"Critical Mass" question

[Bill Mattocks]

I keep reading about 'critical mass' with regard to various things in the nuclear reactors or the cooling ponds in Japan. I'm a little confused about this.

I grew up thinking that 'critical mass' was a bad thing for a nuclear reactor - essentially a runaway chain reaction that was desirable only in a nuclear weapon, not a reactor. And I guess I just assumed it means 'BOOM' and a big mushroom cloud.

If the radioactive material in Japan reaches 'critical mass' in the sense they're talking about, does this not mean a nuclear explosion?

Sorry, I'm a bit uninformed on this, and the media isn't being very expository.

 

[Nomad]

From Wikipedia:

A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (e.g. the nuclear fission cross-section), its density, its shape, its enrichment, its purity, its temperature and its surroundings.

So in order to have a nuclear reaction (and hence, nuclear power), you need to have a critical mass of fissionable materials. This is different from a runaway chain reaction, which occurs when things spiral out of control. My understanding is that a runaway chain reaction shouldn't occur here, as the control rods were engaged and nuclear fission was halted.

A meltdown or partial meltdown is the more likely danger here, as the main problem appears to be getting sufficient cooling water to the reactor to dissipate the significant residual heat.

A nuclear meltdown is an informal term for a severe nuclear reactor accident that results in core damage from overheating. The term is not officially defined by the International Atomic Energy Agency[1] or by the U.S. Nuclear Regulatory Commission.[2] However, it has been defined to mean the accidental melting of the core of a nuclear reactor,[3] and is in common usage a reference to the core's either complete or partial collapse. "Core melt accident" and "partial core melt" are the analogous technical terms, though the severity of these nuclear accidents can vary in the extreme.

Hope this helps!

 

[Empty Hands]

See here for an example of criticality that occurred by accident simply from dropping a neutron reflective half-sphere around a plutonium core. The man received 4 times the lethal dose in a second or two, equivalent to standing 1500 meters away from a nuclear bomb detonation - and yet no runaway reaction or explosion occurred. The term simply means that the nuclear reaction is sustained under it's own conditions due to enough neutrons being kept inside the fissile mass to keep the reaction going. That's how controls rods work - they are made of neutron-absorbing material, and when placed between fissile masses, absorb enough neutrons to keep the mass non-critical, and the nuclear reaction non-sustained.

Basically from what I understand of the Japan situation, they are sitting on top of the world's largest dirty bomb due to the inability to control cooling. However, there is no danger of nuclear explosion or an ongoing reaction. The heat is being generated from radioactive decay of already fissioned uranium atoms, and does not require a nuclear reaction to occur. The decay series of Uranium 235 ends in stable Lead 207, and would take more than 150,000 years to complete (5 half-lives being our standard of safe) mostly due to the protactinium-actinium transition which has a half-life of 33,000 years.

 

[Bill Mattocks]

OK, see, that's what I'm not understanding. I get both of your explanations, thanks.

However, the media is talking about not just a core meltdown, but (in the cooling ponds) a critical mass situation occurring where there is no explosion, but the spent rods reach critical mass and BOIL AWAY, releasing all their radiation into the air as highly radioactive vapor, containing various isotopes of uranium and plutonium. Not an explosion, but a massive release of radiation and radioactive material into the air, such that if anyone gets downwind of it, they're essentially dead meat.

Am I not understanding this? Again, not talking about the core here; but the cooling ponds for the spent rods that have lost the water which covered them, and now the rods themselves are melting together, forming critical mass, and the metal itself is boiling away in a matter of seconds (or is at risk of doing so).

My question would be what makes it not explode, or rather, why does a nuclear bomb explode and this not? Both are 'out of control' chain reactions, not like the core melt-down scenario. The media is distinctly talking about an out of control chain reaction in the cooling ponds.

 

[Empty Hands]

The media is distinctly talking about an out of control chain reaction in the cooling ponds.

They are wrong. What they are describing is a meltdown, but not the formation of a critical mass. Physics-wise, they are not related. A critical mass requires a particular geometry, materials and so forth, a meltdown just requires a lot of heat to turn the fuel rods into slag. Once the fuel is exposed to the air without cooling, the incredible heat will evaporate all volatile radioactive elements into the air, resulting in a large radioactive release. There also could be combustion, but I'm not sure about that. The radioactive release is simply the result of heat, not a nuclear reaction.

Of course, this is the understanding of an interested amateur. What say you, Elder?

ETA: the boiling away of the fuel pond water is a continual process over time, again based only on heat. Within some time of the circulation pumps shutting down, the containment water would begin to heat and eventually boil. After boiling for however long, the water would eventually boil away leaving the fuel rods exposed to the air without cooling. This is probably where the steam release and pressure explosions came from.

 

[KELLYG]

Thanks guys for the tread and the information. The main thing that I do not understand is why they can't water/coolant to where it needs to go. Is it a one shot fill the pond then everything is OK or dose it, due to the excessive heat need a more continuous supply of "fresh" cool water. I can't seem to rap my mind around why it is so difficult to get water there?

 

[Empty Hands]

Is it a one shot fill the pond then everything is OK or dose it, due to the excessive heat need a more continuous supply of "fresh" cool water. I can't seem to rap my mind around why it is so difficult to get water there?

Yes, it requires continual circulation, and the heat is continually generated. It's not hot like a stove is hot after being turned off; the stove is still on. Thus, any water they put in there is only a temporary solution, they need to restore circulation. It takes something like 5-6 years, according to one expert I heard, for the fuel rods to cool enough for dry storage.

As for the difficulties, for one the infrastructure is devastated, so it's difficult to deploy equipment and power. For another, no one can enter the buildings. There are no pictures of the fuel ponds (last I checked) or the reactors, we are extrapolating information from what we can see. I don't know of any personal protection that could fully protect you from the quantity of radiation potentially inside. For the Chernobyl cleanup, a cameraman took video through one of the holes in the building. He died weeks later.

 

[KELLYG]

Thanks, Empty Hands. I am totally uninformed when it comes to nuclear power and how it works.

 

[Bill Mattocks]

They are wrong. What they are describing is a meltdown, but not the formation of a critical mass. Physics-wise, they are not related. A critical mass requires a particular geometry, materials and so forth, a meltdown just requires a lot of heat to turn the fuel rods into slag. Once the fuel is exposed to the air without cooling, the incredible heat will evaporate all volatile radioactive elements into the air, resulting in a large radioactive release. There also could be combustion, but I'm not sure about that. The radioactive release is simply the result of heat, not a nuclear reaction.

Of course, this is the understanding of an interested amateur. What say you, Elder?

ETA: the boiling away of the fuel pond water is a continual process over time, again based only on heat. Within some time of the circulation pumps shutting down, the containment water would begin to heat and eventually boil. After boiling for however long, the water would eventually boil away leaving the fuel rods exposed to the air without cooling. This is probably where the steam release and pressure explosions came from.

http://www.ctvbc.ctv.ca/servlet/an/...-rods-110316/20110316?hub=BritishColumbiaHome

The outer shell of the rods could also explode with enough force to propel radioactive fuel over a wide area, if Gregory Jaczko, chief of the U.S. Nuclear Regulatory Commission, is correct.
He said the problem is at the complex's Unit 4 reactor.

This is just one link. It seems that many are stating that the spent rods can reach critical mass for them to boil - NOT the water but the actual metal that encases the uranium pellets themselves. Metal boiling away, not the cooling water.

Is this wrong? I have to say I'm still confused. I do understand that when they say 'explode' in the above context, they're not talking about a nuclear detonation.

 

[Empty Hands]

This is just one link. It seems that many are stating that the spent rods can reach critical mass for them to boil - NOT the water but the actual metal that encases the uranium pellets themselves. Metal boiling away, not the cooling water.

The fuel pellets are encased in a zirconium alloy tube. I can't find any information on the boiling point of the metal, but the metal is prone to corrosion in reaction with oxygen, and can react with hydrogen and become more brittle. That combined with the heat could lead to an "explosion" of sorts, but it would be heat and pressure based, not nuclear. Again, I'm sure Elder could address the specifics better than I, but I am positive that it is not a nuclear reaction or a critical mass involved in this process.

ETA: In no case could the metal tubes encasing the pellets ever form a critical mass, since the elements in the alloy are not radioactive or neutron emitters.

 

[Empty Hands]

A question of my own (ahem, Elder): whenever you see pictures of fuel ponds or reactor cores, you always see a guy handling a rod or standing over the pool wearing a gown and a surgical mask. Obviously not concerned about radiation. Are they only handling fuel rods prior to insertion into the reactor? Is there some unseen transport mechanism for transferring spent rods to containment? Is the water in the containment pool shielding that well, particularly since some of the uranium series are gamma emitters? Or is there extra shielding around the fuel rods I don't know about?

 

[elder999]

A question of my own (ahem, Elder): whenever you see pictures of fuel ponds or reactor cores, you always see a guy handling a rod or standing over the pool wearing a gown and a surgical mask. Obviously not concerned about radiation. Are they only handling fuel rods prior to insertion into the reactor? Is there some unseen transport mechanism for transferring spent rods to containment? Is the water in the containment pool shielding that well, particularly since some of the uranium series are gamma emitters? Or is there extra shielding around the fuel rods I don't know about?

Bill, EH handled the whole criticality question pretty well...though I do have a few things to add.

Slotin, according to many who knew him, was a death-obsessed tool who courted the very disaster he so heroically and cooly averted. This event was not "critical" but sub-critical....once an assembly like the one he was playing with achieves criticality, well, it's critical, as in sustaining fissions, in this case, in what would have been a decidedly uncontrolled manner. In a reactor, criticality is sustained and controlled.

Now, on to a pet peeve: you don't need a "Critical mass" to sustain criticality. I'm going to illustrate with something that I have to add a few preambles to: this information is entirely available on the internet, and mostly in the public domain.

When we speak of the assembly of a weapon, the actual object that is the core, it's often referred to as acheiving "critical mass" in that two pieces of material are rapidly introduced to each other, or an object is imploded to a certain density, but what we're really talking about with criticality is the propagation of fissions, that is to say, neutrons. If we have a smaller object, say a somewhat spherical (I can say "somewhat spherical," can't draw a picture or get more specific) mass of plutonium of less than the often quoted 10 kg of Pu-239, but it were surrounded by a neutron reflector, like beryllium, and made denser by dynamic detonation (I've always hated the use of the term "implosion" for this, though it is the standard) we can acheive criticality with less than the so-called "critical mass," whic is, after all, dependent upon a variety of factors such as purity and surroundings. Thus it is that the French and Israelis have built wonderfully efficient thermonuclear devices using as little as I can't reall say how many but it's less than 22 lbs of plutonium for the primary.....so I'm told, anyway.....:lol:

As far as nuclear fuel goes, it's important to point out that zirconium, uranium and plutonium are all pyrophoric metals. They make very, very, pretty, and very, very, very, hot fires-this is what should be most feared in japan's situation, and there's some pretrty rampant speculation that that's exactly what's happening at Unit #4.....bad.

The people you see pictured next to fuel assemblies are typically checking the receipt of a shipment of new fuel that hasn't been irradiated. We used to say that if you drove over an assembly that was removed from the reactor at 60 mph you'd be dead before you hit it, as those fuel rods are emitting in excess of 1 million R-while there has been one instant death from fuel radiation in the industry, Chernobyl proved that it's not necessarily true, though that wasn't the same type of fuel at all.

Typically,EH, the refueling cavity is filled with 30 feet of water, as is the spent fuel pool, and the two are connected by a tunnel with a trolley. New fuel is loaded into the pool, into the trolley, sent through to the cavity, upended and placed in the core, all by remote manipulation from a moving bridge with long, water-filled tools. The same goes in reverse for "spent" fuel. THe water does provide shielding for these operations, this is why the tools have drilled holes so that they also are water filled. This is how the one death that I know of took place-a technician was using a boroscope to do fuel repairs (yes, they can do that) and failed to fill it with water, taking that dose directly to the eye and brain, and directly dropping dead....so I'm told....

I'm going to post my analysis of what I know of the events at Fukashima in the other thread. They're bad-unprecedented really, and not quite to Chernobyl level-yet, but well past Three Mile Island, no matter what the Japanese say. Damn things kept me on the phone and emailing for two days, now...

 

[Empty Hands]

THe water does provide shielding for these operations, this is why the tools have drilled holes so that they also are water filled.

Very interesting, thanks. I was mostly familiar with water as shielding for beta emitters, and I didn't think it would be as effective against gamma emitters. I see that some of the u-235 series are gamma emitters. Is the water effectively shielding the gammas, or is the gamma component minor enough to not matter?

 

[elder999]

Very interesting, thanks. I was mostly familiar with water as shielding for beta emitters, and I didn't think it would be as effective against gamma emitters. I see that some of the u-235 series are gamma emitters. Is the water effectively shielding the gammas, or is the gamma component minor enough to not matter?

It's a little over 30 ft of water. The halving thickness of water is about 7 inches. This comes out to about 15 halving thicknesses, and reduces gamma to 1/65536 (1/2 times itself 15 times) of their original strength.

 

[Empty Hands]

It's a little over 30 ft of water. The halving thickness of water is about 7 inches. This comes out to about 15 halving thicknesses, and reduces gamma to 1/65536 (1/2 times itself 15 times) of their original strength.

Interesting, thanks.

 

[Shuto]

According the the WSJ, they are dealing with more than just spent fuel in some of these pools.

WSJ Link

In addition, a standard practice at Japanese nuclear plants—to remove fresh fuel from a reactor and park it for weeks or months in a less-protected "spent fuel" pool during maintenance—appears to have been a significant contributor to the crisis, engineers say.