Transcript Health Physics Aspects of Depleted Uranium (DU)

Depleted Uranium (DU):
More than you want to know . . .
But what every health physicist ought to know
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!)
Historical and General Introduction
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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 1400’s, 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
Natural Uranium
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A dense (ρ = 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
More History
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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
Toxicological Studies
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First toxicological studies by chemist
Christian Gmelin (1824); concluded that
uranium was “a feeble poison”
Gmelin’s work confirmed by other
researchers over the next several decades
Since about 1860 until discovery of insulin
in 1920’s used therapeutically to treat
diabetes mellitus
Toxicological Studies II
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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
Production of Depleted Uranium
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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
Radiological Characteristics of DU
Isotope
Mass
Fraction
Half-life
(years)
Specific
Activity
(Bq g-1)
Activity
Fraction
(%)
U-234
0.0006
2.44 x 105
2.31 x 108
9.9
U-235
0.2
7.04 x 108
8.00 x 104
1.1
U-236
0.0003
2.34 x 107
2.4 x 106
0.05
U-238
99.8
4.47 x 109
1.24 x 104
88.8
Total
100
1.49 x 104
100
Uses of Depleted Uranium
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Pigments
Industrial catalyst
Counterweight for aircraft control surfaces
Ballast for ships
Radiation shielding
Munitions
Armor
Munitions Usage
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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
Toxicokinetics
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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
Biokinetic Models for Uranium
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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
Inhaled Uranium
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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
Ingested Uranium
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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
Percutaneous Absorption
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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
Absorption and Distribution
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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
Bioassay for DU
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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
Chemical Toxicity
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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
LD50
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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
Hard to Believe but True:
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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
Chemotoxic Effects: Kidney
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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.
Chemotoxic Effects: Kidney
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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).
Chemotoxic Effects
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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
Chemical Genotoxicity
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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
Radiation from DU
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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
External Exposure Considerations
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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
Internal Exposure Considerations
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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.
Risk from Inhaled DU
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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
Epidemiology
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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
Epidemiologic Findings
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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