What happened in Chernobyl Ukrain?

Answer 1

On April 26, 1986 the Chernobyl nuclear power plant, in what used to be the Ukranian SSR, the worst nuclear power plant in history occurred.

The day was to coincide with a test using reactor number 4 to try and fix a basic flaw in the plants design. The flaw was that if the plant was ever shut down in an emergency it took about a minute to fire up the desire engine generators to continue the flow of coolant in the reactor. This time gap meant that shutting down the reactor in an emergency would result in a meltdown.

To counteract this problem the engineers came up with a solution. The plan was to use the remaining power in the turbines to run the coolant through the reactor as the backup generators started up. Three previous tests had resulted in failure; that day being the fourth.

The plan for the test was to power down the rector to between 700 and 800 MW. Once there the steam turbines would be run up to full power. Next, the steam supply to the turbines would be shut off. The performance of the turbine would be recorded to see if it would provide sufficient power to circulate the coolant in the case of a power failure. The plan was authorized by the director of the plant. No one ever got the approval of the designer or the scientific manager of the plant; if the approval was gotten chances are they would have stopped the test when safety parameters were broken.

Powering down the reactor began in the day shift on the 25 April 1986. Once at 1600 MW, half power, a different power plant went offline and the power down had to be postponed because the power was needed on the grid. The day shift left and the evening shift was preparing to leave when the call came in to resume the test. The night shift was unprepared for the test expecting only to deal with the end results of the test, but now they would be forced to deal with the test itself.

Five minutes after midnight the reactor finally reached the tests required 700 MW, but the reactor had a natural buildup of the isotope xenon-135(this isotope collects loose neutrons) further powering down further from 700 MW to 500 MW. The operator of the reactor that night was a rookie and he accidentally put the control rods in too far. This absorbed even more neutrons lowering the power level from 500 MW to a measly 30 MW.

This made the test even more dangerous; so the managers decided to extract the control rods to almost there limit in an attempt to raise the power level. The buildup of xenon-135 had continued during the power down so more control rods were removed.

All of these factors, as well as the core temperature and coolant flow becoming unstable, were causing alarms to start going off in the control room. By 1 AM the reactor was finally stabilized at 200 MW and setting up for the test resumed.

During the time between the stabilization of the reactor and the beginning of the test coolant flow exceeded safety limits causing the reactor to cool down and reduced its power again. This made the reactor become more unstable.

The experiment finally began at 1:23:04 AM. Steam was shut off and the turbines began to run down. The diesel generators were started up and started to take the load off of the turbine, but not fast enough. The rate of flow for the coolant began to slow. The lack of coolant caused steam bubbles to form in the reactor. That caused the reactor to increase its power level. The steam bubbles also weakened the ability of the water to absorber neutrons causing the reactor to power up even more. This constant powering up caused yet more steam to form, one can see the loop that's created causing constant powering up. To counteract this some control rods were reinserted.

Thirty-seven second later at 1:23:40 the button to emergency shut down the reactor was pushed. The shutdown put all control rods into the reactor automatically. The control rods began to lower at their normal astoundingly slow speed. The tip of the control rods were designed poorly and actually caused an increase in power production as they entered the reactor. This caused a massive power spike to occur causing the first explosion.

This explosion caused the control rods to become stuck only inserted about one-third of the way. The power generation continued to rise. The instruments stopped recoding data at this point so it is theoretical. Parts in the reactor began to rupture as steam build up continued causing another massive power spike. A steam explosion is thought to have happened next. This explosion is what lifted the 2,000 ton upper plate off of the reactor. With it now open the remaining coolant escaped and the reactor continued its power clime.

A few seconds after the coolant escaping caused another explosion resulting from the release of neutron particles occurred. Graphite from the reactor caught fire; this fire and the explosion greatly increased the spread of radiation. Burning graphite was expelled from the core landing on various roofs of the plant causing more fires. One of the roofs that caught fire was the building housing the cooling unit for reactor three which was still running at the time.

The fire department arrived on scene and started trying to put out the fires. Most of the fires were finally put out around five in the morning. The fire in reactor number 4 continued to burn for two weeks. To put out the fire sand and other material was dropped on the reactor from helicopter.

Pripyat, the nearby town, was evacuated on April 27. The Soviet Union refused to admit there was a problem until a Swedish nuclear power plant had radiation alarms go off.

Underneath the reactor there were two levels of water pools that were used by the emergency coolant system. Above them, molten goo of concrete, graphite, fuel, and other materials had formed into a lava like substance. If the substance burned through the floor and got to the pools another steam explosion would result spreading more radiation. To prevent this from happening three men swam through radioactive water to turn valves to release the water from the pools. The men were successful; two of the men later died of radiation poisoning.

The radioactive debris was collected. A concrete "sarcophagus" was erected around the reactor; then was coved by various materials. 239 people from around Chernobyl were treated for radiation sickness; 31 of which died within three months of treatment. Effects of the disaster are still evident today. Funds for erecting a better shelter than the current "sarcophagus" have been collected since the late 1997; completion of the project is hoped to occur in 2013.