ORNL-MIT-99.txt

 

DATE:

SUBJECT:

AUTHOR:

Consultant:

OAK RIDGE NATIONAL LABORATORY

OPERATED BY
UNION CARBIDE CORPORATION

 

 

 

 

NUCLEAR DIVISION
POST OFFICE BOX X
OAK RIDGE, TENNESSEE 37830
ORNL-MIT-99
March 5, 1970 COPY NO.

Analysis and Scaleup of the Pulsed-Gas Impregnation of Graphite
with Carbon - A

S.H. Rose, A.A. Jeje, and J.M, Ganzer*
C.B. Pollock

ABSTRACT

A gas-pulsed impregnation technique for depositing pyrolytic
carbon in the surface pores of graphite was scaled-up to handle
graphite rods to be used in molten salt reactors. In addition, an
attempt was made to apply an autoradiograph technique to a study of
the degree of attachment of the pyrolytic carbon. However, this
portion of the study was not completed.

*
Revised in part by R.H. Mayer and M.S. Bautista

Oak Ridge Station
School of Chemical Engineering Practice
Massachusetts Institute of Technology

NOTICE This document contains information of a preliminary nature
and was prepared primarily for internal use at the Oak Ridge
National Laboratory. It is subject to revision or correction and
therefore does not represent a final report. The information is
only for official use and no release to the public shall be made
without the approval of the Legal and Information Control Depart-
ment of Union Carbide Corporation, Nuclear Division.
Contents
Page

Y 11111 1 4
Introduction .....iiiniiiiiiiiiiiitnrinioetersnrerentsnceronas 4
2.1 Background ........cciiiiiinnnnn. et etse et ceer et eenas . 4
2.2 ODJECEIVES vvitvrrrnrenenioesneneensoneannnnnnes reeaee .. 5
2.3 Method of Attack ...... et et ee et eeeresace e e reesens ... b
Scaleup of the Pulsed-Gas Impregnation Process ..... [ . b
3.1 Laboratory Apparatus and Procedure .......... cerecaaan e 6
3.2 SCAlEUP tvvvrrnerernnnnnnnnnannns Weeerssctersesssssssssnees D
3.3 Foreseeable Problems in the Sca]eUp Cetere et 12
Development of Autoradiographic Technique .......cvevivennnnnn. 12
4.1 Background C et tec et e ettt aee e eeeree e cesee e 12

4.2 Experimental Design for Determining Migration of Pyrolytic
Carbon into the Graphite ....... feeeseseseesateesenasanes 13
Conclusions and Recommendations ...........ce... eesessensasnass 15
Acknowledgement .....cceecvevvnnnnn e eteeeeaee e seeaes 15
Appendix .......... e teeereea et e teeae et eeaenennnns .. 16
7.1 Design Specifications .....ccivieiiiieeneereeeneeenneenns 16
7.2 Factors Considered in Scaleup .....cveveeveenrrennnnen N Vi
/.3 Additional Background Discussion ..... edesesesasvone e 20
7.4 Analysis of Autoradiograph Results ........ceiievivennnnn . 26
/.5 Penetration and Detection of Tracer ......... erenesees e . 26
/.6 Apparatus and Procedure for Synthesis of Acetylene ....... 3l

7.7 Literature ReferenCes .......eeeeevecocosess Ceeeeeseeeens .. 34
1. SUMMARY

A pulsed-gas impregnation technique has been developed for reducing
the permeation of undesirable gases into the low porosity graphite to be
used in Molten Salt Reactors (1). The technique involves intermittently
exposing the graphite at 850°C to a hydrocarbon gas (0.5 sec) and a vacuum
(+15 sec) in order to deposit pyrolytic carbon in the surface pores. It
was the purpose of this investigation to develop a technique to evaluate the
coating stability and to propose a scale-up of the laboratory equipment for
production of full-size graphite rods.

An attempt was made to characterize the mode of attachment of pyrolytic
carbon b¥ autoradiography. The exper1menXa1 program involved depositing
carbon (14C-1abelled) by pyrolysis of a !"C-labelled acetylene in pores of a
Tow poros1ty graphite sample; taking autoradiographs of internal sections of
the sample; and utilizing optical density methods to determine the concen-
tration of the isotope as a function of position. The concentration profile
can then be mathematically analyzed to calculate the diffusion coefficient
(or more appropriately - migration constant) of pyrolytic carbon into the
graphite. At the time of this report the experimental results were not
available. |

A scale-up was proposed for production of full-size rods (16 ft by 4
in. by 4 in.) under the same conditions found to be opt1mum for impregnating
the laboratory sample (0.5 in. by 0.4 in. OD by 0.126 in. ID).

The following recommendations were made:

1) The proposed experimental program to evaluate the attachment of
pyrolytic carbon to graphite should be completed, and the values of diffu-
sion coefficients of pyrolytic carbon into graphite estimated as a function
of temperature and duration of heat treatment of the finished sample.

2) Further developments in the gas-pulse technique should be made,
and optimization of conditions for the best attachment of pyrolytic carbon
obtained.

3) A full-size rod production process should be developed using the
design specifications given and the optimum conditions obta1ned from recom-
mendation (2). ,

2. INTRODUCTION
2.1 Background

The present concept of the molten salt reactor (MSR) utilizes graphite
arranged in a lattice as flow conduits for the molten salt and as a modera-
tor and reflector in breeder reactors. Fission products sorption in the
graphite must be minimized, because the after-heat they produce necessitates
lTong cool-off periods at the end of the graphite life. _Also, when the MSR
is used to breed fissile material, the accumulation of !39Xe must be pre-
vented because of the large neutron absorption cross section of xenon. It
has been demonstrated that excessive xenon accumulation can be prevented if
the xenon flux per unit graph1te thickness (referred to as permeability)

can be reduced to 10-8 cm?/sec (18) at operating temperatures in the range
700-750°C.,

Vapor deposition techniques have been developed whereby very dense
solids are deposited on a substrate by thermal decomposition of gases.
Beatty and Kiplinger (1) developed the technique to deposit pyrolytic car-
bon in the surface pores of graph1te A graphite sample (porosity ~2.5%,
0.5 in. long x 0.4 in. OD x 0.125 in. ID) was intermittently exposed to
1,3-butadiene (+0.5 sec) and a vacuum (~15 sec) at 850°C. Under their op-
erat1ng cond1t1ons, the helium perneability of the graphite specimen was
reduced to ~10-8 cmé/sec [corresponds to the requisite xenon permeability
(see Sect. 7.3.1 for additional information)] and the average density of
the graphite was increased by 8%.

2.2 O0Objectives

The objectives of this project were to develop experimental techniques
to study the mode of attachment of the impregnant on the substrate and to
use these findings along with previous experimental data (1) to scale-up
the pulsed-gas impregnation process for the impregnation of full-size rods
(4 in.2 x 16 ft long with 0.6 in. diam axial hole).

2.3 Method of Attack

Various techniques have been used to study the structure (12) and
properties (physical and chemical) of pyrolytic carbon (3, 15, 16). MWe
decided to develop a tracer technique using !%C to study “the | mode of attach-
ment of pyrolytic carbon to the substrate graphite since x-ray diffraction
and electron and optical microscopy methods would not be able to distinguish
between atoms of pyrolytic carbon and the substrate graphite.

Theoretical and experimental studies of self-diffusion of carbon atoms
in graphite were conducted in Refs. (8, 10, ]6) These studies were either
on diffusion in natural graphite crystals or in single crysta]s, both of
which are highly anisotropic. However, the present problem is concerned
with diffusion of carbon atoms into the pore walls of an isotropic graphite
sample. (The orientation of the crystallites in the graphite is random.)
It is proposed that this technique be used to evaluate the coefficient of
self-diffusion of pyrolytic carbon into substrate graphite.

The scaleup of the pulsed-gas impregnation process is given in Sect. 3.
The autoradiography technique was not sufficiently developed to assess its
value. The present state of development s given in Sect. 4.

3. SCALEUP OF THE PULSED-GAS IMPREGNATION PROCESS
3.1 Laboratory Apparatus and Procedure

Figure 1 shows a schematic of the apparatus used in the pulsed-gas
impregnation process. A basic feature is a containment chamber that can
be cycled between vacuum and hydrocarbon atmosphere. The chamber is a
silica glass tube connected to on-off solenoid valves powered through a
pulse timer in series with an interval timer. This system permits adjust-
ing the vacuum and hydrocarbon pulse periods to any desired combination and
presetting the number of pulses of thé processing run. [The best cycle
obtained was 0.5 sec in the presence of 1,3-butadiene and 15 sec of evacua-
tion at 850°C (17).] . The system is evacuated by a 2.3-liter/sec mechanical
pump, which achieves about 0.2 torr during each vacuum period. Higher
vacuum levels do not appear either practical or necessary. The hydrocarbon
pulse is 1,3-butadiene supplied at 20 psig. A 1.2-kw KHz inductive genera-
tor heats the graphite specimens.

3.2 Scale-up

The scale-up was mainly concerned with: (1) heating the graphite rods
uniformly and quickly, (2) introducing the hydrocarbon and achieving similar
contact times as in the laboratory specimens, (3) obtaining an acceptable
vacuum as quickly as possible, and (4) combining the first three into a
feasible design. Table 1 summarizes the important factors in scaling-up
from the small-scale model to the prototype; Figs. 2 through 4 schematically
illustrate the design developed. . |

The fundamentals of the proposed design (Fig. 2) are discussed below
and the design specifications can be found in Appendix 7.1. The graphite
rods are initially exposed to an extended vacuum period and then purged with
the hydrocarbon gas to prevent oxidation and other chemical reactions while
they are being heated to the operation temperature. The hydrocarbon gas
inlets are located at each end of the container vessel. The container would
be either a 6-in.-ID, high-tensile steel circular pipe or a 5-in.-square-
cross-section pipe (fabricated from steel plates). Both containers would
be approximately 17 ft long with removable caps at each end. The graphite
rods are supported on a roller platform (Fig. 3) which can be pushed into
the container. Passage of an electrical current directly through the graph-
ite (connections shown in Fig. 4) heats the rods. The power requirements
are calculated from Q = I2R where Q, I, and R are power (watts), current
(amperes), and resistance (ohms) respectively. The hydrocarbon and vacuum
lines are to be connected to a pulse timer just as in the laboratory appa-
ratus. Calculations (see Appendices 7.2 and 7.3.4) have shown that a
ORNL-DWG 68-3993R

 

 

 

 

 

 

 

 

 

 

 

 

 

/SOLENOID VALVE

 

 

=
@ \ < 1,3-BUTADIENE
‘ AT 20 psig
SUPPORT —
GRAPHITE
SUBSTRATE
.
INDUCTION I/
coL—— & 710 %
N R PULSE | | INTERVAL
N TIMER TIMER

 

 

 

SILICA TUBE —

 

 

 

 

 

 

 

 

é@mx}-—] 1VENT
SOLENOID VALVE

 

 

o)

 

 

VACUUM PUMP

Fig. 1. Carbon Impregnation of Graphite by Vacuum=Pressure System. (1)
 

hydrocarbon
container
size of piping

vacuum
hydrocarbon

- number of inlets

vacuum
hydrocarbon

graphite rod support

means of heating

temperature control

purge gas

 

1,3-butadiene

silica tube 16 cm

X 1 cm ID
1.5 in. ID
0.5 in. ID

]

]

vertically hung
induction coil

optical pyrometer

argon

Table 1. Scale-up
Condi tion Laboratory | Prototype |
dimensions of graphite 0.5 in. long x 4 in, 16 ft long x 4 in. square
rod 0D x 0.125 in.-ID x 0.6 ft ID
pore sizes 0.8 u 0.8 u
penetration depth of V10 u V10 u
pyrolytic carbon
pyrolysis temperature 750-950°C 750-950°C
hydrocarbon pressure 20 psig 30-40 psig
vacuum desired 1.0 torr 1.0 torr
time of hydrocarboh pulse 0.5 sec 1.5 sec
time of vacuum period 15 sec 15-20 sec

'1,3-butad1ene

steel pipe 17 ft long
X 6 in, ID

2.5 in. ID
1 in. ID

1

2

horizontal position on
cart

electrical current
directly applied
automatic r-diation
balance sensor

same hydrocarbon at room
temperature

 
 

 

See
Fig. 3
for
details

 

Temp. | :
<:;;) Control Graphite
AAAAAAAAAA PRI VYV NI FIIVYVIIVI YV IVEYY 1/[/‘4

Cylindrical
Tube

ot o 80l e s 2L

Py ryryYyy,

Solenoid
Valve

 

 

 

 

Pglse

Solenoid Valve N

 

*{ T rmr T TmTmTETEERRERRERERRERTTTSESS

‘w P‘Ower 7
Source 2

 

 

 

Hydrocarbon

35 psia
pump

P
14.7 psia

 

Feed et
/|

 

  
 

    

Con-
denser

 

 

 

 

Vacuum
Pump

 

  

 

MASSACHUSETTS INSTITUTE OF TECHNOLOGY
SCHOOL OF CHEMICAL ENGINEERING PRACTICE

AT
OAK RIDGE NATIONAL LABORATORY

 

Scaled-up Pulsed-Gas Impregnation System

 

 

DATE DRAWN BY FILE NO. IFIG.

3-4-70 JMG EPS-X-99 2

 

 
 

  
  
 

 

Graphite Block
4 in. square

High Tensile Steel
Pressure Vessel
ID = ~6 1in.

Insulation

Roller Platform

 

MASSACHUSETTS INSTITUTE OF TECHNOLOGY
SCHOOL OF CHEMICAL ENGINEERING PRACTICE

AT
OAK RIDGE NATIONAL LABORATORY

 

Cross Section of Pressure
Vessel and Graphite Rod

 

 

DATE DRAWN BY lFILE NO. FIG.

3-4-70 JMG CEPS-X-99 2

 

 

 

Ol
 

 

Smooth Contact
(minimize contact
resistance)

 

 

_‘%/, / —/§_ Metal Plate
:\\ W\ ‘ :‘:‘\‘ \

  

Insulated Current Line

 

MASSACHUSETTS INSTITUTE OF TECHNOLOGY
SCHOOL OF CHEMICAL ENGINEERING PRACTICE
AT

OAK RIDGE NATIONAL LABORATORY

 

Proposed Connection for
Electrical Heating

 

DATE DRAWN BY FILE NO. IFiG.

3-4-70 JNG CEPS-X-99 4

 

 

 

L1
12

pulsing sequence of 1.5 sec hydrocarbon and 15-20 sec vacuum are required.

A vacuum pump with a capacity of 70 liters/sec, or 140 ft3/min, is needed

to achieve the above pulsing sequence; a maximum power of 160 kw is required
for heating and maintaining the graphite temperature at 800°C.

Resistance type heating was chosen in preference to induction heating
because of its more simple and economical operation. The power requirements
are smaller since heating is direct. The commercial availability of an in-
duction coil 16 ft-long is not readily known. Induction heating would re-
quire a quartz or silica container which in turn would necessitate a verti-
cally supported arrangement due to the low structural strength of the con-
tainer vessel.

3.3 Foreseeable Problems in the Scale-up

A few problems which will require further investigation are: the mode
of attachment of the electrodes (contact resistance causing non-uniform
heating at the rod ends), the absence of pyrolytic carbon impregnation at
the ends and support areas of the rods, pyrolysis and deposition occurring
on the walls of the container vessel, and the effects of heat losses. The
first problem can be reduced by having the rod ends and electrodes highly
polished and by attaching the electrode plates with external compression.
Since it will be very difficult to notably eliminate the contact resistance,
the rods could be made one foot longer than actual size then cut off the
last six inches at both ends where non-uniform conditions will exist during
processing. The areas of support (Fig. 3) are very small. The rod ends
are not extremely important in the MSR. A radiation stable coating, such
as epoxys or furfuryl alcohols, applied to uncoated areas after the rods
are impregnated may be sufficient. Prevention of pyrolysis at the walls of
the container may be achieved by maintaining the wall temperature below the
pyrolysis temperature simply by externally cooling the container vessel if
necessary. Heat losses by radiation can be reduced by making the container
walls highly reflective.

4. DEVELOPMENT OF AUTORADIOGRAPHIC TECHNIQUE
4.1 Background

The deposition of pyrolytic carbon inside graphite pores and the radia-
tion and thermal stability of the attachment (carbon to substrate graphite)
will be discussed in this section. These are of major importance in estab-
lishing the quality of the carbon impregnated graphite for use in molten
salt reactors.

Carbon formed from the pyrolysis of hydrocarbons at moderate tempera-
tures (750 - 950°C) has a wide range of structures and properties depending
on the deposition conditions, viz, the pyrolysis temperature, the contact
13

time, and the concentration of the hydrocarbon (see Appendices 7.3.2 and
7.3.3 for more information). In this context the term carbon is used to
describe a wide variety of solid structures, many of which contain appre-
ciable amounts of hydrogen and other elements present in the starting com-
pound. The carbon can exist as dense, highly oriented, chemically-defined
solid (pyrolytic graphite) or as an amorphous, low density structure with
micropores and fractions of hydrocarbon decomposition intermediates.
Actually, pyrolytic carbon has structures and properties between these
extremes. Various studies on the structures and properties of pyrolytic
carbon deposited under varying conditions from different gases have been
reported (1, 4, 12, 23).

Thermal expansion of deposited pyrolytic carbon usually differs con-
siderably (3, 15) from that of the substrate graphite. Therefore, carbon
deposited at moderate temperature under zero stress can develop h1gh shear
stresses at high temperatures such as those in the MSR. The internal
stresses can cause either surface cracks of the impregnant or complete
separation of impregnant and substrate. Both of these are undesirable, but
the former is preferred to the latter because less surface of the substrate
1s exposed. The impregnant is attached to the substrate by both weak Van
der Waal forces and weld-Tike connections due to self diffusion of carbon.
The Tatter form of attachment is most desirable. A measure of the degree
to which this form of attachment occurs, hopefully, can be obtained from
autoradiograph measurements. The technique was not completely reduced to
practice during the present study Interpretation of the expected auto-
radiograph results is given in Appendix 7.4.

4.2 Experimental Design for Determining Migration
of Pyrolytic Carbon into the Graphite

Autoradiography was investigated as a potential technique to determine
the concentration profile of pyrolytic carbon in the graphite. Carbon-14
labelled acetylene was used in the pyrolysis to produce a labelled pyrolytic
carbon. The success of the technique, however, depends on the sensitivity
and degree of resolution obtainable on photographic films and emulsions used
to record the low energy beta emitted by 14C. Concentration calibration
curves can be made and therefore concentration as a function of position
can be plotteds provided the optical density of image produced can be accu-
rately measured. Preliminary calculations (Appendix 7.5) show that films
with adequate sensitivity are available. A resolution slightly better than
1 u is expected.

Various authors (4, 23) hypothesized that acetylene is an intermediate
compound in the dehydrogenation of hydrocarbons to produce pyrolytic carbon
(see Fig. 5). However, exper1menta11y it was found that the conditions for
pyrolysis of 1,3- butad1ene is slightly milder than fflr acetylene. On this
basis it is recommended that acetylene tagged with 1%C be mixed with buta-
diene. This mixture is then fed into the pulsed-gas impregnation apparatus
used in the experiments by Beatty and Kiplinger (1) shown in Fig. 1. The
 

 

 

CoHy

Surface decomposition

Carbon nucleus oF fuel ~\\‘
Y

Carbon
particles

 

 

 

A
Saturated Unsaturated Partially
polymer - polymer =% carbonated —d
particles

 

MASSACHUSETTS INSTITUTE OF TECHNOLOGY
SCHOOL OF CHEMICAL ENGINEERING PRACTICE

AT
OAK RIDGE NATIONAL LABORATORY

 

Simplified Version of the Schematic
Representation by Street and Thomas (23)
of the Routes to Carbon Considered in
Hydrocarbon Polymerization Theories

 

 

DATE DRAWN BY FILE NO. FIG.
3-4-70 SHR l CEPS-X-99 5

 

 

14}
15

impregnated sample is then sectioned, perpendicular to a coated surface,
so that an autoradiograph of the edge of the coated surface can be taken.
Autoradiographic results were not completed in time for this report, since
the radioactivity of the specimens was very low and the film used was not
as sensitive as can be obtained.

Acetylene was particularly suitable for-this study for the following
reasons. The carbon-to-hydrogen ratio is 1:1, and therefore the amount of
hydrogen to be vacuumed out of the graphite after hydrocarbon exposure
periods is small. Since previous investigators (15) have concluded that
some -CH groups remain included in the pyrolytic carbon, then even if the
acetylene is not completely pyrolyzed, it can be well accommodated in the
pyrolytic carbon structure in the Tow concentrations present. Thus, this
partially pyrolyzed acetylene will contribute to the radioactivity of the
specimen. However, it probably has a much lower diffusivity than elemental
carbon.

Acetylene, with ]4C backbone, was not available at the time of this
study; therefore, the gas was synthesized using the technique developed by
Cox and Warne (6). The synthesis is briefly reported in Appendix 7.6.

5. CONCLUSIONS AND RECOMMENDATIONS

Filling the surface pores of full-size graphite rods with pyrolytic
carbon by the gas-pulsed impregnation technique was found to be feasible
and a production design was proposed. However, the application of auto-
radiography to a study of impregnant stability to radiation and thermal
stress was not carried to completion. It is recommended that this develop-
ment program be continued. A more sensitive, high resolution, g-sensitive
film should be used.

More small-scale impregnation experiments are recommended to obtain
the optimum conditions for depositing the most heat- and radiation-stable
(as evaluated from autoradiography) pyrolytic carbon in graphite.

6. ACKNOWLEDGEMENT

We gratefully acknowledge the extensive assistance and encouragement
accorded us by our consultant, C.B. Pollock, and W.H. Chilcoat. The
generous cooperation and aid we received from the Metals and Ceramics
Division personnel was greatly appreciated. |
16

/. APPENDIX

/.1 Design Specifications

7.1.1 Container Vessel

 

The vessel in which the graphite block is placed is constructed of high
tensile steel because of the corrosion resistance, especially to hydrogen
and hydrocarbons at the high temperature of pyrolysis. The carbon block is
introduced into either end which is then pressure sealed with steel caps.
Asbestos insulation around the container is used to reduce heat loss to the
surroundings.

7.1.2 Temperature Measurement and Control Device

A radiation balance sensor is recommended for measuring and controlling
the temperature since this instrument will use infrared radiation to monitor,
without contact, the surface temperature of the fixed carbon rod. With this
instrument it is possible to accurately monitor and control the carbon sur-
face temperature, independent of the emissivity of the surface and the am-
bient temperature of the surroundings. The device measures up to 3000°F
temperatures to about 1% accuracy. The block diagram of the industrial
version of the radiation balance sensor can be found in Ref. (13). The
equipment is available from the Honeywell Corp.

7.1.3 Vacuum Pumps

 

A rotary piston vacuum pump is recommended, Edwards' high vacuum pump,
model 1SC3000 with ultimate pressure of 5 x 103 torr and a pumping capacity
of 68.3 1iters/sec (14), is adequate for evacuating the system in less than
one second. Some of the features are large displacement-to-size ratio,
minimum vibration and quiet. The cost of the pump is #1650.

/7.1.4 Heating Device

 

Electrical heating of the graphite is proposed. A direct current is
passed through the graphite block from metal plates tightly pressed against
the ends of the rod. The plates are insulated with a ceramic insulator to
minimize carbon deposition on their surface and reduce heat losses. The
power input, Q, is calculated from |

Q = I%R

where:
17

I = current, amp
R = resistance, ohm
Q = power, watt

The current is controlled by the temperature-sensing device to adjust the
graphite surface temperatures. The power requirements are given in
Appendix 7.2.1.

7.1.5 Additional Equipment

 

A condenser using an ammonia- or Freon-cooled system will condense
1,3-butadiene (boiling point of 5°C) from the mixture, while purging un-
condensed hydrogen gas from the system. Also required are time-controlled
close-open valves in both the hydrocarbon and vacuum lines. Any pump that
can increase the hydrocarbon pressure from 14.7 psi to 40 psi will be re-
quired, as shown in Fig. 2.

7.2 Factors Considered in Scale-up

7.2.1 Power Requirements for Heating

 

The heat necessary to bring the graphite rods to the reaction tempera-
ture assuming no heat loss is determined by:

Q= (Ve)(C)(aT)
where:
V = volume of the rod = 4.84 x 104 cm3
o = density = 1.86 gm/cm3
Cp = heat capacity of graphite = 1.36 joules/gm-°C
AT = change in temperature ~ 800°C
Q = heat = 10° joules

If we require the graphite rod to be heated in 10 min, then the power neces-
sary 1is:

 

8 .
P = power = % JOUIES .\ 3.6y 10°w = 160 k
6 x 10° sec
18

The above calculation is a first appoximation of the power requirement since
there is considerable flexibility in setting the time required for heating
the rod to temperature. Given the average specific resistance is 955 micr-
ohm-cm, the total resistance of the rod, R, is 4.62 microhm, and the trans-
former has to be able to carry current, I = YA//R = 5.8 x 103 amperes.

This calculation assumes R is contant; in the actual case R will vary with
temperature.

7.2.2 Vacuum Capacity

 

The equation which describes the vacuum process is

' 1 dv
Vo C vt (1)
i
where:
Q = capacity of the pump, liter/sec
Vi = container vessel volume - rod volume = 15 liter
V = volume the total initial gas inside the container would
have at pressure P
Considering adiabatic reversible éxpansion, PV¥ = constant. Therefore,
gE- = -y %y- and from Eq. (1)
V
dt = _..-I_..._.I_.g_P_
yv P

Integrating between 1 atm (760 torr) and P¢ (final pressure of vacuum),

o

Vi

v

f

t = - znm

< |—

If we desire Pe = 1 torr and t = 1 sec, then:

v = 70.5 liter/sec = 141 ft3/min
19

7.2.3 Time Required to Fill Container with Hydrocarbon

Estimated variables:

Vi = container vessel Volume - rod volume = 15 liters

Pe = hydrocarbon pressure = 2 atm = 2.04 x 106 gm/cm-sec2
L = «cylindrical inlet pipe length = 100 cm

D = cylindrical inlet pipe diameter = 2 cm

Since the flow is turbulent, we apply

C; = 0.079 Re™'/*
and
Umj - éi E P (17)
‘ where:
Umi = initial mean velocity of entering gas, cm/sec
AP = Pe - Pj, gm/cm-sec2
P;i = container pressure, gm/cm-sec2
Cs = friction factor
o = hydrocarbon density before entering, cm/cm3
Re = Reynolds number

By trial and error,

4

Cc = 8x10 " and Uy, = 7 x 103 cm/sec

f

Considering an ideal gas undergoing reversible free expansion, we have

V'i = (Ve) ('Y)

where:

<
1))
i

volume of gas filling the container at pressure P, and the
temperature of the gas source, cm

 
20

y = €/C, % 1.2 for 1-3 butadiene

Thus the volume of gas entering per unit time is,

o= Uy S

where:
Um = mean velocity of entering gas, cm/sec
S = cross section of inlet conduct, cm2

Applying the ideal gas law to.the hydrocarbon gas before and after
entering the container and integrating for V between 0 and Vg, A

 

 

"\ Vi ] 2Cf Lo
t w ——(2)(5) 57— Vv 1 sec when the numbers are
Y € substituted
/.3 Additional Background Discussion
7.3.1 Permeability
The permeability is defined as (1):
P (1leak rate)(wall thickness)
area of cylinder at wall mid-thickness
or
Lo L '
P % = w5 (2)
where:
L = Tleak rate, cm3/sec
A = cylinder area at wall mid-thickness, cm2
§ = wall thickness, cm

This definition suggests a three-dimensional,wall thickness-dependent
property. The major resistance of the coated graphite should be in the
coating, which will have a constant thickness, independent of the wall
thickness of the specimen. Therefore equal permeabilities measured in two
different size bodies do not mean equal surface coatings for both. The

 

 
 

21

measure which indicates equal surface coatings in two different bodies is
the leakage per unit area at the wall mid-thickness. A calculation can be
made to obtain the perme§b11§ty for a laboratory specimen that corresponds
to a permeability of 107° cm“/sec (18) in the actual size graphite.

The laboratory sample:

(12.7 mm Tong x 10.2 mm OD x 3.2 mm ID)

§' = sample wall thickness = 3,5mm = 0.35 cm
The graphite rod:

(16 ft long x 4 in. x 4 1in.)

§ ~ 2in. ~ 5 cm

It is reasonable to assume that the sorption criterion corresponds to
an equal leak flux (L/A) for the laboratory specimen and graphite rod:

 

|
—k":'LAT:C
and by Eq. (2),
P _ L _ L P
S A A S’
So,
pI = Ps' _ (10_8)(%4859 = 7 X 10-10 cmz/sec
= = = : ,

Thus, based on an equal sorBt1on flux, the permeab111ty of the labora-
tory specimen should be 7 x 10-10 cm?/sec. This would mean that the coat1ngs
obtained by Beatty and Kipplinger (1), with permeabilities of 10-8 cmé/sec,
were insufficient. However, because helium permeability is being used as a
measure of xenon permeability, the permeability may refer to a steric effect
of the resistant layer. In this case the criterion would be that_the perm-

Iy : 8 : ~ 10-
eability of the coating alone equal 10°. Assuming écoating 102 cm,
> _ o Scoating _ 10-8(10"3) v o4, 101!
coating S 0.35

Based on this criterion the laboratory specimens are more than satisfactory.
Thus, it is important that permeability criterion be clearly "established.
22

7.3.2 Impregnation Temperature and Kinetics"

It has been reported (9) that the direct relation between temperature
and kinetics in the pyrolysis reaction plays an important role in the
process of depositing pyrolytic carbon on the graphite substrate. At very
high temperatures (over 950°C) complete pyrolysis will take place as soon
as the hydrocarbon is near the surface of the sample and the pyrolytic
carbon will build a surface coat on the graphite rod without penetrating
the pores. Since the pyrolytic carbon has a different thermal expansion
coefficient than the graphite, the surface coat will easily break off
during irradiation. At lower temperatures the hydrocarbon has sufficiently
Tong time to penetrate the pores of the graphite before the pyrolysis is
complete. |

Therefore, an intermediate temperature range (750-950°C) gives suf-
ficient impregnation in a reasonable time. It is very difficult to predict
theoretically the amount of deposited carbon per unit time, because two
more factors, which were not determined, are to be considered: the contact
time of the hydrocarbon with the graphite and the amount of hydrogen from
the pyrolysis which impedes the continuation of the process. Also, the

number of intermediate compounds that appear during the process of complete -

pyrolysis makes it very difficult to calculate the final amount of carbon
deposited by theoretical considerations of the equilibrium constant and
the kinetics of all the reactions.

In the process of complete pyrolysis the initial hydrocarbon is decom-
posed to molecules of lTower molecular weight which can combine to form
compounds of higher molecular weight and even polymers. Many of these
reactions are thought to occur mainly by free radical mechanism (9).

7.3.3 Equilibrium Constant of Hydrocarbons

A survey of the free energies of formation of hydrocarbons at room
temperature shows they are positive for all but the saturated hydrocarbons.
The table below shows, however, that the free energies of formation for low
molecular weight hydrocarbons are all positive in the range of 750 to 950°C
which means that the equilibrium is toward total decomposition in all cases
The values in the table for AG°, AH°, and Tog Kf® at 25°C were taken from
Ref. (25). The values of log K¢' at 900°C have been calculated applying
Vant Hoff's equation. The three hydrocarbons chosen are the most likely to
be used in future work and represent three different types of aliphatic
hydrocarbons, |

9

 
23

 

 

 

 

AH® AGO lTog K; Tog K%

1,3 Butadiene 26.75 36.43 -26.70 -11.9

Acetylene 54.194 50.000 -36.64 -6.8

Methane -17.890 -12.140 8.9 -3.7
where:

AG® = free energy of formation at 25°C, kcal/gmole

AH® = enthalpy of formation at 25°C, kcal/gmole

K; equilibrium constant at 25°C

K% = equilibrium constant at 900°C

The large negative values of log K. mean that equilibrium corresponds
to practically total decomposition of the hydrocarbon.

7.3.4 Diffusion Into the Pores

The following calculations determine an order of magnitude of the time
needed to fill an average pore with gas to a depth of 1 u. We calculated
the flux by assuming both molecular flow and Knudsen flow to compare their
relative importance. In considering molecular flow the formula for laminar,
fully-developed, steady-state flow was used to determine an order of magni-
tude value of the velocity, whereas in the actual case, unsteady state flow
exists, and there is a counter diffusion of hydrogen. For laminar flow,

 

u = 8 fiPL rez %'
where:
AP = pressure = 1 atm = 1.02 X 105 gm/cm-sec2
u = viscosity, poise
L = Tlength of the pore = 1y = 10°% em
ra = radius of pore entrance = 0.45 um = 0.45 x ]0'4 cm
u = velocity of entrance, cm/sec

v = tortuosity factor N2
24

The viscosity is estimated by,

w o= [0.00333(M T)'/% £(1.33 T)I/V 23 (20)
where:

TC = critical temperature

Tr = reduced temperature

VC = critical volume

M = molecular weight
From the same reference,

T = 425°C V. = 221 cm/gmole

_ 1100 _ -

Tr = 5 - 2.8 M = b4

mT )12 = 151 f(1.33T) = 1.8588
Therefore,

_ -2 .

u = 2.53 x 10 ~ centipoise
Substituting all these values in the exbression for velocity, we find

- 3

u = 4 x 107 cm/sec
Finally,

_ 1074 -8
time = L/u = - 3 2.5 x 10 =~ sec
4 x 10
- _ 1y _ -2 2

flux = Np; = (E)(S)(Wfi = 4.2 x 10°° gmole/cm“-sec

where:
T -

S = density (EE—%fifi—J(TQQ = 6 x 10 4 gm/cm3

M = molecular weight (butadiene)

T and T = 273°K and 1100°K, respectively
After impregnation the diameter, D = 0.1 ym = 10

25

) cm (1). In

this case u becomes 6.2 cm/sec and the flux, Nm2 = 6.5 x 10-4 Ehole/cmz-sec.

For Knudsen flow (21),

where:

and

where:

Thus, for

and for re

1l

it

v 10-1 gmole/sec-cm

Dk e ,’T 2
= — = 9700 — AW cm-/sec

radius entrance pore, cm
tortuosity factor and effect of varying cross section E_ 2
molecular weight

Knudsen diffusion

= effective Knudsen diffusion

D¢ ap
RT L

flux, gmole/cmz-sec

gas constant = 82 atm-cm3/°K-gm01e
pore length = 1 um = 1074 em
pressure = 1 atm

0.4 um = 4 x 1072 cm

825°C ~ 1100°K
2

05um = 5 x 10°% cm

1.2 x 1072 gmo]e/sec-cm2

Comparing diffusions,
26

 

 

N

N 107"

N — = 2.4 for ro = 0.4 um
My 4.2 x 10

N

K -2

2= L2x10 o 3 forr, = 0.05 um
M2 6.5 x 10

Therefore, molecular flow controls and is rapid compared to the time required
to fill the container with hydrocarbon.

7.4 Analysis of Autoradiograph Results

The graphite used in MSR cores are made as isotropic as possible to
equalize dimensional and property changes in all directions. Overall
isotropy, however, does not imply that the isotropy is carried down to the
microscopic scale. However, the assumption of microscopic isotropy will
be made to simplify calculations.

The solution to the counter diffusion of pyrolytic carbon and graphite
is given in Refs. (7, 8). A schematic representation of the expected con-
centration profile is given in Fig. 6. The inflection point corresponds to
the original graphite surface. The solution to the diffusion equation can
be used to obtain a migration constant from this figure. However, as is
shown in Appendix 7.5.1, in all likelihood common treatment procedures
would only give enough penetration to allow one to determine if any migra-
tion occurred.

7.5 Penetration and Detection of Tracer

 

7.5.1 Penetration

To test the sensitivity of experimental technique, it is desired to
estimate the penetration of tracer element into the substrate graphite. A
one-dimensional model is chosen with pyrolytic carbon and graphite divided
by a plane, infinite in either direction.

For isotropic carbon (both graphite and pyrolytic carbon), the dif-
fusion coefficient, D, is a constant independent of direction. This assump-
tion can be made if the crystallites are assumed randomly oriented. The
most probable orientation of a crystallite to any plane will then be at
45° to the plane (Fig. 7).

Let the diffusion coefficients parallel and perpendicular to the base
planes of a crystallite be D7 and Dy, respectively. Then the apparent dif-
fusion coefficient, D3, is obtained by vector summation (Fig. 8). That is,
 

 

Pyrolytic Carbon

Concentration

    

Graphite Pyrolytic Carbon

 

 

 

Distance X

Concentration profile obtained from optical
density measurements of autoradiograph.

 

MASSACHUSETTS INSTITUTE OF TECHNOLOGY
SCHOOL OF CHEMICAL ENGINEERING PRACTICE

AT
OAK RIDGE NATIONAL LABORATORY

 

Anticipated Results from
Autoradiography Measurements

 

 

3-4-70 JSG CEPS-X-99 6

DATE DRAWN BY IFILE NO. IFiG.

 

 

L
 

 

/

. /\ e - . i .

gJ\«;]}/p\, ,’)| ~".. < _Pyrolytic Carbon
( )} e .: '».. * | |

. \ ~ e -
Graphite /4\ /"\/t%)l s
N o YN e

(;- Y17 ‘ ot ¥

¢ fj{*/;;é r } -

— \,-,)- LR

X=-0 &—

8¢

 

MASSACHUSETTS INSTITUTE OF TECHNOLOGY
SCHOOL OF CHEMICAL ENGINEERING PRACTICE

AT
OAK RIDGE NATIONAL LABORATORY

 

Orientation of Crystallites in Graphite

 

DATE TORAWN BY FILE NO. FIG.

3-4-70 JMG CEPS-X-99 7

 

 

 
 

 

 

e\

Crystallite

Surface

 

MASSACHUSETTS INSTITUTE OF TECHNOLOGY
SCHOOL OF CHEMICAL ENGINEERING PRACTICE

AT
OAK RIDGE NATIONAL LABORATORY

 

Crystallite Plane

 

 

DATE DRAWN BY IFILE NO. FIG.

3-4-70 SHR CEPS-X-99 8

 

 

6
30

§3 = 5] + 52

If 8 is the angle made by D3 to the surface, then the perpendicular compo-
nent D (measured on macroscopic scale) is given by D3 cos 8. (@ has been
assumed to be 45°.)

For a one-dimensional diffusion in a semi-infinite solid with initial
concentration zero and the surface concentration maintained constant at Cg,
the concentration at time t is given by (7, 8):

C(x, t) = C, erfc[——=7]

7(pt) /2

The boundary condition assumptions are reasonable in view of the fact that
pyrolysis occurs much faster than diffusion in solids at the temperature of
deposition of carbon.

At the distance of penetration,

n
v

C(x,t) 0

X = L('IT’Dt).I/Z

Adding 1,3-butadiene with traces of ]4C2H2 containing 0.224 mmoles 14¢ with
gross activity of ~10 mc will result in a specific activity of 0.8 wc/mole
gas mixture. Experiments with highly oriented natural graphite show that
D] >> Dz,and therefore 02 will be assumed equal to zero.

Then experimentally determined D = D] cos 45°, From the data of Kanter
(16) at 2347°C,

D, = 1.07 (10°'2) emé/sec

For heat treatment period of 200 hr, the penetration depth
X = 1.5y

This is virtually the limit of film resolution, so we would only be able to
determine if any penetration occurs. Much longer heat treatment times would
be needed to obtain a migration constant.
31

7.5.2 Detection

 

An activity of 10'5 uc/cm2 of an infinitely thin film of isotope gives
a perceptible image on a fast x-ray film (I1ford Nuclear Research Plates
D.1) in 24 hr (6). (Resolution must be checked before using this film.)
Calculations based on these assumptions should show the Tower limit of
activity required in deposits and also duration expected for film exposure
to beta radiation.

The hydrocarbon gas mixture used in this study should produce a pyro-
lytic carbon with a.specific activity of ~3.2 uc/mole carbon. The activity
of the deposited carbon is ~0.3 wmuc/cmé,which is sufficiently intense. How-
ever, the carbon which migrates will be much less intense.

7.6 Apparatus and Procedure for Synthesis of Acetylene

The apparatus used is schematically represented in Figs. 9 and 10. The
available compound containing 14C is solid barium carbonate Ba]4C03 (supplied
by Isotopes Division, ORNL). Acetylene_is most conveniently prepared by the
reaction of water on barium carbide (Bal4C ), which is first prepared by
?e?¥ing barium carbonate with metallic barium. The overall scheme is as

ollows:

H,0

 

14 +Ba metal 14 2 14 14 14 '
Ba CO3 ——HEEE——+-Ba C, + Bal =fTux~ Coflo + traces( CoHy + C2H6)
~600°C ~110-160°C

1. Ba]4C03(f1.5 mg, gross activity 20 uc) is weighed into a thick-

walled Pyrex centrifugal tube which contains barium metal (1 g, shredded).
Inactive barium carbonate (228.5 mg) is then added, giving a total of
1.515 mmoles of the carbonate. A further layer of shredded barium car-
bonate (1 g) is added. The equipment is set up as shown in Fig. 9, and a
slow stream of COp-free argon is passed through the tube to remove traces
of water. On being heated with a full bunsen flame, the reaction mixture
will become incandescent and set to a black mass; the tube is then cooled
in a dessicator. \

2. Next a file mark is made at A (Fig. 9) and a crack started with
a hot rod; the bottom of the centrifuged tube is then broken off inside
the flask shown in Fig. 10. The air in the system is swept out with argon,
and cooling baths are placed around traps E and F (Fig. 10). Five cubic
centimeters (5 cc) of water are added initially,and Tow heating is applied.
Twenty-five centimeters (25 cc) of water are slowly added,and the resulting
solution is then brought to boil in 20 min, and reflux is maintained for a
further 20 min. The acetylene and traces of other hydrocarbons produced
are condensed in the two traps.
 

 

Ar (puge gas)

——— - -, e ———— ——

—

 

"

 

 

 

 

 

 

1 M KOH
(remove COZ)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

BaC

14

 

 

 

 

 

KOH
(remove COQH

 

03

 

MASSACHUSETTS INSTITUTE OF TECHNOLOGY
SCHOOL OF CHEMICAL E#GUNEERING PRACTICE

A
OAK RIDGE NATIONAL LABORATORY

 

 

IApparatus for Synthesis of Barium Carbide

 

DATE

3-4-70

SHR CEPS-X-99

 

DRAWN BY FILE NO. lnc.
9

 

"

¢€
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Methanol and Dry Ice
(E)

 

 

 
 
  

 

 

Liquid
Nitrogen

(F)

 

 

 

 

 

 

 

 

 

 

 
 

CUZC'I2

 

MASSACHUSETTS INSTITUTE OF TECHNOLOGY
SCHOOL OF CHEMICAL ENGINEERING PRACTICE

AT
OAK RIDGE NATIONAL LABORATORY

 

Apparatus for Synthesis of Acetylene

 

 

3-4-70 SHR CEPS-X-99 10

DATE DRAWN BY lFILE NO. FIG.

 

 

 

et
34

/.7 Literature References

1. Beatty, R.L., and D.V. Kiplinger, "Gas Pulse Impregnation of Gra-
phite with Carbon," ORNL-TM-4434 (July 1968).

2. Boyd, G.A., "Autoradiography," Academic Press, New York (1955).

3. Brown, A.R.G., A.R. Hall, and W. Watt, "Density of Deposited
Carbon," Nature, 172, 1145 (1953). |

 

4, Carley-McAuley, K.W., and M. Mackenzie, "Studies of the Deposition
of Pyrolytic Carbon," AERE-R3726 (1961).

5. Catch, J.R., "Carbon-14 Compounds," Butterworth, New York (1961).

6. Cox, J.D., and Warne, R.J., "Syntheses of Isotopic Tracer E]ements,
Part 3," J. Chem. Soc. (London) Part 2, 1893 (]95])

/. Crank, J., "The Mathematics of Diffusion," p. 62, Oxford University
Press, London (1964).

8. Dienes, G.J., "Mechanism of Self-Diffusion in Graphite," J. Appl.
Physics, 23, 1194 (1952).

9. Egloff, Gustav, "The Reactions of Pure Hydrocarbons," pp. 230-277,
Reinhold, New York (1937).

10. Feldman, M.H., W.V. Goeddel, G.J. Dienes, and W. Gossen, "Studies
of Self-Diffusion in Graphite Using C-14 Tracer," J. Appl. Physics, 23,
1200 (1952).

 

11. Girifalco, L.A., "Atomic Migration in Crystals,” Blaisdell, New
York (1964).

12. Harvey, J., D. Clark, and J.N. Eastabrook, "The Structure of
Pyrolytic Carbon," Specia] Ceramics, p. 181 (July 1965).

 

13. Herzfeld, C.M., "Temperature: Its Measurement and Control in
Science and Industry," Part 2, p. 889, Reinhold, New York (1962).

14. "Industrial Research," 1970 Instruments Specific Annual, Indus-
trial Research Co., p. 100, Indiana (November 1969).

15. Johnson, W., and W. Watt, "Thermal Conductivity of Pyrolytic
Graphite," Special Ceramics, 237 (June 1962).

 

16. Kanter, M.A., "Diffusion of Carbon Atoms in Natural Graphite
Crystals," Physical Review, 107, 655 (1957). .

17. Kay, J.M., "An Introduction to Fluid Mechanics and Heat Transfer,"
p. 65, University Press, Cambridge (1957). |
35

18. Kedl, R.J., "Noble Gas Migration in the MSBR Reference Design,"
ORNL-TM-4344, p. 73 (August 1968).

19. Peebles, F.M., "Removal of ]35Xe from Circulating Fuel Salt of
the MSBR by Mass Transfer to Helium Bubbles," ORNL-TM-2245 (July 1968).

20. Reid, R.C., and T.K. Sherwood, ”Propertiés of Gases and Liquids:
{heir)Estimation and Correlation," 2nd ed., p. 396, McGraw-Hi11, New York
1966) .

21. Satterfield, C.N., and T.K. Sherwood, "The Role of Diffusion in
Catalysis," p. 16, Addison-Wesley, Reading, Mass. (1963).

22. Seith, W., and T. Henmann, "Diffusion of Metals: Exchange Re-
actions," AEC-tr-4506, Chem. Trans. (1955),

 

: 235 Street, J.C., and A. Thomas, "Mechanism of Pyrolysis," Fuel, 34,
4 (1955).

24. MWalker, P.T., "Chemistry and Physics of Carbon," Vol. 1, p. 175,
Marcel Dekker, New York (1965).

25. Weast, R.C. (ed.), "Handbook of Chemistry and Physics," 47th ed.,
Chemical Rubber Co., Cleveland, Ohio (1966).
'y

36.
37.

37

ORNL-MIT-99

INTERNAL

R.B. Briggs

K.E. Cowser

W.P. Eatherly

P.R. Kasten

H.E. McCoy

Lewis Nelson

C.B. Pollock

M.W. Rosenthal

J.L. Scott

D.B. Trauger

J.R. Weir

Central Research Library
Document Reference Section
Laboratory Records
Laboratory Records, ORNL R.C.
ORNL Patent Office
M.I.T. Practice School

EXTERNAL

S.M. Fleming, MIT
S.T. Mayr, MIT