Eric Pendry / Physics/ Period 6/ 4-8-15
Chernobyl Disaster (1986)
Events to Disaster
On April 25th, 1986 the Chernobyl Disaster happened. It all began when the reactor crew at Chernobyl 4 began preparing for a test to determine how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply. The same test was done the previous year but this time around new voltage regulator designs were to be tested. Before the test happened, the operator decided to disable the automatic shutdown mechanisms. During the test, very hot fuel mixed with the cooling water and the pressure in the reactor started to increase. The pressure increase caused the 1000 t cover plate of the reactor to become partially detached, rupturing the fuel channels and jamming all the control rods. At this point the operator was trying to stop the reactor on his own but failed. Intense steam traveled throughout the reactor and an explosion happened releasing fission products to the atmosphere. About two to three seconds later a second explosion happened and it is likely to have been caused by the production of hydrogen from zirconium-steam reactions. After the explosion of the reactor, the surrounded area was filled with radioactivity.
Facts and Figures
On the day of the disaster, two workers were killed at the scene of the explosion. In the weeks to follow, 28 more people were killed from radiation. The accident caused the largest uncontrolled radioactive release into the environment ever recorded for any civilian operation, and large quantities of radioactive substances were released into the air for about 10 days. The plant operators' town of Pripyat was evacuated on 27 April (45,000 residents). By 14 May, some 116,000 people that had been living within a 30-kilometer radius had been evacuated and later relocated. Radiation was exposed throughout the whole area and many people were relocated and evacuated. The financial loses were hundreds of billions of dollars form all the effort to repair the damage that was done and the money was supplied by multiple countries. In the diagram below, all the ways radiation can be contracted are shown. A map of the area affected is also shown below.
What was to blame?
Who or what is to blame for this disaster? The answer is that there are multiple things to blame. The first thing to blame is the operator because they disabled the automatic shutdown mechanisms. With the mechanisms shutdown there was not a chance to save the reactor. The other thing to blame was the way the reactor was designed. Due to the design, the pressure was easily able to get by the control rods and destroy the reactor. As described on the website, the RBMK-1000 is a Soviet-designed and built graphite moderated pressure tube type reactor, using slightly enriched (2% U-235) uranium dioxide fuel. It is a boiling light water reactor, with two loops feeding steam directly to the turbines, without an intervening heat exchanger. Water pumped to the bottom of the fuel channels boils as it progresses up the pressure tubes, producing steam which feeds two 500 MWe turbines. The water acts as a coolant and also provides the steam used to drive the turbines. The vertical pressure tubes contain the zirconium alloy clad uranium dioxide fuel around which the cooling water flows. The extensions of the fuel channels penetrate the lower plate and the cover plate of the core and are welded to each. A specially designed refueling machine allows fuel bundles to be changed without shutting down the reactor. The reactor was designed to fail as seen in the diagram below.
Physics Behind the Failure
Physics had to do with the Chernobyl Disaster. With a low power level, xenon built up in the fuel rods. In order to get the reaction level to rise the control rods were almost completely withdrawn. When the turbine switched to start the test, less pumps were working causing fewer neutrons and the reaction to rise rapidly. This caused the xenon to burn away. Water came and increased the reaction rate before the neutrons were absorbed. This caused the power to surge 100 times the normal operating power and for an explosion to occur.
Things That Came From Disaster
An extensive benefits system was established for the populations that were seen to have suffered as a result of the Chernobyl accident, either through exposure to radiation or resettlement. By the late 1990s, Belarusian and Russian legislation provided more than seventy, and Ukrainian legislation more than fifty, different privileges and benefits for Chernobyl victims, depending on factors such as the degree of invalidity and the level of contamination. The system also guaranteed allowances, some of which were paid in cash, while others took the form of, for example, free meals for schoolchildren. In addition, the authorities undertook to finance health holidays in sanatoria and summer camps for invalids, liquidators, people who continued to live in highly affected areas, children and adolescents. The enormous scale of the effort currently being made by the three governments means that even small improvements in efficiency can significantly increase the resources available for those in need. In the end, the Chernobyl Disaster (1986) was a terrible event that can be avoided from the changes in the reactor design.