**Research Activity: Heavy Element Chemistry**

Division: Chemical Sciences, Geosciences, and Biosciences

Primary Contact(s): Lester Morss
([Lester.Morss@science.doe.gov](mailto:Henry.Shaw@science.doe.gov));
301-903-9311

Team Leader: John C. Miller

Division Director: Walter Stevens

**Portfolio Description:**

This activity addresses the scientific principles that form the basis
for the understanding of the chemical behavior of actinide,
transactinide, and fission product elements and their compounds. Areas
of interest are coordination chemistry and reaction kinetics of
actinides in aqueous and non-aqueous solutions; gas-phase and
solid-state chemical bonding; measurements of chemical, thermodynamic,
and magnetic properties; synthesis of actinide-containing materials;
chemical properties of the heaviest actinide and transactinide elements;
and theoretical methods for the prediction of heavy element electronic
properties, molecular structure, and reactivity.

The actinide elements form a unique chemical series in the periodic
table due to the filling of the 5f electron subshell. Their nuclear
properties, and the resulting use of uranium and plutonium isotopes as
energy sources and in weapons, cause some actinide isotopes to be
essential industrial materials. This activity supports efforts to
develop a fundamental understanding of chemical bonding and reactivity
of the actinides and transactinides, and to determine the similarities
and differences between the actinides and the 4f (lanthanide) series or
the 3d, 4d, and 5d transition metal series. There is a close and
synergistic relationship between the heavy element chemistry program and
the separations science program. The heavy element chemistry program
contributes to the maintenance of the scientific infrastructure required
to meet future challenges in heavy element chemistry and to provide
unique facilities for the education of students.

**Unique Aspects:**

This activity represents the only source of funding for basic research
in the actinides, transactinides, and fission products in the United
States. Its major emphasis is on understanding the underlying chemical
and physical principles of the actinide and fission product materials.
The activity is primarily based at the national laboratories because of
the special facilities needed in order to handle these radioactive
materials safely.

**Relationship to Other Programs:**

This activity provides the fundamental understanding of the properties
of the actinides, transactinides, and fission product elements that are
necessary for the Department of Energy missions in nuclear energy,
stewardship responsibilities for defense programs, and environmental
clean-up. The heavy element chemistry program conducts unclassified
basic research on all the actinide and transactinide elements, while the
applied programs generally limit their investigations to the chemical
and material properties of specific elements and systems of strategic,
economic, or programmatic interest. This activity also has close ties to
the DOE BES separations and analysis activities, which have a major
focus on the separation of actinides and fission products from other
elements.

**Significant Accomplishments:**

The heavy element chemistry activity had its genesis in the Manhattan
project. It continues today as the nation's only basic research program
supporting the exploration of the physical and chemical properties of
the transuranium elements and their compounds. The early goal was to
determine the basic inorganic chemistry and physical properties of the
new elements and their compounds and to discover new elements. The
chemical properties of the transuranium elements, especially plutonium,
originally were determined from microscale experiments. The processes
for the separation of plutonium from uranium and fission products on an
industrial scale were developed and scaled up from these results. The
completion of the High Flux Isotope Reactor (HFIR) at ORNL in the 1960's
provided a stable supply of curium and heavier elements that continues
to the present. The inorganic chemistry of the elements through
einsteinium (Es, atomic number 99) has been determined with small but
weighable quantities of the elements. For the elements heavier than Es
in the periodic table, tracer techniques and one-atom-at-a-time
chemistry have been developed and carried out through element 108 to
determine chemical properties.

Taken together, the results from this activity have repeatedly confirmed
the Seaborg hypothesis that the actinides are best represented in the
periodic table as a 5f element series placed under the 4f (lanthanide)
series. Interpretations of spectroscopic results have provided estimates
of thermodynamic quantities such as oxidation-reduction potentials and
enthalpies of sublimation. Specific electronic transitions determined in
this activity have proven useful in developing processes for laser
isotope separation of uranium and plutonium. Structural systematics of
the actinide metals, oxides, and halides as a function of atomic number
have been determined. Magnetic measurements have shown that the light
actinide metals have delocalized 5f orbitals (*i.e.,* the 5f electrons
form bands), but that the f electrons become localized at americium,
element 95. Thus, the magnetic behavior of the first part of the
actinide series resembles that of the d-orbital transition metals but
the heavier actinides exhibit behavior similar to the rare earth metals.

**Mission Relevance:**

Knowledge of the chemistry of the actinide and fission product elements
is necessary for the successful conduct of many of DOE's missions. In
the defense area, understanding the chemistry and material properties of
specific actinides was key to the development of our nuclear deterrent,
and now plays a major role in the stewardship of the nuclear stockpile.
This program conducts the broadly based unclassified research on
actinides that provides the scientific basis for framing the narrower
issues facing the DOE's defense programs. In the area of nuclear energy,
this activity provides the fundamental understanding of actinide and
fission product chemistry that underpins the development of advanced
nuclear fuels, as well as the predictions of how spent nuclear fuels
degrade and radionuclides are transported under repository conditions.
Driven by the necessity to identify possible important species in highly
basic solutions found in the waste tanks at the Hanford and Savannah
River sites, this activity has had a renewed emphasis on the chemistry
of the lighter transuranium elements and fission products. Knowledge of
the molecular speciation of actinide and fission products materials
under tank conditions is necessary to treat these complex mixtures.
Molecular speciation information is also needed to predict the fate of
actinide and fission product elements accidentally released to the
environment. Finally, the analytical methods developed as part of the
basic research funded under this activity have broad application across
all the applied missions of DOE that deal with nuclear materials.

**Scientific Challenges:**

The role of the 5f electrons in bond formation remains the fundamental
unanswered question in actinide chemistry, and hence provides the
central focus for this program. The 5f orbitals participate in the band
structure of materials that contain the light actinide metals and some
of their alloys. Whether the 5f orbitals participate significantly in
molecular compounds is still unclear. Molecular-level information on the
geometry and energetics of bonding can now be obtained from experiments
carried out at the Nation's synchrotron light sources, and from
multi-photon laser excitation studies. These new tools are enabling
studies of actinides in the gas phase, as small clusters and at
interfaces such as between solutions and mineral surfaces or other
well-defined solids. However, actinide and fission product samples have
to be treated with special consideration because of their radioactivity,
which has limited the types of experiments that can be safely conducted.
For example, in the soft x-ray regime new sample preparation techniques
need to be developed so that micro-quantities of well-characterized
actinide samples can be measured using high vacuum techniques, or in
\"wet\" cells to study actinide chemistry at liquid-solid interfaces.

Sophisticated quantum mechanical calculations that treat spin-orbit
interactions accurately will need further development so that they can
be used for predicting the properties of molecules that contain
actinides. Development and validation of such computer codes will
provide a means for obtaining fundamental information about actinide
species that are difficult to study experimentally, will predict the
electronic spectra of important species, and will correlate the optical
spectra with actinide molecular structure. Ultimately, experimentally
validated theoretical calculations will be the key to understanding the
role of the 5f electrons.

**Funding Summary:**

**Dollars in Thousands**

**[FY 2003]{.underline} [FY 2004]{.underline} [FY 2005
Request]{.underline}**

9,974 9,375 9,375

> **[Performer]{.underline} [Funding Percentage]{.underline}**
>
> DOE Laboratories 75 %
>
> Universities 15 %
>
> Other 10 %

These are percentages of the operating research expenditures in this
area; they do not contain laboratory capital equipment, infrastructure,
or other non-operating components.

This activity supports research in heavy element chemistry at
universities and encourages collaborations between university and
laboratory projects in this area. Graduate and postdoctoral students are
trained in this area and provide personnel for the technological
challenges associated with the heavy elements. Twenty-four undergraduate
students, chosen competitively from universities and colleges throughout
the United States, are taught actinide chemistry and radiochemistry each
summer in two programs at BNL and San Jose State University

**Projected Evolution:**

Kinetic, thermodynamic, and electrostatic effects in separations
processes and in synthetic reactions involving the actinides are not
well understood. Modern tools brought about by technological innovations
can now be brought to bear on these topics. Heavy Element Chemistry
research pursues advances in gas-phase chemistry that explore new
reactivity patterns, high-pressure studies of actinide alloys, and
spectroscopic investigations of new actinide materials. Other areas
where new advances are expected are the design, synthesis and
effectiveness of preorganized chelating agents for the separations of
particular actinide ions; characterization of important actinide and
fission product species in separations processes for a more fundamental
understanding of why particular separations processes work; effects of
temperature on actinide solution structure/property relationships;
photophysics and photochemistry of actinide ions in their excited
states; actinide organometallic chemistry; and theoretical work on
actinide molecular clusters and compounds.

More sophisticated quantum mechanical calculations of actinide compounds
and actinide species in aqueous solution are being undertaken. New
facilities for safely handling radioactive materials at the synchrotron
sources will permit more widespread use of techniques such as XANES,
EXAFS, and x-ray scattering on radioactive samples, thus providing more
detailed information on actinide speciation in solutions and in
crystalline and amorphous solids. These data will complement data from
other techniques, and can be used to validate and extend the quantum
mechanical calculations.

Improved modeling of actinide transport requires understanding of the
processes describing sorption on surfaces. Surface complexation models
exist, but experimental validation of these models has not been readily
available. Technological advances now allow molecular characterization
of actinide surface species to be explored. An enhanced program of
characterizing and modeling the nature of the interactions of actinides
with well-characterized liquid-solid interfaces, including mineral
surfaces under environmentally relevant conditions, is needed.

The magnetic and electronic properties of actinide intermetallic systems
are not understood. These effects depend on the hybridization of the 5f
electrons with conduction electrons. The strength of such hybridization
is a direct function of the local metal bond environment around the
actinide ion and much work needs to be done, especially on transuranium
compounds. Work in this area should provide a much better understanding
of the changes in the types of bonding exhibited by the light *versus*
the heavy actinides.

Increased emphasis will be placed on encouraging academic investigators
to enter this field and to address outstanding questions in heavy
element chemistry. Because most academic institutions do not have the
facilities necessary to safely handle radioactive materials, it is
anticipated that most academic investigators will conduct their
experimental work in collaboration with one or more national laboratory.
Increased academic involvement is essential for training the next
generation of scientists with experience in handling radioactive
materials.

Finally, the actinide facilities in the national laboratories are aging.
In order to continue to carry out forefront research, infrastructure
problems at these laboratories must be addressed.