# Presentation: 715357

## Extreme Conditions in the Nuclear Fuel Cycle:From Reactor High Pressure and Temperature to Ligand Radiolysis

**Extreme Conditions ****in the Nuclear Fuel Cycle:****From Reactor High Pressure and Temperature to Ligand Radiolysis**

**Lætitia H. Delmau**

***Basic Research Needs for Advanced Nuclear Energy Systems***

- Chemical Sciences Division
-  Oak Ridge National Laboratory
- MS-6119, P.O. Box 2008, Oak Ridge, Tennessee 37831-6119

- This presentation was prepared in collaboration with Donald Palmer, Ariel Chialvo, David Wesolowski, Miroslaw Gruszkiewicz, David Cole, and Robert Smithwick.

## Overview

**What does “chemistry under extreme conditions” mean?**
- Radiation (internal and external) environments
- High temperature
- High pressure
- Solubility limitations
- Reactive atmospheres
- Unconventional solvents

## Radiolysis

**Irradiation consequences occur throughout the Nuclear Fuel Cycle**
- Reactor: Irradiation from the core: neutron, alpha, beta, and gamma irradiation, either directly or via nuclear reactions
- Primary circuit: 10-100 MR/h near the core, neutron flux 1012-1015
- Cladding of fuel: if using Al cladding, internal irradiation 100 mR/h/L due to 24Na produced by 28Al(n,)24Na
- Secondary circuit:  irradiation mostly from primary circuit water in heat exchanger
- Irradiation of the fuel rods during cooling time (and more if disposed of intact)
- Radiolysis induced during any reprocessing stages

## Radiolysis

**Implications for separation agents**

- Radiation resistant
- Degradation products should not interfere with the separation process
- Design of ligands so that the degradation products can be washed out (specific location for heteroatoms)
- Example: Study of the CSSX solvent (137Cs extraction)
- Calculation of the dose equivalent for the time the solvent is to be used in the process
- External irradiation of the solvent in contact with various aqueous phases using 60Co (irradiation chamber)
- Internal irradiation of the solvent in contact with various aqueous phases using 137Cs (hot cell)

## The physical properties of water change systematically with temperature although the change is far more dramatic at the approach to the critical temperature (374°C).
 The decrease in the dielectric constant to values similar to those of organic solvents (at room temperature) leads to:
Dramatic increases of all ion-ion interactions (i.e., increased association of ions of opposite charge).
Decreases in the pH of the zero-point-of-charge of metal oxides and dramatic increases in the capacity of these surfaces to adsorb ions.
An initial increase in the ion product of water from Kw = 10-14 to 10-11.2 at 250°C, followed by a decrease, but the values are uncertain in the supercritical region.
Coupled with the corresponding increase in the compressibility of water, these factors lead to reactions in solution becoming very pressure (or water density) dependent near the critical temperature as the heat capacities of ions approach infinite values.
The solubility of metal oxides changes with temperature and pH in a manner that is unique to the nature of the metal, although general trends with pH can be rationalized from empirical observations

- The physical properties of water change systematically with temperature although the change is far more dramatic at the approach to the critical temperature (374°C).
-  The decrease in the dielectric constant to values similar to those of organic solvents (at room temperature) leads to:
- Dramatic increases of all ion-ion interactions (i.e., increased association of ions of opposite charge).
- Decreases in the pH of the zero-point-of-charge of metal oxides and dramatic increases in the capacity of these surfaces to adsorb ions.
- An initial increase in the ion product of water from Kw = 10-14 to 10-11.2 at 250°C, followed by a decrease, but the values are uncertain in the supercritical region.
- Coupled with the corresponding increase in the compressibility of water, these factors lead to reactions in solution becoming very pressure (or water density) dependent near the critical temperature as the heat capacities of ions approach infinite values.
- The solubility of metal oxides changes with temperature and pH in a manner that is unique to the nature of the metal, although general trends with pH can be rationalized from empirical observations

## Drastic Differences as T increases

- This example of the solubilities of Ni and Zn oxides with their radically different behavior is relevant to current PWR primary circuit chemistry issues of corrosion and control of radiation migration

## Current PWR’s and BWR’s use chemicals to control pH (e.g., LiOH) and neutron fluxes (e.g., boric acid), but we cannot predict their behavior (hydrolysis and ion association) above 400 and 300°C, respectively.  These properties become very sensitive to T and P near and above the critical temperature of water (374°C).
Current PWR’s in the US experience problems associated with axial offset anomalies when operated at high fuel loadings and high temperatures. The unresolved chemistry issues associated with the onset of AOA are the formation of deposits (crud) are strongly enhanced solubility of the zirconia fuel cladding and ion adsorption interactions with the cladding and crud, as well as suspected solubility limits for lithium borate.  In a reactor unit operating at more extreme conditions the nature of these reactions needs to be understood at a fundamental molecular level.

**Two examples of existing knowledge of nuclear plant chemistry issues that require attention if operating conditions are extended to extremes**

- Current PWR’s and BWR’s use chemicals to control pH (e.g., LiOH) and neutron fluxes (e.g., boric acid), but we cannot predict their behavior (hydrolysis and ion association) above 400 and 300°C, respectively.  These properties become very sensitive to T and P near and above the critical temperature of water (374°C).
- Current PWR’s in the US experience problems associated with axial offset anomalies when operated at high fuel loadings and high temperatures. The unresolved chemistry issues associated with the onset of AOA are the formation of deposits (crud) are strongly enhanced solubility of the zirconia fuel cladding and ion adsorption interactions with the cladding and crud, as well as suspected solubility limits for lithium borate.  In a reactor unit operating at more extreme conditions the nature of these reactions needs to be understood at a fundamental molecular level.

## Understanding the Behavior of Solutions at High T

- Recent advances in molecular modeling simulations provide a direct route to determining water/steam plus dissolved contaminant/treatment chemical properties at extreme conditions, particularly at low densities (pressures) where experiments are difficult to perform.
- High-temperature experimental methods (many of which were pioneered at ORNL) are available or are under development to determine the properties of aqueous solutions and the thermodynamics and kinetics of reactions to extreme conditions with the possibility of now interfacing these results with simulation predictions.

## Impact of Molecular Modeling

-  Used to predict thermophysical and thermochemical properties
- Examples: (1) critical constants; phase equilibria; PVT data; diffusivity, viscosity; (2) molecular structure; Gibbs energies of formation and reaction; dipole moments and other spectroscopic properties; reaction rates.
-  Particularly useful at the process screening level of design
- Physical properties frequently not available, *e.g*., at extreme conditions.
-  Three crucial roles:
- Prediction of fundamental properties used in engineering correlations.
-  critical constants, molecular structure, dipole moment.
- Prediction of required properties directly.
-  phase equilibria of mixtures.
- Providing conceptual molecular-level understanding of properties.
-  developing correlations, evaluation of theories, guide/replace experiments.

## Example: Natural and Industrial Hydrothermal Systems

- pH and corrosive nature of solutions
-  limited/no experimental accessibility  difficult data interpretation
- high compressibility  low dielectric permittivity   ion pairing
- challenging macroscopic modeling  requires molecular-based input
-  molecular-based ion pair association studies (pioneered at ORNL)

- Chialvo & Simonson, *JCP* **118**, (2003)

**Change of density**
- Change of solubility of components, change in absorption mechanism
- Impact on volatility model (based on associated species)

## Molecular dynamics simulations of ion adsorption on rutile have been done in concert with experimental surface studies using synchrotron X-rays. Rutile 110 surfaces in contact with 2048 SPC/E water molecules containing dissolved Rb+ and Cl- ions.  Ion-water interactions from literature, Ion-surface interactions and relaxed surface structure from ab initio calculations

**RbCl, negatively charged rutile 110 surface**

- The pH of 50% adsorption Is a model-independent  indicator of the binding strength of ions on mineral surfaces, so long as the comparison is made at approximately equal surface loadings.

- Rutile 110 surface adsorption sites identified at sub-Å resolution by X-ray standing wave and reflectivity studies  at APS and NSLS

- Temperature dependence of adsorption

***Example of Molecular Modeling***

## Solubility and Hydrolysis Studies at High Temperatures

- The hydrogen-electrode concentration cell and analogous iridium-electrode cell allows precise **measurement of the pH of solutions to 300°C**, while the flow-through versions can operate in the supercritical region

## Solubility and Hydrolysis Studies at High Temperatures

- The flow-through solubility apparatus has been used to measure the solubilities of metal oxides/hydoxides in water and steam to extreme conditions.

- Log (mmetal)

## Up to Supercritical Conditions

**The flow-through**** platinum conductance cell provides the most sensitive tool to investigate ion-association reactions to extreme conditions**

- The flow design is capable of measurements at extreme dilution (at ppb levels) necessary for accurate determination of ion-association constants to supercritical conditions
- At supercritical conditions all ions, including sodium, associate significantly.  Conductance measurements provide quantitative information necessary for building speciated thermodynamic models of high-temperature solutions.

- This new apparatus will be able to operate at vapor-like densities to 760°C and 1000 bar.

- Temperature and pressure dependence of the ion-association constant for NaOH(aq) close to the vapor-liquid coexistence curve. (ORNL: Ho *et al*., 2000)

## Use of MD simulations to determine water/steam plus dissolved contaminant/treatment chemical properties at extreme conditions, particularly at low densities pressures) where experiments are difficult to perform.
Use of advanced flow-through conductance techniques to measure ion-ion interactions in water and steam (interfaced with simulation predictions).
Use of new pH sensors to quantify the hydrolysis (and in some cases, solubility) reactions of chemicals needed to control the pH and neutron flux in the primary circuit of future PWR’s operating at more extreme temperatures and pressures.
Use of special flow-through autoclaves and spectrophotometric cells to determine the solubility of metals, including zirconia and nickel-based alloys, and the solution speciation associated with the metals released into solution.
Use of ORNL potentiometric cells to investigate ion adsorption equilibria at the solution/metal interface.
Use of neutron scattering techniques (currently up to 400°C) to extract crucial information on metal speciation in anticipated crevice and “under-crud” corrosion environments.
Neutron diffraction and EXAFS measurements at high temperatures could be used to establish the relative strengths of hydration interactions among cations. Determining whether ions follow the anticipated Hoffmeister series for complexing strength could be used in designing new separations systems for selective separation at high temperature.

**List of equipment and techniques necessary to conduct studies at high T and P > 1 atm**

- Use of MD simulations to determine water/steam plus dissolved contaminant/treatment chemical properties at extreme conditions, particularly at low densities pressures) where experiments are difficult to perform.
- Use of advanced flow-through conductance techniques to measure ion-ion interactions in water and steam (interfaced with simulation predictions).
- Use of new pH sensors to quantify the hydrolysis (and in some cases, solubility) reactions of chemicals needed to control the pH and neutron flux in the primary circuit of future PWR’s operating at more extreme temperatures and pressures.
- Use of special flow-through autoclaves and spectrophotometric cells to determine the solubility of metals, including zirconia and nickel-based alloys, and the solution speciation associated with the metals released into solution.
- Use of ORNL potentiometric cells to investigate ion adsorption equilibria at the solution/metal interface.
- Use of neutron scattering techniques (currently up to 400°C) to extract crucial information on metal speciation in anticipated crevice and “under-crud” corrosion environments.
- Neutron diffraction and EXAFS measurements at high temperatures could be used to establish the relative strengths of hydration interactions among cations. Determining whether ions follow the anticipated Hoffmeister series for complexing strength could be used in designing new separations systems for selective separation at high temperature.

## Extreme Chemical Conditions: Reactive Atmosphere

**Fluorine Chemistry**

- Use of entirely fluorinated uranium compounds for isotopic separation purposes (upstream)
- Concept: Study of the complexation of major waste constituents with fluorine to investigate the idea of separation of actinides by gaseous separations
- Volatility, oxidation states, complexation constants of elements of interests with HF and F2 would have to be studied.

## Unconventional “Solvents”

**Source:** *Chemistry under extreme or non-classical conditions*, R. van Eldik, C. D. Hubbard, Eds, Wiley and Spektrum, 1996

**Supercritical fluids**
- “tunable” density
- Large variety of reactions (organic and inorganic)
- Easy recovery of solute
- Unusual solubilities (e.g. fluorocarbons in scCO2)
- Could be used for separation purposes

**SCF conditions are quite applicable to the Nuclear Fuel Cycle**

**RTIL**
**Third-phase formation**

## Conclusions

- Chemistry of water under alpha, beta, gamma, and neutron irradiation
- Better understanding of ligand design for better radiation robustness
- Need for a fundamental understanding of the properties and processes uniquely associated with fluid/solid interfaces relevant to nuclear reactor systems.
- Need to develop a comprehensive understanding of the thermophysical properties, structures, dynamics, and reactivities of complex processed-based fluids and molecules (water and other C-O-H-N-S-bearing fluids, electrolytes, and organic contaminants) at multiple length scales (molecular to macroscopic) over wide ranges of temperature, pressure, and composition.
- Need to develop unconventional solvents to look into separation opportunities