|
|
| Fig. 1: Diagram of reactor number four after the explosion with biological lid disrupted and lava flowing below core. [1] (Courtesy of the United Nations). |
Late on the night of April 26th, 1986 in the city of Pripyat, Ukraine the most significant nuclear disaster known to mankind occurred with the nuclear meltdown of reactor number four at the Chernobyl Nuclear Power Plant. The nuclear meltdown was the cause of multiple issues, mostly man made mistakes and flaws in the Soviet RBMK-1000 design.
These mistakes were made when a routine stress test was run to see how long the generators turbine would spin after the power was cut out. The complications began when the tests were run at 200 MW when they were supposed to be run at much higher power levels, 700MW. [1] Both manual and automatic control rods were removed so power levels could increase to compensate for the negative reactivity caused by xenon poisoning of the core from operating at the low power rates. [1] This xenon poisoning occurred when the Xe-135, present after the reactor is shut off, absorbs all of the neutrons being breed by the fuel rods, not allowing power levels to increase. Due to RBMK-1000's positive void coefficient at low reactor power levels, however, the removal of the control rods lead to a rapid increase in power as increased steam voids reduced neutron absorption of the water, which in turn lead to increased reaction rates and power output. [1] Temperatures had risen too high, causing the fuel rods to crack and resulting in the water, being used to cool the uranium fuel rods, to turn to steam. Pressure build up was so immense that it blew the 1,000 ton fastening plate through the roof of the building. [1] The end result from the explosion can be seen in Fig. 1, with the biological shield being lifted and lava melting through the core. There began a fire that would spread nuclear fission products into the atmosphere and Eastern Europe for the next nine days.
 |
| Fig. 2: This is a photograph taken of the "Elephants foot" beneath reactor number four. (© Getty Images, reproduction by permission) |
Reaching estimated temperatures between 1,660°C and 2,600°C and releasing an estimated 4.5 billion curies the reactor rods began to crack and melt into a form of lava at the bottom of the reactor. [2] This lava was not only the uranium fuel rods, but also the graphite moderators and boron control rods and the sand that surrounded the reactor. The molten lava was at such a high temperature that it melted through the steel beams, concrete, and other structures below the reactor. It would then cool down and form a substance scientists call Chernobylite, a silicate, formed from the melted concrete and sand that surrounds the reactor, but also containing up to 10% uranium from the fuel rods. [3] The most infamous example of this mineral is the "elephant's foot", photographed in Fig. 2, right below the core of reactor number four. This mixture of uranium, silicon dioxide and whatever else was absorbed through the process of it eating away the core of the reactor is only 1 meter in size, but weighing an estimated two metric tons. Although the substance is estimated at between 5-10% uranium, it is measured to be releasing 10,000 roentgens per hour, a dose that would kill a human in 300 seconds if you were standing within three feet. [4] When looking at photos there is a noticeable blur in the film. This is because the film is being hit by high energy alpha particles, being able to see where these particles are hitting the film and affecting its quality. There is no way that any living cell, no matter how microbial, could inhabit this area.
 |
| Fig. 3: Pictured is the "New Safe Containment" structure currently being built. (Source: Wikimedia Commons) |
In the coming days after the meltdown, helicopters were sent to the reactor core to drop large amounts of sand, lead and boron, meant to end up in the core and absorb all of the neutrons being given off by the melted fuel rods. These attempts were rather unsuccessful in stopping the spread of radioactive materials, as most of the material dropped never even reached the core, however was relatively successful in stopping the ongoing fires.
As a long term option, the Soviet government built a "sarcophagus" to stop the spread of these radioactive materials into the atmosphere. [5] The idea of this was to enclose the 16 tons of uranium and 30 tons of radioactive dust inside the sarcophagus which consisted of 400,000 cubic meters of reinforced concrete. However, due to the poor construction it has only lasted up until this point and now Ukraine is working on the construction of what they call the "New Safe Confinement" which has construction underway now in 2017, which can be seen in Fig. 3. The new building will serve the same purpose as the old one, but due to less strict time constraints plans to me more structurally sound and aim to last the next 100 years.
© Matthew Gutwald. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Report on the Accident at the Chernobyl Nuclear Power Station," U.S. Nuclear Regulatory Commission, NUREG-1250, January 1987.
[2] "Sources and Effects of Ionizing Radiation, UNSCEAR 2000 Report to the General Assembly, Vol II," United Nations, 2008, Annex J: Exposures and Effects of the Chernobyl Accident.
[3] S. A. Bogatov et al., "Formation and Spread of Chernobyl Lavas," Radiochemistry 50, 650 (2009).
[4] B. E. Burakov et al., "The Behavior of Nuclear Fuel in First Days of the Chernobyl Accident," MRS Proceedings 465, 1297 (1996).
[5] D. Sim, "Chernobyl: Huge Sliding Arch to Block Radiation Is Nearing Completion 30 Years After Nuclear Disaster," International Business Times, 26 Apr 16.
