Flats Environmental Technology Site (RFETS) is an environmental cleanup
site located about 16 miles northwest of downtown Denver (Fig 1).
Two decades of routine monitoring have shown that the environment around
RFETS is contaminated with actinide elements (U, Pu, Am) from site operations,
 and RFETS has been designated by the U.S. Environmental
Protection Agency (EPA) as a Superfund cleanup site. Until December 1989,
the Rocky Flats Plant made components for nuclear weapons using various
radioactive and hazardous materials, including plutonium, uranium and beryllium.
Nearly 40 years of nuclear weapons production left behind a legacy of contaminated
facilities, soils, and ground water. More than 2.5 million people live
within a 50 mile radius of the site; 300,000 of those live in the Rocky
A massive accelerated cleanup effort
began in 1995. The key priority of site management and surrounding community
leaders is the safe, accelerated closure of Rocky Flats. Kaiser-Hill and
the DOE, working in close coordination with Rocky Flats stakeholders, are
working on a plan to substantially complete the cleanup and closure of
Rocky Flats by the aggressive goal of 2006. The closure of Rocky Flats
is estimated to cost between $6 billion and $8 billion. 
The spatial distribution of plutonium
in RFETS soils has been estimated,[3,4]
with plutonium activities in surface soils ranging from 1,450 to 0.05 pCi/g,
with the data showing a clear west-east trend away from an old drum storage
site known as the 903 Pad.  More than 90% of the Pu
is contained within the upper 10-12 cm of soils downwind of the 903 Pad
1. From 1952 to 1989,
Rocky Flats was the production site for nuclear and non-nuclear weapons
components for the nation's nuclear weapons stockpile.
2. The 903 Pad was
used in the 1950s to 1960s for storage, on bare ground, of more than 4,000
drums of plutonium-contaminated solvents and oils. The drums were removed
in 1967 and 1968, and an asphalt pad was installed to control the spread
and co-workers from Los Alamos National Laboratory examined X-ray absorption
spectra at the plutonium LII edge
on a series well-characterized standard compounds in oxidation states 0,
III, IV, V, and VI, followed by samples of contaminated RFETS soils and
concrete collected from the site. [8,9,10]
These studies utilized the actinide facility at SSRL's new Molecular Environmental
Science Beam Line 11-2.
For plutonium studies, researchers
prefer to use the LIII x-ray absorption edge (near 18,060 eV)
because of its high absorption intensity. However,
in RFETS environmental samples there are other elements
(principally strontium) present in the
natural minerals which exhibit x-ray emission in this same region, and
therefore interfere with plutonium measurements. As a result, the plutonium
x-ray absorption studies of RFETS samples were performed instead at the
LII x-ray absorption edge (near 22,270 eV). A calibration experiment
was run at both LIII and LII absorption edges using
plutonium standards in oxidation states 0, III, IV, V, and VI. The
and LII XANES and XAFS spectra of the plutonium standards were
nearly identical, giving high confidence in the use of the plutonium LII
edge for study of RFETS environmental samples. A comparison of LII
XANES spectra of plutonium standards in oxidation states III, IV, V, and
VI, RFETS soil, and RFETS concrete exposed to contaminated smoke (from
a building fire) is shown in Fig 3. From Fig 3, the Pu XANES
spectra of the soil and concrete samples were not only clearly consistent
with Pu(IV), but were identical within the experimental uncertainties to
the PuO2 standard. The XANES measurements on RFETS samples clearly
show that the oxidation state of plutonium is Pu(IV), and the XANES spectral
signatures are very similar to that of PuO2 in both soil and
3. Comparison of plutonium
LII XANES spectra for plutonium in oxidation states III, IV,
V, and VI with RFETS soil and concrete samples.
One of the soil samples was concentrated
enough with respect to plutonium that XAFS data could be analyzed. XAFS
curve fitting and data analysis revealed local structure features nearly
identical to that for Pu in the solid PuO2 standard, the structure
of which is illustrated in Fig 4, with one small exception. In
to interatomic distances of 2.33 and 3.86 Å, the Fourier transforms
from RFETS soil and concrete data show some small peaks at intermediate
Pu-O distances between 2.3 and 3.0 Å which are consistent with additional
Pu-OH or Pu-OH2 interactions which would be expected for hydrated
PuO2 exposed to water in the environment. This was verified
experimentally by examining the XAFS of a number of laboratory-prepared
samples of hydrated plutonium dioxide,
PuO2·nH2O  X-ray Absorption
Spectroscopy (XAS) therefore shows unambiguously that plutonium in RFETS
soils taken from the 903 Pad is in oxidation state (IV), and in the chemical
form of insoluble PuO2·nH2O. For decades it
had been presumed that Pu in RFETS soils existed as plutonium dioxide,
but this hypothesis had never been proven.[3,11]
The Los Alamos study is the first spectroscopic confirmation of the speciation
of plutonium in soils at RFETS. This finding is consistent with the observed
insolubility of plutonium in site-specific waters, and supports a growing
body of evidence that physical (particulate) transport is the dominant
mechanism for plutonium migration at RFETS. This is particularly true for
plutonium, which is largely insoluble and is transported when wind and
water erosion move soil and sediment particles to which the plutonium is
bound. This recognition not only identified the need for the Site to develop
a soil erosion model, but significantly helped in gaining public trust
that an erosion model was the correct model for the Site, and that soluble
transport models are inappropriate for plutonium in RFETS soils. Thus plutonium
XAS measurements have developed into a decision-making tool for Kaiser-Hill
LLC, saved the company and the taxpayers millions of dollars by focusing
Site-directed efforts in the correct areas, and will aid the DOE in its
effort to cleanup and close the RFETS by 2006.
Unit cell of fcc plutonium dioxide.
Plutonium dioxide exhibits the well-known Fluorite crystal structure.
atoms are shown in green, and oxygen atoms are shown in red. This structure
type gives rise to XAFS near-neighbor shells with Pu-O = 2.33, Pu-Pu =
2.38, and Pu-O = 4.52 Å, respectively.
Draft Evaluation of
Existing Data on Actinide Migration at the Rocky Flats Environmental Technology
Site. 1996, RF/ER-96-0048.UN.
Krey, P. W.; Hardy,
E. P. "Plutonium in Soil Around the Rocky Flats Plant", 1970, HASL-235,
US AEC, Health and Safety Laboratory, New York.
Litaor, M. I.; Thompson,
M. L.; Barth, G. R.; Molzer, P. C., J. Environ. Qual., 1994,
DOE, 1995. Phase II
RFI/RI Report for the 903 Pad, Mound, and East Trenches Area in Operable
Unit No. 2, Volume 13, Appendix D (October, 1995)
Clark, D. L., "The Chemical
Complexities of Plutonium," Los Alamos Science, 2000, 26,
Runde, W., "The Chemical
Interactions of Actinides in the Environment," Los Alamos Science,
2000, 26, 338.
Conradson, S. D.;
Al-Mahamid, I.; Clark, D. L.; Hess, N. J.; Hudson, E. A.; Neu, M. P.;
Palmer, P. D.; Runde, W. H.; Tait, C. D., "X-Ray Absorption Edges of
Plutonium in Different Oxidation States," Polyhedron, 1998,
Conradson, S. D., Appl.
Spec. 1999, 52, 252.
S. D. Conradson, D.
L. Clark, M. P. Neu, W. Runde, C. D. Tait, "Characterizing the Plutonium
Aquo Ions by XAFS Spectroscopy," Los Alamos Science, 2000,
Hardy, E. P.; Krey,
P. W., "Plutonium in the Environs of the Former Rocky Flats Plant," Health
Phys. 1996, 71, 796.