|Radiation Plume from Chernobyl|
For the non-technical readers on SLB, a brief deconstruction of the above is in order. The test was to determine if the generators had enough inertia to keep spinning and generate power, even though steam was shut off to the turbine, to keep emergency systems energized until the diesel-powered generators could be started and brought up to speed.
The plan was to run the reactor at part-load, which would have tripped the existing safety systems into a shut-down. The safety systems were therefore disabled. However, the reactor load was far below the planned load. The reactor went into a mode that had more steam bubbles than normal, which is dangerous because steam does not slow down fission products such as neutrons. This is the "positive void reactivity coefficient" mentioned above. Also, the operators pulled the control rods, almost all of them, out of the reactor. The resulting power surge caused the reactor to go critical, which melted down part of the nuclear fuel and caused not only an explosion, but the graphite parts of the reactor to catch fire.
UPDATE - 6/11/2014
So much more could be written about the Chernobyl disaster. In fact, an internet search turns up nearly 5 million websites with the term "Chernobyl." Hundreds of books about Chernobyl have also been written. Until the multiple-meltdowns at Fukushima, Japan in 2011, Chernobyl was the greatest nuclear disaster of all-time.
From an institutional safety standpoint, Chernobyl refutes many of the nuclear proponents' arguments. First, the plant was subject to regulations in its own country, the USSR. International regulations apparently were largely ignored. Who is to say that future nuclear power plant operators will not do something equally devastating, especially as nuclear plants are built in more and more countries?
Nuclear apologists or proponents are fond of saying that modern plants are secure, have safety systems and backup systems, and have designs that would never allow such an event to happen again. That is mere talk; as mentioned in the Conclusion below, it is only too easy for operators to disable safety systems or ignore warnings, and run the plant in manual mode. What is also apparent from the NRC report linked above is the very, very rapid change from quasi-normal operation to reactor criticality, meltdown and explosion. At Chernobyl, the change required only a few seconds. Operators tried desperately to insert some of the control rods, but it was too late.
It is also clear from the NRC even reports that many, if not all nuclear plants in the US run some of their systems in manual mode at times. Nothing can be made to run forever, as parts degrade and fail and must be replaced or repaired. A control system normally has an automatic mode and a manual mode, and only well-trained operators should be allowed to run the systems in manual mode.
What is also apparent from Chernobyl is the industry did not speak out in a timely manner about what happened and the risk to other countries from the radioactive cloud that was headed their way. It is true that the operators in the plant had more things on their mind right about then, if they were still alive after the explosion. However, it was radiation detectors in other countries that first gave the alarm internationally. The extent and magnitude of the event was not known for days. The psychological impact on billions of people was not small. What of the parents of small children, who needed to drink milk? What worries did couples have about future children? What worries did other people have about radiation sickness, or long-term illnesses such as thyroid cancer and other cancers?
The next two articles in TANP discuss two more disasters involving core meltdowns: Three Mile Island and Fukushima. In both instances, like at Chernobyl, a combination of bad design and human error caused major disaster. Fukushima was a bit more complex because a natural disaster, and earthquake with tsunami initiated the events.
-- end update
Part Fourteen - A Few More Reasons Nuclear Cannot Compete
Part Fifteen - Nuclear Safety Compromised by Bending the Rules
Part Sixteen - Near Misses on Meltdowns Occur Every 3 Weeks
Part Seventeen - Storing Spent Fuel is Hazardous for Short or Long Term
Part Eighteen - Reprocessing Spent Fuel Is Not Safe
Part Twenty Two - Fukushima The Disaster That Could Not Happen
Part Twenty Three - San Onofre Shutdown Saga
Part Twenty Six - Evacuation Plans Required at Nuclear Plants
Part Twenty Seven - Power From Nuclear Fusion