Vaporization of Simulated Nuclear

Waste Glass

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

High Temperature Processes Group

Inorganic Materials Division o Washington, DC 20234

June 1 983

Interim Report

#</*CAU O*

U S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NATION JU. BTJf-A-J OF STANDARDS

’ IBHARF SEP 3 0 1983

rr^acc-occCL <^ voo

v

NBSIR 83-2731

1*1 VAPORIZATION OF SIMULATED NUCLEAR

WASTE GLASS

J. W. Hastie, E. R. Plante, and D. W. Bonnell

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Materials Science

High Temperature Processes Group

Inorganic Materials Division

Washington, DC 20234

June 1 983

Interim Report

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Table of Contents

3

Page

ABSTRACT 1

1. Introduction 1

2. Knudsen Effusion Mass Spectrometry 5

2.1 Basis of Technique 5

2.2 Apparatus 6

2.3 Instrument Calibration 8

2.4 Detection Limits 10

2.5 Glass Vaporization in Vacuum 11

2.6 Glass Vaporization in ^0 Atmosphere 19

3. Transpiration Mass Spectrometry 21

3.1 Basis of Technique 21

3.2 Apparatus 22

3.3 Instrument Calibration 25

3.4 Detection Limits 28

9 3.5 Glass Vaporization in Ng Atmosphere 29

3.6 Glass Vaporization in Atmosphere. . . 32

4. Thermodynamic Estimates 35

4.1 Glass Vaporization in Inert Atmosphere 35

4.2 Glass Vaporization in HgO-Og Atmosphere 38

5. Conclusions 40

6. Acknowledgements 41

7. References 42

8. Figure Captions 44

9. Figures 47

a

In the interests of accuracy and clarity in describing various items

of equipment or apparatus, mention has been made of commercial sources

or brand names. This in no way implies endorsement of such products

by the U.S. Government.

Vaporization of Simulated Nuclear Waste Glass

J. W. Hastie, E. R. Plante, and D. W. Bonnell

Center for Materials Science

National Bureau of Standards

Washington, DC 20234

ABSTRACT

Industrial development of nuclear waste glass processing

requires basic data on glass vaporization thermodynamics.

Detailed mass spectrometric experiments and thermodynamic

estimates have been made for vaporization of a nonradioactive

borosilicate glass containing representative nuclear waste

isotopes. Alkali metaborates were observed to be the dominant

vapor species and their partial pressures indicate significant

vapor transport under likely process conditions. The results

indicate the following order of significance for vapor transport of radionuclide species, Cs ~ Re (y Tc) > Ru » Sr.

Key words: Glass, mass spectrometry, nuclear waste, thermodynamics, vaporization.

1. Introduction

Industrial plans for nuclear waste storage require the waste to be

incorporated into a borosilicate glass. Vapor pressure data are needed

for these borate-waste forms in order to optimize the process conditions

(e.g. , temperature, composition) and minimize losses of radionuclides

1
by vapor transport during the high temperature mixing treatment. The

primary objective of the present study was to define the vaporization

thermodynamics of a simulated nuclear waste (SNW) borosilicate glass,

containing non-radioactive isotopes, as prepared by DuPont-Savannah

River. Also, measurements of partial pressure for each vapor species

allow thermodynamic activities and related partial molar functions to

be derived. Such basic data are pertinent to the development of thermodynamic models of other nuclear waste properties such as leaching and

storage durability. Special emphasis was given to vapor species containing the following elements which in actual nuclear waste form are

highly radioactive: Cs, Sr, Ru, and Re—present as a technetium simulant.

Table 1 gives the analytical and nominal compositions of the glass

sample used. The analytical composition was determined by inductively

coupled plasma spectroscopic analysis [Soper, 1982] for all elements

except Cs, Ru, and Re which were determined by neutron activation

analysis. For the thermodynamic estimation of species pressures the

nominal glass composition data were used.

No literature data exist for a glass of this type. However, there

are indications from vapor transport measurements on metaborate glasses,

both in inert and H

2

0 atmospheres, that alkali metaborate and metaboric

acid are the likely vapor species [Cable, 1978; Wenzel and Sanders, 1982].

The thermogravimetric data of Gray [1980] for radioactive waste glasses,

with higher concentrations of radionuclides than for the present system,

is not molecular-specific and hence cannot be used for extrapolation to

our conditions.

High temperature vaporization data were required for vacuum, inert

N

2

, and reactive H

20, C02

, CO, H

2

, 0

2 atmospheres, the latter mixture

2
Table 1

Simulated Nuclear Waste (SNW) Glass Composition3

Compound

Nomi nal

Weight %

Composition

Mole %

Analytical

Weight %

Compos itionb

Mole %

S10

2

52.00 57.01 53.76 59.40

Fe2°3 12.96 5.35 11.49 4.78

ai

2

0

3

4.86 3.14 4.75 3.09

B2°3 7.29 6.70 6.49 6.19

Li

2

0 5.10 11.25 3.96 8.80

Na

2° 10.13 10.76 8.59 9.20

Mn0

2

3.25 1.36 3.57 2.72

NiO 1.05 0.93 1.80 1.60

CaO 1.59 1.87 1.01 1.19

MgO 0.68 1.11 0.54 0.88

Zr0

2

0.70 0.38 3.79 2.04

SrO 0.09 0.06 0.05 0.03

Cs 2° 0.10 0.03 0.07 0.02

Ru0

2

0.09 0.05 0.11 0.05

Re

2

0

7

0.10 0.01 0.03 0.01

a [Soper, 1982]

b

See text for selection

3
representing the off-gas condition in the industrial glass processor.

For the temperature range of interest, 850 to 1150 °C, this glass is in

a molten state. Under the conditions of interest we can expect a

priori an exceedingly complex vapor phase composition—containing many

types of polyatomic species—and measurements over a wide dynamic range

of pressure and temperature are required for an unambiguous thermodynamic

analysis. Specialized state-of-the-art mass spectrometric techniques

are required to characterize such complex systems. Three techniques,

Knudsen effusion Mass Spectrometry (KMS), Gas Inlet KMS (GKMS), and

Transpiration Mass Spectrometry (TMS) were selected for the present

study. These techniques were supplemented by thermodynamic calculations

which are at present approximate owing to assumptions concerning solution

activities.

The experimental and theoretical results indicate that complex

oxide vapor species dominate the vaporization behavior of this glass,

with the following order of significance for the radionuclides

Cs Re (y Tc) > Ru » Sr.

Also, significant losses of the host glass components Li

2

0 , Na

20, and

B

2

0

3

occur due to the high volatility of alkali metaborate species.

Our experiments and thermochemical calculations suggest the reactive

atmosphere to be of secondary importance, though additional experiments

and detailed modeling are required for a definitive picture to emerge.

4
2 . Knudsen Effusion Mass Spectrometry

2.1 Basis of Technique

Knudsen effusion mass spectrometry (KMS) combines the classical

Knudsen method for vapor pressure measurement with mass spectrometric

identification of individual molecular species. The rate of effusion

is related to the pressure within the effusion cell by the expression:

= P.-C-a UjiRT/M.)'

55 (2.1)

where, for species i, m. is the mass loss for time At, P. the partial

pressure, C the Clausing factor, a the orifice area, R the gas constant,

T the temperature, and M. the molecular weight. The partial pressure

is determined from the mass spectrometric positive ion intensity I.

using the expression:

Pi = kl.T, (2.2)

where k is the combined mass spectrometer-chemical species sensitivity

constant.

These relationships are based on the condition of molecular effusion

where the molecular mean free path within the cell is much larger than

the orifice diameter. For practical orifice sizes of -v 0.5 mm and

typical temperatures and molecular weights, the upper limit in cell

-4

pressure is about 10 atm for molecular effusion.

5
2. 2 Apparatus

A schematic of the apparatus used is given in figure 1. Details

of this apparatus have been given elsewhere [Plante, 1979]. To discriminate against scattered gas in the mass spectrometer ion source, beam

modulation was used. In this method of operation, molecules issuing

from the effusion orifice are interrupted at a selected frequency

(about 200 Hz) by a slotted disc mounted on a motor. A light beam

passing through the disc, and detected by an optical switch, serves as

the reference signal for a lock-in amplifier. The mass-analyzed positive

ions, formed by electron impact in the source of the mass spectrometer,

are amplified by the lock-in detector only if they bear a specific and

definite fixed phase relationship to the reference signal. This signal,

which is called an ac spectrum, is relatively free from interference at

the same m/e ratio from other ions which are formed in the source of

the mass spectrometer but have not passed through the chopping wheel.

The quadrupole mass filter (QMF) shown in figure 1 is mounted

about 1-1/2 inches off center on a 6-inch flange in a horizontal position.

The water-cooled motor (M) which drives the modulation disk (MD) and

the optical switch (OS) are located below the mass filter. The location

of ion gauges (I.G.), and a shutter (S) for protecting the window (W)

are also indicated. A prism (P) on the top flange deflects the radiant

energy emitted from the Knudsen cell into an optical pyrometer for

temperature measurement. However, for the relatively low temperatures

of the present study a thermocouple (T.C.) was used for temperature

measurement.

Figure 2 shows a schematic diagram of a typical Knudsen cell and

furnace. The Knudsen cell was centrally located, both axially and

6
vertically, in the furnace. This arrangement was contained within a

water-cooled copper chamber to reduce outgassing effects at elevated

temperatures. The Knudsen cell was made of 0.025 cm Pt sheet and was

of welded construction. It was 0.80 cm o.d. by 1.90 cm high and was

welded to a 0.32 cm o.d. platinum mounting tube about 2.5 cm long. The

top of the cell was made from 0.015 cm Pt sheet, the outer edge of

which was bent in a mold to form a lip so that the lid fits snugly

inside the effusion cell body (note detail in fig. 2). An effusion

orifice diameter of 0.5 cm was used. For the present study, the cell

was modified to include a reentrant thermocouple support tube located

in the sample region (not shown in fig. 2).

For measurements involving injection of water vapor through the

cell bottom, the Knudsen cell was modified by insertion of a Pt leak

tube connected to a gas inlet system. A 0.025 cm (0.010 in.) orifice

at the top of the leak tube allowed for water vapor injection into the

cell. The leak rate into the cell was controlled by a valve external

to the vacuum system. With this cell arrangement, temperatures were

measured using a thermocouple spot welded to the side of the effusion

cell.

The Ta sheet furnace, as shown in figure 2, was 7.6 cm long and

0.005 cm thick, with heavier gauge support rings connected to electrodes.

High amperage low voltage power was supplied to this radiation furnace

through water-cooled electrodes.

7
2.3 Instrument Calibration

2.3.1 Mass Discrimination

The quadrupole mass spectrometer may discriminate against

higher mass ions, depending on tuning, and a mass discrimination cal i bra

tion is necessary. For species i in a gas mixture, the ion current is

given by the equation:

VarVTi (2.3)

where f. is the fraction of species i in a gas mixture, the isotopic

fraction, cr.. the electron impact ionization cross section, the mass

spectrometer transmission factor, and the instrument geometric

factor. Each term in equation (2.3) is known except the transmission

and geometric factors, the latter being independent of* ion type.

Relative transmission factors were obtained from equation (2.3) using a

gas mixture consisting of about 95% Ng and 1% each of He, Ar, Ne, Kr,

and Xe.

2.3.2 Sensitivity Constant

Two approaches are used to determine the sensitivity constant

k. defined in equation (2.2). These approaches rely either on a determination of the ion current corresponding to the evaporation of a known

mass of material, or of the ion current corresponding to a known pressure

in the effusion cell.

To determine the calibration constant by a mass calibration

method the pressures from equations (2.1) and (2.2) are equated, giving

8
the relationship:

k, = [m

i A.(27tR) 1/2 ]/[Cat lt(M.T) 1/2

]. (2.4)

In practice, the sum of weight losses for a number of experiments at

different temperatures and for different periods of time (At) are

observed so that eq. (2.4) is cast in the form,

k. = [A.(2rtR)!s 2}n

1.]/[ aC (2.5)

T T

In some instances more than a single vapor species, or more

than two vapor species linked by a known stoichiometric requirement,

are formed. In such a case (which includes the simulated nuclear glass

vaporization measurements) it is necessary to use measured or estimated

ionization cross sections to relate the various sensitivity constants.

In this case, the total mass loss (? m.) can be used to estimate the

various mass spectrometer constants for individual ions. Rearrangement

of equation (2.5) for an individual species, i, gives:

= yc-a^R)*5 £ (S yD-Atm-r5 )

. (2.6)

Using this equation, it is possible to calculate the contribution of

each vapor species to the total estimated or measured weight loss and,

thereby, determine if the estimated or measured cross sections are

consistent with the sample stoichiometry.

Calibration of the mass spectrometer by passing a known

amount of calibrating gas through a leak system is in principle the

same as the weight loss method. For measurements where water vapor

from an external source was passed over the glass using the KMS system

9
this calibration was obtained by passing Ar from a calibrated leak into

the gas inlet tube of the Knudsen cell. Since the Ar leak rate is

•6 3 -

known (1.61 x 10 cm s ), the equivalent pressure in the Knudsen cell

can be calculated and a value of k

Ar

determined from the Ar ion intensity.

Values of k. can then be calculated from k

Ar using equation (2.3).

For the various calibration and normal experimental procedures

a 30 eV ionizing electron energy was used. The production of doubly ionized

species for the calibration gases is minimal at 30 eV and the total

ionization cross sections are a good approximation to the cross section

for single ionization. Also, for more complex molecular species, the

amount of fragmentation is less at 30 eV than at higher ionizing energies.

In the case of electron impact fragmentation of molecular species it

was necessary to either correct the estimated total cross section for

this effect or to include in I. all ions formed by fragmentation of a

parent molecule. In this work, we have chosen to correct the total

estimated cross section to obtain a partial ionization cross section

valid for the observed species.

2.4 Detection Limits

The ability of the modulated beam mass spectrometer to detect

different species depends on a number of factors. For mass positions

relatively free of hydrocarbon or permanent gas background, such as at

23 amu, a signal of 1 pV can be detected with an uncertainty of ± 20%.

For unfavorable mass positions, such as at 18 and 44 amu, signals of up

to 5 pV may be required for detection within ± 20% uncertainty. Although

it is possible to obtain greater total amplification by increasing the

multiplier gain, the signal-to-noise ratio is a more important factor.

10
Improvement in this factor is usually achieved by decreasing the residual

gas pressure in the mass spectrometer section of the vacuum system.

Other factors of importance include, the distribution of isotopes among

the nuclides, the instrument mass discrimination, and the cross section

for molecular ion formation.

Generally, for the present experimental conditions, species could

be detected in unfavorable cases at partial pressures of 10

7

atm and

_o

in favorable cases at 10 atm. In unusual cases, the sensitivity of

the instrument can be somewhat less as, for example, with NaReO^ vaporization. Here, the NaReO^ molecule fragments under electron impact to

yield mainly Na+ and only about 0.1% of the ions produced are NaRe0

4+

.

As the NaEK^ glass vapor species also produces a high intensity of Na

+

,

this ion cannot conveniently be used as a measure of NaReO^ and a -5

partial pressure of NaReO^ as high as 10 atm may be required for

detection as NaReO^ . This problem could possibly be circumvented in

future studies by using an electron ionizing voltage high enough to

produce fragments such as ReO^* at less obscured mass positions.

2.5 Glass Vaporization in Vacuum

2.5.1 Observations on Li, Na, and Cs-containing Species

Several series of measurements were made on the simulated

nuclear glass. A series represents a sequence of experiments (runs)

using the same portion of the glass sample. Results for each series

were reasonably reproducible and two representative series are reported

here in detail. A number of experimental runs were made for each

11
series, each run consisting of experimental pressure-temperature points.

For each run the sample temperature was increased from room temperature,

or a lower idling temperature where vaporization was negligible, and

data points were then obtained with an increasing temperature chronology.

Mass spectral ions observed in the temperature range from

800 to 1150 °C, at 30 eV ionizing energy, included Na

+ (23 amu), NaBO* (66),

Li +

(7), LiBO^ (50), Cs+ (133), 0^ (32), and the associated isotopic

species. A typical mass spectrum in bar-graph form is given in figure 3.

Note that the intensities are plotted on a logarithmic scale. Also,

isotopic peaks at m/e of 6, 48, 49, and 65 amu have been omitted for

the sake of clarity. Both Li + and Na

+ ions are fragments from metaborate

precursors. This assignment is supported by the invariance of the ion

intensity ratio of M

+/MB02+ with temperature and by the high appearance

potentials of Li + and Na+

. As shown by the ionization efficiency

curves in figure 4, the observed appearance potentials of Li + and Na+

are 10.0 ±0.5 and 9.0 ± 0.5 eV, respectively, as compared with the

corresponding values of 5.4 and 5.1 eV for ionization from the elements.

Also, the form of each curve is indicative of a single molecular precursor.

By analogy with the electron impact behavior of NaB02 and IJBO

2

, it is

considered likely that Cs+is also produced by electron impact fragmentation of the metaborate precursor, CsBC^. Gorokhov et al. [1971] observed

M

+/MB0

2

+ ratios at 70 eV for Li, Na, and Cs metaborates of 0.48, 3.30,

and 32.7, respectively. These ratios are likely to be higher than

those for 30 eV. However, the trend suggests that even at 30 eV the

low Cs + intensities observed in the present study would preclude direct

observation of CSBO

2 as CsB02

+

.

The release of O

2

from the glass melt (see fig. 3) is attributed

mainly to reduction of the Fe

2

0

3

-Fe

3

0

4

dissolved in the melt.

12
A small amount of Cl (0.1 wt.%) was indicated in the nominal

glass composition. Therefore a search was made, at a reduced temperature,

for the relatively volatile alkali chlorides. No significant signals

were noted for LiCl + (40 amu), NaCl

+ (58), and Cs+ (from fragmentation

of CsCl). It is possible that most of this Cl was lost during the

glass preparation.

Conversion of ion intensity to species partial pressure data

was made using the parameters defined by equation (2.3) and which are

listed in Table 2. For series I, the first four runs were obtained

using a single mass spectrometer sensitivity calibration and mass

discrimination curve. After the fourth run, the spectrometer tuning

was changed to expand the observable mass range. The fifth run of the

Series I data was obtained using a second mass calibration and the new

mass discrimination curve. These new calibrations were also used for

the series II data. We have assumed that LiB0

2

, NaB0

2

, and CsB0

2

each

have the same total ioization cross section value, estimated as 2.4

(7ta

Q

units). The partial cross sections (corrected for fragmentation)

are given in table 2, except for CsB0

2

where no CsB0

2 was observed.

Values of k. were obtained using the mass spectrometric constants given

in table 2, together with the experimental II^At-T values and the

observed weight loss data.

A check on the internal consistency of partial pressure data

can be made by comparison of calculated weight loss data with the upper

limit imposed by the sample composition. Table 3 lists the calculated

weight loss by vaporization (using eq. 2.6) of the major oxide components

together with the initial concentration of each oxide. The calculated

weight losses are reasonably consistent with the sample composition

except for 0 2> In this case more 0

2 was lost than was initially present,

13
Table 2

Mass Spectrometric Constants Used for Ions

Over Simulated Nuclear Waste Glass

ion m/e

Series

A

I (Runs

T

1-4)

a a

Series I (Run 5)

and Series II

t a

Li 7 0.925 0.97 1.98 0.83 2.26

Na 23 1.00 0.97 2.14 0.97 2.33

°2 32 1.00 0.87 1.20 0.91 1.20

LiB0

2

50 0.742 0.64 0.42 0.88 0.14

NaB0

2

66 0.802 0.53 0.26 0.87 0.08

Cs 133 1.00 0.29 2.40 0.70 2.40

a 2 Units of 7ta Q

, e.g. , see Bonnell and Hastie (1979).

assuming reduction of to Fe^O^ as the only source of O

2

. This

effect was also noted in the TMS studies (section 3). It is likely that

a secondary source of O

2

is present in this glass. For the series II

data the calculated amount of CsB0

2

lost is slightly greater than the

total amount present. Considering the uncertainties in ionization cross

sections and sample composition, the agreement between observed and

calculated losses is satisfactory.

The experimental partial pressure versus temperature data are

plotted in figures 5 to 10. Several distinctive features are common to

each of these figures. Note in figure 5, for instance, that the initial

run 1 data show anomalously high pressures at low temperatures. Also,

note the tendency for the observed pressures to decrease slightly with

time e.g., as in the run 5 versus run 1 data. The initial high pressure

effect may be due sample inhomogeneity. It is likely that small surface

regions exist with higher concentrations of volatile components than for

14
Table 3

Comparison

of

Calculated

Weight

Loss

Data

with

Sample

Composition

(in

mg)

as

co

O)

co

03E as -o P

•r— c X as CO CO CM tH

CO o US • • in 00 rH as CM tH • • • r— s- CM O o tH

ns Q_ co P p

-C o

o> l—

•f— P

as

C 5 as

as r— CM CM o os as

as

a ns 00 P*. in vo CO r— •r° > • • • • • CL X •I— in fH o o CO E o 3 tH

ns O’

us LU , ns

•r- as P p US VO CM as

•r~ ns US co r** VO 1 CO C o . • « 1 . t—

o _l CM m o 1 CO

03 tH

as

p

-a c

•r* as X us VO o as tH CM CM tH

s- • • « • • f— a. VO CO o as tH

ns CM rH tH Po

t”

ns ao CO

ns US CO

O o m *0" P us • • • • • o o m CM o o CM t— —

rH Pc

as as TJ r— in in CO P»

O •r* ns in VO o CM o X > .» . • . o •r— tH o o CO rH /—

Lf) 3

as <T E c LU 3

r-t ao • as CO p

O VOO VO ns US CO tH o 1 o CM s- US , • • 1 . o O CO CM o 1 rH p CQ _J

-COS

•r-<U Pc

as as as a r— tH

O

m CM CO r— •r— ns r>- CO o CM a. X > • • . . E *3- o •r* CO tH o rH

(0 1 3

us tH CT

UJ

US ns c

•r" 3 as P cc p r— ns US as CO VO CO c s- US ao CO o i t—( o o • • • i • ao i"* 'Cf o t tH o O o CO CM CM CM o ns •r— U> CM CM z —1 C_> CQ O Pc

as

'ns>

•r—3

CT

<1)

a> O•r—Xo

ini

</) 3

CCCo

•1“ p

nsSE3

CO

OS

15
the bulk sample. The decrease in NaB0

2 pressure with time results from

the significant depletion of the bulk B^^ concentration due to metaborate

vaporization. This observation further confirms that Na vaporization

occurs as the metaborate since the B 20 .j concentration is reduced much

more than the Na

2

0 content. The partial pressure curve shown in figure 5

results from a least squares analysis of the runs one to four with the

anomalous three lowest temperature data points from run 1 omitted. The

pressure of NaB0

2 (g) over pure NaB02 (£) obtained from the JANAF [1971]

tabulation is also given in figure 5 for comparison. From the ratio of

pressures from these curves, the activity of NaB0

2

in the simulated

nuclear glass is approximately 0.01 with a slight temperature dependence.

The vaporization curves for LiB0

2

and CsB0

2 species are given

in figure 6. A least squares curve through the Li B0

2

data was obtained

from the run 1-4 data neglecting the two lowest temperature points of

run 1. Comparing this curve with that for pure LiB0

2 (£), obtained from

JANAF [1971], indicates that the activity of LiB0

2

in the SNW glass is

about 0.1, with some temperature dependence. The CsB0

2

data in figure 6

were obtained in series I, runs 4 and 5. We did not observe Cs

+ until

the fourth run in this series and it is likely that the sample was

somewhat depleted in CsB0

2 prior to the measurements. This depletion

a effect is very apparent at 10 /T = 7.50 (see fig. 6). The least squares

curve through these data is based on the four lowest temperature points

obtained from run 4. At 1200 K, the activity of CsB0

2

in the melt is

about 4 x 10

6 based on the CsB0

2 pressure over pure CsB0

2 as given by

Gurvich and Veits [1983].

Figure 7 shows the observed 0

2 pressure versus temperature

data. The least squares curve was obtained by neglecting the five lowest

16
temperature points. From the reasonable comparison with the literature

curve [JANAF, 1971] for reduction of Fe

2

C> 3(c) to Fe^O^Cc) it appears

likely that reduction of Fe20 3

(in solution) to Fe^O^On solution) is a

primary source of over the SNW glass.

Similar data were obtained for the series II experiments, as

shown in figures 8 to 10. In these figures, the series II data points

are compared with the least square curves from series I (except for 0

2

)

to indicate the degree of experimental reproducibility for separate

glass samples. While the O

2

data for the two series, as given in

figures 7 and 10, are similar, an exact comparison is meaningless owing

to the variation of Fe oxidation state with time, i.e., run chronology.

Note in figure 8 that the run 1 data show the characteristic

high NaB02 pressures at low temperatures. The positive deviation (from

the least squares curves) in pressures during the initial heating is

characteristic of all the data. During the high temperature component

of run 2 an instability was observed in the Na

+ signal. This effect

was characterized by signal increases of a factor of two, or more, for

short periods of time (tens of seconds) but would return to the original

level. These instabilities could be associated with the concomitant

release of a noncondensible gas from the sample as was indicated by

pressure increases on the ionization gauge. Note however, that the run

3 data was well behaved and lies quite close to the least squares line

from the series I data. Also, the LiBt^ data shown in figure 9 agree

well with the series I least squares curve.

The data comparison in figure 10 for CsB0

2

indicates the

series II data to be about a factor of four greater than those based on

the least squares line from the series I data. Since the amount of

17
CsB0

2

calculated to have been lost from the sample is slightly greater

than the amount originally present (see Table 3) we conclude that the

Series II CsB0

2 pressures are anomalously high. However, the series I

data are probably too low because the Cs

+ data was not observed until

runs 4 and 5. Hence we consider an average of these two series to best

represent the CsB0

2 partial pressure data. This procedure increases

the pressure from the series I data by a factor of two and the accepted

curve is shown as the dashed line in figures 6 and 10. This curve also

agrees well with that obtained from the TMS measurements (see Section 3).

2.5.2 Search for Ru, Re, and Sr-containing Species

Attempts were made during the vacuum vaporization experiments

to detect the ions Ru+

, Ru0

+

, Ru0

3

+

, RuO^

+

, and Re+, Re0

+

, ReQ

2

+

,

Re0

2

+

, and Re

20y

+ without success, even at temperatures as high as

1500 °C. No attempt was made to observe Sr0

+ or Sr+

, based on thermodynamic arguments presented in Section 4.1.

From the thermodynamic arguments presented in Section 4.1,

NaReO^ is indicated as the most likely vapor form of Re. Also, this

species is likely to have a sufficiently high partial pressure to

completely remove Re from the glass during the vacuum vaporization

measurements. However, the NaReO^ species fragments extensively on

electron impact to form Na

+ and parent ions form only a small fraction

of the total ion production. The small Na

+ ion signal resulting from

fragmentation of NaReO^ would be completely masked by the large Na

+

signal resulting from fragmentation of NaB0

2

- A similar problem exists

for fragmentation of LiReO^.

18
The thermodynamic analysis given in Section 4.1 for Ru vaporization indicates RuOg and RuO^ as the likely vapor species but their

partial pressures are predicted to be below the KMS detection limit.

2.6 Glass Vaporization in Atmosphere

Volatile hydroxide species are known for all of the pertinent

oxides in the SNW glass system, e.g. , see JANAF [1971], Jackson [1971],

and Hastie [1975], However, the extent to which water vapor will react

with these oxides, in the reduced activity glass form, to yield hydroxide

vapor species cannot be accurately predicted from existing thermodynamic

data. Also, our previous experience with reactive gas-glass systems

indicates that thermodynamic equilibrium may not be attained [Hastie,

et al. 1982]. Hence, gas inlet KMS (GKMS) experiments were carried out

to determine if water vapor would enhance the vapor transport of the

-4

SNW glass. Water vapor pressures up to about 10 atm were used for

these measurements.

2.6.1 H

2

O-B

2

O

2

Test Case

The GKMS approach was tested using the well known H20-B20

3

reaction system (JANAF, 1971). Prior to H

2

O introduction into the

cell, containing pure the mass spectrometer sensitivity calibration

was checked using the known (JANAF, 1971) vapor pressure of B20

3 (g)

over B20

3 (£). Our results agreed to within about 20 percent of the

JANAF [1971] evaluation of previous literature data.

19
(2.7)

In the presence of h^O, the principal reaction is

^B

2

0

3 U) + *sH 20(g) = HB0

2 (g) .

The degree of reaction was monitored mass spectrometrical ly using the

H

2

0+ and HB0

2

+ ions as indicators of the H

2

0 and HB0

2 partial pressures,

respectively. Isothermal measurements of the dependence of the HB0

2

+

ion intensity on that for H

2

0+

, with variable H^O pressure, were used

to demonstrate attainment of a steady state, as shown in figure 11.

The experimental slope of 1.1 ± 0.1 compares favorably with the theoretical slope of 1.0.

Equilibrium constant data for reaction (2.7) are shown in

figure 12. The experimental data compare favorably with the JANAF

[1971] curve, which is based on the mass spectrometric results of

Meschi et al. [I960]. For this comparison, we used the same ionization

cross section estimates as Meschi et al. [I960].

2.6.2 H 2Q-Glass Experiments

Experiments analogous to the B 20

3

-H

2

0 test case gave negative

results and the thermochemical calculations presented in Section 4

indicate that any hydroxide species formed are likely to be below the

instrumental detection limit. It is also possible that the water-SNW

glass system does not readily attain equilibrium, as we have found in

other glass studies (Hastie et al., 1982).

20
3. TRANSPIRATION MASS SPECTROMETRY

3.1 Basis of Technique

The Transpiration Mass Spectrometry (TMS) technique has been

described in detail elsewhere [Bonnell and Hastie, 1979]. Basically,

the method combines the high pressure sampling mass spectrometry and

classical transpiration techniques. High pressure sampling is accomplished

by free-jet expansion from a static region into a region of high pumping

speed [Kantrowitz and Grey, 1951]. This is the only technique with the

potential for preserving the chemical integrity of the gas sample

during the expansion and molecular beam-forming process. The free-jet

expansion occurs through a small orifice (diameter range, 0.008 to

0.03 cm) of controlled geometry into a region of low pressure. During

this expansion process, most of the internal thermal energy converts to

translational energy, i.e., to directed motion along the jet axis. For

very high expansion ratios, this process occurs on a shorter time scale

than for chemical reaction rates, thus generating a beam free from

intermolecular interaction. This chemically "frozen" beam can then be

conveniently analyzed by mass spectrometry.

Analysis of the gas dynamic aspects of the TMS process yields

a simple relationship between the mass spectrometric ion intensities

and species partial pressures. This relationship has the same form as

equation (2.2) used in Knudsen effusion mass spectrometry. However,

with TMS the sensitivity constant k has a slight temperature dependence

and an internal calibration is made at each data point using the carrier

gas signal and pressure for reference. Use of the classical transpiration

21
equation can also be made for partial pressure measurement once the

realtive concentrations of different vapor species is established.

The primary equation for transpiration is

^ = [n./(2n. + lnc

)] P t (3.1)

where , for species i and carrier gas c, Pi is the partial pressure, n

the number of moles, and P. the system total pressure (*>* P ).

L> L An upper pressure limit to TMS is determined mainly by experimental limitations in maintaining vacuum in the expansion chamber. We

have used the technique to 10 atm but the normal optimum pressure range

-4

is in the region 0.1 to 0.5 atm. Thus TMS extends the 10 atm limit

of KMS by many orders of magnitude and allows for higher temperature

and reactive gas pressure studies. Typical gas residence times for TMS

and KMS are 20 s and 0.04 s, respectively. This difference is particularly

useful in revealing nonequilibrium phenomena by comparisons of TMS and

KMS data.

3.2 Apparatus

The basic arrangement of the High Pressure Mass Spectrometer (HPMS)

system with the transpiration inlet system is shown in figure 13. The

vacuum system is aluminum walled and is partitioned into two differentially pumped stages. Stage I contains the transpiration reactor.

This stage is pumped by a 6 inch diffusion pump (Varian/NRC VHS-6) with

a Granvi lle-Phi Hips model 270-6 high-throughput cold trap close-coupled

to the base of the vacuum chamber for maximum conductance. The backing

pump system for this stage consists of a Roots blower (Leybold-Hereaus

22
model WA 250) placed in series with a two stage rotary vane pump (SargentWelch 1375) to maintain the critical backing pressure of the diffusion

pump in the presence of high gas loads. This arrangement assures that

the diffusion pump stack will continue to pump at near its rated speed

~5

into the 10 atm range. A single- axis adjustable combination valve/

shutter separates the two vacuum chambers. The differential pump

aperture used is 0.05 cm diameter, and is located approximately 4 cm

from the sampler orifice. This aperture is located in a shutter which

also acts as a stage isolation valve.

A tantalum foil (0.005 cm) radiation furnace, driven at AC currents

to 400 amperes, provides heat to the transpiration sampler. With a new

element, temperatures as high as 1700 K can be achieved in the sampler.

Adjustable passive tantalum radiation shields are used to maintain a

near constant temperature profile in the boat to skimmer region. The

furnace region is surrounded by a water-cooled copper shroud to prevent

condensibles (oil, high temperature species, etc.) from entering the

mass spectrometer chamber.

In the Stage II mass spectrometer section, a high vacuum is maintained by a 4-inch diffusion pump stack similar to the 6-inch stack of

stage I, but without the Roots blower. Full -load vacuum in this stage

-9

is better than 5 x 10 atm. The beam modulation chopping system is

also located in this chamber. A 24 tooth wheel driven by a degassed ac

syncronous motor is used to provide variable frequency beam modulation.

Usually, chopping frequencies of about 700 hertz are chosen to minimize

syncronous interference at the 60 hertz line frequency and to provide

species sensitive phase information. The mass spectrometer used is an

Extranuclear Laboratories Component quadrupole system, based on an EL

23
model 270-9 quadrupole mass filter, a crossed beam configuration EL ion

source (Model 041-1) —modified to improve isolation, collimation and

ionizing energy spread—and a Galileo Electro-Optics model 4801 channel

tron multiplier. The mass spectrometer assembly is mounted on a semikinetic 3 point mount which allows for three axis alignment of the

ionizing region with the molecular beam.

Figure 14 shows the essential details of the transpiration reactor

with its removable boat-carrier assembly. With the exception of the

thermocouple alumina insulator, which is located upstream from the gas

baffle to minimize reaction with the sample vapor, the entire assembly

is constructed of platinum metal. For glass sampling, the conical

orifice and skimmer assembly shown have been replaced with a more

robust capillary nozzle detailed in figure 15. A typical temperature

profile within the sampler is shown to scale with the sampler. The

capillary sampler has proven to be as faithful a beam generation device

as the more traditional conical orifice [Bonnell and Hastie, 1979].

For preparation and control of gas mixtures, we use a group of

Tylan Corp. mass flow meters and controllers (FM360 and FC260, respectively) attached to an NBS-designed backflow protected mixing manifold.

Individual range selection allows gas mixtures to be prepared at room

-6

temperature with the reactive gas mole fractions varying from 10 to

10

1

. For water vapor additions the dry mixed gas is fed to commercial

water calibrators (either Ondyne models 10 or 14). The water content

of the prepared mixture is monitored by two high sensitivity aluminum

oxide/gold hygrometers (Ondyne model DY143T/model 1400) with a calibrated

accuracy of better than ± 2 °C (about ± 20% in vapor pressure). Total

system pressure is monitored at the gas inlet to the sampler mount

24
using a Rosemount model 1332A4 capacitance manometer transducer excited

and read by an NBS built readout system.

3.3 Instrument Calibration

Calibration of the TMS system is, for the most part, an analogous

process to that used for the KMS system as described in Section 2.3.

For calibration of mass discrimination effects an identical procedure

to that for KMS was used. To convert ion intensity and mass- loss data

to partial pressures using the transpiration equation (3.1), the following

calibration procedures are necessary.

Calibration of the system pressure gauge [P

t

in eq. (3.1)] is

accomplished by comparison with a known standard gauge, and is ©

reproducible to better than 0.003 atm. The term in moles of carrier

gas [n

£

in eq. (3.1)] is determined by direct measurement of the mass

flow of each gaseous component, and a separate measurement of the total

mass flow into the system. Several independent calibration methods are

used for the flow meters and controllers: (1) comparison with previously

calibrated mass flow meters; (2) direct volumetric calibration against

a well characterized "wet-test" meter; (3) direct volumetric calibration

of flow into a mercury- sealed piston gas burette; and (4) direct volumetric collection over saturated fluid. These various techniques

suffice to assure reproducible flow measurements to within the accuracy

specifications of the Tylan meters used, i.e., well within 1% of full

scale. Most mixtures are made using at least 10% of full scale, hence

component composition and n c are known to within a few percent.

25
For vapors containing more than a single type of molecular species,

such as those over the present glass system, a partitioning of the

total mass transport is needed. This partitioning is achieved by

integration of the observed ion intensities with the constraint that

the sum of individual species mass transport equals the gravimetric

total mass loss. This procedure leads to n. values for use in eq. (3.1).

The individual species molar transport partitions q. are given by

where, for species i,

q. is proportional to the number of moles of species i transported

during an entire series of experiments;

Aq

n

- (T) is an individual species molar partition at temperature T;

C. is the factor, (cr x )/(a.x.), which accounts for relative transmission

i cc ii

and ion cross sections effects, where ct is the total ionization

cross section (where fragment ions are involved, direct summation

or average ion ratios are used to allow use of total ionization

cross sections) and x is the mass dependent relative transmission

factor for the quadrupole mass spectrometer and gas expansion system;

R is the universal gas constant 82.057 (cm atm/mol K);

fc is the product of isotopic fraction and gas fraction of component c

in the gas mixture, i.e. the actual fraction of gas observed at

mass position c;

I is the ion current of carrier gas component c—this creates approximate correspondence between q and n, and normalizes beam characteristics which change with temperature and pressure;

i T \

(3.2)

26
I. is the species ion current, appropriately converted (via fragment

ion summation, or scaling by ion ratios) to a total ion current

for species i;

V

s is the flow rate in standard ( P=1 atm, T = 273.15 K) cubic centimeters of total carrier gas per minute (seem);

273 is 1 atm/273.15 K, the reference condition for flow calibration;

At(T) is the elapsed time in minutes for a run segment at constant

temperature, T.

In eq. (3.2), each variable is considered a function of the system

temperature, T, and the summation is carried out over every observed

temperature point. When the total mass loss of the system is measured

at the conclusion of a series, the above partitions can be converted to

pressure by computing a proportionality factor, k, from the equation:

weight loss (gms) = k 2^ q^'M- (3.3)

where is the species molecular weight and k the factor which yields

actual moles from the q^ obtained from ion current measurements.

Individual values of An.(T) are obtained by multiplying the calculated

Aq.j (T) of eq. (3.2) by k. These values are then used in eq. (3.1) to

compute P . (T) from the ion intensity observations.

A second approach to converting ion intensity to partial pressure

data is used when relative ionization cross sections are considered

more reliable than total mass loss data. Here, eq. (2.2) is used but

with k^ measured at every data point using the relationship

27
where S is an apparatus dependent small correction factor (= 0.6) which

results from scattered carrier gas reentering the molecular beam [Bonnell

and Hastie, 1979]. For the present studies, where the glass vapor mass

spectra were relatively complex, preference was given to the calibration

approach based on equations (3.1) to (3.3).

3.4 Detection limits

TMS detection limits are strongly controlled by beam formation

characteristics and the amount of background gas present in the beamforming and mass spectrometer chambers. The beam, as formed, is narrowly

collimated by Mach focussing and by restrictive orifices. However, the

beam formation process leads to a large increase in background pressure

_g _ o in the mass spectrometer chamber, generally in the 10 to 10 atm

range. Beam spread during transit to the mass spectrometer reduces the

12

total beam number density at the ion source to the order of 10 particles

3 cm , about four orders of magnitude above the background number density.

Modulation of the beam allows reliable detection of signals two to

three orders of magnitude below prevailing background DC signals. In

favorable cases, this leads to detection limits of approximately 0.3 to

1 ppm (relative to 1 atm). In unfavorable cases, e.g. , at mass positions

corresponding to scattered carrier gas components or strong hydrocarbon

background levels, detection limits lie in the 10 to 100 ppm range.

Mass positions coincident with major carrier gas isotopes are totally

masked. However, carrier gas selection is made to avoid mass coincidence

with minor species of interest. Mass positions adjacent to the major

carrier gas isotopes usually evidence some residual modulation due to

28
incomplete resolution. For instance, ppm detection at 27 amu requires

a resolution with better than 0.001% cross talk in the presence of

28 amu N

2

carrier gas. In the present study, unit resolution conditions

were used over the whole mass range, with adjacent peak cross talk of

less than 0.1 percent.

In practice, sensitivity limits can be reliably estimated from the

magnitudes of DC ion intensity and non-phase-locked AC signals. For

the results presented here (Section 3.5), the lowest partial pressure

values represent signal-to-noise (or background) levels in the range 2

to 5. At the higher partial pressures given these noise levels are an

insignificant source of error.

3.5 Glass Vaporization in N

2 Atmosphere

TMS measurements of SNW glass vaporization were made using ultra

high purity N

2

carrier gas (at 0.3 atm) and higher temperatures than

those for the KMS vacuum vaporization experiments. The mass spectral

features were generally similar to those found by KMS, as outlined in

Section 2.5. Several distinctly new features were also noted and these

can be attributed to the beam isentropic cooling and reduced mass

transport effect characteristic of the TMS method. As we have noted in

earlier studies [Bonnell and Hastie, 1979], the distribution of parent

to fragment ion intensities can differ between KMS and TMS studies due

to beam cooling with the latter technique. Thus in the present study

we observe, for instance, Li +

(7 amu)/LiB0

2

+ (50 amu) ratios of 30 for

TMS as compared with 9 for KMS. However, the molecular precursor

assignment, as LiB0

2

, and the value of the total ionization cross

section are not influenced by this effect.

29
The mass transport rates characteristic of TMS are much lower than

for KMS and this leads to differences in the early vaporization stage

of a new glass sample, as noted elsewhere [Hastie et al., 1982].

Examples of such effects, which are not characteristic of the bulk

glass properties but rather of impurities, are revealed in figures 16

through 18. Note in figure 16 that the Na /NaB0

2

ratio varies significantly with temperature and time. If NaB0

2 was the sole precursor of

Na

+

, this ratio would be invariant with temperature and time. The

results can be analyzed in terms of two additional sources of Na as

manifested in the Na

+ signal. During the early stages of vaporization,

as represented by the Na+/NaB0

2

+ data points at 1400 to 1500 K in

figure 16 and the Na atom data points at 1300 to 1500 K in figure 17,

most of the observed Na+ results from Na. The most likely source of

this Na is from decomposition of residual Na

2

C0

3

, i.e.,

Na

2

C0

3 U) = 2Na + C0

2 + h02

.

Observation of C0

2

and 0

2 during this period supports this argument as

shown in figure 18. We have shown in earlier silicate glass vaporization

studies that this effect results from a finite Na

2

C0

3 solubility in

glass which results during glass preparation with alkali carbonate

reactants [Hastie et al .

,

1982]. At the higher temperatures and later

vaporization times, when the carbonate impurity is essentially depleted,

a second Na-producing reaction occurs,

Na

20 (in solution) = 2Na + k02

-

The curve labeled Na

2Q in figure 17 shows the Na pressure determined

for this process. No analogous processes were noted with the Li data,

i.e.

,

both Li + and LiB0

2

+ are attributed to a single precursor, LiB0

2

.

30
The observed borate ion signals were corrected for fragmentation

to alkali ions by assuming that the minimum fragmentation ratio (see

fig. 16) corresponded to the borate fragmentation process alone. These

ratios were corrected for isotopic abundance and the borate ion intensities multiplied by the corrected ratio to obtain a total borate ion

signal. As was the case with the KMS results, the Cs + precursor is

assigned as CsBC^. In order to convert ion current to partial pressure

data, ionization cross section corrections for the relative efficiency

of ionizing various species must be made. For the alkali metaborates,

which are the major vapor transport species, an estimated a. value of

2 2.4 was used [units 7ia Q

, see Bonnell and Hastie, 1979]. As noted in

the KMS studies, considerable fragmentation occured even at 30 eV

ionizing energy.

The alkali metaborate partial pressures obtained by this analysis

are summarized in figures 17 and 19. Note in figure 19, the excellent

agreement between the TMS and extrapolated KMS data for each metaborate

species. Such agreement supports the TMS analysis of the Na

+ signal in

terms of the three competing processes. Also, the linear fit to both

the TMS and KMS data sets is consistent with the thermodynamic basis of

these Clausius-Clapeyron type plots and allows for accurate extrapolation

to temperatures beyond the experimental results. The agreement between

the TMS and KMS data is also good evidence of thermodynamic equilibrium

and of the absence of surface segregation effects. These arguments are

based on the different gas residence times and material transport rates

characteristic of the two techniques, as discussed in Section 3.1.

Surface segregation of volatile species have been reported in a sodium

borosilicate melt [Wenzel and Sanders, 1982].

31
It is conceivable that the Cs

+ signal may also result from several

processes analogous to those for Na

+

. However, at the relatively low

temperatures of the KMS experiments no evidence for additional sources

of Cs

+ was found. Also, the good agreement between the KMS extrapolated

CsB0

2

curve and the higher temperature TMS data is reasonable evidence

of the absence of competing reactions leading to additional Cs + signals.

It is interesting to note, though thermodynamically fortuitous,

that the CsB0

2 partial pressures of SNW glass are similar to those for

Cs over SYNROC (Ba

Q g

Cs

Q ^

Al

2 Tig O^g • Ti02

) as reported very recently

by Carpenter, et al. [1983]. Also, the value of P(CsB0

2

) at 1075 °C

lies between that of P(Cs) over the minerals CsAlSiO^ and CsAlSi

20g

[Odoj et al. 1979].

3.6 Glass Vaporization in H 2

0-0

2

-C0

2

-N

2 Atmosphere

In the industrial glass process the off-gas contains components

which are potentially reactive with the glass constituents. These

include, with mole fractions in parentheses, H

2

0 (0.85), 0

2 (0.02), CO

(0.008), H

2 (0.008), C0

2 (0.01), and minor amounts of N- and S-containing

species. This room temperature composition is clearly nonequil ibrated.

At high temperatures we need only specify the H 20, 0

2

, and C0

2

concentration at thermodynamic equilibrium. This result can be seen in table 4

where the calculated composition of an equilibrated off-gas mixture is

given as a function of temperature. Note that at a representative

temperature of 1250 K, C0

2

, 0

2

, H

20, and N2 are the only thermodynamically significant components.

32
Table 4

Calculated Equilibrium Composition (mole fraction)

of Off-gas Mixture at 1 atm

a

Input Composition (moles)*

3

:

H

2

0 (0.8555), N

2 (0.09023), C02 (0.01244),

0

£ (0.02378), CO (0.008359), H£ (0.007588).

Temperature K

2500 2000 1500 1250 1000 333

CO 3. 77-3C 2.09-4 7.80-7 8.59-9 9.69-12 0.00

C0

2

1.66-2 2.07-2 2.09-2 2.09-2 2.09-2 2.09-2

H

2

2.99-2 1.92-3 1.26-5 2.29-7 5.81-10 0.00

h

20(L) 0.00 0.00 0.00 0.00 0.00 8.39-1

h

2o 8.01-1 8.64-1 8.69-1 8.69-1 8.70-1 3.09-2

°2 2.56-2 1.67-2 1.68-2 1.69-2 1.69-2 1.68-2

N

2

8.79-2 9.12-2 9.18-2 9.18-2 9.18-2 9.19-2

NO 2.81-3 7.81-4 1.28-4 3.00-5 3.41-6 1.19-15

N0

2

7.97-7 3.55-7 1.85-7 1.10-7 5.12-8 5.39-11

Calculated using the NASA CEC 76 computer program of Gordon and

.McBride [1971].

Cased on Dupont analysis of off-gas composition at room temperature

[Soper, 1982]. _ 3

Computer notation, 3.77-3 = 3.77 x 10 mole fraction.

33
The following mole- fraction mixture was used in the TMS experiments

to approximate the off-gas reactivity: N

2 (0.67), 02 (0.18), C0

2

(0.15), and H

2

0 (0.1). The TMS results with this gas mixture indicated

no significant enhancement of alkali vapor transport, due to hydroxide

formation, with detection limits of 5 ppm for Li OH and 20 ppm for NaOH.

The higher NaOH detection limit, resulted from the unavoidable presence

of Ar impurity (also 40 amu) in the gas mixture. Vaporization of the

lithium and sodium metaborates was essentially the same as for the pure

N

2

runs, to within the mass spectrometric reproducibility of ca. 20%.

No increase in either sodium or lithium ion currents attributable to

fragmentation from a hydroxide was found. From our previous experience

with TMS studies of alkali hydroxides [Hastie, et al., 1982, 1983],

only moderate fragmentation occurs, and the detection limits at the

parent ion can be considered reliable. -

With these reactive atmosphere conditions, a higher than normal

hydrocarbon background was present, particularly at high temperature.

This background raised detection limits by as much as an order of

magnitude at the Cs+ion mass position. Some suggestion of an atmosphere

induced increase in Cs + intensity was noted, but the results are considered inconclusive due to the increased background level. This hydrocarbon

background could have originated as a low level relatively tightly

bound constituent of the glass, or, as an abnormal apparatus background.

This TMS observation of alkali vapor transport insensitivity to

the presence of H

2

0 vapor is consistent with a similar observation from

the KMS experiments (Section 2.6). Previous experiments on a sodium

borosilicate glass [Wenzel and Sanders, 1982] and various sodium,

potassium borosilicate glasses [Gray, 1980], also showed an unexpected

non-effect of H

2

0 on alkali vapor transport.

34
4. Thermodynamic Estimates

4.1 Glass Vaporization in Inert Atmospheres

The only radionuclide element observed by the present mass spectrometric studies is Cs, present in the vapor as CsB0

2

- From the mass

spectrometric detection limits, partial pressure limits for the other

radionuclides at temperatures as high as 1500 °C are in the region of

10

7

atm with an inert atmosphere and 10 ® atm with H

2

0 or the reactive

gas mixture present. Using the experimental activity results for the

various glass components, together with auxiliary thermodynamic data

from the literature, it is possible to calculate approximate radionuclide

concentrations in the vapor phase, as .follows.

4.1.1 Cs

As this radionuclide was observed experimentally, the data

can be used to indicate the quality of the thermodynamic estimates.

The KMS results indicate the activity of CsB0

2 as

a(CsB0

2

) obs = 4 x 10” 6

at 1200 K.

If we assume that Cs is present primarily as CsB0

2 (£) in the glass,

and that this component mixes ideally, then from the glass composition

(see Table 1) the activity would be

a< CsB°

2 )calc ' 4 x 10

' 4

On this basis, the vapor phase concentration of Cs (as CsB0

2

) would be

two orders of magnitude higher than observed, i.e.

P(CsB0

2

) calc ~

10~ 6

atm at 1200 K

35
whereas

P(CsB0

2

) obs

lo"

8

atm at 1200 K.

4.1.2 Sr

An upper limit vapor concentration can be calculated assuming

ideal mixing of SrO (£) in the glass. Thus the activity of SrO (£) is

0.0006 and, from JANAF [1971],

P(Sr0) < 1.4 x

10' 11

atm at 1800 K.

-5

Also, in the presence of 1 x 10 atm 0

2

, as produced by Fe

2

0

3 decomposi

tion (see Section 2.5), we calculate

P(Sr) < 3 x 10" 12atm at 1800 K.

4.1.3 Re -

The compound Re^ has a . boiling point of 635.5 K [Wicks et

al. 1963]. If we assume that Re

2

0

7

is dissolved in the glass (with a

mole fraction of 0.0001) then the vapor pressure at this temperature

-4

would be 1 x 10 atm. Hence, Re^ would be lost from the glass

during the initial stage of an experiment. It seems more reasonable

that Re would be incorporated in the glass as Re0

2

, ReO^ or, more

likely, as LiReO^ or NaReO^. In the case of LiReO^ and NaReO^, the

Re

20^ activity would be reduced far below the value given by the formal

mole fraction.

Battles et al . [1968] have published definitive studies on

Re0

2

and ReO^ vaporization processes. Both of these phases vaporize to

36
produce a gas containing predominantly Re

2

0

y

. From their vapor pressure

curve for the reaction

7/2 Re0

2

(s) = 3/2 Re(s) + Re

2

0

y (g) (4.1)

an extrapolated vapor pressure at 1500 K is 7.46 atm. As an approximation, we can neglect the thermodynamic difference between Re0

2

(s)

and Re0

2 (£). Taking the mole fraction of Re0

2

in the solution to be

0.0002 and the activity of Re(s) equal to unity, the calculated partial

-13

pressure of Re

2

0

y

at 1200 K is 7.31 x 10 atm. This pressure could

be increased significantly if Re dissolves in the glass melt thereby

reducing the activity significantly below unity.

The vaporization behavior of NaReO^ has been established by

mass spectrometry (Skudlarski et al., 1967). This salt evaporates to

produce NaReO^ monomers as well as dimers in the temperature range 700

to 800 K. Extrapolation of the monomer data to 1200 K and using 0.0002

as the mole fraction of NaReO^, the calculated pressure is

P(NaRe0

4

) ^ 1.5 x 10* 7

atm at 1200 K.

4.1.4 Ru

For condensed Ru0

2

in the presence of 0

2

the important vapor

species are RuO^ and RuO^, as shown by Schafer et al. (1963). From

their vapor pressure equations, with an Ru0

2

mole fraction of 0.0005,

-5

an oxygen pressure of 10 atm and a temperature of 1500 K, the predicted

-9

partial pressure of RuO^ is 5.3 x 10 atm. Similarly, the calculated

-12

partial pressure of RuO^ is 3.1 x 10 atm. Thus vapor transport of

Ru under the KMS experimental conditions is predicted to be undetectable.

37
However, for higher 0

2 pressures, such as the likely off-gas value of

0.02 atm, the partial pressures of Ru0

3

and RuO^ are calculated to be

-7 -9

2.35 x 10 and 6.13 x 10 atm, respectively. Thus under melt conditions

most of the Ru vapor transport would be due to loss via Ru0

3

.

Results of these thermodynamic estimates are summarized in

figure 20.

4.2 Glass Vaporization in H 2

0-0

2 Atmospheres

Within the experimental detection limits of KMS and TMS, the

presence of H

2

0 or 0

2

had no effect on the SNW glass vaporization

process. Thermodynamic estimates of the likely influence of these

reactive gases on the glass vaporization are as follows. Unless indicated otherwise, the thermodynamic tables for hydroxide vapor species

given by Jackson [1971] and the JANAF [1971] tables have been used in

the following calculations. Also, to simulate the likely off-gas

composition we set P(H

20) =0.1 atm and P(0

2

) = 0.02 atm.

4.2.1. Cs

The major influence of H

2

0 on Cs vapor transport can be

expected to result from the reactions

MB0

2 U) + H 20(g) = M0H(g) + HB0

2 (g),

where M is Li, Na, or Cs. We can show thermodynamically that the Li

case greatly outweighs the Na and Cs cases. Hence the Li reaction can

be used to fix the system HB0

2 pressure. Using this pressure, the

38
experimental activity of CsB0

2 (£), the literature thermodynamic functions

for CsBO^C^) [Gurvich and Veits, 1983], and the reactant thermodynamic

functions, we calculate P(CsOH) data as given in figure 21.

4.2.2 Sr

The most volatile strontium hydroxide is Sr(0H)

2

. For the assigned

water vapor pressure of 0.1 atm, and an Sr0(£) mole fraction of 0.0006,

-9

the calculated Sr(0H)

2 partial pressure at 1500 °C is 3.5 x 10 atm.

In this calculation it was assumed that a negligible difference would

result from assuming that the properties of Sr0(£) are the same as

Sr0(s) at the reduced activity. Partial pressures of Sr(0H)

2 as a

function of temperature are summarized in figure 21.

4.2.3 Re

Calculated partial pressures for the HReO^ vapor species,

from NaRe0^(£) with an activity of 0.0002, are given in figure 21. The

partial pressure of product NaOH was fixed by the reaction

NaB0

2 U) + H2

0 = NaOH + HB0

2 ,

shown in figure 21.

4.2.4 Ru

The Ru( 0H)

2 species is predicted to be the main hydroxide of

Ru under the assigned conditions. However, as indicated in figure 21,

the calculated partial pressures are extremely small. The possible

39
existence of higher oxidation state, more volatile, hydroxide species

should not be discounted as the thermodynamic literature is sparse in

this area.

5. Conclusions

The KMS and TMS experiments provide complementary, self consistent

data for the major volatile species, NaBC^, LiBt^, and CsBO^ as summarized

in figure 19. Estimates based on thermodynamic arguments for the most

likely oxide forms for the other radionuclides of interest are given in

figure 20. Note that the NaReOd species may be even more important

than CsB02- Both Ru and certainly Sr are indicated to have very low

oxide volatility. In the presence of H^O and O

2 a number of additional

hydroxide species are predicted to be significant as shown in figure 21.

However, the partial pressures of these species are still less than

those of the oxide forms. As an interim conclusion from these observations, it appears likely that the glass off-gas atmosphere is of secondary

importance to the condensed phase composition in controlling vapor

transport. In view of the importance of metaborate vapor species, the

concentration or, more particularly, the thermodynamic activity of

boron oxide in the glass is of key significance.

The thermodynamic predictions of figures 20 and 21 should be

refined in future studies to include

0 competition between competing reactions

0 effects of departure from ideal mixing

This refinement will need to incorporate direct experimental observations

of radionuclide species. Figures 20 and 21 provide a useful guide to

40
the conditions needed in future mass spectrometric studies. Increased

glass doping concentration levels of the radionuclide isotopes would

increase the oxide and hydroxide partial pressures to well above the

mass spectrometric detection limits. From the radionuclide partial

pressure variation with glass concentration extrapolation to the composition of the present glass sample would be possible. Application of

the ideal mixing of complex phases solution model [Hastie et al . , 1981,

1983] to generalized predictions of SNW glass volatility and phase

behavior should also be investigated.

6. Acknowledgments

A significant portion of this work was supported by contract with

DuPont Atomic Energy Division, Savannah River Laboratory. The glass

sample used was prepared and analyzed by DuPont. Discussions with

Dr. Paul D. Soper of DuPont are acknowledged. Mr. Art Sessoms provided

valuable technical assistance to the project.

41
7. References

Battles, J. E. , Gunderson, G. E. , and Edwards, R. K. (1968). J. Phys.

Chem. 72, 3963.

Bonnell, D. W. and Hastie, J. W. (1979). "Transpiration Mass Spectrometry

of High Temperature Vapors," in Characterization of High Temperature

Vapors and Gases , Hastie, J. W. , ed. , NBS-SP 561, p. 357, U.S. Government

Printing Office.

Cable, M. (1978). Materials Science Research 12, 399.

Carpenter, J. H. , McMullen, J. C. , Olmscheid, B. A., Chezick, P. A.,

and 01 ig, C. (1983). "Cesium Vaporization Losses from the SYNROC Mineral

Barium, Cesium-Hoi landite," Proc. Second Int. Conf. Ceramics in Nuclear

Waste Management, Am. Cer. Soc. , in press.

Gordon, S. , and McBride, B. J. (1971). "Computer Program for Calculation

of Complex Equilibrium Compositions, Rocket Performance, Incident and

Reflected Shocks, and Chapman-Jouguet Detonations," (NASA Lewis Research

Center, Cleveland, OH) NASA SP-273.

Gorokhov, L. N. , Gusarov, A. V., Makarov, A. V., and Nikitin, 0. T. (1971). Teplofiz. Vysok. Temp. 9, 1082. (English Trans.).

Gray, W. J. (1980). Radioactive Waste Management 1, 147.

Gurvich, L. V., and Veits, I. (1983). Thermodynamic Properties of

Individual Substances, Vol. 4, Book 1, p. 525.

Hastie, J. W. (1975). High Temperature Vapors, Academic Press, NY.

Hastie, J. W. , Horton, W. S. , Plante, E. R. , and Bonnell, D. W. (1981).

"Thermodynamic Models of Alkali Vapor Transport in Silicate Systems,

IUPAC Conf., Chemistry of Materials at High Temperatures, Harwell,

U.K. , August, 1981; High Temp. High Press, in press.

Hastie, J. W. , Plante, E. R. , and Bonnell, D. W. (1982). "Alkali Vapor

Transport in Coal Conversion and Combustion Systems," in Metal Bondi ng

and Interactions in High Temperature Systems with Emphasi s on A1 kal

Metals, ACS Symp. 179, p. 543, American Chemical Society, Washington,

DC.

Hastie, J. W. , Bonnell, D. W. , Plante, E. R. , and Horton, W. S. (1983).

"Thermodynamic Activity and Vapor Pressure Models for Silicate Systems

Including Coal Slags," in Thermochemistry Today and Its Role in the

Immediate Future, M. A. V. Ribeiro da Silva, Ed., Reidel Publ. Boston,

in press.

Jackson, D. D. (1971). Thermodynamics of Gaseous Hydroxides, UCRL-51137.

JANAF (1974). Joint Army, Navy, Air Force Thermochemical Tables, 2nd

ed. , NSRDS-NBS 37, U.S. Government Printing Office, Washington, DC.

See also later supplements for 1971-1982.

42
Kantrowitz, A. and Grey, J. (1951). Rev. Sci. Instru. 22, 329.

Meschi , D. J. , Chupka, W. A., and Berkowitz, J. (1960). J. Chem. Phys.

33, 530.

Odoj, R. , Hilpert, K. , and Gerads, H. (1979). "Investigations of the

Volatility of Cesium from Aluminosilicates by Mass Spectroscopy," in

Scientific Basis for Nuclear Waste Management, 1, 227, G. J. McCarthy,

Ed., Plenum Publishing Co., New York.

Plante, E. R. (1979). "Vapor Pressure Measurements of Potassium over

ICO-SiOp Solutions by a Knudsen Effusion Mass Spectrometric Method," in

Characteri zati on of High Temperature Vapors and Gases , Hastie, J. W.

,

ed. NBS-SP 561, p. 265, U.S. Government Printing Office.

Schafer, H. , Tebben, A., and Gerhardt, W. (1963). Zeit. Anorg. Allg.

Chem. , 321, 42.

Skudlarski, K. , Drowart, J. , Exsteen, G. , and Auwera-Mahieu, A. Vander.

(1967). Trans. Faraday Soc. 63, 1146.

Soper, P. D. (1982). Private communication.

Wenzel, J. T. and Sanders, D. M. (1982). Phys. Chem. Glass. 23, 47.

Wicks, C. E. , and Block, F. E. (1963). Thermodynamic Properties of

65 Elements and Their Oxides, Halides, Carbides, and Nitrides, U.S.

Bur. Mines, Bull. 605.

43
8.

Figure Captions

1. Schematic of vacuum enclosure and KMS mass spectrometric system.

2. Schematic of furnace and Knudsen cell.

3. Typical mass spectrum for SNW glass vapor at 1000 °C.

4. Ionization efficiency curves for Li +

, Na

+

, and Ar

+ reference.

4

5. P(NaB0

?

) vs 10 /T(K) for SNW glass vapor, series I data, compared

with P°(NaB0

2

) from JANAF [1971].

6. P(LiB0

2

) and P(CsB0

2 vs 10*Vt(K) for SNW glass vapor, series I data. Least squares line for LiB0

2

is based on runs 1 through 4

omitting two lowest temperature points from run 1. Curve through

CsB0

2 data is based on four low temperature points from run 4.

The P°(LiB0

2

) reference pressure curve is from JANAF [1971];

P°(CsB0

2

) is from Gurvich and Veits (1983).

4

7. Oxygen pressure vs 10 /T(K) for SNW glass vapor, series I, run 1-5

data; the five lowest temperature points were rejected in the

least squares analysis. Comparison curve is pressure for Fe«0^(c) *

Fe

3

0

4 (c) from JANAF [1971].

*

4

8. Pressure of NaB0

2 vs 10 /T(K) for SNW glass vapor, series II data.

Line is least squares line from series I data (see fig. 5).

4

9. Pressure of LiB0

2 vs 10 /T(K) for SNW glass vapor, series II

compared with least squares line from series I data (see fig. 6).

4

10. Pressure of CsB0

2

and 0

2 vs 10 /T(K) for SNW glass vapor from

series II data compared with least squares curve from series I data (for CsB0

2

). Dashed curve is considered the best compromise

between series I and II data for pressure of CsB0

2

.

11. Isothermal (1265 K) dependence of log I*

3

(H

10

B0

2

+

) vs 1/2 log I*

8

(FL0

+

) showing expected stoichiometric behavior for the equilibrated

reaction

1/2 B

2

0

3 Q>) + 1/2 H20(g) = HB0

2 (g).

4

12. Equilibrium constant vs 10 /T(K) for B

2

0.-t-L0 reaction shown as compared with JANAF [1971] curve.

13. Detailed schematic of assembled transpiration mass spectrometer,

drawn approximately to scale.

14. Transpiration mass spectrometer inlet system showing internal

details and relative location of components

not to scale.

44
15. Detailed schematic of integral capillary nozzle section of transpiration inlet system. Relative locations are given approximately to

scale, except the nozzle channel, which has been enlarged for

clarity. A typical experimental temperature profile along the

axis of the system, obtained without the boat present, is also

shown.

16. Variation of ion current ratios with temperature and time (increasing

temperature run chronology). For sodium borate, 123^*66 and for

lithium borate, I^/I*

0 are indicated, respectively.

17. TMS partial pressure data for metaborate and alkali species over

SNW glass, with pure N

2

carrier gas at ~ 0.3 atm total pressure

and a flow rate of 6 seem. LiB0

2

data have been multiplied by a factor of 10 for clarity. The lines labeled Na

2 C0g and Na^O are

for the Na decomposition pressures of these components in the SNW

glass (see text).

18. Noncondensible gases observed in pure N^ SNW glass experiments.

The solid curves indicate the time as well as temperature history.

Note that the curves marked C0

2

and 0^ follow the suggested Na

from Na^O^ curve from figure 17, whereas H

2

0 appears to result

from a process more representative of the bulk glass. At the

higher temperatures, possible formation of 44 amu HB0

? may interfere with the C0

2 signal.

19. Comparison of metaborate partial pressure data selected from KMS

and TMS experiments The solid curves shown are least squares fits

to the entire ensemble of data with equal weighting, except for

the CsB0

2

data where the first five KMS and four TMS points were

selected as most representative of this species. KMS CsB0

2

data

is from Series I multiplied by factor of two for an average of

series I and II. Least squares analysis of the TMS + KMS metaborate

data yields:

log P(atm) { LiB0

2

} = (4.73 ± 0.2)

log P(atm) {NaB0

2

) = (4.09 ± 0.2)

log P(atm) {CsB0

2

} = (3.45 ± 0.3)

20. Comparison of experimental metaborate pressures with thermodynamic

estimates for the major volatile Re, Ru, and Sr species. Calculations assume an oxygen partial pressure of 0.02 atm. The nominal

glass composition was used with activities set equal to oxide

component mole fractions.

- (1.292 ± 0.02) x 10

4

/T

- (1.190 ± 0.02) x 10

4

/T

- (1.288 ± 0.04) x 10

4

/T .

45
21. Predicted metal hydroxide partial pressures, calculated for the

reactions shown. Selected best thermodynamic functions have been

used, with the assumption that activity equals mole fraction (or

actual activity from experiment in borate cases). The interdependency between species has been approximated by assuming

PCHBC^) = PCLiBO^) for all reactions involving HBf^, with that

value set from the LiOH pressure line. Similarly, P(NaOH) was set

by the reaction

NaBC^ (solution) + = NaOH + HBC^.

46
Figure 1.

Figure 2.
AC SPECTRUM

SIMULATED NUCLEAR GLASS 1000*C

m/e (amu)

Cs (133) -

Figure 3
IONIZATION EFFICIENCY OF METABORATES

ARGON REFERENCE

>=<

C£t

QQC£<

4*

CORRECTED eV (VOLTS)

Figure 4.
0

2

PRESSURE

(ATM)

TEMPERATURE (K)

1429 1250 1111

Figure 5
CsB0

2

AND

LIB0

2

PRESSURE

(ATM)

TEMPERATURE (K)

1429 1250 1111 1000

Figure 6.
NaB0

2

PRESSURE

(ATM)

TEMPERATURE (K)

1429 1250 1111 1000

Figure 7.
NaB0

2

PRESSURE

(AIM)

TEMPERATURE (K)

1429 1250 1111

104 /T (K)

Figure 8
LiB0

2

PRESSURE

(ATM)

TEMPERATURE (K)

1429 1250 1111

Figure 9
CsB0

2

AND 0

2

PRESSURE

(ATM)

1429

TEMPERATURE (K)

12S0 1111

104/T (K)

Figure 10
LOG

!

Figure 11.
1429

TEMPERATURE (K)

1250 1111

7 8 9

104 /T (K)

Figure 12.

BEAM

Figure 14.
+ 2 r

Figure 15.

BEAM
TEMPERATURE K

1600 1500 1400

10

4 /TEMPERATURE K

Figure 16
PARTIAL

PRESSURE

(atm)

TEMPERATURE K

10

4 /TEMPERATURE K

Figure 17.
TEMPERATURE K

Figure 18.
TEMPERATURE C

1400 1200 1000 900 800

Figure 19.
PARTIAL

PRESSURE

(atm)

TEMPERATURE C

Figure 20.
PARTIAL

PRESSURE

(atm)

TEMPERATURE C

Figure 21
NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 83-2731

4. TITLE AND SUBTITLE

VAPORIZATION OF SIMULATED NUCLEAR WASTE GLASS

5. Publication Date June 1983

t. PerforrolntOrgantzation Code

7. AUTHOR(S)

J. W. Hastie, E. R. Plante, and D. W. Bonnell

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, state, ZIP) 13. Type of Report & Period Covered Interim Report

15. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or leae factual summary of moat significant information. If document includes a significant bibliography or

literature survey, mention it here.) Industrial development of nuclear waste glass processing requires basic data on glass vaporization thermodynamics. Detailed mass spectrometric experiments and thermodynamic estimates have been made for vaporization of a nonradioactive borosilicate

glass containing representative nuclear waste isotopes. Alkali metaborates were observed to be the dominant vapor species and their partial pressures indicate significant vapor transport under likely process conditions. The results indicate the following order of significance for vapor transport of radionuclide species,

Cs 'x* Re (-v Tc) > Ru » Sr.

17. KEY WORDS ("six to twelve entries ; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

boron; glass; nuclear waste; processing; radionuclide; silicon; thermodynamics;

vaporization

18. AVAILABILITY [^Unlimited

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20402, SD Stock No, SN003-003-

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