Clean Energy Prof: The Fermi Chronicles - Part 22: Nuclear Events

If Three Mile Island (TMI) was thought to be the worst nuclear accident in history (even though no one was injured or killed), Chernobyl drove a stake into the nuclear industry. Up to that point in time, only 3 people had been killed in a nuclear accident and that happened at the SL-1 event (see below). Chernobyl changed the nuclear safety statistic for all time. The Chernobyl reactor design was in a word horrendous. That, coupled with nothing short of shear stupidity, caused the disaster. The Chernobyl reactor design is like nothing in the U.S. The reactors were RBMK reactors, for which there is a Wikipedia entry:

RBMK is an acronym for the Russian reaktor bolshoy moshchnosti kanalniy (Russian: Реактор Большой Мощности Канальный) which means "High Power Channel TypeReactor", and describes a class of graphite-moderated nuclear power reactor which was built in the Soviet Union for use in nuclear power plants to produce nuclear power from nuclear fuel. The RBMK reactor was the type involved in the Chernobyl accident. In 2008, there are at least 12 RBMK reactors still operating in Russia and Lithuania, but there are no plans to build new RBMK type reactors (the RBMK technology was developed in 1950s and is now considered obsolete) and there is international pressure to close those that remain.

What makes the RBMK nothing short of dangerous is for one, they are graphite moderated, which itself is just a bad idea as I have mentioned before (see the Windscale event below), and also because at low power levels, the reactor tends to have a positive feedback aspect to it, meaning that it could run away with power excursions at any time.

This is in large part what resulted in the Chernobyl disaster.

At 1:23 am local time on 26 April 1986, reactor 4 at Chernobyl underwent a massive power excursion due to its positive-feedback tendencies at low power coupled with the circumvention of several safety systems by inexperienced engineers. This resulted in a steam explosion that blew the roof off the reactor building. It should be noted that the reactor building had no containment and was housed in a metal warehouse similar to Costco or Sam's Club. After the steam explosion, hydrogen generated by the unstable reactor exploded and exposed the reactor core to the atmosphere. The graphite moderator burned for some time and was largely responsible for radioactive material being discharged into the air by the resulting smoke. The radioactive particles were then carried by the wind well beyond the borders of Lithuania.

The entire series of events was initiated by a test that was to be run while the reactor was at low power. What made this event worse was that the reactor was at the end of its fuel cycle, which meant that there was a lot of highly radioactive fission products in the fuel assemblies. It was simply the worst possible time to risk anything going wrong. The test to be run was to see how much power the turbine could generate purely by its inertia should steam no longer flow into the turbine. This power was seen to be necessary to run cooling water pumps in the event of an external power failure. Thus, this simulated blackout test was to determine the adequacy of the voltage regulation system as the turbine spun down. In theory, power should have been available to drive the pumps for some 45 seconds, giving the backup diesel generators plenty of time to come up to full power.

Constant flowing water is a necessity of the RMBK design, even when the reactor is totally shut down. There was simply too much decay heat to be cooled by natural circulation alone, especially at the end of the fuel cycle when highly radioactive fission byproducts were at their maximum. This test was supposed to be performed before the reactor went commercial, but it was not done. This is a disturbing entry over at Wikipedia:

Apparently, the test had not been completed successfully by March 1984 when the unit was brought into commercial operation ahead of schedule and celebrated as a "labour victory". Under pressure, the director of the Chernobyl station Viktor Bryukhanov signed an acceptance document on the last day of 1983, in order to declare that works planned for that year had been fulfilled. Had he not done so, thousands of workers, engineers and his own superiors would have lost bonuses, awards and other extras. Records were falsified to hide this fact.

The Chernobyl power plant had been in operation for two years without this important safety feature. The station managers must have wished to correct this at the first opportunity. This could explain why they were so determined to carry out the test, even when serious problems arose, and why the requisite approval for the test was not sought from the Soviet nuclear oversight regulatory body.

The test was to be conducted by reducing the power level to 20% of normal, then the steam to the turbine was to be cut off temporarily to test the turbine spin down. However, when the reactor was gradually brought down to the 50% level, a regional power station went off line and a Kiev grid controller requested that power not be brought down further so that demand could be satisfied. The test was delayed into the night of April 25. The test crew were electrical engineers with no special nuclear training, and were likely exhausted by the time 11:00pm rolled around, when the Kiev controller gave the green light to continue the power down. The night shift of plant workers were set to come in at midnight, and had no knowledge of the test. The night crew, which was a skeleton crew at best, had little overall experience. Here is a disturbing entry over at Wikipedia on this aspect:

In Valeri Legasov's posthumous article, he maintains that the operators did not know what the test was about:

I have in my safe a transcript of the operators' telephone conversations on the eve of the accident. Reading the transcript makes one's flesh creep. One operator rings another and asks: What shall I do? In the programme there are instructions of what to do, and then a lot of things are crossed out. His counterpart thought for a while and then replied: Follow the crossed out instructions.
The operators of the power plant and the conductors of the experiment on the No. 4 reactor held too much faith in the reactor; to them, a catastrophe was simply inconceivable. Because of this, they had no qualms about disabling the safety features of the reactor and taking unnecessary risks to carry out the experiment.

Unfortunately, as the power-down commenced, the control rods were inserted too far too fast. This resulted in nuclear byproduction of Xenon-135, which is known as a fission poison since it absorbs neutrons. All of a sudden, instead of bringing the reactor down to 700 MW thermal (power output is about a third of that), it went down to a mere 30 MW. This output was far too low to run the test. The output had to be increased. However, the personnel were unaware of the level of Xe-135 poisoning. Control rods were pulled out of the core beyond the position necessary for 100% output, which had to be done manually to override safety systems. Even beyond any safety regulation. Even so, the reactor power only increased to 200 MW. Unbelievably, despite the low power, the test was actually continued by the operators!

At 1:05am, the water flow through the core was increased until it was beyond safety regulations. The core temperature decreased because of the increased water cooling and the high concentration of fission poisons. To overcome this, the operators pulled out all of the manual control rods. In addition, they disabled a backup emergency shutdown system that would have SCRAMmed the reactor under such conditions. At this point, the reactor was basically an cone standing straight up on its tip, a position that is inherently unstable.

At 1:23am, the steam to the turbine was shut off to conduct the test. The water flowrate to the core decreased as a result of the turbine spin down. Boiling started taking place. Because of the positive feedback aspect of the RBMK reactor, a bad situation was about to take place. The steam voids don't absorb neutrons like liquid water does, so the reaction rate increased. This lead to the fuel rods becoming hotter, which created more steam voids, which lead to the rods becoming hotter, etc. Positive feedback. At each feedback cycle, more and more neutrons were available for fission which eventually overcame the fixed amount of the fission poison Xe-135 in the coolant.

At 1:23:40, just 40 seconds into the test, the operators initiated a SCRAM, although the reason for this will never be fully known as the operators perished. However, damage to the core had already taken place as the fuel rods overheated resulting in a loss of geometry. Because of this, the control rods only went down 1/3 of the way before getting blocked by the distorted fuel elements. Worse yet was the fact that the tips of the control rods were made of graphite, a fission moderator, which increased the reaction rate and at the same time displaced water that absorbed neutrons. A mere 7 seconds later, at 1:23:47, the reactor thermal power was at 30 GW (that's right - 30,000 MW!). The huge thermal load vaporized water, increasing the pressure so fast that pipes ruptured. The fuel rods melted and reached the cooling water in the now flooded basement. The first steam explosion occurred at 1:24:00, blowing the lid off of the reactor. Two seconds later a more powerful hydrogen explosion occurred that blew the entire roof off the building, discharging material with it, including red-hot graphite chunks.

Radiation dosimeters were not available that were capable of large-dose readings, so workers couldn't have known the danger they were really in. By 4:30am, a dosimeter that gave an extremely high reading was dismissed as faulty. (ignorance can be bliss, no?) The firefighter that arrived on scene were also unaware of the danger. Amazingly and unfortunately the graphite fire in Chernobyl reactor 4 burned until May 10! That despite the fire suppression techniques including the injection of liquid nitrogen and the helicopter dropping of boron and lead onto the fire.

The aftermath of the disaster was nothing short of calamity, especially since the USSR kept it a secret, not warning of the radiation cloud that was spreading throughout the atmosphere. It wasn't until several days later that the scale of the disaster was to be discovered by the world. By that time, radioactivity had spread across much of Europe and even some over the U.S.

Here's a Discovery Channel documentary on Chernobyl:

Part 2
Part 3
Part 4
Part 5
Part 6

Here's a video made by a cameraman shortly after the accident. The cameraman died a few weeks later from radiation poisoning:

And video of the first helicopter flyover showing the glowing core (I don't see how this crew wasn't exposed to a lethal dose):

Some good animations in this series of vids:

Part 2
Part 3
Part 4
Part 5
Part 6
Part 7
Part 8

Much more on Chernobyl at YouTube and elsewhere.

UPDATE (12/14/2010): Visit Chernobyl: Ukraine to open exploded reactor for tours

Want a better understanding of the world's worst nuclear disaster? Tour the Chernobyl nuclear power plant.

Beginning next year, Ukraine plans to open up the sealed zone around the Chernobyl reactor to visitors who wish to learn more about the tragedy that occurred nearly a quarter of a century ago, the Emergency Situations Ministry said Monday.

...The so-called exclusion zone, a highly contaminated area within a 30-mile radius of the exploded reactor, was evacuated and sealed off in the aftermath. All visits were prohibited.

Today, about 2,500 employees maintain the remains of the now-closed nuclear plant, working in shifts to minimize their exposure to radiation. Several hundred evacuees have returned to their villages in the area despite a government ban. A few firms now offer tours to the restricted area, but the government says those tours are illegal and their safety is not guaranteed.

Emergency Situations Ministry spokeswoman Yulia Yershova said experts are developing travel routes that will be both medically safe and informative for Ukrainians and foreign visitors. She did not give an exact date when the tours were expected to begin.

"There are things to see there if one follows the official route and doesn't stray away from the group," Yershova said. "Though it is a very sad story."

United Nations Development Program Chief Helen Clark toured the Chernobyl plant Sunday and said she supported the plan because it could help raise money and tell an important lesson about nuclear safety.

"Personally, I think there is an opportunity to tell a story here, and of course the process of telling a story, even a sad story, is something that is positive in economic terms and positive in conveying very important messages," said Clark, according to her office.

The ministry also said Monday that it hopes to finish building a new, safer shell for the exploded reactor by 2015. The new shelter will cover the original iron-and-concrete structure hastily built over the reactor that has been leaking radiation, cracking and threatening to collapse.

The new shell weighs 20,000 tons and will be slid over the old shelter. The new structure will be big enough to house the Notre Dame Cathedral in Paris or the Statue of Liberty in New York.

The overall cost of the project, financed by international donors, has risen from $505 million to $1.15 billion because of stricter safety requirements, according to Ukrainian officials.

Clean Energy Prof

Monday, July 6, 2009

The Fermi Chronicles - Part 22: Nuclear Events - Chernobyl, 1986

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