed
  3. Four out of eight main circulating pumps were to be supplied with off-site power, while the other four would be powered by the turbine
  4. When the correct conditions were achieved, the steam supply to the turbine generator would be closed, which would trigger an automatic reactor shutdown in ordinary conditions
  5. The voltage provided by the coasting turbine would be measured, along with the voltage and revolutions per minute of the four main circulating pumps being powered by the turbine
  6. When the emergency generators supplied full electrical power, the turbine generator would be allowed to continue free-wheeling down

    Test delay and shift change

The test was to be conducted during the day-shift of 25 April 1986 as part of a scheduled reactor shutdown. The day shift had been instructed in advance on the reactor operating conditions to run the test, and a special team of electrical engineers was present to conduct the electrical test once the correct conditions were reached. As planned, a gradual reduction in the output of the power unit began at 01:06 on 25 April, and the power level had reached 50% of its nominal 3,200 MW thermal level by the beginning of the day shift.
The day shift was scheduled to perform the test at 14:15. Preparations for the test were carried out, including the disabling of the emergency core cooling system. Meanwhile, another regional power station unexpectedly went offline. At 14:00, the Kiev electrical grid controller requested that the further reduction of Chernobyl's output be postponed, as power was needed to satisfy peak evening demand.
Soon, the day shift was replaced by the evening shift. Despite the delay, the emergency core cooling system was left disabled. This system had to be disconnected via a manual isolating slide valve, which in practice meant that two or three people spent the whole shift manually turning sailboat-helm-sized valve wheels. The system had no influence on the disaster, but allowing the reactor to run for 11 hours outside of the test without emergency protection was indicative of a general lack of safety culture.
At 23:04, the Kiev grid controller allowed the reactor shutdown to resume. The day shift had long since departed, the evening shift was also preparing to leave, and the night shift would not take over until midnight, well into the job. According to plan, the test should have been finished during the day shift, and the night shift would only have had to maintain decay heat cooling systems in an otherwise shut-down plant.
The night shift had very limited time to prepare for and carry out the experiment. Anatoly Dyatlov, deputy chief-engineer of the Chernobyl Nuclear Power Plant, was present to direct the test. He was one of the test's chief authors and he was the highest-ranking individual present. Unit Shift Supervisor Aleksandr Akimov was in charge of the Unit 4 night shift, and Leonid Toptunov was the Senior Reactor Control Engineer responsible for the reactor's operational regimen, including the movement of the control rods. 25-year-old Toptunov had worked independently as a senior engineer for approximately three months.

Unexpected drop of the reactor power

The test plan called for a gradual decrease in reactor power to a thermal level of 700–1000 MW, and an output of 720 MW was reached at 00:05 on 26 April. However, due to the reactor's production of a fission byproduct, xenon-135, which is a reaction-inhibiting neutron absorber, power continued to decrease in the absence of further operator action, a process known as reactor poisoning. In steady-state operation, this is avoided because xenon-135 is "burned off" as quickly as it is created, becoming the highly stable xenon-136. With reactor power reduced, large quantities of previously produced iodine-135 were decaying into the neutron-absorbing xenon-135 faster than the reduced neutron flux could "burn it off". Xenon poisoning in this context made reactor control more difficult, but was a predictable phenomenon during such a power reduction.
When the reactor power had decreased to approximately 500 MW, the reactor power control was switched from local automatic regulator to the automatic regulators, to manually maintain the required power level. AR-1 then activated, removing all four of AR-1's control rods automatically, but AR-2 failed to activate due to an imbalance in its ionization chambers. In response, Toptunov reduced power to stabilize the automatic regulators' ionization sensors. The result was a sudden power drop to an unintended near-shutdown state, with a power output of 30 MW thermal or less. The exact circumstances that caused the power drop are unknown. Most reports attribute the power drop to Toptunov's error, but Dyatlov reported that it was due to a fault in the AR-2 system.
The reactor was now producing only 5% of the minimum initial power level prescribed for the test. This low reactivity inhibited the burn-off of xenon-135 within the reactor core and hindered the rise of reactor power. To increase power, control-room personnel removed numerous control rods from the reactor. Several minutes elapsed before the reactor was restored to 160 MW at 00:39, at which point most control rods were at their upper limits, but the rod configuration was still within its normal limit, calculated as equivalent to having more than 15 rods inserted. Over the next twenty minutes, reactor power would be increased further to 200 MW.
The operation of the reactor at the low power level was accompanied by unstable core temperatures and coolant flow, possibly due to instability of neutron flux. The control room received repeated emergency signals regarding the low levels in the steam/water separator drums, with accompanying drum separator pressure warnings. In response, the operators triggered rapid influxes of feedwater. Relief valves opened to relieve excess steam into a turbine condenser.