Depleted Uranium DU: More than you want to know . . . But what every health physicist ought to know

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Depleted Uranium (DU)More than you want to
know . . . But what every health physicist
ought to know

• Ronald L. Kathren, CHP

Ronald L. Kathren, CHP Professor
Emeritus Washington State University at
Tri-Cities Presented at a breakfast meeting
of the Columbia Chapter of the Health Physics
Society Richland, WA November 15, 2007 (at the
ungodly hour of 7 AM!)
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Historical and General Introduction

• Human experience with uranium goes back two
  millennia the Romans used uranium oxide to
  impart a yellow color to ceramic glazes
• At least as early as the 1400s, pitchblende,
  extracted from the Joachimsthal mines, was used
  to color glass
• Uranium was discovered by German chemist Martin
  Klaproth in 1789 named after planet Uranus

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Natural Uranium

• A dense (? 18.68 g cm-3), silvery white weakly
  radioactive and chemically toxic heavy metal
• Ubiquitous in the environment (heaviest natural
  element)
• NatU contains Three isotopes
• U-238 99.27 wt 50 of activity
• U-235 0.72 wt
• U-234 0.0057 wt 50 of
  activity
• Several valence states (4, 5, 6) compounds
  range from soluble (UF6, UCl4) to insoluble
  (typically oxides UO2, UO3, UO4, U3O8
• Pyrophoric particulates oxidize quickly in air

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More History

• Uranium metal isolated in 1841 by French chemist
  Eugene-Melchior Peligot
• Radioactivity of uranium was discovered in March
  1896 by French physicist Henri Becquerel
• Fission of U discovered in 1939 by German
  chemists Otto Hahn and Fritz Strassmann process
  explained by physicists in exile Lise Meitner and
  Otto Frisch

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Toxicological Studies

• First toxicological studies by chemist Christian
  Gmelin (1824) concluded that uranium was a
  feeble poison
• Gmelins work confirmed by other researchers over
  the next several decades
• Since about 1860 until discovery of insulin in
  1920s used therapeutically to treat diabetes
  mellitus

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Toxicological Studies II

• Extensive studies of U toxicity in Manhattan
  District confirmed low level of toxicity
• Low order of toxicity further butressed by
  studies of accidental exposures and epidemiologic
  studies of U workers
• Use of DU in Gulf Wars and Kosovo generated new
  interest and intensive study but many questions
  still remain

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Production of Depleted Uranium

• A byproduct of the enrichment process
• Two basic methods
• Gaseous diffusion
• Gas centrifugation
• U-235 content reduced to 0.2 wt
• Specific activity about 60 of NatU
• 88.8 of activity from 238U, 9.9 from 234U
• and only 1.1 from 235U

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Radiological Characteristics of DU
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Uses of Depleted Uranium

• Pigments
• Industrial catalyst
• Counterweight for aircraft control surfaces
• Ballast for ships
• Radiation shielding
• Munitions
• Armor

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Munitions Usage

• High density and self sharpening characteristics
  provide excellent armor piercing capability
• High density and strength also make it excellent
  for tank armor
• DU penetrators produce airborne particulates of
  typically high fired and hence insoluble DU which
  fall out close to the point of impact and produce
  localized ground contamination

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Toxicokinetics

• Oral absorption poor few per cent
• Percutaneous absorption has not been observed in
  humans
• Absorption following inhalation a function of
  solubility, particle size ICRP lung model
  applies
• Approximately 70 of absorbed U is excreted in
  first 24 hours, largely via urine

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Biokinetic Models for Uranium

• Several biokinetic models for U have been
  proposed over last 50 years
• ICRP Publication 69 (1995) likely most generally
  used and accepted model
• Other models have been proposed in general are
  similar to and basically refinements of ICRP
  model
• Models may not fit well solubility, route of
  intake are important factors

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Inhaled Uranium

• Rely on ICRP lung model
• Type F Compounds UF6, UO2F, UO2(NO3)2 100
  absorption of fraction not excreted via GI tract
  with half time of 10 min
• Type M Compounds (UO3, UF4, UCl4, U3O8) 10
  absorption of fraction not excreted via GI tract
  with half time of 10 min remaining 90 of
  nonexcreted fraction has half time of 140 days
• Type S Compounds (UO2, U3O8) low and slow
  absorption of the fraction not cleared via GI
  tract, 99.9 absorbed via respiratory tract with
  half life of 7000 days ( 19 years)
• Note Some compounds such as U3O8 may behave as
  Class M or Class S under specific circumstances

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Ingested Uranium

• Gut absorption poor f1 1-2 for soluble U,
  order of magnitude less for insoluble forms
• ICRP uses 0.02 for soluble forms and 0.002 for
  highly insoluble forms, the latter specified as
  UO2 and U3O8.
• Gut absorption may be affected by food

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Percutaneous Absorption

• Not likely no evidence to support transfer
  through unbroken human skin
• May enter blood through cuts and abrasions
• Depositions in wounds may remain for years,
  slowly releasing U and progeny into systemic
  circulation kidney concentration
• builds up with time

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Absorption and Distribution

• Once absorbed into systemic circulation, most U
  is quickly excreted via the kidneys
• Small fractions are deposited in skeleton, liver
  and kidney
• Depots may have both long and short term
  residence compartments

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Bioassay for DU

• Fecal analysis useful for inhaled material,
  especially after acute accidental intake
• In vivo counting useful reasonable LLD (few mg)
  for lung counts
• Urinalysis (KPA, ICPMS) most practical and has
  excellent sensitivity
• Hair analysis not established technique and
  subject to interferences and error
• Blood concentration insufficiently sensitive and
  may not be practical

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Chemical Toxicity

• U is a heavy metal and exhibits heavy metal
  toxicity
• Low enrichment U ( 15 U-235) is primarily
  chemically toxic
• Toxic effects (including death) have been clearly
  demonstrated in animals but not in humans
  sensitivity variable among species

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LD50

• Variable depending on animal species humans seem
  to have lower sensitivity to U toxicity than
  animals
• No data for humans, but limited human data and
  extrapolation from animals suggests that the
  acute LD50 for ingested or inhaled U is at least
  several grams

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Hard to Believe but True

• There has never been a documented death
  attributable to U ingestion or inhalation in
  humans
• BUT . . . This does not mean that DU is
  without toxic effects . . .
• More study of DU and its possible effects in man
  is clearly indicated

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Chemotoxic Effects Kidney

• Kidney is the most sensitive organ for chemotoxic
  effect of DU
• Histopathology and apparent mild functional
  changes may occur days after large acute intakes
  (tens to hundreds of mg) and manifest as injury
  to renal tubular epithelial cells
• Biomarkers of tubular effects include glucosuria,
  enzymuria, albuminuria, and elevated blood
  creatinine or NPN/BUN.

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Chemotoxic Effects Kidney

• Threshold for renal effects from acute exposure
  likely 2 µg U/g kidney but 3 µg U/g generally
  accpted as of now
• Effects are typically transient but are more
  serious and longer lasting with higher exposures
• Studies of chronic occupational exposure in
  workers and Gulf War veterans suggests
  concentrations 1µg/g kidney may produce mild
  tubular dysfunction (Thun et al. 1985 Squibb et
  al. 2005).

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Chemotoxic Effects

• Effects on other systems -- CNS, cardiovascular,
  ocular, liver, immune system -- typically not
  observed in animal studies or in humans, but any
  such effects may have been overshadowed and
  masked by far more severe renal effects
• No evidence of reproductive system dysfunction or
  sperm abnormalities in male Gulf War veterans,
  nor any evidence of excess fetal abnormalities or
  mortality. Placental transfer equivocal

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Chemical Genotoxicity

• Has not been seen (or even looked for) in animal
  studies
• A 10 y followup study of Gulf War veterans
  classified as high exposure based on urine
  biomarkers was equivocal (McDiarmid et al. 2004)
• Recent in vitro studies with Chinese hamster
  cells suggests chromosome breakage and
  mutagenicity associated with heavy metals
  including U (Stearns et al 2005) but such studies
  have not been confirmed nor are they necessarily
  applicable to human exposures

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Radiation from DU

• All three U isotopes in DU are alpha emitters
  (plus associated photons)
• Specific activity of DU 14.9 kBq g-1
• (4.1 x 10-7 Ci g-1 )
• Decay chain products emit betas, photons
• Surface dose rate from infinite slab of DU is
  2.55 mGy h-1, 90 from beta and 10 from photons

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External Exposure Considerations

• Potential for significant external exposure is
  small to zero
• Dose rate 10 cm from an infinite slab of DU is
  only 0.1 mGy h-1
• Unless DU is in contact with the skin (or very
  nearly so) it is virtually impossible to get a
  dose sufficient to produce any deterministic
  effect

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Internal Exposure Considerations

• Risk of deterministic effects from radiation
  virtually zero as chemotoxicity effects would be
  overwhelming and possibly fatal irrespective of
  route of entry
• Ditto for carcinogenic risks from ingestion of
  soluble DU ingestion of 1 g of soluble DU
  produces a total stochastic risk of 3.3 x 10-5
  including both carcinogenesis and genetic
  effects.

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Risk from Inhaled DU

• Inhalation of soluble DU compounds dose not pose
  significant stochastic or deterministic risk from
  radiation the chemical effects are overwhelming
• Inhalation of insoluble aerosols may pose
  significant stochastic risk e.g. inhalation of
  1 mg of a 1 µm AMAD Class S aerosol produces a
  lifetime stochastic risk of 5.6 x 10-6 primarily
  from lung cancer
• Lung cancer risk may be potentiated or increased
  by other insults such as smoking

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Epidemiology

• Numerous radioepidemiologic studies have been
  done over the past three decades of uranium
  miners, millers, and workers and of populations
  exposed to high background levels of U
• DU studies are under way in military and civilian
  populations in areas where DU munitions were used

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Epidemiologic Findings

• Results are at most equivocal and certainly
  comforting no study has shown a statistically
  significant excess for total cancer (so if there
  are in fact effects they will be minimal)
• No study has shown long term kidney or other
  somatic effects associated with acute or chronic
  exposure to uranium