The explosion of a nuclear power plant in Chernobyl, Ukraine on April 26, 1986 was the first of its kind. Unlike Three Mile Island incident of 1979, the explosion and radiation release was difficult to contain, spreading widely and rapidly. Suddenly a new type of disaster was possible; nuclear bombs were no longer the only nuclear disaster to consider. It tends to be common knowledge that the Chernobyl reactor exploded, releasing radiation to a wide range of communities. The area was then evacuated and cancer rates were watched closely. But what really happened on that fateful day in 1986? And what implications does it have on the world, and the nuclear power industry?
What happened?
The explosion at the Chernobyl reactor number 4 is often accredited to a combination of two things: poor Soviet Union reactor design and human error by plant operators. The weight that reports give to each factor varies. Regardless of which factor was more influential to the explosion, they did both have large effects on reactor design and overall operating safety practices. While further information can be found on the World Nuclear Association‘s website (WNA), or McFarland et. al.’s book Energy, physics, and the environment (these are the sources I used for my information), I will account a summarized version of what I understand of the events.
The reactor that exploded was a boiling water reactor (BWR) with a graphite moderator. Graphite, as I have mentioned in previous posts, is one of the less stable moderators that have been used. The building being used to house the reactor at the time had a standard industrial roof (McFarland et. al., 1994), which is important to note as much of the explosion was directed up and out through the roof. At the time of the event, a group of operators were preparing for a test of the plant, attempting to determine “how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply” (WNA). In order for the test to work without the cooling system turning on (which would effectively stop any reactions occurring at that time), the operators turned off the emergency core-cooling system (McFarland et. al., 1994). The operators then miscalculated the correct energy level instructed to the reactors computer. Combined with pulling the shutdown rods out too far, the rods were unable to return fast enough to quell the energy levels that were being produced (McFarland et. al., 1994). On top of this, the graphite cooler has what is called a “positive void coefficient“, which exacerbated the sudden increase of heat and energy. The steam pressure that was produced as a result blew the roof of the building, sending debris and radioactive material into the sky and adjacent reactor buildings.
Result of the Explosion
As a result of the explosion, readiation was airborne for up to ten days following the accident, with strong deposits in other parts of Ukraine, Belarus, and Russia. Radiation was detected to varying degrees in other nearby countries, such as Poland, and even reached Western Europe in small doses (McFarland et. al., 1994). Reports list two plant workers died as an immediate result of the explosion, while another 28 died in the following weeks and months due to acute radiation sickness. (WNA). At least 19 more people related to the event died in later years (between 1987-2004), but it is unclear whether these deaths were because of ARS (WNA). There was also mass evacuation and relocation of nearby residents, with about 116,000 evacuated immediately after and about 220,000 were similarly relocated in the following years as the contaminated zone was increased (WNA).
The main cancer concern from the explosion was that of thyroid cancer from radioactive iodine. This often crept up as people drank milk from cows eating radioactive grass, a fairly unknown problem at the time (discussed by the World Health Organization). The concerns and perceptions at the time also lead to a combination of mistrust of official information, as well as potentially incorrect assumptions connecting various health problems in later years to radiation exposure. In the grand scheme of the disaster, most people (that didn’t live in the immediate vicinity) affected by the event were exposed to relatively low radiation, especially when compared to normal background radiation levels (WNA).
McFarland, E. L., Hunt, J. L., & Campbell, J. L. (1994). Energy, physics, & the environment. Winnipeg: Wuerz Publishing.