Chernobyl Nuclear Disaster: A Retrospective
The Chernobyl Disaster
Twenty-three years have passed since the fire and explosion in reactor number 4 at the Chernobyl Nuclear Power Plant. The accident, which occurred at 1:23 a.m., released massive amounts of radioactive material into the atmosphere. This contamination significantly impacted large areas of Belarus, Russia, and Ukraine, severely affecting the local population. The accident was initiated by turbine operators conducting an experiment. The reactor’s state at the time—a higher-than-normal cooling flow rate and excessive extraction of neutron poisons—created a supermoderación state. This resulted in a sharp reactivity increase that couldn’t be compensated for. The automatic reactor protection system, partially disabled, failed to respond. The ensuing explosion destroyed the reactor and the deck. The energy release was so powerful that plutonium particles reached an altitude of 2 km.
In the decade following, considerable efforts were made to assess and mitigate the effects of this accident, rooted in human errors, design flaws, and political factors. Below is a summary of the major events before and after the accident, based on recently concluded research.
- What exactly happened in Chernobyl?
- Why did it happen?
- What was the ecological impact?
The accident, occurring on the morning of April 26, 1986, resulted from a combination of human error and plant design flaws. It stemmed from a series of tests intended to improve safety during reactor startup. The goal was to verify that the turbine’s inertia could sustain the cooling system in case of an abrupt power interruption, until emergency diesel generators took over.
“Western” reactors incorporate this possibility into their design, accepting a delay of up to 30 seconds for diesel generators to activate. Such testing is prohibited or strictly regulated elsewhere. Unit 4 at Chernobyl had undergone this experiment after a successful trial on Unit 3. The reactor needed to be at 30% of its operating power (3200 MW thermal).
On April 25, at 1:00 a.m., power reduction began. By 1:00 p.m., the reactor was at 50% power when one turbine was disconnected. System authorities requested maintaining power due to grid demands. Authorization for the experiment came at 11:00 p.m.
At 11:10 p.m., reactor power decreased. Due to an operational error (first error), power dropped to 1%, causing steam condensation in the core. Water absorbs more neutrons than steam, introducing negative reactivity.
Zero reactivity means a self-sustaining core reaction with a constant neutron population (critical reactor). Positive reactivity increases the neutron population and core power. Negative reactivity decreases the neutron population, causing the reactor to fade. Lowering the reactor power increased Xe131 concentration, a fission product and strong neutron absorber, introducing further negative reactivity. Concerned about the reactor shutting down, operators withdrew all control rods (second mistake), enabled by a design lacking a locking mechanism.
With almost no control rods, the reactor reached 7% power in a highly volatile state. (Control rods absorb excess neutrons, stabilizing the reactor. Their removal introduces positive reactivity).
The reactor had automatic flow control. At low power, this system tended to stop. Operators disabled it and initiated manual control (third mistake), again enabled by a lack of interlocks.
All refrigerant was condensed in the core. At 1:23:04 a.m. on April 26, 1986, the turbine was disconnected from the steam line to start the test. Operators also disconnected other emergency systems (fourth mistake).
Disconnecting the turbine caused pumps to rely on the generator’s voltage during inertia braking. Lower voltage reduced pump speed, forming vapor bubbles in the core, increasing reactivity and power.
At 1:23:40 a.m., the operator tried to insert control rods. Too late! The reactor’s power far exceeded its rating.
Rapidly rising pipe pressure caused ruptures, lifting the core shield. Burning fuel and graphite fragments were ejected, igniting a fire on the adjacent turbine roof. Firefighters extinguished most of it by 5:00 a.m., suffering severe radiation exposure.
After failing to flood the core, Soviets covered it with neutron and gamma-ray absorbent materials (lead, sand, clay, dolomite) from helicopters between April 28 and May 2. A tunnel was dug under the station for a concrete floor to prevent groundwater contamination, stopping major radioactive releases.
A concrete “sarcophagus” now encloses the reactor, providing shielding for surrounding work. Fans and filters manage residual heat.
The accident directly killed 31 people: two from the explosion and 29 from radiation, all plant staff. Radioactive material rendered many hectares unusable. Mathematical models predict a less than one percent increase over the normal cancer rate (20%) for the 135,000 surrounding inhabitants.
Conclusion
This century saw the discovery of nuclear energy. Countries worldwide contributed to its peaceful application, developing nuclear power plants for electricity.
This collaborative effort, starting in the 1950s, provided a virtually unlimited energy source, addressing the limitations of conventional fuels and reducing their use for irreplaceable purposes.
Nuclear plants have proven reliable, safe, and economical for electricity generation.
Fusion energy research continues in advanced countries but isn’t an immediate solution to energy problems.
Nuclear energy production has matured technically, scientifically, and in terms of safety for plant operators, the public, and the environment. It can replace fossil fuel-generated energy, benefiting our planet.
Most doubts about nuclear waste have been addressed, though it remains a challenge until we can reuse it or dispose of it permanently.