# What is the reasoning behind using depleted uranium as counterweights in the 747?

I recently found out that some of the earlier models of 747 had between 600 and 1100lbs of depleted uranium as counterweights. These counterweights were found in 747s manufactured between 1968 and 1981.

Given that the study linked above and the well-known radiation exposure for workers (and passengers) even prior to the 1968 introduction of these counterweights, why did Boeing (and McDonnell Douglas) decide to use such a hazardous (and potentially dangerous from a military perspective) material?

I understand that the weight/density allows for a smaller package, but it seems that the tungsten weights were integrated after 1981 without any space issues in existing aircraft. Was depleted uranium in such high supply that it was also more economical?

Eventually, the counterweights were replaced with these tungsten replacements:

• DU is only hazardous when you come into contact with particles; especially when inhaled. In a crash severe enough to compromise the integrity of the weights; this would be your last concern. My guess is that a more safety conscious (litigious) industry and availability of alternatives led to other materials being used. – Simon Feb 8 '17 at 9:35
• This was also found on the L-1011. – Sports Racer Feb 9 '17 at 22:44
• @Simon : and in this regard DU is not in any way more hazardous than any other heavy metal, like lead or tungsten. Why it is falling out of fashion today is not because of safety concerns, but because of the hysterical public thinks "uranium = dangerous radiation or nuclear explosion hazard". However, depleted uranium, and even regular (non-enriched) uranium emits so little radiation that it's completely negligible. In fact, uranium can be used to shield against radiation (it's much denser than lead). You'll probably get more radiation from eating a banana than holding a piece of uranium. – vsz Jun 19 '17 at 6:28
• @vsz: The (much) greater safety concern is that uranium has very potent chemical toxicity, much greater than that of, say, tungsten. Also, while DU is quite good at shielding against most ionising radiation, it isn't an option for anything producing significant quantities of fast neutrons, as these fission the uranium atoms and produce lots of secondary radiation. – Sean Jun 6 '18 at 20:40

## Boeing

Boeing would have used DU because it had the right combination of physical characteristics and cost. Their tests showed that the radiation exposure for workers was low (2.6% of the statutory "safe" level). In most cases the exposure was so low as to be not detectable.

Passengers on aircraft are exposed to cosmic radiation at much higher levels than people on the ground.

On the ground the average American is exposed to 620 mrem/year from all sources.

Those flying from Washington DC to Los Angeles would be exposed to close to 2 mrem from cosmic radiation. This is not an issue for passengers but is something that airline crew and other very-frequent flyers are aware of.

Exposure to aircraft occupants from cosmic radiation is 600 times higher than from DU counterweights in the early 747 aircraft.

### Depleted Uranium

DU is Uranium from which the most radioactive parts have been removed. It is less radioactive than naturally occurring Uranium.

Heavy metals like Lead, Tungsten and Uranium are toxic if ingested. This toxicity is not due to radioactivity. Aircraft crew and passengers are not exposed to these metals in any way that could cause them to be ingested.

## Boeing Tests

Boeing carried out safety tests on the material.

### Workers

Boeing has conducted two dosimetric studies of exposures to workers. For both studies, whole body exposures were measured with film badges which were provided and processed by Landauer, Inc. These badges have a minimum detectable exposure of 10 millirem per issue period (monthly). In the second study, extremity exposures were measured with finger rings, also provided by Landauer, having a minimum detectable exposure of 30 millirem per issue period (also monthly). The periods of the study were December 1968 to February 1970 for the first study and September 1977 to April 1978 for the second.

Both of these studies showed all worker whole body exposures were less than 2.6% of the exposure limits for occupationally exposed employees (5000 millirem per year) and less than 26% of the limits for members of the general public in effect at that time (500 millirem per year).

### Crew and Passengers

Based on the data from National Lead, reported in the section on exposures to the general public, dose rates to flight crew will be less than 0.8 microrem per hour. During a 2000 hour working year, this results in a maximum potential exposure of 1.6 millirem, less than 2% of the 100 millirem per year limit for members of the general public. This is only 1/600th of the 500 microrem per hour increase in dose rate from cosmic radiation flight crew experience at 39,000 feet.

Boeing Use of Depleted Uranium Counterweights in Aircraft. 1984. Retrieved from NRC

## References

• It still seems like a lot of effort to go through, studies, exposure tests, etc to save a few dimensions or dollars over tungsten. Eventually Boeing and MD decided that the risk was worth removing the counter weights so I can't agree that it was found entirely safe. In the ELAL 1862 crash something like 150kg of DU was never recovered. – Ron Beyer Feb 8 '17 at 12:41
• @RonBeyer Perception of safety matters almost as much as actual safety to an airline. They may have removed it because the public perceived it to be unsafe, rather than because it was actually unsafe. – Notts90 Feb 8 '17 at 12:47
• The 1992 El Al incident highlighted the risk to rescuers. There is no significant risk to workers or passengers, which is what Boeing had checked, but while correct that still is an incomplete conclusion. – MSalters Feb 15 '17 at 15:11
• TL;DR just because it's called uranium doesn't mean it's dangerous. Soldiers sit in vehicles "made of" (ish, a bit) depleted uranium, next to stacks of ammunition made of depleted uranium. And it turns out it's more radiologically perilous to just fly in a plane at all. (Or, possibly, lay on a beach without sunscreen.) – Grimm The Opiner Apr 4 '18 at 9:07
• @GrimmTheOpiner: Ironically, tanks and APCs are about the only places where you would have reason to shy away from using DU; in a nuclear attack, DU-armoured vehicles would become deathtraps for their occupants, due to the high fast-neutron flux inducing fission reactions in the uranium, releasing lots and lots of secondary radiation in the process. – Sean Jun 6 '18 at 20:45

[W]hy did Boeing (and McDonnell Douglas) decide to use such a hazardous (and potentially dangerous from a military perspective) material?

Because it's not particularly hazardous and it's not very dangerous from a military perspective.

Uranium comes as a mixture of two major forms (called isotopes). Uranium-235 accounts for about 0.7% of the mix and is the isotope that supports nuclear chain reactions so is used in nuclear fuel and atomic bombs. (Actually, almost all nuclear weapons since the bomb dropped on Hiroshima have used plutonium but that's a different story.) Almost all of the rest is Uranium-238, which is only mildly radioactive. It doesn't support nuclear chain reactions, so it can't be used in a bomb. Depleted uranium (DU) is uranium that has been processed to remove almost all of the Uranium-235. In other words, DU is almost entirely U-238.

U-238's radioactivity is mild in two senses. First, it has an extremely long half-life of about 4.5 billion years, which is about the age of the earth. Second, most of the radiation it emits is alpha particles. Alpha particles are trivial to stop with even the most negligible "shielding": a piece of paper or even a few centimetres of air is enough. Human skin is good shielding against alpha radiation: the particles are stopped by the outer layers of the skin, which are already dead, and which are shed in a few days anyway. You're not going to get skin cancer from low-intensity alpha-sources.

U-238 is only really hazardous to health if it gets inside your body. When that happens, it's bad for you in two ways. First, it's toxic in much the same way as any other heavy metal. Second, if it's inside your body, then the alpha particles it emits are hitting living cells of your body, rather than your dead skin, so now it can cause cancers. Inhaling dust is a big risk, here, and that's why soldiers who've worked with DU weaponry can have problems. In the case of the counterweights, this risk is mitigated by coating the DU so people aren't coming into contact with it.

The military implications are small. DU has two main military uses: nuclear weapons ("Waaaait, I thought you said it couldn't be used for that!") and non-nuclear weapons.

Because of its density and the fact that it burns vigorously when finely powdered and exposed to the air, DU is used in some anti-tank weapons. According to Wikipedia, the coalition used more than 1000 tons of DU-based non-nuclear weapons in Iraq in a three-week period in 2003. Each 747 contained between a third of a ton and half a ton of DU, which corresponds to about ten to fifteen minutes' worth of ammunition. That doesn't seem to be a big concern.

Application of DU to nuclear weapons is, like everything else in this answer, in two ways. First, it's used in the "tamper" of a nuclear weapon, which reflects neutrons back into the weapon's core and allows smaller weapons to be made. Second, you can put U-238 in the right kind of nuclear reactor and make the exact kind of plutonium that's used for bombs. However, neither of these is a big concern because any organization that's capable of turning DU into nuclear weapons must already have enough infrastructure that they will already have their own large supplies of DU and not need to pillage it from 747s. In particular, they must already have nuclear reactors to convert DU to plutonium and isotope separators to extract the plutonium.

• DU is also used in ammunition without refinement. Either way at some point (as in my comment on the other answer), Boeing and MD decided to not only stop using the DU counter weights, but remove them from older aircraft, so somebody decided that there was enough of a concern to go through that process. – Ron Beyer Feb 8 '17 at 12:44
• @RonBeyer: "DU without refinement" doesn't make sense; refinement or enrichment is what splits natural Uranium into Enriched Uranium and Depleted Uranium. – MSalters Feb 15 '17 at 15:15
• @MSalters I meant without further refinement. I understand there is a process to create DU, however it doesn't need a lot of further processing to be used in that regard. – Ron Beyer Feb 15 '17 at 15:44
• @RonBeyer I don't really understand. Once you've made DU, you have DU. You don't need to refine it any more because it already is what it is: depleted uranium. – David Richerby Feb 15 '17 at 15:53
• The DU tamper of a nuclear weapon doesn't just serve as a neutron reflector; if it's a thermonuclear weapon (as essentially all nukes are nowadays), a large portion of the total explosive yield comes from the fission of the DU tamper by the fast neutrons generated by fusion. – Sean Jun 6 '18 at 21:08

Depleted Uranium is 68% denser than lead and costs much much less as DU is a discarded material. Therefore, there were both space requirement considerations as well as cost considerations when using DU as a ballast or counterweight in aircraft.

Although precautions are taken, there are no substantial health concerns when using DU in this way. The radioactivity of the DU is no longer a factor as it has been depleted.

Former Fighter Pilot / Private Jet / Corporate Jet Pilot

• Googling for "cost of lead" and "cost of depleted uranium" suggests that lead costs about costs about \$1/lb, compared to \$5/lb for DU. So, although bigger, lead counterweights would be significantly cheaper. DU's much cheaper than tungsten (\\$25-45/lb) and easier to machine, but it's plenty more expensive than lead. – David Richerby Feb 8 '17 at 20:19
• +1 for Density, which is the key design factor. It is not just flutter mitigation, but harmonic damping (or snubbing) that has to be achieved by internal placement of the DU, sometimes at extremities where space is at a premium. DU makes a far more compact flywheel than lead, for the same reason. I agree the ionising radiation risk is negligible considering the background counts at altitude. – mckenzm Feb 9 '17 at 0:20
• So you're not going to edit your claim that depleted uranium is "costs much much less" than lead, even though it's actually five times more expensive? – David Richerby Feb 10 '17 at 0:57
• TBH you'd have to check the historical prices. – MSalters Feb 15 '17 at 15:17
• @DavidRicherby: Some more googling on my part suggests that, in 1968 (when the DU counterweights were first installed), depleted uranium had a lower price per unit mass than lead. – Sean Jun 6 '18 at 21:05

I'd say reason for using non-radioactive Uranium in counterweights is same as in perforating bullets: it has one of the highest specific gravity available, and this means a high weight, a higher inertia, momentum, within same volume, compared to other materials, including Lead.

• The question already states that DU has a very high density. What does your answer add to that? (And, by the way, DU is still slightly radioactive.) – David Richerby Feb 15 '17 at 14:40
• I wanted adding examples of Depleted Uranium use with the same rationale, but please feel free to delete my entry. Depleted Uranium is not radioactive, at least no more than an ordinary granite soil, your observation may come from activists who overacted regarding dangers of Uranium in perforating bombs and other warfare, but we all know that, as G Bush said: 'War is a dangerous place', no need to remark remote and long acting combat risks for those fighting. Do you have a control-review command from the site? – Urquiola Feb 16 '17 at 20:59
• @Urquiola: DU from munitions is dangerous to those exposed to it, but this is almost entirely due to its considerable chemical toxicity, rather than its (extremely mild) radioactivity. – Sean Jun 6 '18 at 20:46
• I'm not that sure. The 1972 edition of OIT: 'Encyclopedia of Medicine, Hygiene, and Safety at Work', tells, about use of Uranium in catalysts: 'Radiation risks from catalyzers are low. The Uranium radiation is of low penetration, no need exists to provide protective screens. The Uranium used is not soluble, and the risk of chemical intoxication is small. Cautions must be addressed against retention of non-soluble Uranium inside lungs and protection of skin against the surface radiation' – Urquiola Jun 8 '18 at 7:24