Hope College, Holland, Michigan, B.A. (’77)
Northwestern University, Evanston, Illinois, Ph.D.
Tel. (574) 631-5561
Fast Kinetics of Radiation-Initiated Chemistry
Fast Kinetics of Free Radical Reactions
Free radicals are generated in virtually all radiation-initiated
processes, and are responsible for most of the permanent chemical
changes. The recombination reactions are often diffusion limited
or nearly so, but also depend on pairing of spin to produce stable
singlet products. This gives rise to the fascinating Chemically
Induced Dynamic Electron Polarization (CIDEP) phenomenon in their
time-resolved EPR spectra, and Chemically Induced Dynamic Nuclear
Polarization (CIDNP) in NMR spectra of the recombination products,
where some lines appear with negative phase due to population inversions.
Radiation Chemistry and Photochemistry
of Water To ionize water molecules in the gas phase
requires at least 12.6 eV of energy, but dissociation of water to
produce (H+)aq, (e-)aq, and OH radicals can be accomplished in liquid
water with 6 eV photons in a photochemical event that is still not
well understood. What is the nature of electronically excited liquid
water, and how can we explain the escape yields of H atoms, OH radicals,
and solvated electrons?
Solvent Effects on Reaction Rates in Supercritical
Water — Supercritical water is proposed as the coolant
for efficient Generation-IV nuclear reactors, and is the medium
for an important advanced oxidation technology for hazardous waste
destruction. The properties of water change dramatically in the
supercritical region as the water density changes continuously between
zero and 1 g/cc. The primary free radicals in water – hydrated
electrons, H atoms, and OH radicals – are respectively ionic,
hydrophobic, and dipolar, providing opportunity to investigate nearly
all possible solvent effects using radiolysis excitation. Many strange
effects are being found, such as rate constants that decrease as
the temperature is raised.
Radiation-Enhanced Corrosion — Three mechanisms can be postulated for the larger corrosion rates observed in nuclear power plants relative to non-irradiated systems. (a) The water is irradiated to produce products like H2 and H2O2 which will change the electrochemical corrosion potential. (b) The surface metal-oxide layer is directly excited to cause “photochemical” corrosion. And (c) the neutron field produces displacement damage to accelerate movement of oxygen ions through the surface oxide to the neat metal below. Present research funded by the nuclear industry is aimed at sorting out the relative importance of these various effects in hopes of finding new mitigation strategies
Recent Accomplishments | Top |
Hydrated Electron Optical Absorption in Supercooled and Supercritical Water the
intense red absorption spectrum of the hydrated electron shifts
strongly to the infrared as the temperature is raised. In supercritical
water there is a slight red shift as the density decreases. By integrating
the spectra we can estimate the rms size of the electron wavefunction
and its kinetic energy at different phase points. This was compared
with absolute thermodynamic energies derived for hydrated electron
solvation. Surprisingly the rms size of the wavefunction barely
changes in supercritical water, even though the density changes
a factor of six in this study. A far more difficult task is determination of an absolute extinction coefficient for the short-lived species. Our studies demonstrated that the value accepted for forty years was 20% too low. The implication is that the hydrated electron spectrum “borrows” some of its intensity from the water solvent molecules. Simulations of the solvated electron with state-of-the-art computational techniques are still not able to reproduce all of its experimental properties.
Radiolysis Yields in Supercritical Water — Of tremendous practical importance is the radiolysis escape yield (G value), defined as the number of product molecules divided by the total radiation energy deposited. Our recent measurements of H atom, solvated electron, and molecular hydrogen yields in supercritical water found a minimum in yields at intermediate density and nearly “gas phase” escape yields at 0.1 g/cc water density. To explain this we need to postulate that almost no geminate recombination occurs, and that the photophysics of the water molecules change dramatically with density.
Small Free Radical Recombinations in High Temperature Water — Near room temperature, recombination of small free radicals like H and OH are nearly diffusion limited in aqueous solution, i.e. once they meet their reaction is certain. We have been surprised to learn that above about 200C, “barrierless” reactions involving H and OH are no longer limited by diffusion. Diffusion becomes so fast that the solvent “caging effect” fails to average over all possible angles of approach, and the reaction rate is limited by a “steric effect”. The great surprise has been that the rates measured in water, where hydrogen bonding was assumed to be important, are identical to the “high pressure limit” rate in the gas phase. Water is “merely” a very effective third body for energy transfer.
Modeling of Nuclear Reactor Chemistry — Surprisingly the radiation chemistry occurring in nuclear power reactors has not been successfully modeled until recently. A review of all reaction rates and radiolysis product yields was prepared in collaboration with John Elliot of Atomic Energy of Canada in 2008, which included all of the new high temperature information generated in our laboratories. Simulation of the "Critical Hydrogen Concentration" or excess added hydrogen needed to suppress radiolysis in the reactor cores was still not successful. Additional experiment and modeling shows that radiolysis yields due to neutron radiation has not been correctly measured in laboratory experiments. This key missing information is now a primary target of research.
Selected Publications | Top |
Bartels, D.M., Henshaw, J. and Sims, H.E.
Modelling the critical hydrogen concentration in the AECL Test Reactor
Radiat. Phys. Chem. 2013 82, 16-24 link
Kanjana, K., Haygarth, K.S., Wu, W. and Bartels, D.M.
Laboratory studies in search of the critical hydrogen concentration
Radiat. Phys. Chem. 2013 82, 25-34 link
Haygarth, K.S. and Bartels, D.M.
Neutron and beta/gamma radiolysis of water up to supercritical conditions. 2. SF6 as a scavenger for hydrated electron
J. Phys. Chem. A 2010 114, 7479-7484 link
K.S. Haygarth, T.W. Marin, I. Janik, K. Kanjana, C.M. Stanisky and D.M. Bartels
Carbonate Radical Formation in Radiolysis of Sodium Carbonate and Bicarbonate Solutions up to 250 degrees C and the Mechanism of its Second Order Decay
J. Phys. Chem. A 2010 114, 2142-2150 link
Hare, P.M., Price, E.A., Stanisky, Janik, I. and Bartels, D.M.
Solvated electron extinction coefficient and oscillator strength in high temperature water
J. Phys. Chem. A 2010 114, 1766-1775 link
Comment on the possible role of the reaction H.+ H2O -> H2 + OH. in the radiolysis of water at high temperatures
Radiat. Phys. Chem. 2009 78, 191-194 link