Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

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420 Charged Particle and Photon Interactions with Matter

temperature while the spur model suggests an increase with temperature (Laverne and Pimblott,

1993; Swiatla-Wojcik and Buxton, 1995). All of these should be examined by experiments with

better

time resolution.

Recently

a kinetic analysis of the hydrated electron was performed up to 350°C in the picosec-

ond time range (Baldacchino et al., 2006). The results showed increasing competition between

the initial size of the spurs, the accelerated diffusion of the species and the temperature-activated

recombination. Thus, the radiolytic yield of the hydrated electron remains stable between 100ps and

3ns

at 350°C (Baldacchino et al., 2006).

15.8 ConCluding remarks

Under subcritical or supercritical conditions, the radiolytic yields of water decomposition products,

the reaction rate constants, and the spectral properties of transient species show signicant dif-

ferences from those of water at ambient condition or even at elevated temperatures below 300°C.

These properties are not only dependent on temperature but also greatly affected by water density

in SCW, or in other words, they exhibit nonlinear behaviors or even non-monotonic functions with

temperature at a xed pressure. This would strongly suggest a reconsideration of the current water

radiolysis model in nuclear reactors, especially for the future SCW reactors. On the other hand,

these unusual behaviors are certainly related to the peculiar water properties and water structure of

SCW such as dielectric constant, hydrogen bonding network, clustering, and density inhomogeneity.

A thorough understanding of this eld will require the involvement of gas-phase chemistry, cluster

chemistry, computational chemistry, etc. For radiation chemistry, the fundamental studies of tem-

perature and density effects of electron solvation, spur reaction processes and other more general

chemical interests in these intriguing reaction media are fairly meaningful. To achieve these goals,

higher performance equipment such as picosecond or sub-picosecond pulse radiolysis systems and

sophisticated theoretical calculations are essential.

aCknowledgments

We would like to thank Professor Y. Hatano for his continuous support of our studies on the radia-

tion chemistry of high temperature and supercritical uids as well as his encouragement to write this

chapter. We are also grateful to T. Ueda and Professor M. Uesaka for their technical assistance in pulse

radiolysis experiments and encouragement. We are also thankful to Professor M. Mostafavi, Professor

J.P. Jay-Gerin, Dr. Y. Muroya, Dr. G. Wu, Dr. T. Miyazaki, Dr. H. He, Dr. Z. Han, and many other

students who have contributed to these studies.

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425

Radiation Chemistry of Water

with

Ceramic Oxides

Jay A. LaVerne

University of Notre Dame

Notre

Dame, Indiana

16.1 introduCtion

One of the new challenges in radiation chemistry is to understand the radiolysis of water under the

various and often extreme environmental conditions associated with nuclear technology. The radia-

tion chemistry of liquid water and aqueous solutions has been thoroughly examined and some of the

basic aspects have been summarized in recent reviews (Buxton, 2004; Garrett et al., 2005). Energy

deposition by ionizing radiation is sufcient to populate a variety of ionic and excited states leading

to the formation of radicals that decay by a range of chemical reactions over a large timescale. Initial

water decomposition occurs on the ultrafast timescale making water radiolysis studies very difcult

and many details have yet to be resolved (Garrett et al., 2005; De Waele et al., 2009). The radiolytic

decomposition of water adsorbed on solid surfaces can be even more demanding than bulk water

because of the heterogeneity of the system. One of the fundamental questions in heterogeneous water

radiolysis is whether the radiation-induced decomposition of water is different for molecules adsorbed

on surfaces or in a near-surface layer as compared to the bulk liquid. The presence of a solid interface

can lead to catalytic, steric, or other effects that alter the water decomposition. There is also the het-

erogeneous nature of the energy deposition since energy can transfer between the solid phase and the

water to enhance or hinder the water decomposition, modify product yields, and even alter the solid

surface. Migration of energy can lead to problems in determining the “effective” dosimetry, which is

equivalent to the quantity of energy available for radiation effects in the adsorbed water. The transport

of charge, excited states, even atoms or molecules to or through the water–solid interface can lead to

signicant chemical consequences beyond that observed in bulk water alone.

Differences between the radiolysis of bulk liquid water and that of water in the vicinity of solid

interfaces have long been observed, but not vigorously examined until recently. This chapter will

present some of the newer results on the mechanisms that explain how the presence of a ceramic

Contents

16.1 Introduction ..........................................................................................................................425

16.2 Applications..........................................................................................................................426

16.3 Fundamental

Processes ........................................................................................................427

16.3.1

Energy

Deposition .................................................................................................... 427

16.3.2

Water

at Interfaces.................................................................................................... 428

16.4

Radiolysis of Aqueous Suspensions....................................................................................... 430

16.5 Radiolysis

of Adsorbed Water .............................................................................................. 433

16.6

Radiolysis

of Conned Water............................................................................................... 437

16.7

Conclusions........................................................................................................................... 439

Acknowledgments.......................................................................................................................... 439

References...................................................................................................................................... 439

426 Charged Particle and Photon Interactions with Matter

oxide interface can affect the outcome of water radiolysis. Most of the discussion will concern the

radiolytic effects on water absorbed on or very near to the surface of a ceramic oxide interface.

Athorough review of the radiolytic effects of organic molecules absorbed in or on solid oxides such

as silica, alumina, and the zeolites has been given elsewhere (Thomas, 1993). Low-weight-percent

aqueous suspensions of the solid essentially behave like bulk water or dilute solutions and will not

be covered here because of the minimal contribution to the overall radiolysis by the heterogeneous

nature

of the system.

Most

of the systems discussed in this chapter consist of small particles in the nanometer size

range. These so-called nanoparticles are used because of their large surface area and rarely because

of the quantum effects observed at small dimensions. A recent survey discusses the radiation effects

of nanoparticle suspensions, so the observations on those systems will not be discussed here as thor-

oughly as in that work (Meisel, 2003). Only ceramic oxides as the solid phase will be covered here

because these are commonly encountered in the nuclear technology eld. Numerous summaries

have been published on the radiolysis of water associated with metals, especially the noble metals,

and their applications range from solar cells to photography and nanoparticle fabrication. Ceramic

oxides can be good insulators and they often do not follow the same redox chemistry as observed

with semiconductors. In addition, the surface of ceramic oxides is rarely inert and it can contribute

signicantly

to the radiation chemistry of surrounding water.

16.2 appliCations

There are a number of very important practical reasons for performing radiolysis studies on water

with ceramic oxides. Many parts of some nuclear reactors, including fuel rods (ZrO

), control ele-

ments (BeO), and cooling coils (Al

, Fe

, etc.), contain ceramic oxides that are in constant con-

tact with water. Water decomposition products such as the OH radical or H

are responsible for

initiating and propagating much of the corrosion of reactor components and yet there is little infor-

mation on the radiation-driven processes occurring at the critical water–solid interfaces (Roberto

and de la Rubia, 2006). Heterogeneous radiation chemistry studies are required to predict the aging

of the existing reactors and evaluate how long they may safely operate. The next generation of reac-

tors must have radiation chemistry data to be correctly engineered against radiation damage in the

harsh

environments expected to be encountered.

Decommissioned

weapons and recycled fuel elements will require long-term storage facilities

that are often enclosed and constructed of stainless steel and a variety of other oxides (National

Research Council, 2001, 2002). Shipping containers are almost always sealed and held to higher

standards than storage facilities. These containers may pass through populated areas and they are

required to be able to withstand extreme temperature ranges and mechanical stress. Water from

the humidity in the air will settle on the surface of the waste materials during processing and

packaging. Self-radiolysis will then lead to water decomposition over the lifetime of the container.

Variations in the yields of the potentially explosive product H

(or the ratio of H

to O

) can lead to

long-term

management problems.

Nuclear

power is being promoted more regularly as a source of the energy that will be required

to meet global energy needs. No toxic gases are produced by using this technology in its intended

manner and the contribution to global warming is minimal. However, radiation effects still present

serious challenges to an expansion in the use of nuclear power reactors. A variety of workshops

sponsored by the U.S. Department of Energy have contained signicant discussions related to the

problems due to the radiolysis of water in the presence of a solid phase (U.S. Department of Energy,

2003, 2006, 2008). Very little information can be found in the open literature on the interaction of

the solid interface with water in a radiation environment. However, this research topic has recently

become more popular. Practical solutions have been made in some instances within the nuclear

power industry, but the underlying mechanisms describing the role of water radiolysis in the corro-

sion processes are still to be resolved. Concurrent with corrosion is the dissolution of the solid phase

Radiation Chemistry of Water with Ceramic Oxides 427

into the liquid (Nechaev, 1986; Christensen et al., 1994; Christensen and Sunder, 1996; Corbel et al.,

2001; Sattonnay et al., 2001). This process degrades fuel elements and disperses radioactive materi-

als throughout the complex. Much time and money has been spent to slow dissolution processes.

Dissolution may be a critical obstacle in the containment of waste materials in wet storage environ-

ments.

Water in these scenarios has an important role as a solvent and also in the redox chemistry.

Targeting

radiation specically to tumor sites has long been a goal in radiation therapy in order

to minimize damage to normal cells. Nanoparticles are now being examined with this goal in mind

(Juzenas et al., 2008). The scheme is to selectively deposit nanometer-sized materials at the site of

a tumor and irradiate. The high local density of the nanoparticle will absorb more energy than the

surrounding tissue. Low-energy electrons can then escape the nanoparticle to locally irradiate the

nearby tumor, thereby minimizing damage to normal surrounding tissue. Optimization of these

techniques will require much more information on the mechanisms of energy transfer between

phases

in radiolysis.

16.3 Fundamental proCesses

16.3.1 energy depoSition

Rutherford was the rst to determine the fundamental interactions of ions as they pass through

matter (Rutherford, 1911). The simple cross section developed by Rutherford is not sufcient to

calculate the rate of energy loss or stopping power (−dE/dx) because it does not take into account

the properties of the medium. A classical formula for the stopping power of matter was developed

by Bohr, which was followed by a quantum-mechanical description by Bethe (Bohr, 1913; Bethe,

1930). Thorough discussions of the passage of ions in matter and the tracks they form are given

in several reviews (Bohr, 1948; Bethe and Ashkin, 1953; Fano, 1963; Mozumder, 1969; LaVerne,

2000). All charged particles interact mainly with the electrons of the medium. The exceptions are

very

low-velocity, highly charged ions, which undergo nuclear ballistic collisions (Bohr, 1948).

Most

of the inuence of the medium on the stopping power is contained in a parameter called

the mean excitation energy, but this dependence is logarithmic. Even though mean excitation ener-

gies can vary widely for different media, the effect on the stopping power is relatively small (ICRU

Report 37 1984). For example, the energy corresponding to the median density of energy loss events

for 1MeV electrons in SiO

is calculated to be 27eV as compared to 22 eV for liquid water even

though the mean excitation energies are 139.2 and 71.6eV, respectively (ICRU Report 37 1984;

Milosavljevic et al., 2004). The distribution of secondary electrons is shifted to slightly higher ener-

gies in silica than in water, but the effect on the chemistry is expected to be negligible. The individ-

ual energy loss events are nearly the same in either phase. However, the stopping power formalism

dictates that the number of events depends directly on the electron density of the medium. Electron

densities are very nearly the same for most media, for instance relative to water = 1.00, SiO

= 0.90,

and ZrO

= 0.82. Therefore, the energy loss of ions in each component of a heterogeneous medium

is roughly proportional to their weight fraction. At a relative humidity of less than about 75%, the

weight fraction of adsorbed water on many ceramic oxides is one percent or less, so energy loss is by

far

more predominant in the solid phase (LaVerne and Tandon, 2002).

The

average gamma ray energy from the cobalt-60 sources typically used in radiation chem-

istry studies is about 1.25 MeV. Compton scattering is the dominant energy loss process for pho-

tons of this energy and leads to the production of two secondary electrons. These electrons lose

energy in the different components of the medium by Coulombic interactions as discussed above.

However, because many ceramic oxides are composed of very high Z materials, the gamma ray

cross sections will be slightly different than in the aqueous media often used for dosimetry, the

Fricke dosimeter for instance. Studies with cobalt-60 gamma rays used ionization chambers to

show that the energy absorbed by ZrO

is about 10% greater than that with the Fricke dosimeter

(Sagert and Robinson, 1968; Wong and Willard, 1968). Monte Carlo calculations were used to

428 Charged Particle and Photon Interactions with Matter

estimate that a correction factor of 15% should be added to the dose measured using the Fricke

dosimeter to arrive at the dose in pure SiO

(Rotureau et al., 2005). Accordingly, a suitable cor-

rection to the dose determined with aqueous dosimeters must be applied to systems of water and

ceramic oxides. No thorough study seems to have been made using mass attenuation coefcients

for the different compounds. A cursory examination of the mass attenuation coefcients for a

variety of ceramic oxides suggests that a correction in dose of 10%–15% to the Fricke dosimeter is

appropriate (Hubbell and Seltzer, 1996).

Energy deposition in the different phases is relatively straightforward to predict, but an esti-

mation of the ionization produced in each medium is more problematic. Monte Carlo track cal-

culations predict an ionization yield of 4.11 per 100eV of energy deposited in SiO

, which is

very similar to that of liquid water (Milosavljevic et al., 2004). The energy required to produce

an electron-hole pair in SiO

is estimated to be between 17 and 30 eV (Petr, 1985). These values

give a range of ionization yields of 3–6 per 100 eV, which is comparable to the Monte Carlo

predictions. A previous work proposed that the average energy to produce an electron-hole pair

is approximately equal to 2.73E

+ 0.55 eV, where E

is the band gap (Alig and Bloom, 1975).

The band gap for SiO

is 11 eV, resulting in a predicted ionization yield of 3.3 per 100 eV. ZrO

has a band gap of 5 eV and the above formula would estimate an ionization yield of 7 per 100 eV.

Experiments have suggested the ionization yield in ZrO

to be 10 per 100 eV, but the chemistry

of those systems may be misleading (Meisel, 2003). The ionization yield is probably not solely

dependent on the band gap energy due to a variety of other processes such as trapping and

recombination. Clearly, better estimates of the ionization yields in various ceramic oxides are

required to gain a complete understanding of charge production and transport in the radiolysis of

heterogeneous media.

16.3.2 water at interfaceS

Two very detailed compilations have been published on the adsorption of water on a variety

of surfaces (Thiel and Madey, 1987; Henderson, 2002). Physisorbed water may be expected to

behave similarly to that of bulk water especially if there is a large amount of water present.

However, water fragments such as OH, H, and O will have quite different chemistry if the amount

of water available is not sufcient to support complete hydration. Water dissociatively adsorbs on

many ceramic oxide surfaces (Boehm, 1971). The two OH groups formed by the water molecule

are not equivalent on the surface. The OH anion will be bound to a surface cation site to form a

terminal OH group while the proton will attach to a surface oxygen anion to form a bridged OH

site, as shown in Figure 16.1. Even more types of OH groups are possible by binding to multiple

sites or defects at the surface.

H H

Zr Zr ZrOO

Zr Zr ZrOOZr Zr ZrOO

Figure 16.1 Schematic for the dissociative water addition to ZrO

and formation of ice-like layer.

Radiation Chemistry of Water with Ceramic Oxides 429

A variety of complementary techniques can be used to make and probe the OH sites on the

surface of ceramic oxides. Clean surfaces can be made in ultra high vacuum (UHV) chambers and

water vapor slowly deposited and examined. Surfaces can be completely reduced with H

or D

and the addition of D

O can be used for isotopic studies. UHV chambers allow the use of electron

spectroscopy and electron scattering for the examination of OH groups on particle surfaces (Freund

et al., 1996; Hendrich and Cox, 1996; Vickerman, 1997; Henderson, 2002). Some of the oxides

examined for surface water dissociation are listed in the compilation of Henderson and include:

, CaO, CeO

, MgO, SnO

, SrTiO

, TiO

, UO

, and ZrO

(Henderson, 2002). Several of these

studies examine the kinetics of the reversible and irreversible water dissociation and give valuable

information on the species identity and their expected stability on the ceramic oxide surface. Many

of the early studies were interested in water on single crystal metal oxides and much of the discus-

sion concerned whether the most reactive sites are actually defect sites and if water dissociation

is associated with these sites only (Hendrich and Cox, 1996). Studies have shown that water can

dissociate at non-defect and defect sites, although the latter seem to be more active. Most water

dissociation on ceramic oxides is reversible, which means that heating leads to the reformation and

desorption

of the water molecules.

variety of infrared spectroscopic techniques can be used to examine oxide surfaces includ-

ing transmission/absorption, total reection, and diffuse reection (Busca, 1996). The oxides may

be in high vacuum or under controlled atmosphere conditions. Diffuse reection infrared Fourier

transform (DRIFT) techniques have proven to be very useful for the surface examination of ceramic

oxide powders. Powder surfaces are different than single crystals in that they have a variety of facets

and defect sites. They are probably much more representative of the surfaces encountered in practi-

cal applications. In the DRIFT technique, the physisorbed water is driven off by heating to several

hundred degrees to reveal the underlying surface OH groups. In many respects, this technique is

complementary to thermal desorption studies that examine the release of gases as a function of

temperature programming. DRIFT spectra have been measured for Al

, SiO

, TiO

, and ZrO

powders (McDonald, 1958; Yates, 1961; Peri, 1965; Lavalley et al., 1988; Merle-Mejean et al., 1998;

Szczepankiewicz et al., 2000; Al-Abadleh and Grassian, 2003; Roscoe and Abbatt, 2005; Takeuchi

et al., 2005; Murakami et al., 2006; Thomas and Richardson, 2006; Carrasco-Flores and La Verne,

2007). Typical DRIFT spectra for ZrO

are shown in Figure 16.2. The broad OH stretch band

4000 3500 3000 2500 2000 1500 1000 500

120

160

Transmittance

Wavenumber (cm

–1

)

OH stretch

OH bend

Libration

Combo

Zr–OH

Zr–OH–Zr

Amorphous

ice

“wet” ZrO

ZrO

-25°C

ZrO

-400°C

Figure 16.2 DRIFT spectra for amorphous ice (shifted up) (Hudgins et al., 1993) for ZrO

at 25°C and

400°C

(Carrasco-Flores and La Verne, 2007), and for “wet” ZrO