Tuesday, July 5, 2011

Science Behind Depleted Uranium

In the past I’ve made a number of posts and videos mentioning the fact that uranium is a rather common mineral and that it’s been used in a number of consumer products.   Indeed, thousands of kitchen cabinets contain uranium-glazed dinnerware, some of which was mass produced as recently as the 1980’s.

This has been met with a curious response on numerous occasions.   Many concede that uranium is not all that harmful when touched or even ingested but then say “but what about the nano-particles.”   The dust, or “nanoparticles” resulting from uranium combustion are one thing that seems to come up again and again.  They are often credited with nearly magical properties, like the ability to stay suspended in the air indefinitely or to cause horrible health problems even in those far from the location where the uranium projectile was fired.

Indeed uranium tends to be more hazardous when inhaled than when exposure is by other routes, but that’s the extent of the truth to these statements.  Uranium is hardly unique in this respect.  Exposure to dust in general can cause respiratory problems, and certain metallic particles, such as beryllium, are well known to be especially hazardous if inhaled.   By comparison, uranium less dangerous, though it can be a hazard in high concentrations.

What’s so special about depleted uranium projectiles:

Uranium is used for armor-penetrating munitions because it has a number of properties that make it the most ideal material available.  Its use is generally confined to rounds intended to be used against armor, as the difficulties in machining uranium make it more expensive than other materials and its unique physical properties are of less use against softer targets.   Uranium rounds are generally of the kinetic energy penitrator variety, meaning they contain no explosive and simply use their own energy to punch through armor.

Uranium is very heavy, with a density of 19.1 grams per cubic centimeter.  That’s nearly twice as heavy as lead.   The increased mass means that the round has more kinetic energy than a lighter round moving at the same velocity.

Uranium metal also has a very unique property in how it reacts on impact.  Uranium is pyrophoric and auto-ignites when it is ground or ablated.   Solid samples of uranium will not burn under normal circumstances, but granular uranium or uranium turnings will.  When a round strikes armor, tiny particles of uranium break free from the surface and ignite.  The friction of being pushed through the target effectively grinds off a layer of material from the round, which burns, creating an aerosol of burning uranium which surrounds the round and cuts through the armor like a plasma lance.

In addition to this, uranium is extremely hard.  The hardness of uranium combined with he pyrophoric qualities of the metal give depleted uranium rounds a very unique property known as self-sharpening.   Rather than “mushrooming” as most metals do on impact, uranium rounds actually keep their sharpness.   As material peels away from the round, it retains its taper and even becomes sharper.   Maximizing this property is achieved through specialized (and sometimes classified) alloying methods.

Some have described the effect of depleted uranium as being like a laser in its ability to punch a hole through armor.  The projectiles do not themselves contain explosives, but explosions often result from the fuel or munitions stored within a vehicle.   In addition to being highly effective, depleted uranium rounds have the advantage of avoiding unexploded ordinance left on the battlefield or causing extreme collateral damage if the round misses the target.  They also won’t explode in the breach of a gun.

All of these properties make uranium the best material available for armor penetrating munitions.

What happens to the uranium after the round has been fired:

In most cases, the majority of the uranium in a round stays relatively intact, and only a small portion of the metal peals off and burns.   Uranium metal tends to oxidize relatively quickly when exposed to the atmosphere, so the round and any large portions of it will rapidly acquire a layer of uranium oxide.  Once the layer forms, the oxidization process will slow, as the metal is no longer in direct contact with the air.   Because the uranium oxide layer is not a perfectly impermeable material, oxidation will continue, although at a much slower rate.

The process is analogous to a how a piece of iron corrodes when left to weather.   It will quickly form a layer of rust and, over time, more and more of the iron will rust away, but it may take many years for the metal to be completely reduced to oxides.  Like iron, given enough time, uranium will return to a state similar to how it is found in nature.  The most common form of uranium ore is composed of uranium dioxide. This stable form of uranium is ultimately what uranium metal will revert to.

The uranium which aerosols or combusts when the projectile strikes will also revert to an oxide.   The combustion of uranium produces stable uranium oxides, such as uranium dioxide as well as some intermediate products, including uranyl.   Uranyl is a polyatomic ion of uranium in its +6 oxidation state.

Because uranyl is an ion, it will no be found on its own in nature.   Uranyl itself is fairly stable but due to its positive charge, it behaves like a free-radical and will form a compound almost immediately.   In most circumstances, this will be uranium trioxide (also known as uranyl oxide).   Occasionally, a small amount of uranyl nitrate or uranyl carbonate will be created.   Uranium trioxide is also found in uranium ores, though it is not as common as uranium dioxide.  Other less common forms of uranium oxide, such as U3O8 or U3O7 may be present in insignificant amounts.

Therefore, it can be said that when uranium combusts, it is converted to uranium oxides, primarily uranium dioxide, which are very similar to those found in uranium minerals.

Where the dust goes:

The material which does burn is atomized, reduced to dust or “nanoparticles” – which is really just another way of saying “really small dust.”   Some of the uranium particles may bond to the still-molten metal of the armor that the round has penetrated and become embedded in it.   However, much of it will be expelled into the environment.

There is already quite a lot of dust in the earth’s atmosphere.   Tiny dust particles help give the sky its blue color and seed clouds to produce precipitation.    Uranium dust, however, does not tend to stay suspended in the atmosphere as well as other types of dust.  The fact that uranium is so heavy means that it will settle out faster than nearly any other particle of similar size.

The vast majority of dust from a uranium projectile will settle to the ground quite quickly.   Once the “smoke” (composed of uranium dust as well as other dust and debris) has cleared, all the larger particles of uranium will have settled to the ground.  Most of the uranium oxide dust settles out of the air within a maximum of about fifty yards of the impact.   The area around the impact may therefore have a small amount of increased uranium content, but this is not all that significant considering that uranium is already quite abundant in the earth’s crust and therefore already present in soil.   Aside from absence of most of the uranium-235 a few specks of uranium dust mixed in with local dust and soil is generally identical to the uranium already found in the environment.

Only a tiny fraction of the dust produced by the combustion of a uranium projectile will have any chance at remaining suspended in the atmosphere for any significant period of time.   Some of the dust may also be kicked up again after it has settled from suspension.  As distance increases from the site of the impact, the concentration of uranium particles will become lower and lower until they are, for all intents and purposes, negligible.

The odd particle of uranium in the atmosphere is hardly unusual.  Any time that mineral dust is kicked up into the atmosphere, there is likely to be some uranium mixed in.

In addition to the vast amounts of uranium already present in the soils and sands of the world, human activity does contribute some to the amount of uranium present in the atmosphere.   The single largest contributor (by far) is the burning of coal.   In addition to numerous other heavy metals released each year, coal fired power plants blow hundreds of tons of particulate uranium into the atmosphere each year and leave hundreds of thousands of tons more in the ash produced. Though the concentration of uranium in coal is low, the sheer amount burned produces significant uranium releases.

In the event that any tiny particles of uranium manage to stay suspended in the atmosphere for an extended period of time, they will eventually be cleared by the same natural process that removes all other forms of dust from the atmosphere: precipitation.   Tiny particles of dust form the seeds onto which water vapor can condense in the upper atmosphere.  This allows the formation of clouds and eventually rain or snow.  Upon raining out of the atmosphere, the uranium will find its way into the hydrosphere and eventually the world’s oceans, which already have quite a bit of uranium dissolved in them.

Health Effects of Uranium Dust:

There is no doubt that the inhalation of significant quantities of uranium dust poses a health hazard.   Indeed, inhaling any dust at all in large enough quantities is something that should be avoided.  Chronic exposure to metallic or non-metallic dust, even of non-toxic materials can cause damage to the lungs.  Those in occupations that involve grinding, sanding or scraping are therefore advised to wear some type of respirator or dust mask.   Of course, some materials are worse than others due to their toxic or carcinogenic nature.  In such cases, greater precautions may be necessary.

It would certainly not be healthy to be in the immediate area of a depleted uranium round impact when it occurs.  The impact would not only produce uranium dust, but also various metallic and non-metallic debris from the target it hits and any explosives it contains.   Of course, for those who are so close to the target, there would be much greater concerns.  At greater distances, the possibility of inhaling any uranium from such an impact is remote and if any is inhaled, will be of an extremely small quantity.

It is known that sufficient uranium inhalation can cause lung damage and, in some cases, lead to lung cancer, but this has only been shown in cases of fairly large and chronic exposure.  There are other materials which are of far greater hazard when compared to uranium.  Beryllium is well known for its inhalation hazards, which are far greater than uranium.   If beryllium is to be machined, it is important to have proper ventilation and respiratory protection;.   Still, when beryllium is ground or machined, nobody worries that it will contaminate whole regions of the earth’s surface.

By contrast, uranium is less toxic than lead and, although radioactive, its long half-life assures that only a minuscule amount of radiation is produced by a tiny particle.   In circumstances where uranium is being worked with and inhalation is a possibility, general purpose respiration protection and dust control measures are recommended.   Even when uranium dust is produced by activities, it is not necessary to resort to extreme measures of protection, such as negative pressure glove boxes or fully isolated protective suits.   Such measures are may be with materials like plutonium, but uranium is not toxic or hazardous enough to warrant anything beyond standard measure of protection.   Handling of solid uranium metal, in circumstances where it is not being cut, machined or drilled, requires no special protection at all.

Should a particle of uranium be inhaled, one of three things will happen.   Either the particle will be trapped in the mucus membranes of the respiratory system, the particle will be exhaled or the particle will become embedded in the tissue of the lungs.  Most dust particles, especially larger ones, never make it to the lungs if inhaled.  The human body has a very effective system to filter air that is inhaled through the use of mucus and small hairs that line the sinuses and trachea.   Should the dust particle be stopped here it will either be expelled from the body or ingested, in which case, it will pass through the digestive tract with little absorption.   In most circumstances, about 50% of inhaled uranium is swallowed, rather than being deposited in the lungs.  If the particle is tiny enough to avoid being filtered out by the body, it may well remain suspended and be exhaled when the individual takes their next breath.

If the particle becomes embedded in the lungs, it may cause some very minor irritation or damage to surrounding tissue.   The damage from a single particle is generally insignificant, however significant damage can occur if enough uranium is inhaled.   Uranium is primarily an alpha emitter, so radiation produced will only effect the most localized portion of the body, and the long half life of uranium means that it will only produce a tiny amount of radiation exposure.   This exposure, along with the chemical toxicity to cells, is reason for concern, but only if the amount of uranium inhaled is fairly significant – more than the occasional tiny particle.

Tiny particles of uranium which manage to make it all the way to the alveoli of the lungs cannot be as easily cleared from the body.  Some of the larger particles may still be cleared by phlem, while those which are deeply embedded or which are are less than about a half a micron in size are absorbed into the bloodstream. The process of absorbing the uranium typically takes a period of days, for the tiniest “nano”-particles, up to months for larger particles of uranium dust, but once in the body it clears the bloodstream and is passed in urine quickly.

NOTE:  This is being discussed in the context of possible exposure to a tiny amount of uranium, as might occur from the impact of a projectile at a distance of hundreds of meters or more.  In these circumstances the total level of possible exposure is minuscule and thus damage to the lungs is not a major issue.  This should not be taken to mean that more direct exposure to uranium particles is not a health hazard.

The kidneys are quite effective in the removal of uranium from the body.   Concern has been expressed about the danger of kidney damage due to uranium exposure, and kidney damage remains the most prevalent health effect observed in humans as the result of uranium exposure.   The kidneys are, however, capable of handling a certain level of toxic metals before damage occurs.  Renal tube damage has been observed in those exposed to uranium, but only in circumstances where the exposure is extremely high and generally of a chronic nature.   In all but the most extreme examples, the damage is temporary and heals once the uranium exposure ends.  It takes about eight milligrams of uranium absorbed by the body to produce even mild, temporary effects on the kidneys.

Other effects, such as disposition in the skeleton or damage to the reproductive system has only been observed at very very extreme exposure levels.

Standards for inhaled uranium exposure:

In the US, the maximum occupational exposure for uranium by the NRC (of 5% U-235 or less) is .2 milligrams per cubic meter, for a typical 40 hour work week.  OSHA’s upper limit is .25 milligrams per cubic centimeter.  This level is significantly lower than the level at which any health effects are detectable, but is much higher than the regional exposure one would expect from the use of uranium projectiles.

For soluble uranium compounds, which are more readily absorbed, the standard is lower at .05 milligrams per cubic centimeter. This is also higher than the average exposure from a projectile used the general region one lives in, but it does not apply anyway, because the dust produced by uranium combustion is not a “soluble” form of uranium.

The WHO considers the standard of one .1 milligrams of uranium per cubic meter of air to be acceptable for the general population.   Surveys by the WHO of sites where depleted uranium has been used have shown that an increased concentration of uranium is only detectable within a very small area around the impact.  The general increase in environmental uranium and background radiation is described by the WHO as “negligible.”   A full survey of several areas in Kosovo concluded “the probability of significant exposure to local populations was considered to be very low.”

It is estimated that the average human absorbs up to 1.1 micrograms of uranium per day, primarily from natural enviornmental sources. At any given time, the average person’s body contains 90 micrograms of uranium, although some may contain significantly more due to the levels in their localized environment.   A total body burden of up to few hundred micrograms is not generally considered to be abnormal or reason for concern.

Sources of Additional Information:
Uranium Lung Solubility – LANL Report [PDF]
Chemical Toxicity Of Uranium
Depleted Uranium Health: Facts and Helpful Suggestions, by Glen Lawrence
Health Physics Society Fact Sheet on Depleted Uranium


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