The Chernobyl Disaster

1. The Chernobyl Disaster Katherine Culp Erin Meyer http://mt.sopris.net/mpc/military/v/reactor.chernobyl.jpg

2. “This accident [Three Mile Island] could only happen in a capitalistic society where they put profits ahead of safety.” Professor Alexandrov, president of the Soviet Academy of Sciences “Good Evening Comrades. All of you know that there has been a incredible misfortune – the accident at the Chernobyl nuclear plant. It has painfully affected the Soviet people and shocked the international community. For the first time, we confront the real force of nuclear energy, out of control.” Soviet President Mikhail Gorbachev The Irony

3. Background: How a Nuclear Reactor Works • Uranium-235 can easily undergo induced nuclear fission; free neutron + U-235  fission • Plutonium-239 can also be used • Reactor operates at critical state – one neutron ejected from each fission  another fission • Enriched uranium pellets arranged in long rods • Bundle of rods submerged in coolant (water) • Bundle must be at supercritical state

4. Nuclear Reactors • Supercritical state – if left alone, uranium bundles would overheat and melt • Control rods – control rate of reaction • Made of neutron-absorbing material • Can be raised or lowered into bundle • Raising  more heat generated • Lowering  less heat • Completely lowering  shut down reaction

5. Nuclear Reactors… • Bundle – heats water to form steam • Steam drives a steam turbine • Turbine spins a generator to produce power • Containment vessel – prevents leakage of radioactive material

6. RBMK Reactor – Reactor Bolshoy Moshchnosty Kanalny http://classroom.psu.ac.th/users/ssmarn/pplant/Nuclear/NPP8_files/image002.gif

7. RBMK Reactors • Only in Russia • Produce power and plutonium for military use • Design Flaws • In order to remove the fuel rods for reprocessing and obtaining plutonium, a crane is needed. However, this makes the reactor too tall for a containment shell, which would keep radioactive material from spreading in case of an accident. • Carbon moderator used instead of water. Carbon moderators have a tendency to catch fire. Moderators are used to slow down rapid neutrons.

8. Positive Void Coefficient • Positive void coefficient – if the water in the reactor boils, steam bubbles are produced. In other reactors, excess steam pockets reduce nuclear activity and slow down the nuclear reaction. But in RBMK reactors, the nuclear reaction speeds up. • With a positive coefficient, a reactor is unstable at low power and is susceptible to a rapid power surge that is uncontrollable. • In reactors with positive void coefficients, the moderator and coolant are in separate circuits or are different materials. However, RBMK reactors have no modifications to prevent the rapid, uncontrollable power surge that can be produced by a positive void coefficient. • There are 13 RBMK reactors in the world still operating today.

9. Construction Flaws • Gross deviations were present in the construction of the RBMK reactors at the Chernobyl plant. • The foundation was not poured properly, so there were gaps in the foundation which could allow for leakage. • Roof placement did not conform to design specifications. • There was also damage to the waterproofing, which could allow for radioactive contamination of groundwater. The damage was backfilled instead of repaired.

10. An Experiment • Reactor 4 was shut down for routine maintenance, and workers decided to run a test to determine if, during a shutdown, enough electrical power could operate the emergency equipment and core cooling pumps until the diesel supply returned. • Communication between group in charge of test and group operating the nuclear reactor broke down. • The emergency core cooling system was turned off; reactor operated at ½ power. • The reactor stabilized at 1,000 MW, but operator error caused the power to drop to 30 MW, a power at which the positive void coefficient can be a problem.

11. April 26, 1986 • Operators stabilized the system by withdrawing almost all the control rods. A minimum of 30 control rods is required; only 6-8 were left in place. If a power surge were to take place, operators would only have 20 seconds to lower the rods and to shut down the reactor • Reactor became extremely unstable; operators had to make adjustments every few seconds in order to keep the power constant. • Operators decided to reduce the flow of water so that they could maintain the steam pressure. Turbine slowed  pumps not providing as much cooling for the reactor. • More steam was created in the cooling channels  power surge about 100 times the normal power. • Power surge  steam explosion which blew off the top off of the reactor. A second explosion took place a few seconds later; its origin is not understood. • Carbon moderator caught on fire; burned for nine days.

12. Initial Release of Radiation • When Reactor 4 went supercritical and exploded there was a ‘mechanical discharge of dispersed radioactive fuel’, meaning that a plume of radioactive elements was released into the atmosphere. This release was made up of isotopes that closely corresponded to the fission products of spent fuel, including 89,90S, 131I, and 134,137Cs, which made up the largest part of the initial emissions.

13. The Days After the Accident • From April 26th to May 1st the rate of radioactive element release decreased because the core was covered with boron, lead, and sand in an attempt to extinguish the fire that had begun during the explosion. • This covering of the core, however, led to temperatures in the core increasing and to release of more radiation in the form of 131I. Estimates indicate that 50% of the core’s 131I was released.

14. Soviet Disclosure • The Soviets did not initially inform other nations of the disaster. The first indication European nations had that a large-scale nuclear disaster had taken place was when nuclear plants in Scandinavia began to register unusually high levels of background radiation. The Soviets denied all knowledge about the accident for several days. Full disclosure was not made until August of 1986

15. Soviet Evacuation • The Soviet authorities did not evacuate the villages that surrounded Chernobyl until April 27. At that time they also provided iodine supplements for the evacuated residents. These supplements are supposed to help prevent thyroid cancer by filling the thyroid with nonradioactive iodine so it doesn’t take up the radioactive forms.

16. Soviet Response to Chernobyl • The Soviet authorities sent in cleanup crews to supplement the workers already in Chernobyl. Of those who were sent in to contain the fires and radiation damage, 29 died from intense β burns. There were also reports of 237 cases of acute radiation sickness in the surrounding areas. Later research has questioned these numbers, and there is some evidence that the Soviets grossly underestimated the number of people made ill by the explosion and the subsequent release of radioactive isotopes.

17. Effects on the World • Chernobyl released radiation high into the atmosphere, where it eventually was carried across Europe. The amount of radioactive elements varied country by country, according to their proximity to the Ukraine and also meteorological patterns. Countries that received a lot of rainfall in the days after the explosion tended to have higher levels of radiation due to the elements being sent out in ‘fine aerosol form’ that was best carried by a liquid medium.

18. Long-term effects • Using risk models, researchers have estimated that the risk of developing cancer is doubled in those who were within a thirty-mile radius of Chernobyl. • The modeled risks for those in Europe are considered statistically insignificant.

19. Health Effects • Researchers in 1996 indicated that, out of 235 villagers in the area of Chernobyl at the time of the explosion, 35 had died of cancer. Many survivors suffer from possibly radiation-influenced conditions such as goiter and diabetes. • A Russian Health Ministry report from the same time indicates that half of the children in the areas irradiated by the fallout show signs of radiation sickness, including anemia, kidney failure, and lymph gland inflammation. • A Ukrainian biologist’s study indicates that 8500 of those who went in to clean up the radiation hot spots had died by the mid 1990s.

20. The Chernobyl Legacy • After covering Reactor 4 with a cement sarcophagus that was intended to seal up the radioactive elements, the Soviets continued to operate the other reactors at Chernobyl. • By the mid-’90s the sarcophagus was crumbling and international workers had to go into Chernobyl and reinforce it. • The last reactor was shut down in late 2000. • People are still not allowed to live in an eighteen mile radius from Chernobyl.

21. Sources • Anspaugh, Lynn R., et al. “The Global Impact of the Chernobyl Reactor Accident.” Science, 242(Dec., 1988), 1513-1519. • ApSimon, Helen, et. al. “Analysis of the Dispersal and Deposition of Radionuclides from Chernobyl Across Europe.” Proceedings of the Royal Society of London. 425(Oct, 1989), 365-405 • Brain, Marshall. “How Nuclear Power Works.” http://science.howstuffworks.com/nuclear-power.htm • “Chernobyl: Assessment of Radiological and Health Impact.” Nuclear Energy Agency. http://www.nea.fr/html/rp/chernobyl/c01.html • “Chernobyl Nuclear Disaster.” http://www.chernobyl.co.uk • Dahlburg, John-Thor. “Study Finds Chernobyl Radiation Worse than Originally Reported.” Los Angeles Times. April 14, 1992, pg. 2 • “Fire near Chernobyl reported out.” CNN.com, April 1996. http://www.cnn.com/WORLD/9604/23/chernobyl.htm • McCarthy, John. “Chernobyl.” http://www-formal.stanford.edu/imc/progress/chernobyl.html • “The RMBK Reactor” World Nuclear Association. http://www.world-nuclear.org/info/inf31.htm • Wilson, Richard. “A Visit to Chernobyl.” Science, 236(June, 1987), 1636-1640.