The 1986 explosion at the Chernobyl Nuclear Power Plant in Ukraine remains the most severe nuclear accident in history. This catastrophic event resulted in the immediate evacuation of surrounding areas and the establishment of the vast, restricted Exclusion Zone. The explosion released immense quantities of radioactive material, contaminating soil, water, and air across a wide area. Decades later, the zone continues to draw scientific interest as researchers seek to understand the long-term effects of radiation. The most radioactive place ultimately lies deep inside the ruins of the destroyed Reactor 4, where the original molten core material now resides.
How Radiation is Measured
Understanding the peril within the Exclusion Zone requires familiarity with the units used to quantify radiation exposure and dose. The Sievert (Sv) expresses the equivalent dose, measuring the biological effect of radiation on human tissue. This is distinct from the Gray (Gy), the unit for absorbed dose, which quantifies the energy deposited by radiation per unit mass. The Roentgen (R) is an older measurement describing the amount of ionization in the air caused by X-rays or gamma rays.
Radiation exposure is often discussed as a dose rate, such as Sieverts per hour, indicating immediate danger. The total accumulated dose dictates the overall health risk to an individual. The long-term danger is exacerbated by the half-life of radioisotopes, the time it takes for half of a radioactive substance to decay. Isotopes like Cesium-137 and Strontium-90 have half-lives of about 30 years, meaning contamination will persist for centuries.
The Elephant’s Foot and Corium Masses
The most intensely radioactive location in Chernobyl is within the basement levels of the destroyed Reactor 4. Here, the molten core solidified into a glassy, ceramic mass called corium. This material formed during the meltdown when superheated nuclear fuel mixed with concrete, sand, and metal from the reactor structure. The corium flowed through pipes and fissures, eventually pooling in a maintenance corridor beneath the reactor vessel.
The most infamous formation is nicknamed the “Elephant’s Foot,” named for its wrinkled appearance and massive size. This structure, located in Room 217/2, is only a portion of the total corium mass distributed throughout the sub-reactor levels. When discovered in December 1986, radiation levels near the Elephant’s Foot were estimated between 8,000 and 10,000 Roentgens per hour (roughly 80 to 100 Grays per hour). Exposure at this level was lethal, delivering a fatal dose in approximately three to five minutes.
The extreme danger of the corium stems from the high concentration of fission products, including highly radioactive Cesium-137, which emits penetrating gamma radiation. The corium also contains uranium, an alpha emitter. While alpha particles are easily shielded, the material’s slow degradation into dust poses a severe internal contamination risk if inhaled. Although the immediate dose rate has decreased significantly due to the decay of short-lived isotopes, the corium remains a formidable gamma radiation source. Other corium flows deeper within the steam distribution corridors may exhibit radiation levels comparable to or higher than the Elephant’s Foot due to variations in their exact composition.
High-Contamination Hotspots Beyond the Reactor
While the corium masses represent the peak danger, significant zones of high contamination exist outside the immediate reactor building due to the initial fallout. The most well-known external hotspot is the Red Forest, an area of pine trees located just west of the power plant. These trees absorbed such high doses of radiation from the initial explosion that they died, their needles turning a rusty-ginger color.
The Red Forest remains one of the most contaminated terrestrial ecosystems globally. Radioactivity is concentrated mainly in the topsoil and the remains of the buried trees. Contamination in this ten-square-kilometer area is dominated by long-lived radionuclides like Cesium-137 and Strontium-90. Radiation levels in hotspots can still measure up to 10 millisieverts per hour, a dose rate thousands of times higher than natural background radiation. Waste burial trenches created during the initial cleanup can exhibit even higher local dose rates if the contaminated material is disturbed.
Containing the Peak Radioactivity
The ongoing challenge at Chernobyl centers on containing and stabilizing the highly radioactive corium and the damaged structure of Reactor 4. The initial containment effort was the Sarcophagus, a concrete and steel structure hastily completed in late 1986. This temporary shelter was intended to limit the release of radioactive dust, but it quickly began to degrade.
To address the Sarcophagus’s instability and facilitate future dismantling, the New Safe Confinement (NSC), or “Arch,” was constructed and slid over the old shelter. The NSC is designed to contain the radioactive materials for at least a century, protecting the environment and allowing for the safe removal of the original structure and the corium masses. The corium is slowly cooling but remains structurally active, with continued chemical changes posing a risk to the fragile ruins. Monitoring systems track radiation levels and structural integrity, managing the long-term threat posed by the sealed material.