Chernobyl Disaster

Chernobyl Disaster

The Chernobyl disaster happened on the 25th of April in 1986. It was a mixture of disregard to safety and The Cold War policy on sharing between the communist U.S.S.R and the U.S.A... While being the worst commercial nuclear disaster in history, it was also one of the easiest to prevent.

The disaster in Chernobyl, Ukraine was a nuclear meltdown of the Chernobyl-4 reactor. The problem started with the test that was to be performed before shut-down. The test was to see how long the turbines would spin after a loss of the main electrical power supply. This test in itself defies logic. These reactors become very unstable when power supply is lowered. Part of the problem was caused by turning off the ECCS (Emergency Core Cooling System). Another player in the catastrophe was the Grid controller, who controls electrical power to the various parts of the plat, refused to lower power for this reactor. For this test the reactor should have been stabilized at 1000MWt (Megawatts thermal) but due to an error in the system the system fell to 30MWt. This is where something called the positive void coefficient comes into effect. This coefficient is the increase in steam production in the fuel channels, allowing the neutrons to produce increased fission instead of being absorbed by the denser water. This is fine at normal operating status, but when the power output drops below twenty percent of the maximum the reactor becomes unstable and prone to sudden power surges. This is the basis of what happened at Chernobyl. The operators tried to raise the operating power to 700-1000MWt which would have been a relatively safe area, but were only able to stabilize it around 200MWt. Another problem was the fact that the RBMK model reactor is suppose to function with a minimum of 30 control rods; the crew was running Chernobyl-4 on 8 rods. This was due to the buildup of xenon which reduces power. It takes about 20 seconds to lower all the control rods in the case of a surge, but still the test went on. An increase in water flow resulted in a drop in steam, at this point the auto shut-off mechanisms would have come into play except that they were turned off for this test. The decrease in steam was answered by the operators with the removal of even more control rods. The result was a very unstable core and constant changes had to maintain stabile power. The point of the test, the slowing turbines, was the final problem in this ordeal. The slowing turbines were pumping less cooling water which in less volumes is easier to turn to steam, causing an uncontrollable surge to over one hundred times the nominal level. The surge caused a breach in the fuel elements caused a rush of steam that lifted the cover plate, releasing nuclear material into the atmosphere. A second explosion expelled fuel and graphite from the core, also allowing air to flow into the core where it reacted in a combustion reaction with the remaining graphite which instantly burst into flames.

The next problem was getting the fires under control. The conventional fires were quickly taken care of though they resulted in 31 deaths caused by radiation poisoning. The real problem was the graphite fires. At this time there was no real experience in graphite fires, especially those involving radiation. There were many variables to look at. A major problem is what will happen to the radionuclides. They could be dispersed in steam or whatever is attempted could cause another explosion resulting in a major dispersal of radiation. The decision was finally made to dump an assortment of different chemicals to combat different aspects of the fire. The total amount of materials used was around five thousand tons. This included forty tons of boron compounds, two-thousand four hundred tons of lead, eighteen hundred tons of sand and clay, and six hundred tons of dolomite. One problem arose with the helicopters hovering over the reactor were exposed to far too much radiation. The loads had to be dropped as the helicopter passed over the drop zone. This had the negative effect of causing more damage which in turn led to more contamination. It was later found that much of the material was not even dropped on the core if was believed to act as a thermal barrier which caused an increase in the release of radiation due to an increase in temperature.

There were a total of fifty-six deaths that can be directly linked to the Chernobyl disaster according to the United Nations. There has also been an increase in thyroid cancer especially with children that were in the surrounding area during the explosion. The worst of the radiation was received by the rescue workers shortly after the explosion. There levels reached twenty thousand millisieverts. The sievert is used to measure the biological effects of nuclear radiation as opposed to the physical effects,

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