|Nov20-06, 11:37 PM||#1|
Chernobyl - Can someone answer a few questions please?
Can anyone answer a few questions I have about chernobyl and related. I am doing some research on something related, and its hard to get good information about this. Here they are.
1) - Can someone explain the steps of the disaster? I mean, lets say is the cooling system failed, what were the physical factors that happened. Did the core heat WAY up? I thought it had something to do with steam bubbles?
2) - This is a question based on the answer to the first question - How is radiation released, what causes the Uranium to release this radiation, and how come it is not released under normal opperation conditions?
3) - Can anyone explain any extraordinary effects this radiation had? Can you explain how this type of radiation effects humans? And how it effects/kills them so fast (They said direct exposure kills under 1 hour!! ? )
That would be great if you could answer these. Thank you and take care.
|Nov21-06, 05:57 AM||#2|
If you haven't, take a look a this web site. It has some great info on Chernobyl and really is a great read.
She gives a brief description of the events here:
|Nov21-06, 05:58 AM||#3|
well the last one is basically doin youre assignment for you but ill give you some pointers
1.) not entirely sure, i used to know but i forgot the details. all i know is it wasnt a nuclear explosion. the pressure built up and the dome imploded as the concrete melted, i think. it then exploded.
2.) radiation in this situation is due to nuclear fission, a term which you can easily look up and get oodles of information. radiation is emitted under normal conditions from uranium as it undergoes radioactive decay, but fission releases far more.
3.) radiation damages humans because the energy particles/waves interact with and damage DNA molecues in a way that causes mutation and/or irrepairable damage
EDIT: damn fred submitted his as i was typing mine :P
|Nov21-06, 08:49 AM||#4|
Chernobyl - Can someone answer a few questions please?
The Chernobyl reactor, the RBMK; is a BAD design. It was a scale-up of a Soviet
nuclear weapons production reactor. The RBMK was dual-use; it produced fuel for
nuclear weapons as well as power.
Even so; the main cause of the Chernobyl accident, as with the Three Mile Island
accident; was STUPID operators!! The operators had planned running an experiment
on the Chernobyl reactor [ a poorly planned experiment, at that]. They lowered the
power in preparation for the experiment.
However, the grid load controller in Kiev [ the people that run the electric system ]
called the plant and requested that they remain online at the reduced power because
Kiev needed the electricity. It was 12 hours later before the load controller let the
plant go offline.
When you shutdown or reduce power on a nuclear reactor, there is a temporary
build-up of a neutron poison; Xenon-135. This is called a "Xenon transient" - the
effects of which will disappear in under 24 hours if the reactor is left shutdown.
However, when the load controller released the Chernobyl reactor to go offline - they
were right in the middle of this Xenon transient. The Xenon makes it difficult for the
reactor to operate. But the operators wanted to run the experiment.
So they bypassed all the safety systems!!! They pulled the control rods out farther
than the safety systems would have allowed them to do otherwise. Because of the
design of the RBMK control rods that have a "non-poison follower"; this is a
particularily DUMB thing to do. It means that if you request an emergency shutdown
or SCRAM of the reactor; the control rods will cause power to INCREASE before
they DECREASE power. [ Such a design is NOT PERMITTED in the USA. ]
The reactor was in a VERY UNSTABLE condition due to the low power and Xenon transient.
Then the operators ran their experiment!!! KABOOM. They had a fast release of
energy which blew the reactor open. The RBMK is graphite moderated. Most of
the volume of the reactor is graphite - the stuff that's in the "lead" of your #2 pencil.
It's like charcoal, and when exposed to the air at high temperature; the graphite
caught fire and started to burn. It was the heat and convection of the fire that
spread the radioactivity.
The main problem at Chernobyl was that the operators weren't THINKING!!
They were so intent about running their experiment, they didn't stop and
think about what was really happening in the reactor. The safety systems
tried to stop them; and they overrode the safety systems because the
safety systems weren't going to let them do their experiment!!!
radioactive fission products, the remanants of Uranium atoms that have split can't
move because they are surrounded by Uranium atoms. [ Uranium is hardly radioactive
at all. You can safely hold unirradiated Uranium in the palm of your hand. It is the
remnants of split Uranium atoms that are so radioactive.]
Additionally, the fuel is encased in Zirconium tubes that prevent the escape of the
radioactive atoms. The entire core of Zirconium-encased Uranium is locked in a
sealed reactor vessel.
The whole building around the reactor, called a "containment building" - the big
cylindrical buildings you see at a reactor plant, is also sealed. Courtesy of
Wikipedia; an aerial view of the Braidwood nuclear power plant in Illinois:
[Zoom in on the two cylindrical builings in the center of the picture. Those are
the containment buildings for the two reactors at Braidwood. ]
These buildings are shaped / constructed to take great pressure; so they can
"bottle up" any accident. They are the final layer in a multi-layer defense that
protects the public and environment.
that will kill in under an hour, the radiation affects your nervous system. Your body
is controlled by an "electrical system" called the nervous system. This system
conveys the control signals that regulate the proper operation of your body. With a
high enough radiation field, those signals are disturbed, and your body doesn't
operate properly. Can you imagine how long an airliner would continue to fly if
someone damaged all the electrical and hydraulic systems the pilots use to control
Dr. Gregory Greenman
|Nov21-06, 11:30 AM||#5|
1) As Morbius indicated, the decay of U releases little radiation. On the other hand, the fission of U creates the radiation in a reactor. The absorption of a neutron (capture) causes U-235 to become an excited U-236, which is unstable and emits a gamma ray or fissions.
The fission process creates two new nuclides (fission products), which are themselves radioactive (hence radionuclides), and releases prompt neutrons (usually 2 or 3). These prompt neutrons are necessary to fission more atoms, but some are absorbed by the structure, which then becomes radioactive. Most fission products decay by beta-emission (and a few by positron emission or electron capture) and gamma-decay. Some fission products release neutrons, and it is these delayed neutrons that allow for the control of the nuclear reaction. The Chernobyl accident was an example of a prompt critical excursion, which happens in fractions of a second and there is no time for humans to respond. As Morbius indicated, the experiment should never have been permitted.
2) In most power reactors, U is in the form of ceramic grade UO2 which is usually clad in an alloys of Zr. The UO2 is usually in the form of right circular cylinders or pellets. Most fission products are solid, although some have low melting points or are volatile (e.g. I, Br, Cs), and others are gaseous (Xe, Kr - fission gases).
As long as fission products stay in the fuel, they simply emit the beta and gamma radiation - through the cladding. This is why spent nuclear fuel is stored underwater (which also provides a cooling medium) and after a while the fuel may be stored in canisters of steel or concrete.
Ultimately the fuel may be reprocessed or sent to a repository for burial. If the fuel is reprocessed, the fissile/fertile material can be reused, but the fission products must be calcined into a solid form, which is then encapsulated, and that form is buried in a repository.
In US, most of Europe, and elsewhere, power reactors are contained in a containment structure as Morbius mentioned. This structure is several meters thick - stainless steel lined inside reinforced concrete. It is designed to 'contain' any accident of the reactor and primary cooling system. Chernobyl had no effective containment - it was housed in a conventional metal building structure, which blew apart in explosion.
For thorium - see - http://www.world-nuclear.org/info/inf62.htm
Any fission process whether conventional fission or accelerator driven reactor will produce radioactive fission products - that is inherent in the process - that is where the thermal energy originates.
|Nov21-06, 11:33 AM||#6|
Wow, thank you all for the VERY fast reply! Also, thanks you Morbius for the detailed reply :)
EDIT: Thank you also Astronuc for the detailed reply :)
I just have 2 more questions, One for my research and one for my own personal interest.
1- (research) - RBMK design - sorry if this is a vauge question - but how does the RBMK design work? How does it get energy form the Uranium?
2 - (Personal Interest) - In the RBMK, or any reactor they have to keep the core cool correct? They use distilled water as a coolent? My question is, if the coolent system for some reason fails, and there is no way to cool the reactor, can you simply turn off the reactor? If not, and the core continues to heat up, what are the possible outcomes of this?
Thank you all again for the great help. Take care.
|Nov21-06, 11:38 AM||#7|
showed a diagram of the RBMK reactor. The portion of the building above the
reactor floor has WINDOWS at the top!!! There was absolutely NO pressure
containment capability in the RBMK reactor building.
EDIT: Here's the graphic refered to above, look under the heading of "Infrastructure Development":
You may need to enlarge the graphic, but look above the red bridge crane, up next to the red truss
at the top of the building; the wall has WINDOWS!!!
sub-critical system, or a self-driven critical system is immaterial vis-a-vis the
The energy to melt the core comes from decay heat, not fission heat; and decay
heat is inherent in either system.
Dr. Gregory Greenman
|Nov21-06, 05:02 PM||#8|
The pressure tubes allow water to flow through the assemblies and away from the graphite. The uranium in the fuel is fissioned by neutrons, and each fission releases about 205 MeV of energy of which about 168 MeV are in the form of kinetic energy of the two radionuclides (fission products) formed by the fission of the U (Pu) nucleus. The thermal energy from the fission process is conducted through the fuel to the water, and the water is heated as it passes through the fuel. Heat from the primary system can be sent to a large heat exchanger where it is transfered to more water which is boiled at a lower pressure. The steam is passed to a turbine (which converts thermal energy to mechanical energy) and turbine then turns an electrical generator (convers mechanical to electrical energy).
A reasonable description - http://en.wikipedia.org/wiki/RBMK
RBMK - Реактор Большой Мощности Канальный
http://en.wikipedia.org/wiki/Nuclear_fuel - shows RBMK fuel assembly.
At commissioning of a core, neutron sources (Sb-Te photoneutron source, or Pb-Be source) are necessary to initiate the fission process. The sources also allow the reactor to approach criticality under control. Once sufficient transuranic isotopes accumulate in re-inert fuel, one can do a 'sourceless' startup using the neutrons from spontaneous fissions.
Control rods containing neutron absorber materials are inserted in the core to slow down or shutdown the reactors. The RMBK and VVER-440's use control fuel assemblies, and I concur with Morbius that it is a bad idea.
Control materials are boron compounds (e.g. B4C), Ag-In-Cd (common in western reactors), Dy-titanate (Russian idea) and Hf. Hf was tried in western reactors, but it absorbed hydrogen from the coolant and the Hf swelled causing the stainless steel (SS304, or SS316) structural material to crack. Cracked control elements is a big NO-NO in the industry.
If the cooling system fails, the control rods are inserted rapidly into the core to shutdown the system. In water cooled systems, there are safety injection systems attached to the primary cooling system that allow borated water to be injected into the core (part of the Emergency Core Cooling System - ECCS).
If the cooling system fails, the core could heat up - as happened at TMI. If the core heats to a certain point and pressure builds up, then the primary system may fail and the fuel and fission products could escape into the containment building. If there is a power excursion like Chernobyl or SL-1, and explosion may occur with the resulting dispersal of the fuel. Western plants have concrete and steel containment buildings to 'contain' the consequences of reactor/primary system breaches - and also to keep out large objects (generically - missiles) that could harm the reactor system.
|Nov21-06, 08:59 PM||#9|
Once again, thank you very much for the informative reply - You have answered all my questions :)
I wanted to ask this one last thing:
I have heard (Again, sorry for being vauge) that is the core heats up far past where it should be, and nothing can cool it down, and melts through the reactor, it would hit the earth and either split the earth or melt its way through a good portion of the ground. Is that possible?
|Nov21-06, 10:40 PM||#10|
http://www.spaceman.ca/gallery/chernobyl - good images
http://www.spaceman.ca/gallery/chernobyl/f421 - lava
http://www.spaceman.ca/gallery/chernobyl/fruin11m - elephant's foot
http://www.spaceman.ca/gallery/chernobyl/sark3b - another view
There is concern that material from a molten core could cause a steam explosion, however the water would likely boil, and the steam boiling on the surface of the molten core would solidify it and reduce the heat transfer rate. If one has seen lava in the ocean, it boils very locally as the lava oozes into the water. In order to have a steam explosion, the would have to be somewhat finely dispersed.
|Nov22-06, 08:04 AM||#11|
One of the key words in your question is "IF".
One can postulate a scenario like you phrase above; however when you do that, you
are totally discounting all the measures taken to prevent the scenario.
As Astronuc explained, there are design features of nuclear power plants designed
to stop the scenario you envision.
One of my Professors tells the story that he gets asked that type of question all the
time; what if the core melts, and the containment fails, and...
He replies "What if two 747 airliners were to collide over Pasedena, and they crashed
into the Rose Bowl stadium, and it happened to be New Years Day while the
Rose Bowl game was being played with tens of thousands of people in attendance,
and the flaming wreckage of the 747s kill all the people in the stadium..."
At which point the person asking the question usually gets upset and says something
like "Don't be ridiculous - that scenario is totally contrived!!". The Professor would
then reply, "Not as contrived as the one YOU gave!!"
The point being, is that one can dream up strange scenarios - and that's good,
because that tells us what we have to protect against. However, you also have
to be mindful of the the probabilities of that event happening, and the measures
taken to prevent it.
If the probability of the event is so low as to be one-millionth the probability that the
Earth gets clobbered with a big asteroid that wipes out all life on the planet, then you
should spend your time worrying about the asteroid and not the power plant.
Dr. Gregory Greenman
|Nov22-06, 09:10 AM||#12|
As Morbius points out, one can ask "what if . . ." until sheep fly, but if one is asking "what if" about a highly unlikely, improbable or impossible situation, e.g. "what if gravity reversed" or "what if the earth suddenly stopped spinning?", then we stop dead in our tracks.
Instead we spend our time thinking about what might actually happen and we engineer ways to deal with it and mitigate the consequences. Then there are further studies and experiments to better understand accidents and prevent them or at least mitigate the consequences. That's part of being a good engineer or scientist.
|Nov22-06, 09:32 AM||#13|
One of the Bay Area radio stations has a talk show host who answers questions on
science. He is a former Professor at Berkeley and was formerly on staff at the lab
where I work.
When people ask him some of these "what if" questions; especially the ones that
don't really have an answer, his reply is:
"I want you to IF in one hand, and SPIT in the other; and tell me which hand has the most"
fantasies that are meant to scare people; is what being a good scientist or engineer
is all about.
Dr. Gregory Greenman
|Nov22-06, 12:25 PM||#14|
Morbius and Astronuc, your first two posts here were very informative and interesting. I had followed all the news on Chernobyl as it unfolded in the papers and television, but it's nice to have an accurate post-mortem analysis. I had never heard about Xenon and the power grid. Thanks!
|Nov22-06, 02:53 PM||#15|
Here's a rather technical report on Chernobyl from the IAEA:
There's a good summary of the causes of the accident starting on page 23
as marked in the report.
On page 19 of the report; the issue of the ad hoc modification of the
experimental procedures is addressed:
"When the reactor power could not be restored to the intended level of
700 MW(th), the operating staff did not stop and think, but on the spot
they modified the test conditions to match their view at that moment of
the prevailing conditions."
When explaining the "Xenon transient", I have oft used what I call my
"two bathtub analogy"; two bathtubs, one above the other representing
the levels of I-135 and Xe-135. The two bathtubs obey the same
form of the diffential equations as does the I-135 / Xe-135 balance
equations. I found a paper that also uses a form of this analogy in
some class notes from McMaster University at:
See pages 6/14 and 10/14
Dr. Gregory Greenman
|Nov22-06, 09:56 PM||#16|
Thank you very much the for details and informative replies.
It is human nature for us to think of the worst possible outcome, nevermind if that outcome is possible or not, we as human must worry about something. I unfortunately am in the business of risk and analysis, so this is something I have to deal with everyday. If people take the time to learn something, they might not need to worry as much as they presently do.
Rational minds cannot save us I am afraid :)
The reason people do not worry about something like an asteroid is because we did not make the asteroid, and even though an asteroid could wipe out our plannet, we still worry about a malfunction in a nuclear reactor, and even if measures have been taken to limit certain negative outcomes, it is still something that we control and have made, so by nature, it has flaws.
I find it funny how people think. How many people has Uranium killed in our lifetime? 500k, 1M ? - The general public still fears it, yet smoking kills 5million per year!
|Nov23-06, 03:30 PM||#17|
I have another question about radiation, mainly the type that was given off in Chernobyl (Reactor)
some of you gave a good example of what it does by effecting out CNS, so it's like the electrical system of a car shutting down.
Can you give me an example of a healthy person, walking into an area of very high radiation levels, much like the fireman that tried to put out the fire at Chernobyl and what that person would experience? I understood they died within hours, so what would be the first thing that would happen to them?
I am planning on going to Chernobyl (well, to whatever check point I can get to) because it has been a great interest of mine for quite some time.
Thank you all for the great help :)
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