Jeffrey LewisFOGBANK

Rob Edwards in New Scientist writes about a material called FOGBANK, which is used in US nuclear weapons such as the W76. (Ian Sample in the Guardian also picked it up.)

Edwards’s article contains some speculation on the use of FOGBANK, which is something I’ve been looking into since Frank Munger started asking uncomfortable questions about delays in the W-76 Life Extension Program in January (January 24, January 25, February 12, and March 6).

I believe that FOGBANK is an aerogel used as the interstage material — Howard Morland’s exploding styrofoam — in three thermonuclear designs: the W76, W78 and W80. I believe it is recycled, and will be produced, at the so-called Purification Facility at Y-12.

John Field thinks FOGBANK is an aerogel based on a hypothesis he has about how a thermonuclear secondary works. I suspect he is right for a more mundane reason. Aerogels (that’s one on the right, with a brick sitting on top of it) are extremely low-density materials that feel like polystyrene and look like smoke or fog. Indeed, the nicknames for aerogels include “frozen smoke” and “San Francisco fog.”

Witty bastards in our nation’s nuclear weapons complex, eh?


There are not many official references to FOGBANK, but I’ve collected them for you here. I think they link FOGBANK, ACN, interstage material and the Purification Facility very tightly.

  • A variety of DOE Nuclear Explosive Safety documents describe FOGBANK as a material “used in nuclear weapons and nuclear explosives” along with Lithium hydride (LiH) and Lithium deuteride (LiD), Beryllium (Be), Uranium hydride (UH3), and Plutonium hydride.
  • A Y-12 employee was paraphrased as saying “They’re starting to make things with FOGBANK again after many years of not using it, and it’s a big concern.”
  • NNSA Administrator, Tom D’Agostino, has mentioned FOGBANK twice, linking it to the interstage material of a Navy nuclear warhead and the flammable chemical, Acetonitrile (ACN):

Finally, there is a material that we currently use and it’s in a facility that we built … at Y-12. It’s a very complicated material that — call it the fog bank. That’s not classified, but it’s a material that’s very important to, you know, our life extension activity. And we are spending a lot of money as part of the [LEP] in making — trying to … produce that material, and we are not out of the woods yet. And it’s a material that uses a cleaning agent that is extremely flammable. And in fact, we had to build a separate spillway, external, because if this stuff ever caused a problem we would want — we would have to put it in this area. It’s expensive to operate and maintain that facility.

My ideal world would be — I don’t have to make that material anymore. I don’t have to deal with these chemicals anymore. I can take advantage of outsourcing as — in fact, one of the things on the RRW — in a closed session I would talk about what we would outsource on this — what we think we can outsource on this weapons system that would reduce cost. Want to take advantage of all those things.

[Emphasis mine. Hearing of the Energy and Water Development Subcommittee of the House Appropriations Committee, March 29, 2007]

There’s another material in the — it’s called interstage material, also known as fog bank, but the chemical details of course are classified.

That’s a facility that we currently have right now. It’s a very complicated process. I use that to support the Navy’s program. It takes a tremendous effort to operate this facility. It’s dealing with toxic materials — hazardous to our workforce — but it’s required. It’s the way we did things back in the Cold War. The RRW will allow us to not have to develop and maintain that capability. And that’s very important because that’s got a long-term cost to run and it’s got an impact on our workforce, just like the case material.

[Emphasis mine. Hearing of the Strategic Forces Subcommittee of the Senate Armed Services Committee, March 28, 2007]

We have another material that requires a special solvent to be cleaned. It’s — the chemical term is ACN. But that solvent is very volatile. It’s very dangerous. It’s explosive. And I’m required to use it because that’s what we used 30, 40, 50 years ago, when we made this special material. And so these are the kinds of things that I can eliminate.

[Emphasis mine. Remarks by Thomas D’Agostino at the Woodrow Wilson Center, June 15, 2007]

  • NNSA released a chart (below) on features of the RRW, including the replacement of an “expensive ‘specialty’ material … eliminating need for unique facilities” in the interstage.

  • I believe the so-called “Purification Facility” at Oak Ridge replaced building 9404-11 and is used to recycle and produce FOGBANK. Dennis Ruddy, then-president and general manager of BWXT Y-12, the government’s contractor, said the Purification Facility was used to refurbish a classified material:

“It reprocesses a material that we’re taking out of weapons so that we can reuse it in refurbished weapons. That’s probably all I can say.”

“The material is classified. Its composition is classified. Its use in the weapon is classified, and the process itself is classified,” Ruddy said.

  • John Ainsle, in an excellent backgrounder, quotes a Los Alamos document as stating that the “evaluation of internal gas generation of a Fogbank in a neutron environment was started in FY95.”
  • The Purification Facility uses ACN. On three separate occasions in March 2006, workers evacuated the Purification Facility after alarms went off. According to DOE documents, the Purification Facility is alarmed to monitor for acetonitrile (ACN) levels.
  • Contrary to D’Agostino’s claim that FOGBANK is used to “support the Navy’s program,” I would think three warheads — the W76, the W78, and the W80 — use FOGBANK based on when those warheads were produced.


  1. Pavel (History)

    Yes, they should just outsource the produciton. To China.

  2. Ben (History)

    Sounds very plausible – but isn’t it a basic principle of picking a codename that it should be unrelated to what it’s describing? Why did they choose such a descriptive name?

    I hope they haven’t picked all their codenames the same way. “The weapon has a fissile core of a material classified as SHINY METAL DEATH.”

  3. Siddharth

    The Guardian story came out first, I believe.

  4. Allen Thomson (History)

    Very interesting. I’m kind of sympathetic to the RRW idea, actually. Probably have to give a little on yield/weight and form factor, but it seems worthwhile.

    BTW, the “toxic, brittle material” in the NNSA slide is most likely beryllium or an alloy of it.

  5. Carey Sublette (History)

    The public theoretical and experimental literature on radiation implosions shows that there is an optimal driving radiation power curve required at the secondary to achieve extremely high compressions. This curve is an exponentially increasing power curve. In inertial confinement fusion systems this power curve is approximated by a series of shaped pulses, the net profile is one of a relatively long period of low power followed by a very rapid ramp-up at the end. It seems likely that the “natural” radiation emission profile from the primary does not closely match the desired optimal power curve.

    It is also well known that pre-heating of the secondary fuel by neutrons emitted by the primary interferes with high compression. In a compact light-weight system, limiting neutron pre-heating would be a challenge.

    Based on these general design considerations it could be anticipated a priori that there is some set of design features – a structure – in the most compact nuclear explosive systems that has the role of modulating the transmission of energy from the primary to the secondary, both the thermal photon and neutron emissions. (In fact I concluded this in the mid 1990s, before I ever heard of the existence of the interstage.)

    It can be surmised then that the interstage is this postulated energy modulation structure. The use of beryllium (almost certainly the undesirable “toxic, brittle material” of the RRW slide) and possibly lithium hydride suggests that the interstage absorbs neutron energy, thus reducing pre-heating. The complexity of FOGBANK suggests a material (possibly an aerogel) with an exacting complex structure, perhaps a material of graded density, which could shape the flow of radiation particularly when the power is relatively low.

    By the way, aerogels have some other remarkable properties besides just very low densities, such as a very low speed of sound transmission, high sound attenuation, superb thermal insulating properties, very high strength-to-weight ratios, etc. that could be valuable structural properties in the warhead. Thus aerogels may be used for more than one purpose, or may have multiple benefits in a single application.

  6. kme

    The points about recycling / reusing / refurbishing of the material are interesting – I can’t see why you would do this for a silica aerogel, it’s not like the raw material is hard to come by.

  7. Stephen Young (History)

    Jeffrey, a clarification: You say “Y-12 Facility 9404-11 is used to make FOGBANK,” then later that 9404-11 was replaced by the Purification Facility. I guess you mean “was” used, as 9404-11 was torn down sometime after BWXT took over Y-12 in 2000.

    “In this General Ordering Agreement for the U.S. Department of Energy, Conti performed the demolition of two facilities and auxiliary buildings within the high security area of the Y-12 National Security complex . . . Building 9404-11 was a 2-story building formerly used in a solvent extraction and purification process supporting Y-12 weapons production missions and was located immediately adjacent to electrical lines and high voltage transformers.”


    So, it seems they are having trouble starting up a new facility to produce FOGBANK, presumably as you say in the Purification Facility.

  8. Stephen Young (History)

    Ah, I see John Ainsle’s excellent backgrounder covers this – 9404-11 was torn down in 2004. So, you should say “was” rather than “is” . . .

  9. Jeffrey Lewis (History)

    Thanks Stephen, you are right.

    We no longer make FOGBANK, at least not as long as the Purification Facility experiences trouble.

    I’ve updated the post.

  10. Allen Thomson (History)

    Carey Sublette said:

    “the net profile is one of a relatively long period of low power followed by a very rapid ramp-up at the end.”

    Hmm. Pure speculation based on that profile, but I wonder if FOGBANK isn’t doped with some x-ray absorbing species so it can act as a bleaching filter. I.e., it’s translucent (even foggy) to x-rays until the absorber gets saturated, then becomes transparent.

  11. Carey Sublette (History)

    Allen Thomson said:

    “…I wonder if FOGBANK isn’t doped with some x-ray absorbing species so it can act as a bleaching filter. I.e., it’s translucent (even foggy) to x-rays until the absorber gets saturated, then becomes transparent.”

    Yes, that’s the notion I have about it exactly.

    Here’s some more speculation: John Ainsle makes the observation that:
    “The explosive safety controls in the Purification Facility are more than would normally be required for handling Acetonitrile. This could be due to the amount of Acetonitrile which is involved in the process, or the explosive risk associated with the process.”

    Aerogels are made by extracting the liquid phase of a gel under super critical conditions. ACN is an excellent general purpose organic solvent widely used in polymer chemistry. it would be a suitable fluid phase for making a polymer aerogel doped with organometallic compounds (for example). The critical point for ACN is 272 °C and 4.87 MPa (roughly 49 atmospheres). A production vessel holding a large volume of ACN under such conditions could be quite a fire and explosion hazard, much more than say, a system using it simply as a solvent for purification.

  12. pdz (History)

    Silica aerogel is used for a lot of purposes in high energy physics, where it is a superb Cerenkov light radiator for discriminating electrons from pions and yet heavier particles. It is extremely difficult to make and does use some unpleasant solvents. I used to use a lot of it my lab days. ACN is one of the possible range of solvents.

    The final material is not particularly toxic, any more than glass or beach sand, but I suspect that it would not be good to eat since the shards are pretty sharp and insoluble.

    Silica aerogel is incredibly brittle, more so than beryllium. For the most part, I think some of the other speculations are barking up the wrong tree. But I cannot say more having the disadvantage of having held a Q clearance for a long time.

  13. John Ainslie (History)

    Is the description of the work done in the purification facility consistent with the aerogel theory ?

    The processes are:

    1) dissolution, filtration, and recrystallization;
    2) powder processing in a nitrogen atmosphere,
    and 3) drying, machining and inspection.

    Most of the references to the facility would suggest that what is happening is remanufacture. However Frank Munger at Knoxnews has an old quote which indicates that the facility is to recycle material withdrawn from warheads.

    Sorting this out is difficult. The original process in 9404-11 was described as solvent extraction and purification.

  14. Nuk-U-Lar


    Regarding your last point –

    Contrary to D’Agostino’s claim that FOGBANK is used to “support the Navy’s program,” I would think three warheads — the W76, the W78, and the W80 — use FOGBANK based on when those warheads were produced.


    I think that the dates of production are of less importance than the origin of the designs ( i.e. which lab designed which system ) when you are trying to infer where this mystery material might or might not have been used.

    The old joke around the NW complex is that there are things that we keep secret from our enemies ( i.e. Russians and Chinese ) and then there the really important things that we keep secret from the competition ( i.e. the other design lab ).

  15. Carey Sublette (History)

    John Ainslie asked: “Is the description of the work done in the purification facility consistent with the aerogel theory ?”

    It looks to me like no smoking gun for actual manufacture (e.g. an autoclave for liquid phase removal). Gels are made by precipitation or polymerization.

    But does the prefatory phrase: “Operations performed within the Purification Production Facility will include…” mean that other important operations may not be named (i.e. the list is not exhaustive)?

    Most of the named processes might have a role in processing an aerogel. For example purification of new or old aerogel (solvent extraction, dissolution, filtration) and fabrication (glove boxes, powder processing in a nitrogen atmosphere, drying, machining and inspection). Recrystallization is a stretch though I think.