What happened at the nuclear power plant in Chernobyl Russia in 1986?

Answer 1

1. The reactor was to be shutdown for maintenance and refueling.
2. Management decided this was a good time to do a safety test that had been deferred to meet construction schedule: can left over steam keep turbines turning in an emergency until diesel generators start to keep plant powered?
3. In order to facilitate repeating the test if needed, management ordered emergency water pumps turned off, emergency water supply valves turned off and padlocked, and several other emergency systems inhibited (including the automatic SCRAM).
4. Original planning was for the test to happen in the latter part of day shift and these operators were instructed on procedures and began test setup.
5. Shortly before the test could start, the Kiev power grid controller called requesting more power temporarily. The test was put on hold.
6. The hold lasted about 10 hours before Kiev released them. By this time the reactor had built up a large excess of xenon (a fission product and neutron poison). It was also graveyard shift, with operators who had had no instruction on the test.
7. When they began shutting down the reactor, after a while the xenon poisoning caused a more rapid decline in reactivity than planned and control rods needed to be removed to counteract this (instead of inserted as in normal shutdown).
8. Before they knew it they had removed more control rods than design safety limits given in the operators manual allowed, yet reactivity kept dropping due to the xenon poisoning. So more controls rods were removed!
9. Finally enough xenon decayed to allow some increase in reactivity and the reactor was temporally stabilized but at a power level too low to start the test and one that was so unstable the operators manual required shutdown.
10. Management ordered a power increase to the level to start the test at. The graveyard shift operators refused. Management told them to do as ordered or they would be replaced with operators that would!
11. More control rods were removed to do this, suddenly an alarm triggered indicating a sudden unsafe rise in reactivity (which would normally have triggered the automatic SCRAM and emergency cooling water). The operators checked instruments and agreed, one of them slapped the manual SCRAM button. All withdrawn control rods began inserting at once.
12. Note: the control rods on an RBMK reactor have a very odd construction, a 3 foot graphite (moderator) tip before the actual control rod began.
13. As the graphite tips of all these control rods reentered the reactor they caused a brief but dramatic rise in reactivity. The heat released caused flash vaporization of the cooling water first causing loud knocking noises in the steam separator tanks, followed by a large steam explosion blowing the top off the reactor building and destroying the reactor core.
14. The hot graphite in the shattered core, now exposed to air ignited, giving a large graphite fire.

It took a long time and many many loyal firefighter's lives before the graphite fire was out. Then additional effort to seal the damaged reactor. Now it will take much monitoring and work at periodic resealing it.

Answer 2

The Chernobyl nuclear power plant in Ukraine experienced a catastrophic meltdown on April 26, 1986, resulting in a massive release of radioactive materials. The explosion and resulting fire released large amounts of radioactive particles into the atmosphere, causing widespread environmental contamination and forcing the immediate evacuation of nearby residents. It is considered one of the worst nuclear disasters in history.