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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Wed Mar-16-11 07:18 PM
Original message
Radiation level falls at Japan Fukushima plant: agency
Reuters) - The level of radiation detected at the Tokyo Electric Power Co Fukushima plant has fallen steadily over the past 12 hours, an official at Japan's Nuclear and Industrial Safety Agency said on Thursday.

A level of 752 microsieverts per hour was recorded at the plant's main gate at 5 p.m. (0800 GMT) on Wednesday, said the official, Tetsuo Ohmura. The monitoring point was then changed to the plant's west gate and readings were taken every 30 minutes, he said. At 5 a.m. the reading was 338 microsieverts per hour.

That level was still much higher then it should be, but was not dangerous, and that by comparison absorption of a level of 400 was normal from being outside over the course of a year, Ohmura said.

http://www.reuters.com/article/2011/03/16/us-japan-quak...
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drm604 Donating Member (1000+ posts) Send PM | Profile | Ignore Wed Mar-16-11 07:29 PM
Response to Original message
1. Am I misunderstanding this?
It seems to be saying that the reading was lower at the west gate that it was at the main gate. Can you measure at two different points and then state that the level is falling?

Has it fallen at the main gate? Was it previously higher at the west gate? I hope that it is falling, but unless I'm missing something this doesn't prove it.
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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Wed Mar-16-11 08:09 PM
Response to Reply #1
2. I thought the same thing.
But it is relevant that the level continued to fall after it was moved.
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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 09:52 AM
Response to Reply #1
6. It isn't written clearly but yesterday the reading at west gate was 1500 uSv.
Edited on Thu Mar-17-11 10:19 AM by Statistical
So apples to apples from 1500 uSv to <400 uSv.

Mot sure why the article is written so badly. Maybe the author is confused too.
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Peace Patriot Donating Member (1000+ posts) Send PM | Profile | Ignore Wed Mar-16-11 08:34 PM
Response to Original message
3. The nuke industry needed a positive headline? "Move along, nothing to see here"?
Jeez. So this is why they are trying very last resort, never before used methods of cooling, with water jets from fire trucks? (--as someone said, visualize trying to fill a bathtub on the other side of the yard with a hose). Several of these plants are so "hot" workers can't go near them--and a "safe" distance, according to U.S. authorities, is 50 miles (or maybe Mars?). There are at least two core partial meltdowns, two outer containment structure roofs blown off, another with a wall blown out, and at least four plants with grave "spent fuel pool" overheatings and fires, with two such pools wide open to the sky. This has already been designated a Level 6 nuclear disaster, beyond "Three Mile Island" (Level 5) and heading toward Level 7 (Chernoybl). This contrived announcement--whether faked up or not--doesn't begin to describe the situation, nor what could happen in the next 24 to 48 hours. There are simply no "good" scenarios. The "best" scenario is Level 6--a disaster that will go on probably for years, with various radiation releases to air and ocean.
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SpoonFed Donating Member (801 posts) Send PM | Profile | Ignore Thu Mar-17-11 12:25 AM
Response to Reply #3
4. +1

nice reality check for those who need one.
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joshcryer Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 05:54 AM
Response to Reply #3
5. JAPAN needs positive headlines. The question is if it's TRUE.
If it's TRUE then your entire post is just insulting.
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Peace Patriot Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 11:37 AM
Response to Reply #5
9. "Insulting"? You are inhabiting "Alice in Wonderland," it seems to me.
There is NO WAY that this disaster is going to be "contained." Maybe "armageddon" can be prevented (six Chernoybls--the whole site and all six of its reactors going up in flames, turning Japan into a waste land, destroying swaths of Pacific sea life and spreading lethal clouds of radiation around the Pacific Rim and wherever the wind blows). But we're already at Level 6 and there is really no way out of this. Nuclear disaster has struck. And today bears out the REALITY I was talking about yesterday. Water-bombing the #3 and 4 reactors/spent fuel rods didn't work. Radiation levels are rising despite all the water dropped and hosed onto the reactors and spent fuel rods. They are almost out of options for preventing Level 7 (which may be far worse than Chernoybl). Burying the whole facility in a literal mountain of boron (or concrete or lead?) might prevent release of all that nuclear material to the atmosphere but it will likely seep underground and the region around Fukushima will remain irradiated for centuries.

They say they are close to getting the electricity restored to the site. But core meltdowns and spent fuel fires are happening too fast--so that, even if they can get the earthquake/tsunami damaged water pumping system back up, the core and spent fuel have become so "hot" that the water creates irradiated steam which is venting right to the atmosphere (units #1 and 3 roofs blew off, and another unit lost a side wall), with big risk of yet more explosions.

I was assessing these FACTS yesterday. I saw no way that anything you could call "containment" was possible. Preventing multiple Chernoybls, maybe. Chernoybl had a massive fire. That was the biggest problem. The spent fuel pools in every one of these six reactor buildings at Fukushima, which have no containment structure around them (plus outer roofs/walls ripping off), can cause massive fire that can ignite all the nuclear material at the site. Chernoybl didn't have any kind of "safety net" under the reactor, to prevent the nuclear material fanning out into the water table. Fukushima's reactors do have a structure under the reactor cores, but who knows if they will work--whether they got damaged in the earthquake or will melt in a massive fire. And who knows if the water pumping system can be restarted, even if electricity is restored? Its pumps and valves and vents and gages have all been damaged.

Conditions are dire. Nuclear cores are damaged. Spent fuel rods are without water and once fires start they become increasingly hard to put out (and inevitably release radioactive steam). They can't get near several plants. They are too "hot." Best case scenario: They get the regular water pumps working again, and have to keep pumping thousands of tons of seawater on six nuclear reactors and six stores of spent fuel rods, with irradiated steam releases for months and years to come, with on-going risks of more meltdowns and fires if anything goes wrong. (And where are they pumping all that sea water TO, after it cools all this nuclear material? Impact on fisheries, food supplies, people?)

This situation is already a nightmare. And I think what is insulting is to downplay the horror that the Fukushima workers are dealing with, and the current and potential impacts on millions of people. There is no "good" outcome--only levels of terrible outcome.
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 11:49 AM
Response to Reply #3
10. It is a positive development
or "less terrible", if you choose to see it that way.

I agree that the nonsense about it not being "harmful" is itself, harmful. Comparing standing at the gate for one hour to one *year* of normal exposure? :eyes:
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SpoonFed Donating Member (801 posts) Send PM | Profile | Ignore Thu Mar-17-11 10:38 PM
Response to Reply #10
50. living in a fantasy world
Edited on Thu Mar-17-11 10:39 PM by SpoonFed
It has come out today that 2 hours before you wrote your opinion that this was a positive development, that TEPCO went to the PM to ask if they would be allowed to evacuate all of their personnel from the plant.

There are now many reports about workers with serious poisoning and deaths.

It is not possible for someone to believe this is getting better at this point,
but keep talking.

Reports are coming out that outside the 30km zone, there are readings equivalent of 40 chest X-rays per hour...
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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 11:15 PM
Response to Reply #50
54. Can you link to any of those reports of workers with RADIATION poisoning?
I haven't seen anything close "serious poisining and death" from radiation. It's presumed that a couple died in an explosion, but the highest reported exposures that I've seen are in the "somewhat higher chance of cancer later in life" range.

It has come out today that 2 hours before you wrote your opinion that this was a positive development, that TEPCO went to the PM to ask if they would be allowed to evacuate all of their personnel from the plant.

They've evacuated and returned multiple times. A high percentage of the radiation that has been released has been something they knew about in advance because they were the ones releasing it. They wisely got out of the way each time.

It is not possible for someone to believe this is getting better at this point

We are just about past the point where there is any danger of the 1-3 reactors themselves breaching containment (depending on how you "score" the assumed breach in the torus). The fuel pools are the remaining danger. That's by no means insignificant, but I think the greatest risk was when we thought that #2 had lost containment and was in full meltdown.

Reports are coming out that outside the 30km zone, there are readings equivalent of 40 chest X-rays per hour...

Those have been transient readings. You can't get that dose in an hour if the radiation is much lower by then... and 40 xrays is not exactly a concern. It means that if that level hung around for ten hours you would get the equivelent of a CT scan.
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SpoonFed Donating Member (801 posts) Send PM | Profile | Ignore Fri Mar-18-11 02:53 AM
Response to Reply #54
59. Dear Mr. Baggins

I'm just going to ignore you, since every time I have said anything since the beginning of this, you have down played the situation and consequent results and more than oh I've lost count of how many times, you've been wrong in your wonderful optimistic supposition.

You believe it or not, I don't care. I think your position on nuclear power has been quite clear for all to see over the course of the past number of days. What's your background again? Do you work for the nuclear industry in any way, shape or form?

a) NHK itself on the live stream I'm watching as I type, has just said the workers are getting mSv levels of exposure and that the report I was mentioning was > 0.2 mSvs/hr > 30km away from the plant.

b) 18 hours ago TEPCO asked the prime minister to abandon the facility due to danger to their workers. The PM refused to let them, apparently.

c) I think you're an apologist or a shill at this point.

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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Fri Mar-18-11 05:38 AM
Response to Reply #59
60. A simple "No. I made that sh1t up" would have sufficed.
Edited on Fri Mar-18-11 05:39 AM by FBaggins
If you want to pretend to ignore me so you can avoid dealing with your errors... That's fine.

a) NHK itself on the live stream I'm watching as I type, has just said the workers are getting mSv levels of exposure and that the report I was mentioning was > 0.2 mSvs/hr > 30km away from the plant.

And at that level they'll start to get sick and even die after "only" seven months or so. Yeah... That SO backs up your claim.

You said there were "now many reports about workers with serious poisoning and deaths."

Let's see some of those reports.
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hatrack Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 10:00 AM
Response to Original message
7. I'd hazard a guess that levels are going to rise and fall for days, maybe weeks to come
Lots going on in multiple damaged facilities.
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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 10:05 AM
Response to Reply #7
8. Yep. "Lots going on" and shifting wind patterns.
It's worth noting, however, that if there were a full breach of containment (particularly, heaven forbid, if fission had restarted), or if a fuel pool was entirely dry and on fire, that these numbers would be much higher.
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 02:23 PM
Response to Reply #8
11. Uncontrolled fission has been ongoing
That's what's keeping everything so hot.
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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 02:35 PM
Response to Reply #11
12. Sorry. That's incorrect.
Edited on Thu Mar-17-11 02:36 PM by FBaggins
It's simply decay heat, not fission. Fission stopped within a few seconds of the earthquake.


If it was uncontrolled fission going on (particularly in those fuel pools), this would suddenly be comparable to Chernobyl... and would likely be worse.
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 02:51 PM
Response to Reply #12
14. You're wrong.
Edited on Thu Mar-17-11 02:51 PM by wtmusic
Fission is happening in every fuel pellet in the core, all the time. The reason the fuel assemblies stay hot is because of nuclear decay heat - aka fission. They are hot because there is no water between them to absorb neutrons, and they will stay hot for years.
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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 02:58 PM
Response to Reply #14
16. Decay and fission are NOT the same thing.
Didn't you have this corrected a couple days ago? You've had time to look it up.
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 03:00 PM
Response to Reply #16
17. Here you go.
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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 03:03 PM
Response to Reply #17
18. That's an image of fission alright... it just ISN'T the source of the heat here.
Sorry.
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 03:06 PM
Response to Reply #18
19. OK. What is the source of heat? nt
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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 03:09 PM
Response to Reply #19
20. The link is right there below the image you linked.
It's called "radioactive decay".

Also an exothermic event.
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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 03:16 PM
Response to Reply #19
22. Here.
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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 02:43 PM
Response to Reply #11
13. Nope.
Uncontrolled fission would mean the reactors would have melted in a matter of minutes without cooling.
Second there is no neutron radiation detected on the site.
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 02:57 PM
Response to Reply #13
15. The fuel assemblies have at least partially melted
and since that isn't an operating mode of this reactor it would belong in the "uncontrolled" cateogory (uncontrolled doesn't imply criticality has been reached).
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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 03:15 PM
Response to Reply #15
21. Uncontrolled? Yes. Fission? No.
Heat for active fission is about 100x that of current decay heat. Reactors would be a pool of molten metal right now if uncontrolled fission without cooling was happening.

I don't think anyone would consider what is going on to be controlled. :)
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 03:29 PM
Response to Reply #21
23. Spontaneous fission doesn't occur in U-235?
"The total prompt fission energy amounts to about 181 MeV, or ~ 89% of the total energy which is eventually released by fission over time. The remaining ~ 11% is released in beta decays which have various half-lives, but begin as a process in the fission products immediately; and in delayed gamma emissions associated with these beta decays. "

http://en.wikipedia.org/wiki/Nuclear_fission

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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 03:40 PM
Response to Reply #23
24. Spontaneous fission is very rare ...
http://en.wikipedia.org/wiki/Spontaneous_fission

All fissile material undergoes SF. Even uranium in the ground and by definition it can never be controlled. This given its extreme rarity it isn't what is causing the heat in the cooling ponds.

Your quote is refering to nuclear decay. 89% of the energy is released in the reactor. The remaining 11% will occur via nuclear decay and produce heat for thousands of years. That heat decays off exponentially so output falls very fast. After about 5 years spent fuel is still undergoing nuclear decay (as it will for thousands of years) but it is cool enough to put in dry cask and only be warm to the touch.
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 03:45 PM
Response to Reply #24
26. The 89% refers to fission...
"The total prompt fission energy amounts to about 181 MeV, or ~ 89% of the total energy which is eventually released by fission over time."

I admit I'm unclear on this. I thought that even though spontaneous fission events were rare, that because they were so energetic they made up the bulk of the energy released.

And the water in the "cooling ponds" cools (mostly) by moderating neutrons between spontaneous fission events. :shrug:
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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 03:56 PM
Response to Reply #26
27. Re: the pools
No. The water cools in much the same way that water always cools.

Once the atom throws off a subatomic particle (nuclear decay), the heat has been created.

The water DOES provide shielding that keeps most of the particles from leaving the pool.
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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 03:59 PM
Response to Reply #26
28. Not exactly.
Edited on Thu Mar-17-11 04:02 PM by Statistical
In each fission 89% of energy is initial, 11% come later as decay.

In a nuclear reactor you are causing active (or forced fission) so thousands of fissions per second. So you are using that 89% times millions of fissions to heat water. However you are also "banking" the 11% of nuclear decay. The reason why spent fuel is hot is all those nuclear decays (from the fissions that happened in the reactor) are occurring.

SF occurs so rarely it doesn't contribute to significant amount of heat. Take a uranium rock. It is undergoing SF but it is very rare. The rock doesn't get hot. It doesn't melt. What do I mean by rare. For Uranium it is 7.0 x 10^-11. In decimal that is 0.000000007% of the time.

Water is a moderator but that isn't what cools the cooling pond. The water getting hot is what cools the fuel. That warm water is pumped through a heat exchanger where the heat is shed (to another body of water) and the cooler water in returned to the pond. With no pumps the decay heat kept "injecting" thermal energy into the water causing the temp to rise until it boiled off.

For most discussions spontaneous fission can be ignored. It occurs and always has but so rarely that we weren't even aware of it 100 years ago. In matters in nuclear weapons though. Too much spontaneous fission can cause the weapon to go off "Early" before it can become super critical. This is why you need to use weapons grade plutonium.

Take a look at the chart here.
http://en.wikipedia.org/wiki/Spontaneous_fission
Which Plutonium do you think is weapons grade?
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 04:08 PM
Response to Reply #28
29. If SF events are so inconsequential how do they initiate
a chain reaction in a nuclear weapon in under a microsecond?

And a rock wouldn't get hot anyway, because the U-235 isn't concentrated enough to spark bombardment events in surrounding atoms.

I've got two obviously smart people disagreeing with me, but this is challenging my long-held understanding.

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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 04:16 PM
Response to Reply #29
30. It isn't that it WILL start a chain reaction early it is that it could.
Edited on Thu Mar-17-11 04:28 PM by Statistical
Imagine a sub critical mass being compressed by explosives. Now imagine it in extreme slow-mo. The mass is slowly compressing. It reaches critical mass, still compressing it needs a few more nanoseconds to get to a super critical mass. Right now at this very millisecond self sustaining fission can occur the mass is critical. All you would need is a moderated neutron. One neutron could start chain reaction right now. The problem is the mass isn't super critical yet. If a fission chain reaction happens right now the heat and pressure produced will blow the critical mass apart and it will never go super critical.

Pu-239 has is 10,000x as many fissions. Any nuclear weapon can technically fizzle but the chance with Pu-239 the chance is 10,000x higher. You theoretically could get really unlikely with Pu-240 and have a SF happen at the wrong millisecond, likely nobody would ever know exactly why the nuclear weapon failed to go super critical. Likewise you could get have Pu-239 not SF in that critical window. Still the odds are 10,000x better with Pu-240.

Technically in that critical window any neutron could cause fission. A stray neutron flying across the universe could strike an atom in the weapon and begin fission too early.

As far as uranium not being hot. Even pure U-235 isn't hot to the touch. This is because SF is so rare any heat output is negligible.
This is a uranium fuel pellet enriched to about 3%.

One fission isn't that energetic. U-238 fission is about 200 MeV (million electron volts) however that is only about 0.00000000003 Joules.
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 04:29 PM
Response to Reply #30
31. Let's say there's a pool of nuclear fuel at the bottom of the reactor
Although it's a relatively low concentration, it's many times critical mass. You're saying that's not generating any significant heat from fission?

Not adding up.
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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 04:35 PM
Response to Reply #31
32. If there is an fission chain reaction that would be active fission not SF.
Edited on Thu Mar-17-11 05:07 PM by Statistical
I thought we were talking about spontaneousness fission?

Active fission would produce tremendous heat not because one fission has a lot of energy but because the rate of fission would be insanely high, trillions of fissions per second. So that low energy amount per fission be multiplied by a very large number. That is why I said if active fission is going on without coolant the reactor would be a molten pool of liquid metal right now, or it would have exploded within minutes of losing coolant.

Two things prevent active fission.
1) Control rods are absorbing neutrons. Control rods are placed between fuel rods to absorb neutrons from any SF preventing start of fission chain reaction.
2) If fuel melted and formed into a critical mass that wasn't near a control rod fission *could* occur. This is why they are injecting boron into the reaction along with water. Boron acts as a liquid control rod.

Thus active fission can't happen in the reactor. Believe me if active fission happened you would know it. Instantly and it would be very bad. The heat in the reactor and the spent fuel pond is from nuclear decay.

Simplified version:
1) The fission of a single atom releases a relatively low amount of energy.
2) In a chain reaction (active fission) trillions of fissions are occurring every second thus total energy is high.
3) While heavy isotopes can undergo spontaneous fission the rate is very low and thus total energy is low.
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 04:56 PM
Response to Reply #32
33. I don't know what "active fission" is.
Edited on Thu Mar-17-11 05:06 PM by wtmusic
If we're assuming that fission, although relatively rare, is ongoing - that means some of those spontaneous, spare neutrons are striking other nuclei, with each collision generating a tremendous amount of energy.

The more material we have in closer proximity, the more that's going to happen, and the more heat is generated. Say each SF neutron is responsible for knocking off another neutron 1/3 of the time - that's fission, it's generating heat, and whether you want to call it "active" or not is an arbitrary distinction.

If we were able to hold that mass together for any length of time we'd have an explosion (the physicists at Los Alamos found out that's the hard part).
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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 05:05 PM
Response to Reply #33
34. Yeah that is called active fission and THAT is NOT HAPPENING IN THE REACTOR.
Edited on Thu Mar-17-11 05:20 PM by Statistical
As you pointed out.

"If we were able to hold that mass together for any length of time we'd have an explosion (the physicists at Los Alamos found out that's the hard part)."

Yet neither reactor core nor the spent fuel has exploded wouldn't that indicate to you that no chain reaction is occurring?

"If we're assuming that fission, although relatively rare, is ongoing - that means some of those spontaneous, spare neutrons are striking other atoms, each with each collision generating a tremendous amount of energy."

First wrong assumption. Each collision generate as tiny amount of energy, extremely tiny. Fission is only powerful because it has the potential for trillions of fission to occur each second via chain reaction. Each individual fission is not powerful. Look up amount of energy in each fission and convert it to a unit of energy you are use to kwh, joule, btu, etc.

"The more material we have in closer proximity, the more that's going to happen, and the more heat is generated. Say each SF neutron is responsible for knocking off 3 more neutrons - that's fission, it's generating heat, and whether you want to call it "active" or not is an arbitrary distinction."

What you described is active fission.

Spontaneous fission - one fission occurs for not reason (at least no reason we can explain), neutron release. Nothing else happens.

Active fission - a SF event occurs (once again without any reason that we can explain) the neutron hits a fissile atom (U-235 or Pu-240) it splits, release neutrons, they strike other atoms, etc.

So spontaneous fission - 1 atoms splits .............. long delay ........ 1 atom splits.

Active Fission 1 atom splits -> 2 atoms split -> 4 atoms split -> 8 atoms split -> 16 atoms split -> 33 atoms split etc.


"it's generating heat"
Yes each fission does produce heat. An absolutely tiny amount that is undetectable outside laboratories. It is the chain reaction of trillions of fissions that produce the nuclear energy. That chain reaction is active fission. When an atom splits because a neutron hits it that is active fission (doesn't occur in nature). If an atom splits on its own it is spontaneous. It is hardly arbitrary. Active fission (sustaining the chain reaction) can can only happen when you have a critical mass and a moderator.

This is why nuclear fuel before it goes in a reactor can be held in your hand. Spontaneous fission is happening (just as it has for billions of years) but there is no chain reaction, and a negligible release of energy.
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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 05:09 PM
Response to Reply #34
35. Guys... Let's drop this one for now and pick up on another thread
I've enjoyed the conversation, but now were bumping an old thread that says that radiation levels are falling. I don't think it's still true.
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 05:20 PM
Response to Reply #34
36. A chain reaction doesn't always result in an explosion.
http://en.wikipedia.org/wiki/Manhattan_project

Fission events release fifty times as much energy as decay events - 200 MeV vs 4.3MeV. You can call it tiny, but neither of us (maybe no one) knows what part of the heat generated in that pressure vessel is coming from fission vs. decay, especially if a pool has built up.

If a significant amount of radioactive material has built up, fission will occur. Maybe by "active" you meant "self-sustaining", but even still - just because these events are not self-sustaining doesn't mean they're not happening, and not generating heat.
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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 05:27 PM
Response to Reply #36
37. You are 100% wrong. Nuclear decay in aggregate produces 200 billion times as much heat.
Edited on Thu Mar-17-11 05:32 PM by Statistical
SF events are very rare. Larger amount or not it produces negligible energy. Period. This is physics 101.

"Fission events release fifty times as much energy as decay events"
Yeah but 1 fission event vs trillion decay events. Which produces more energy. SF is very rare.

For each decay of U-235 http://en.wikipedia.org/wiki/Spontaneous_fission

0.000000007% of the time it will result in SF.
99.999999993% of the time it will result in decay.

So we even though fission produces 50x the energy it occurs one 14 trillionth as often so it produces negligble energy.

Come on I can't explain it an simpler than this:
Spontaneous Fission - 50x the energy. occurs 1/14 trillionth as often
Nuclear Decay - 1/50th the energy. occurs 14 trillion times as often.


SF produces a rounding error of energy. We can say that with absolute certainty. There is no mechanism to change the rate of SF either intentionally or not. You can't make it happen more often. Virtually all the heat from spent fuel comes from nuclear decay.

What makes fission powerful (in nuclear weapons and nuclear power) is the chain reaction. The ability to wait for a spontaneous fission to occur and use that neutron to FORCE a fission and use that neutron to FORCE two more fissions, etc. The Spontaneous fission is simply the "trigger". Without the chain reaction a nuclear reactor couldn't warm a cup of tea.

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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 05:46 PM
Response to Reply #37
38. The idea you have to "wait" for a SF is nonsense.
The designers of "Little Boy" had confidence to a very high degree of probability that at least one SF would pop at the right microsecond (not millisecond) in their critical mass before it fell apart, and initiate a nuclear explosion (it did).

Nuclear decay may produce 200 billion times as much heat as SF in nature, but what about in nuclear fuel when the mass approaches criticality? How do you know this situation doesn't exist at the bottom of the pressure vessel?

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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 05:55 PM
Response to Reply #38
39. SF is the same regardless of criticality, regardless of location.
Edited on Thu Mar-17-11 06:17 PM by Statistical
Criticality has absolutely nothing to do with spontaneous fission. Sub critical, critical, supercritical. Spontaneous fission always occurs, and always at the same rate. It never changes. It is always exactly the same. There is nothing you can do to prevent it, nothing you can do to force it to happen. You can't alter the rate. It just happens that is why it is called spontaneous. It has been happening since the beginning of the universe (or at least until unstable istopes formed).

Criticality just indicate if a chain reaction can be sustained.
Subcritical - no chain reaction possible.
Critical - sustainable chain reaction possible.
Supercritical - exponentially growing chain reaction possible.

When I used the term "wait" it wasn't like wait 20 minutes for the bus. More like wait a fraction of a second. Still you DO have to wait. You can't prevent SF, can't delay it, and you can't make it happen faster. That is why I said "wait".

For nuclear weapons where timing matters they didn't rely on SF to kick off the fission. The SF could occur to late and no nuclear detonation would happen. Instead once the core was super critical they generated trillions of neutrons which caused millions of fission chain reactions to start simultaneously.

http://en.wikipedia.org/wiki/Urchin_ (detonator)

In a nuclear reactor where exact timing isn't needed and a slow rampup of fission is preferred SF is used as a "starter".


Decay heat produces 285 billion times more energy than spontaneous fission. So we can say without absolute certainty that spontaneous fission is not a factor in this reactor or any other reactor. For all but academic discussion you can simply ignore spontaneous fission. If there was no spontaneous fission at all the reactor would be 0.00000035% cooler. It would hardly matter.


SF was of academic value until they figure out that neutrons could be used to FORCE fision. It is this active or induced fission that powers a nuclear reactor. SF couldn't even heat up a cup of tea.

Come on I can't explain it an simpler than this:
Spontaneous Fission - 50x the energy. occurs 1/14 trillionth as often
Nuclear Decay - 1/50th the energy. occurs 14 trillion times as often.
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 06:25 PM
Response to Reply #39
40. About needing to induce fission in a reactor
It was my impression that they pulled the control rods out and it just happened. Do you have some support for that?

They certainly didn't need to induce it in Little Boy:

"The design used the gun method to explosively force a hollow sub-critical mass of uranium-235 and a solid target spike together into a super-critical mass, initiating a nuclear chain reaction."



You're adamant that at subcritical masses, chain reactions don't occur. That is equivalent to saying SFs, which happen millions of times/second in a near-critical mass, never strike another nucleus on their way out. Is that what you are saying?

And again: how do you know there isn't a near-critical mass of pooled, concentrated fuel in the reactor?

I do understand, and believe you, in regards to individual U235 atoms in nature (not in close proximity to another).
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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 06:45 PM
Response to Reply #40
41. They did pull out the control tods and it did "just happen".
Edited on Thu Mar-17-11 06:54 PM by Statistical
Without control rods in place a critical mass formed. The neutrons from SF stuck other atoms and split them which then struck atoms and split them. By formed I don't mean the fuel moved I mean the environment supported sustained fission.

If an atom is hit by a neutron and splits it isn't spontaneous fission, it is induced or active fission. SF "starts the chain reaction but the chain reaction isn't spontaneous fission it is millions of induced fissions. SF occurs to rarely to boil a cup of tea much less power a reactor. Also SF is a constant it will never get faster. Without induced fission the reactor would never get hotter.

About little boy.
True and that is why this design is no longer used. There is a risk of failed detonation. A SF would need to not only occur but also strike another nucleus durring that critical window. Not too early and not too late. They did use tungsten carbide (a neutron reflector) to increase the chance of a chain reaction during the critical window. There that nuclear detonation wouldn't occur because of timing. All they could do is increase the chances by reflecting neutrons back at the core.

"You're adamant that at subcritical masses, chain reactions don't occur. That is equivalent to saying SFs, which happen millions of times/second in a near-critical mass, never strike another nucleus on their way out. Is that what you are saying?"

Chain reactions can't be sustained. Will sometimes one fission result in a second yes but they are very rare. Remember SF are already rare so a 2 bounce fission is even more rare. Sometimes it might even have a triple chaiin reaction but that is now three magnitudes as rare. Absolutely never? No. But not common enough to sustain a reaction. Eventually the chain will fail and you will need to wait for the very rare SF event to "try again". Kinda like trying to roll only a six. If you roll a six then keep rolling, otherwise stop. You may get some sixes, a few doubles, maybe the occasional triple. but 20 in a row, 400 in a row, 50,000 in a row. Unlikely. So most of the time yes a neutron will hit nothing, sometimes it does cause a secondary fission but then nothing. Each chance of sustaining fission is another roll of the dice and in a subcritical mass and the reaction can't be sustained.

"how do you know there isn't a near-critical mass of pooled, concentrated fuel in the reactor? I do understand, and believe you, in regards to individual U235 atoms in nature (not in close proximity to another)"

If there is a critical mass in the reactor then fission CAN occur BUT THAT ISN'T SPONTANEOUS FISSION. That is induced fission. Even if some fissions do occur they aren't frequent enough to be more than a rounding error. Remember decay doesn't happen slightly more often or 10x as often or even 100,000x as often it occurs 15 trillion times as often. So for fission to produce even 10% of the heat you would need to consistently get 20 billion fissions in a row (no misses). Like rolling 20 billion sixes in a row consistently.


I think you are confusing spontaneous with induced.

Spontaneous fission - atom sitting there, absolutely nothing happening and it splits all on its own. This rate can never speed up and it is a tiny fraction of the energy compared to decay heat.

Induced/Active fission - atom sitting there is hit by a neutron and splits. This is not spontaneous.

Spontaneous fission is always a rounding error in total energy. Why? Because the rate never changes.


COULD INDUCED FISSION OCCUR IN THE REACTOR? It could and that would be catastrophicly bad. Without any control trillions upons trillions of fission would happen in a matter of seconds creating a massive spike in pressure and destroying the reactor. Still that wouldn't be spontaneous fission. Given the reactors haven't exploded yet this hasn't happened yet.

Why hasn't induced fission happened? Control rods and boron. They absorb enough neutrons to prevent the formation of a critical mass. Critical mass isn't just a measure of size it is a measure of neutron density. How many neutrons in how small of any area. Hitting an atom is rare. Most (like 99.999999999999999999999%) of neutrons miss so you need a lot of neutrons to sustain a reaction. With enough boron in the reactor no matter how compact the fuel gets it won't be able to achieve a critical mass.

What happens if a critical mass is achieved?
Boom. Chernobyl. Very bad.






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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 08:30 PM
Response to Reply #41
42. Thanks for your explanation..bear with me.
Edited on Thu Mar-17-11 08:31 PM by wtmusic
I think this question gets to the heart of the matter.

Accompanying this photo, you said, "Even pure U-235 isn't hot to the touch."



We have literally tons of spent fuel rods burning up. Vaporizing water instantaneously. If all of that heat is from decay, why isn't this little nugget burning this gentleman's hand off?

Thousands of cold nuggets do not a hot group make - unless together, en masse, there is fission occurring.
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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 09:11 PM
Response to Reply #42
45. Nope. I think this one will finally do it.
U-235 has a very long half life (which means fewer decays per second) it also doesn't have a very high decay energy.

However in the reactor Uranium gets split into all kinds of stuff called actinides and fission products. Some of this "stuff" has very short half life and very high decay energy.

http://en.wikipedia.org/wiki/Fission_product_yield

If you could take the spent fuel rods and completely remove the remaining U-235 it would still be cool to the touch just like it is in the photo above.

However some of the fission products have very short half lifes and insanely high decay energies. You end up with a isotope soup. Some have half lives measured in years, some in months, some in days or even hours. There are a few with half lifes in seconds. Imagine that every few seconds half of the mass decays releasing huge amounts of decay energy. In a ton of fuel that can be trillions of decays a second.

This is also why spent fuel gets "cooler" over time. As the short half life isotopes burn up (decay to stable isotopes) they no longer contribute to the aggregate heat. So say after a year has passed all the very short istopes are gone and only the longer ones remain. Eventually after say a 1000 years only the longest lived isotopes remain.

Simple version:
The reason spent fuel is "hot" is not because uranium is hot it is because some of the uranium has been changed into shorted lived and higher decay energy isotopes.


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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 10:36 PM
Response to Reply #45
49. Thanks.
It seems that criticality can occur locally (or this is misinformation)


"The threat of a fission explosion at the Fukushima power facility emerged today when the roof of the number three reactor exploded and fears that a spent fuel pool, located over the reactor, has been compromised."

http://www.dcbureau.org/201103141303/Natural-Resources-...

"As pools fill, there are major problems. If the rods are placed too close together, the remaining nuclear fuel could go critical, starting a nuclear chain reaction. Thus, the rods must be monitored and it is very important that the pools do not become too crowded. Also, as an additional safety measure, neutron-absorbing materials similar to those used in control rods are placed amongst the fuel rods. Permanent disposal of the spent fuel is becoming more important as the pools become more and more crowded."

http://library.thinkquest.org/17940/texts/nuclear_waste...


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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 10:55 PM
Response to Reply #49
51. Criticality can happen in cooling pond.
It is unlikely for the reason described there and because spent fuel is "spent" because it no longer is a good environment for criticality (too many built up neutron poisons).

That being said under the right conditions it is possible. Still if criticality occurred and a fission chain reaction was sustained it would be induced fission not spontaneous fission. Second there would be no doubt about it. Uncontrolled fission will generate tremendous amounts of heat and pressure enough to cause an explosion. Not a nuclear detonation but a large conventional explosion from the massive increase in hydrogen and/or steam.
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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 11:56 PM
Response to Reply #51
57. Here's what I don't get
Say we have a mass of pooled reactor fuel at the bottom the vessel. If it's subcritical (but close to criticality) can it not generate significant heat from fission anyway?

Intuitively, I would think that SFs would maintain a chain reaction much longer, resulting in a rise of energy output. :shrug:
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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Fri Mar-18-11 07:14 AM
Response to Reply #57
62. Not long enough.
For fission to make up 10% of heat the average fission chain from a single SF would need to be 28 billion induced fissions before the chain breaks.
For fission to make up 1% of heat the average fission chains from a single SF would have to be 2 billion induced fissions.

Not occasionally but consistently. In a subcritical mass you will get <2 fissions on average from each SF. A negligble rounding error to the heat output.
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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 09:15 PM
Response to Reply #42
46.  It isn't the U-235 making all the heat. Spent fuel has previously been part of a fission reaction.
Some of the resulting elements are themselves radioactive. Some of them have MUCH shorter half-lives (and are thus MUCH more radioactive on an ounce-by-ounce basis). These elements are caused by the fission reaction itself. Only a small portion of the heat from a core that was just "turned off" is from the uranium.

That's why the spent fuel rods cool off over the course of a few years. It isn't because the uranium has significantly decayed (it has a pretty long half-life), but because those more-active elements are dying out.

Some of them die off VERY quickly. That's one of the reasons that the fuel-pool issues are considered a bigger deal right now. Not because they are many times more dangerous in general, but because the cores of these reactors are putting out far less heat than they did a week ago. Those first two or three days were the most critical period. Now there's still a chance of additional core damage, but the chances of a full-blown meltdown are greatly reduced.
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PamW Donating Member (566 posts) Send PM | Profile | Ignore Thu Mar-17-11 09:22 PM
Response to Reply #40
48. Nope - doesn't happen that way.
It was my impression that they pulled the control rods out and it just happened.
==================

NO - a reactor is started with a neutron source - a neutron emitting material.

When you start up a reactor, you put the source in it. One reason for doing this
is that the instrumentation works by detecting neutrons.

Suppose you had no source. Suppose you had no neutrons in the reactor and you
started pulling the control rods. Without neutrons, your instruments would read ZERO.
You would keep pulling rods and the system would actually go critical, but without
any neutrons to start the reaction. You keep pulling the rods since your instruments
read zero, and you get the reactor significantly super-critical.

THEN some cosmic ray gives you a neutron to start the reactor, but now that reactor
is way super-critical - you overshot your intention of having an exactly critical,
i.e. just self-sustaining system.

So you always have a source in the system to give your instruments a neutron level to
monitor so that you can see when the system is approaching the critical ( i.e. normal )
condition. < Critical doesn't mean blow-up - it means a steady state, constant power [br />condition ]

PamW

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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 10:57 PM
Response to Reply #48
52. Thanks for that insight.
I was unaware the reactor used a neutron source. I just figured they used spontaneous fission to act as a trigger. Your explanation on how that could potentially be unsafe makes sense.
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PamW Donating Member (566 posts) Send PM | Profile | Ignore Thu Mar-17-11 09:15 PM
Response to Reply #36
47. 100% Wrong - it's not just the amount.
If a significant amount of radioactive material has built up, fission will occur.
========================

First, fission is not dependent on radioactive material; it is dependent on
"fissile" material. You may have a lot of radioactive Cobalt-60 but you are
not going to get fissions.

Suppose you have a lot of fissile material, does that automatically mean you will
have a system that supports chain reactions? NO!!! A reactor that is shutdown has
a lot of fissile material, but it doesn't support chain reactions.

In order to have chain reactions, you have to have a very special set of conditions.
Neutron transport is determined by the Boltzmann transport equation, which is an
"eigenvalue" equation. In order to have a non-trivial ( non-zero ) solution to the
equation, you must make the transport operator "singular".

That is you have to have very special conditions of geometry and material composition.
Not just any old mix and geometry of fissile material gives you a chain reacting system.

A reactor with the control rods inserted has a lot of fissile material, but does not
support a chain reaction.

Remove the control rods, and if the reactor designer had done his/her job right, the
resultant materials and geometry will be a critical system that supports a self-sustaining
chain reaction.

PamW

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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 11:04 PM
Response to Reply #47
53. Oh please.
Edited on Thu Mar-17-11 11:07 PM by wtmusic
When I said radioactive material "built up" (in this case, fissile U-235) I meant "together". "Not separated by control rods". :eyes:

Also in this case, it is just a matter of the amount, and it's related to the inverse square of the density.



Put enough of it together, and you have criticality.
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PamW Donating Member (566 posts) Send PM | Profile | Ignore Thu Mar-17-11 11:27 PM
Response to Reply #53
55. OK - now you are 200% Wrong
Put enough of it together, and you have criticality.
================================

One of the problems with people quoting formulas that they don't understand,
is that they don't know when the formula is valid and when it isn't.

For any material, there is a quantity that you can calculate called
"k-infinity". It tells you what the neutron multiplication would be if
you had an infinite amount of the material. If that value is less
than unity ( i.e. one ) then even if you have an infinite amount
of the material, you don't sustain a chain reaction:

http://books.google.com/books?id=EpuaUEQaeoUC&pg=PA102&...

On page 102, is a description of k-infinity:

The tendency of a neutron population in an infinite system to change is
expressed in terms of the infinite multiplication factor....


If k-infinity for the material is less than one, it doesn't matter how
much you have, you won't get a chain reaction because that material won't
sustain a chain reaction even if you have an infinite amount.

For example, you do know that it is impossible to get a light water reactor
to run with unenriched uranium fuel. In order to use unenriched fuel, you must
use either graphite or heavy water as the moderator.

But any mix of light water and uranium has a k-infinity of less than one, so
you won't get a chain reaction with light water and unenriched uranium.

Same thing with just pure unenriched uranium. No matter how much of it you
have, you will not get a chain reaction because k-infinity is less
than one.

Better leave neutron transport to the scientist ( me ).

PamW

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wtmusic Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 11:43 PM
Response to Reply #55
56. You know, I really enjoy learning things from some DUers
Edited on Fri Mar-18-11 12:01 AM by wtmusic
others, their insufferable self-righteousness makes it less pleasurable.

You've diverted the discussion from U-235 to all kinds of exotic blends of light water, unenriched uranium, and possibly cornflakes.

I'm not that impressed, but thanks for your input. :thumbsup:
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PamW Donating Member (566 posts) Send PM | Profile | Ignore Fri Mar-18-11 12:21 AM
Response to Reply #56
58. You do know that we don't have pure U-235 here....
Edited on Fri Mar-18-11 12:26 AM by PamW
You've diverted the discussion from U-235 to all kinds of exotic blends of light water,
===================================

We don't have pure U-235 here. When you talk about criticality, you can't just
talk about one material in the mix. You have to include ALL the materials because
they all have an effect.

The uranium in a light water reactor is only about 3-4% U-235 when it is fresh.
Depending where the reactor was in its burn-up cycle, the percentage of U-235
is lower.

So the uranium here is at least 96% U-238. U-238 is "fissionable", but not "fissile".
That is U-238 only fissions with high-energy neutron. But we don't have high energy
neutrons here, because we don't have a critical system. We just have slow or thermal
neutrons. Therefore, the U-238 is just a neutron absorber.

Lets throw 100 neutrons at our mixture. 96 of them will hit U-238 and be captured.
Let's assume that all the neutrons that hit U-235 nuclei will cause fissions. So
only 4 of those 100 neutrons will cause a fission. However fissions give you multiple
neutrons - about 2.5 ( that's the "nu" in the formula for k-infinity if you looked at
my previous link). So those 4 fissions will give us 4(2.5) = 10 new neutrons for the
next "generation.

We started with 100 neutrons, and those produced only 10 for the next generation - a
ratio of only 10%. Those 10 neutrons will give rise to only 1 neutron in the 2nd generation,
and then the chain will die.

It doesn't matter how much of the stuff we have. Adding more material only counteracts
the effects of leakage - the fact that another way to lose neutrons is escape. However,
the above analysis is assuming the leakage is zero - which is what it would be for an
infinite system. You can't escape or leak from something that's infinite in size.

So even with an infinite amount of 4% enriched uranium, you can't get it to go critical.

In fact, you can't get even fresh reactor fuel to go critical if you have a "homogeneous"
fuel reactor. The only reason a light water reactor can go critical, is that neutrons can
escape the fuel and slow down in the water channels where there is no U-238 to absorb them,
and then re-enter the fuel as low energy thermal neutrons.

Google the term "thermal utilization". It is one of the four factors in the "four factor
formula" for k-infinity as derived by Enrico Fermi.

http://en.wikipedia.org/wiki/Four_factor_formula

"In an infinite medium, neutrons cannot leak out of the system and the
multiplication factor becomes the infinite multiplication factor,
k = k_{\infty}, which is approximated by the four-factor formula.


PamW
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Statistical Donating Member (1000+ posts) Send PM | Profile | Ignore Fri Mar-18-11 07:09 AM
Response to Reply #53
61. No it is NOT just density.
That is what 3 different people have told you in about 20 posts.

You could have the most ultra dense uranium slug ever created and it could STILL not be critical.

Criticality requires a certain level of neutron density AND it also requires a moderator to slow down fast neutrons into thermal neutrons.

Density of fissile material is an important factor because it helps to improve the neutron economy. For example if there is boron in the water it will block far too many neutrons for criticality to be achieved.

"Put enough of it together, and you have criticality."
This is a false assumption you keep going back to over and over and over despite being corrected. Saying it over and over doesn't make it any less false.

Think about it a fuel rod can't be critical all the time. Otherwise criticality would occur in the fuel fabrication plant. If the only thing that mattered was density (which never changes) then as soon as you created a fuel rod it would go critical. The reality is you can hold a fuel rod with your hands. No criticality, no significant build up of heat.

Likewise in spent fuel only 5% of the fuel has undergone fission and no significant change in density yet the composition of that fuel has changed (buildup of neutron poisons) that it can no longer sustain criticality.
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PamW Donating Member (566 posts) Send PM | Profile | Ignore Thu Mar-17-11 09:07 PM
Response to Reply #33
44. An individual fission is not a tremendous amount of energy
If we're assuming that fission, although relatively rare, is ongoing - that means some of those spontaneous, spare neutrons are striking other nuclei, with each collision generating a tremendous amount of energy.
--------------------------

When you are talking about individual reactions, it's not a "tremendous amount of energy",
at least not macroscopically.

One gets on the order of 200 MeV per fission. However, do the units conversion to a
macroscopic unit of energy like the Joule. 1 MeV is 1/100,000,000,000 of a Joule
1 MeV ~ 1.0E-13 Joules. That's a million times greater than a single chemical reaction,
but it's a paltry amount of energy. You don't get tremendous amounts of energy unless you
have LOTS and LOTS of fissions from a "chain reaction". If the system is sub-critical, which
the reactors are since the control rods were inserted; it won't support a chain reaction and
any spontaneous fission that attempts to start one will have the chain die out.

PamW

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PamW Donating Member (566 posts) Send PM | Profile | Ignore Thu Mar-17-11 09:00 PM
Response to Reply #29
43. They use a "neutron generator"
If SF events are so inconsequential how do they initiate a chain reaction
in a nuclear weapon in under a microsecond?
-----------------------------------

Nuclear weapons designers don't count on spontaneous fissions to initiate
a nuclear weapon. In fact, the don't want spontaneous fissions in
the assembly. They want the device initiated at the proper time, and not
at some random time due to a spontaneous fission. That's the difference
between "weapons grade" and "reactor grade" plutonium. The weapons grade
is relatively free of isotopes that have substantial spontaneous fission.
That's why it's so difficult to make nuclear weapons with reactor grade
plutonium. The DOE has said it is possible, but only by using techniques
that a novice bomb designer wouldn't know about or have the skill to use.

So how do they start the reaction in a nuclear weapon? They use a device
that Sandia National Labs makes called a "neutron generator". It's a small
accelerator that induces a neutron producing reaction.

http://en.wikipedia.org/wiki/Neutron_generator

http://www.fas.org/nuke/intro/nuke/design.htm

The nuclear chain-reaction is normally started by an initiator that injects a burst of neutrons into the fissile core at an appropriate moment. The timing of the initiation of the chain reaction is important and must be carefully designed for the weapon to have a predictable yield. A neutron generator emits a burst of neutrons to initiate the chain reaction at the proper moment �- near the point of maximum compression in an implosion design or of full assembly in the gun-barrel design.

PamW
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FBaggins Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Mar-17-11 03:40 PM
Response to Reply #23
25. Maybe a millionth as much.
I'm just guessing... but the answer is "technically? Yes... in any real sense? No"

Each time a neutron happens to hit an atom of U-235 and split it... that's technically "fission", but only the tiniest percentage of decay events end up hitting a nucleus.

I'll see if I can find some data, but if you insisted on strict accuracy, you might say that decay represents 99.99999% of the heat (probably more) and there's a tiny bit of fission going on, but it isn't relevant to the conversation.
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