# Research Activity: Heavy Element Chemistry

# Division: Chemical Sciences, Geosciences, and Biosciences

Primary Contact: Lester R. Morss (lester.morss@science.doe.gov;
301-903-9311)

Team Leader: John C. Miller

Division Director: Eric Rohlfing (Acting)

# Portfolio Description:

This activity supports research in the chemistry of actinide elements (a
family of radioactive elements that includes uranium and plutonium),
transactinides, and long-lived radioactive nuclear reactor fission
products such as technetium. Areas of interest are the chemical bonding
and reactivity of actinide ions in solids, solutions, and gases;
synthesis, structure, and other properties of actinide materials;
theoretical methods to predict heavy element electronic and molecular
structure and reactivity; and chemical properties of the transactinide
elements. The central theme is the unique chemical properties due to the
electron orbitals available to and occupied by these elements,
especially the 5f orbital.

The Heavy Element Chemistry program has determined the chemical
properties of the transuranium elements since its genesis in the
Manhattan Project. The present emphasis is the chemistry of technetium,
uranium, neptunium, plutonium, and americium, driven by the necessity to
characterize and control long-lived radioisotopes found at Department of
Energy (DOE) legacy waste sites. Knowledge of the molecular speciation
of stored actinide and fission products is necessary to predict the
properties of these complex mixtures and to deal with them. This
activity is coupled to the BES Separations and Analysis activity, to
actinide and fission product chemistry efforts in the DOE Environmental
Management Science Program, and to nuclear fuel cycle research of the
DOE Office of Nuclear Energy, Science and Technology. This activity
represents the nation's only funding for basic research on the chemical
and physical principles governing actinide and fission product
chemistry. The education of undergraduates, graduate students, and
postdoctoral researchers at DOE national laboratories and universities
is an important responsibility of this activity.

**Unique Aspects:**

This activity represents the only source of funding for basic research
in the actinides and transactinides in the nation. Its major emphasis is
to understand the underlying chemical and physical principles that
determine their behavior. The activity is primarily based at 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
support DOE missions in advanced 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 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 BES
Separations and Analysis activity, which has 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. The early goal was to determine the chemical
and physical properties of the new elements 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 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. Organometallic chemistry has been
enriched by discovery of many unique organoactinide compounds.

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
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. Magnetic measurements have
shown that the light actinide metals have delocalized 5f orbitals and
resemble d-orbital transition metals, whereas the f electrons become
localized at americium, element 95, so that the heavier actinide metals
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 success of many of the DOE missions. 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. In the defense area, understanding the chemistry and
material properties of specific actinides were key to the development of
our nuclear deterrent, and now plays a major role in the stewardship of
the nuclear stockpile. Driven by the necessity to identify and treat
radioactive species in highly basic solutions found in many waste tanks
at DOE sites, this activity has had a renewed emphasis on the molecular
speciation of the lighter transuranium elements and fission products.
Molecular speciation information is also needed to predict the fate of
actinides and fission products accidentally released to the environment.
Finally, the analytical chemistry methods developed 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
topic in actinide chemistry, providing 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 enable studies of actinides in the
gas phase, as clusters, and at interfaces between solutions and mineral
surfaces. Actinide and fission product samples must be handled with
special consideration because of their radioactivity, which limits 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
liquid-solid interfaces.

Sophisticated quantum mechanical calculations that treat spin-orbit
interactions accurately 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. 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. Ultimately, experimentally
validated theoretical calculations will be the key to understanding the
role of the 5f electrons.

**Funding Summary:**

Dollars in Thousands

[FY 2005]{.underline} [FY 2006]{.underline} [FY 2007
Request]{.underline}

10,506 9,354 17,128

> [Performer]{.underline} [Funding Percentage]{.underline}
>
> DOE Laboratories 70%
>
> Universities 20%
>
> 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, encouraging collaborations between university and DOE
national laboratory projects in this area. 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 Brookhaven National Laboratory (BNL) and San
Jose State University. Graduate and postdoctoral students are educated
to provide personnel for the technological challenges associated with
the heavy elements.

**Projected Evolution:**

At the frontier of the periodic table, theoretical chemists predict the
properties of actinides and transactinides in gaseous molecules, liquid
clusters, and solid species using modern calculation tools such as
density functional theory. Because most actinide species have partly
filled 5f electron subshells and all have highly charged nuclei, both
spin-orbit and relativistic effects must be included in the
calculations. More sophisticated quantum mechanical calculations of
actinide compounds and actinide species in environmental media are being
developed. Heavy Element Chemistry theoretical research pursues advances
in gas-phase chemistry that explore new reactivity patterns,
photophysics, and photochemistry of actinide ions in their excited
states, and in organoactinide chemistry.

Support of research to understand the chemical bonding of elements that
have 5f electrons will lead to fundamental understanding of separations
processes and to the design and synthesis of more effective preorganized
chelating agents for the separations of particular actinide ions.
Research in bonding, reactivity, and spectroscopic properties of
molecules that contain heavy elements and of actinides in
environmentally relevant species aids the development of ligands to
sequester actinides in the environment and to remove toxic metals from
the human body. Better characterization and modeling of the interactions
of actinides with well-characterized liquid-solid interfaces, including
mineral surfaces under environmentally relevant conditions, is needed.

Research on synthesis, crystal structure, and bonding in actinide solids
leads to materials that are designed to be especially stable in
environments such as nuclear fuels and nuclear waste forms.
Spectroscopic investigations of new actinide materials and high-pressure
studies of actinide metals elucidate the unique bonding properties and
electronic characteristics attributable to 5f electrons. New facilities
for safely handling radioactive materials at the synchrotron sources
will permit more widespread use of techniques such as x-ray absorption
spectroscopy and scattering on radioactive samples, providing detailed
information on actinide speciation in crystalline and amorphous solids
such as spent fuel and radioactive waste forms.