Last night I began reading Tom Zoellner's excellent book: Uranium: War, Energy, and the Rock that Shaped the World.
The book helped me better understand the events at Fukushima. What follows is my effort to understand how Fukushima continues to actively produce radiation in the absence of a nuclear explosion. I am drawing upon Zoellner's book and also Wikipedia's account of a Nuclear Chain Reaction found here (http://en.wikipedia.org/wiki/Nuclear_chain_reaction)
Direct quotes are in quotation marks.
Uranium is naturally unstable and Uranium-235 is the
most unstable isotope.
Uranium-235 must be distilled and “enriched” to
create nuclear fuel.
U-235 is uniquely equipped to release energy when
its nucleus is torn apart because it is the “heaviest element that occurs in
nature, with ninety-two protons jammed into its nucleus” (p. ix)
A sphere of enriched U-235 the size of a grapefruit
can destroy an entire city.
Uranium naturally loses protons because it is
unstable and decays into radium, then radon, and then polonium. It eventually
rests as lead.
In order for the Uranium-235 to sustain a nuclear
chain reaction it must be enriched to more than 20%
How is Uranium-235 split? That is, how does fission
occur?
The answer is that Uranium is split by aiming
neutrons at the atom’s nucleus. Protons and electrons are charged and therefore
do not penetrate the nucleus but neutrons have no charge and therefore can
penetrate the atom.
When an atom of enriched Uranium-235 is split by one
neutron, it emits two neutrons. The process of this occurring in an
uninterrupted chain form is a nuclear chain reaction.
Thus, enriched U-235 can sustain a nuclear chain
reaction.
Wikipedia describes a nuclear chain reaction:
“A nuclear chain reaction
occurs when one nuclear reaction causes an average of one or more
nuclear reactions, thus leading to a self-propagating number of these
reactions. The specific nuclear reaction may be the fission of heavy isotopes
(e.g. 235U) or the fusion of light isotopes (e.g. 2H and 3H).
The nuclear chain reaction releases several million times more energy per
reaction than any chemical reaction.” (http://en.wikipedia.org/wiki/Nuclear_chain_reaction)
First Sustained Nuclear Chain Reaction from
Wikipedia:
“Enrico Fermi
created the first artificial self-sustaining nuclear chain reaction, called Chicago
Pile-1 (CP-1), in a racquets court below the bleachers of Stagg Field
at the University of Chicago on December 2, 1942.
Fermi's experiments at the University of Chicago were part of Arthur
H. Compton's Metallurgical Laboratory facility, which
was part of the Manhattan Project” (http://en.wikipedia.org/wiki/Nuclear_chain_reaction)
Wikipedia also has
some propaganda about the impossibility of a nuclear reactor having an
explosion:
“Nuclear power plants operate by precisely
controlling the rate at which nuclear reactions occur, and that control is maintained
through the use of several redundant layers of safety measures. Moreover, the
materials in a nuclear reactor core and the uranium enrichment level make a
nuclear explosion impossible, even if all safety measures failed…” (http://en.wikipedia.org/wiki/Nuclear_chain_reaction)
Nuclear fission of U-235 doesn’t just free neutrons,
it also releases gamma rays and neutrinos, thereby releasing energy that had
been “stored” in the atom
So, I conclude from this discussion that fission is
most definitely occurring at Fukushima.
The question is what is “the effective
neutron multiplication factor” and are the chain reactions occurring at
Fukushima criticalities or supercriticalities.
According to Wikipedia, the effective neutron
multiplication factor or “k” “is the average number of neutrons from one
fission that cause another fission.” The value of k determines whether (I'm paraphrasing here):
k < 1: the system cannot sustain a chain reaction
(subcriticality)
k = 1: the system sustains a constant level of
fission so that every fission causes an average of one more fission, creating a
constant level of fission (criticality)
k > 1: the system achieves “supercriticality”
which means “for every fission in the material, it is likely that there will be
‘k’ fissions after the next mean
generating time. The result is that the number of fission reactions
increases exponentially” (http://mfile.akamai.com/127380/live/reflector:51361.asx)
Wikipedia explains
that there are 2 variants of supercriticality, prompt and delayed
“The region of supercriticality between k = 1
and k = 1/(1-β) is known as delayed supercriticality (or delayed criticality). It is in this region that
all nuclear power reactors operate. The region of supercriticality for k >
1/(1-β) is known as prompt supercriticality (or prompt criticality), which is the region in
which nuclear weapons operate…”http://en.wikipedia.org/wiki/Nuclear_chain_reaction)
MAJIA HERE: the question is, what is k at Fukushima?
It seems that the debate about fission that occurred
over the last week was really a debate about the level of criticality.
It seems to me that there is considerable evidence
of k=1 and perhaps delayed supercriticality.
I think that currently the scientists weighing in
think that a prompt supercriticality situation has not yet been achieved,
although it may be feasible.
Potrblog has been discussing this possibility but states that they don't have enough data to determine the likelihood of a nuclear explosion (i.e., prompt supercriticality). http://www.youtube.com/watch?v=3DKFmiUI-5g&feature=youtu.be&t=13m20s
Enenews ran this headline Nov 9: Co-chair of Russia’s Ecodefence and others
“still fear criticality” at Fukushima — “I don’t think there is a reason to say
situation has improved much actually”http://enenews.com/co-chair-of-russias-ecodefence-and-others-still-fear-criticality-at-fukushima-i-dont-think-there-is-a-reason-to-say-situation-has-improved-much-actually
here is the original link: http://www.bellona.org/articles/articles_2011/japan_nochain
This article has some confusing quotes that might make more
sense if we decode them using the vocabulary I’ve outlined above
For example, the article states: “Spontaneous
fission should not be confused with nuclear criticality, said Aoki, especially
since both the temperature and the pressure levels have remained stable in the
reactor. But he did say that “the chances of criticality taking place is not
zero.”
MAJIA HERE: The article doesn’t make
a lot of sense to anyone who has been following the radiation releases and knows
that radiation is continuing to be released in large quantities and that some
of the radionuclides such as Iodine-131 indicate recent fission.
However, if we interpret Aoki’s
comment in relation to the levels of criticality above we might conclude that
what he really was saying is that k=1 (criticality) or perhaps even delayed
supercriticality has been achieved where “The region of supercriticality between k = 1 and k = 1/(1-β) is known as delayed
supercriticality (or delayed criticality)”
Given this
interpretation of what is going on, the real debate that is occurring in a
veiled way among and between the world’s nuclear physicists is about whether or
not PROMPT SUPERCRITICALITY is likely to occur.
Prompt
supercriticality could result in a nuclear explosion.
However, even in
the absence of prompt supercriticality, short-to-medium duration criticalities that
fail to reach supercriticality (fail to reach k > 1) are still producing
radiation and that radiation is still contaminating Japan and the rest of the
northern hemisphere.
PLEASE POST COMMENTS AND LET ME KNOW IF THERE IS AN ERROR IN MY INDUCTIONS OR DEDUCTIONS
"MAJIA HERE: the question is, what is k at Fukushima?"
ReplyDeleteAll the talk of criticality is just a smokescreen. Even a k of less than one, which means an infinite chain reaction is not possible, can still release plenty of nuclear trash. A chain reaction can still be long even if it isn't infinite (a k value of slightly less than one). If the fuel does reach a k of one, the fuel will get very hot very quickly, and this will stop the chain reaction as the fuel vaporizes or otherwise spreads itself out. :-(
Good luck on educating yourself, as the media is not going to help you much, if at all. Yay, internets!