Mathew King is a Reactor Operator at the Nuclear Radiation Center at Washington State University. When Mat isn’t safely operating a nuclear reactor, he can be found commenting and contributing here at Babeled. Today he will help us understand the complex array of factors that contributed to the worst nuclear disaster in human history, Chernobyl.
In the last fifty years, most of the industrial world has become increasingly reliant on safe, renewable, clean, and nearly limitless energy. I’m not talking about unreliable wind turbines, or the chemical filled solar panels touted by the ‘green’ movement. Rather it’s nuclear power that fits these descriptors as well as any, and yet it is one of the most controversial forms of energy production in the world.
The pro-nuke side usually comes to the table armed with facts, statistics, and physics, while the anti-nukes have one big poster boy for their cause: Chernobyl.
Chernobyl was an RBMK-1000 Soviet power/weapons production/testing reactor near Pripyat, Ukraine which destroyed itself in the worst nuclear power disaster in history. Some basic nuclear theory is required to fully understand the event, so feel free to click on the links to learn more about the nuclear theory.
Most reactors use either water or graphite to moderate the reactor core. Chernobyl was a graphite moderated reactor that used water only for cooling. United States (and now Russian) reactors are all required by law to be slightly under-moderated. This way, if a catastrophic event occurred where all coolant was lost the reactor would shut itself down (i.e. Three Mile Island).
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Another contributor to this event was an effect called a prompt-temperature coefficient (which can be either positive or negative). In short, a positive temperature coefficient means that when a reactor gets hotter, it gets more reactive and power increases. A negative temperature coefficient means exactly the opposite. The hotter the core gets the more it wants to shut itself down. All reactors are now required by law to have an engineered negative temperature coefficient. Unfortunately, Chernobyl’s was positive.
The design of the RBMK style reactors also had a large flaw in the Control Rods. The control rods contain a poison (Boron or Hafnium) that are inserted into channels between the graphite moderator and Uranium fuel to absorb neutrons and slow down the reaction. The problem was that the tips of these control rods was not Boron poison, but graphite, a moderator. This means that control rods briefly increased power before bringing power down when they were inserted.
On April 25th of 1986, all of these design flaws came to a head when there was a scheduled test on reactor 4 of the Chernobyl complex. They were trying to find out how long they could cool the reactor if they shut down using only the residual steam and coasting turbines for power to run the coolant pumps. United States law does not allow such experiments at nuclear power plants.
In order to run this test they had to bypass several automated safety systems which was in violation of their own protocols. This was strongly objected to by the reactor operators, but unfortunately, an engineer was in charge. An American reactor operator or engineer could be sent to jail for such behavior.
This experiment had already been done several times unsuccessfully, and the only difference this time was a delay and the disabled safety systems. The reactor was run down to a low power and held there for the experiment. But shortly after that, a call came through ordering the plant to delay the experiment for several hours because of a sudden increase in power demand on the grid. The reactor was run at this low power for nearly twelve hours.
At low power, with fewer neutrons in the core, Xenon-135 built in at an accelerated rate, and the operators noticed that power was dropping further. They began withdrawing motorized control rods further and further in an attempt to stay at power, until the control elements were in their fully withdrawn position.
To make matters worse, once the experiment began, they turned on some extra cooling pumps, making the core cooler causing power to drop even more. At that point they had a reactor held at low power by cold water and burnable poison, with all of its control elements fully removed, and they were about to turn off the cooling pumps.
Compare this scenario to a game tug of war. You are pulling as hard as you possibly can but going nowhere. Then suddenly the other team lets go and you are sent flying out of control.
When they shut off the steam supply to the turbines, the flow of water slowed within the core increasing heat and raising reactor power because of their positive temperature coefficient. The reactor power increased so rapidly that someone hit the big-red button called the SCRAM(emergency shutdown).
Now, the graphite tips of the control rods (a moderator) quickly drove into the reactor, the moderator section filled the space between the fuel and further increased the reaction. This sudden increase in power and heat, led to another increase in power and heat and there was a runaway chain reaction known as a feedback loop.
The heat and power increased so rapidly that there was a steam explosion that blew the 1,200 ton reactor head off the core and through the thin roof of the building. Without the reinforced concrete and steel containment structures of American reactors, the highly radioactive molten fuel and burning graphite was ejected into the atmosphere.
The rest is just a mess, firefighters were sent in unaware of the deadly radiation levels, and the Soviet government refused to even acknowledge the incident for several days. This caused the death of all firefighters and workers sent in to put out the fires and clean up the mess. It also resulted in an impossible to count number of Thyroid Cancer cases that often result in death. The entire nearby pine forest died from radiation poising within a few weeks.
It was the inherent design flaws and human error combined with a culture at the plant that placed results ahead of safety that caused this disaster. If you remove any one of those three elements from the equation, this could not happen. The Three Mile Island scenario was similar to Chernobyl in that several problems occurred simultaneously and destroyed the reactor. However, the designed fail safes and containment structure of the Reactor functioned as designed and no one was hurt, and no one received an excessive dose of radiation.
Chernobyl was a hard lesson learned by the Soviets and indeed to the entire global nuclear industry. The US has also taken some cues from the incident and placed additional limits on safety culture and operator training. Now with more need for reliable energy than ever, I hope my future kids grow up in a nuclear world.
Image used in this Post
Chernobyl photo courtesy of Flickr user Kamil Porembinski published under the CC license.
For what reason were some early reactors designed with a positive temperature coefficient in the first place? Was it a cost saving measure? Did it yield more energy under ‘normal’ conditions?
Russian nuclear technology lagged behind the U.S. by a significant margin until only very recently. The positive temperature coefficient was a design flaw that was simply overlooked as a safety feature by Russian engineers.
Based on your experience, was that an acceptable oversight or something the Russians should have easily identified? Being very much a lay person I am dismayed the engineers something that seems so intuitive and obvious.
Either way, great job on the post Mat and thanks for taking the time to write it for us!
My experience is very biased. Having entered the field with so much already known about reactor technology, and not being an engineer, I don’t know how something like that looks in the design phase.
I would have to say that the design itself would have been fine if they had followed their own procedures and kept their safety systems engaged. The fault was more with human error than anything else, although a better design could have prevented this accident entirely.
The RBMK reactor design did not have a positive temperature coefficient ALL of the time. The temperature coefficient of reactivity is not a constant, but a number that varies based on the combination of at least six factors in a differential equation. Power history, neutron poison concentrations, coolant flow, control rod position, operating temperature, and several other factors all come into play.
As I understand the history, the specific conditions at the time of the accident combined to provide one of the narrow points in the operating profile that would lead to the positive temperature coefficient.
There are times when my most cynical nature wonders just how accidental it was that a large nuclear power station blew up in a nation where more than half of its hard currency income came from oil and gas sales to its already nervous about nuclear neighbors.
Rod Adams
@ Rod,
Good explanation. So it was more of a perfect storm of bad luck, poor timing, coincidence, or more nefarious motives that allowed the disaster to occur. Weighing that explanation I can see where your cynical inclinations can gnaw at you.
Hey Rod,
Nice catch on the Temperature Coefficient.
While I doubt that the Russians had any deliberate intention to do this, history certainly does show that the Russian government was willing to tolerate grotesque levels of human suffering on the part of its own citizens to achieve its ends. Look no further than WWII and the Soviet space program as evidence of that. 50 or so fire fighters and an untold number of cancer deaths is certainly below their historical allowances for civillian deaths.
Mr Adams, you ask the most basic question. The article was very detailed on the “how”, but the “why” was never mentioned. In the reactor description, there are some very good clues, however, no one except the engineer in charge, or perhaps the government, would know for sure. I myself was an unwitting subject of this very grave and damaging Soviet experiment. I was enjoying an adult beverage on a sidewalk cafe, west of the incident. My immune system was compromised and my health suffered as a result. The Soviet Union was indeed, the “evil empire”, as President Reagan called it. Only a Godless communist would have even attempted such an exercise, as the ends always justifies the means.
Graphite versus water-moderated reactors: is there a difference in the safety and efficacy? Which is optimal, or is that a matter of design difference and opinion?
There is one major reason for using water, and that is: graphite burns. Part of the reason Chernobyl was so devastating was because of the burning graphite that was released into the environment carrying highly-radioactive fission products into the atmosphere with the smoke.
Water also makes a better moderator because it contains more hydrogen. A carbon nucleus is relatively large compared to a neutron, so when a neutron hits it, it slows down some but recoils with a significant portion of its original energy. This is analogous to throwing a ping-pong ball at a billiard ball, the ping-pong ball will bounce off.
A hydrogen atom is close to the same size as a neutron, so when they collide it is more like 2 ping-pong balls colliding, they recoil with equal force. This is why graphite is typically used as a neutron reflector, and the moderating is left to the hydrogen in the water or fuel matrix.
That all makes good sense. Thanks for the response.
I’ll add that in a US reactor water acts as a moderator AND a coolant. In the RBMK, water was only a coolant, meaning as the water boiled off, no moderation was lost so the fission kept warming the reactor, raising power, warming the reactor, raising power….. In an American reactor (i.e. TMI Unit 2) when the water coolant boils off, the moderation is lost along with the coolant, so the reactor shuts down and only meltsdown from decay heat instead of blowing up from an out of control fission reaction.
Nuclear reactors don’t kill people. Stupid people in poorly designed nuclear reactors kill people.
Nice picture.
The picture shows the sarcaphogaus (sp?) that was built around the destroyed reactor building. You can see the scaffold that was abandoned in place because no one would bother with the extra radiation dose it would take to remove it.
The actual destroyed reactor building was very erie. You could see the contaminated smoke from the burning graphite pouring out of the wreckage. The fire fighters who went in to put out the fire had no idea what they were running into and they all died horrible deaths from radiation sickness.
did you really say ‘renewable’?
fail.
I beg to differ. Using U-235/238 as fuel you can produce Pu-239 which is yet another useful fuel in MOX (mixed oxide fuel), proliferation fears aside.
There are also U-233 and thorium breeder reactors that can produce more fuel than they consume. So Nuclear technology is renewable in every way that matters.
Is there a clear cut ideal fuel? Or are there pros and cons to each type that you mentioned above?
It really depends on what you are trying to do with your reactor. Pound for pound Pu-239 is a much more efficient fuel than U-235, but it is much more expensive, harder to produce in quantity, and easier to weaponize.
U-233 is also actually more ‘potent’ than U-235 but again it is much less plentiful (completely absent in natural uranium I believe).
Thorium is one of the most plentiful elements on the planet, and can by converted into U-233 by absorbing a neutron. The technology to make a thorium reactor is still in its experimental phase, but is a very exciting future prospect.
“Nuclear technology is renewable in every way that matters.” – Mathew King
Everything renewable and recyclable has some kind of end date at which it cannot be reused again. Everything.
What is the end date for recyclable nuclear fuel? I don’t care if it is a millennium from now (that time will come eventually)….at what point in the far future will our descendants have to deal with the repercussions of nuclear fuel that can’t be recycled anymore…..or will that never ever happen?
How does Yucca Mountain fit into this?
Doesn’t matter. Most (if not all) of our species going to die from some environmental change precipitated by solar radiation fluctuations, a polar shift, change in deep ocean currents, or massive tectonic-related issues long before any of this is a concern. Those who may survive will be put back to the stone age with no technology because we are so compartmentalized that no one person has enough know-how to reproduce a substantial portion of our current technology-base. Apocalyptic rant over.
It does seem inevitable, doesn’t it.
Waste is an inevitable problem, but not as much of a problem as many anti-nukes make it out to be.
A few statistics I have picked up over the years:
ALL of the nuclear waste produced from reactors since the dawn of nuclear technology could fit onto one football field in a layer 9 feet thick. That isn’t much for 60 years of waste.
What is typically called “spent” nuclear fuel is actually just so full of undesireable byproducts of fission that it cannot be used anymore. In reality roughly 80% of the originaly fissile material still remains in a “spent” fuel rod. This entire assembly is then discarded as waste because sadly reprocessing is illegal in this country. Reprocessing could remove nearly 80% of the problem alone.
The vast majority if ‘nuclear waste’ is low-level, and consists of rubber gloves, paper, bits of plastics and metals, none of which is particularly hazardous (I know I have generated a lot of this type of waste). Yet it is still lumped into the category of nuclear waste.
The remaining byproducts from reprocessing are the only real problem. The Hanford site here in Washington is actually working on a method of vitrifying (turning into glass) dangerous nuclear waste. This would contain the problem into neat little chunks of glass that could be easily buried to decay for a couple thousand years.
Yucca mountain is a bust. The encombant president but the kai-bosh on that one.
Thanks, that was an informative.
“Fail” is as technical an argument as I’ve ever heard from an anti. But it certainly breaks new ground on the level of lame associated with an already renewably lame movement. Congratulations.
It’s the new “Not.”
@Mathew King – As far as my research tells me, reprocessing is no longer illegal in the United States. President Carter finalized the ban on reprocessing but President Reagan lifted the ban. I’ve not read anything that indicated any president since has reinstated it. That said, the current system of cooling the rods in pools and then to dry cask storage is relatively cheap and takes little space so there is little motivation to add costs when fresh fuel product shows no sign of running scarce anytime soon.
Interesting point. I am wondering if it is worth the cost to implement spent fuel recycling simply for the good PR and to acquiesce the fears of anti-nukes so that we can move into an era of new nuclear construction? I think it’s worth it, but it seems utility companies are reticent to invest capital in any type of infrastructure upgrades for the sake of their stock price and EPS. Very sad.
The nuclear industry has already paid $24Billion into the Nuclear Waste fund to have Yucca Mt. built. The govt did not keep up it’s end of the bargain.
Now the DOE is spending hundreds of millions on dry cask storage above ground at more than 80 sites around the country. These casks have a 50 year lifespan.
Why not spend less on Dry Casks (band aid solution), and use some of the 24 Billion that has already been paid to the waste fund to build a recycling facility? The money is already there, it’s not like we would need to raise taxes or bill anyone for it.
So we agree that spent fuel should be recycled?
I leiu of a permanent repository, which appears to be long term at best – yes. The real solution to closing the fuel cycle is reprocessing combined with a permanent repository. Both are political landmines unfortunately.
Not knowing anything about Dry Casks that seems far more dubious than storing waste at an established and predetermined location. Why have numerous casks at numerous locations? But, as far as our government goes it is par for the course I guess.
Dry Cask storage is the ultimate political indecision. Safe, expensive, temporary, doesn’t offend anyone, let somebody else worry about it.
A Russian islonic/notize fundamentlest pulled delibrit opearter earrior. pulling the hot water out the reactear , then shoving cold water in steaming off the reactor core lid.
casper wineburger us secatary of defance told vaughn nebeker . that the govit would pay him eney thing as long as his desine put out cheranobyl. vaughn nebeker desine’s had cherobyl out in ten day’s instead of it burning a 120 day’s beatting the reactear a 110 day’s. saving 58,000,000 lives. the way Casper winebuger put it.
Use up whaat people you need. for humen life is renewable . how ever a planit is not
if you lose it. Casper winebuger wanted cherobyl out on a 99.99/10 percent cench.
an that what i gave him… I put up a 4220.0 billion at a trillion to one odd’s at a dubble or nuthing cherobyl would go out. Reality cherobyl went out an I won.