ORNL-2349.txt

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Features of Aircraft Reactors

i 355

3 Y45t D358522 5 |

AIRCRAFT REACTOR EXPERIMENT -
METALLURGICAL ASPECTS

W. D. Manly

G. M. Adamson, Jr.
J. H. Coobs

J. H. DeVan

D. A. Douglas

E. E. Hoffman

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ORNL-2349

This document consists of 67 pages.
Copy/jb/ of 283 copies. Series A.

 

Contract No. W-TLO5-eng-26

METALLURGY DIVISION

ATRCRAFT REACTOR EXPERIMENT - METALLURGICAL ASPECTS

Manly
Adamson, dJr.
Coobs

DeVan
Douglas

. Hoffman

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DATE ISSUED

DEC 201957

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OAK RIDGE NATIONAL LABORATORY 3 yy5kL 0358522 5
Operated by
UNION CARBIDE NUCLEAR COMPANY
A Division of Union Carbide and Carbon Corporation
Post Office Box X

 

 

 
 

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ORNL-2349

C-8k-Reactors-Special Features

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EXTERNAL DISTRIBUTION

 

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H. “L. Yakel, Jr.

W. Zobel

ORNL - Y-12 Technical Library,
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AtomicsEHPE MM 1onal
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Buresu, Qfi,Aeronautlcs .
Bureau,of Aeronsutics: Generalf épresentative
BAR, Aerojet-feferal; “Agusa.. w‘:ff““fih,
BAR, Convair, San Diego TR

 
 

 
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207. Office '

208, bns 0ffice <

209. : Power Laboratory

210. Roffice :

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212. HrE Jjraft Company

213. 18 WM Y

"21k. National Advisory dommlttee ‘'for Aeronautics, Cleveland
215. National Advisory Committee for Aeronautics, Washington
216. Naval Air Development CentZr
217. Naval Air Material Center

218. Naval’ é% ine Test Station
219. Naval’ _

 
   
     
   
  

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220. Nuclea plopment Corporation of America
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222. Nuclear:Mgtdl$, Inc. o

223. Office of*N vgl Research ©

224, Office @f“ heiChlef of Naval Operétions (op- 361)

225. Patent Brda -j Washington oo F 5

226-229. Pratt 'and Wi
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234, Techniéa )
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236. USAF Pgoj , 55; Yo

237. U. S. Za, Jiologicay Defense Laboratory
238-239. Univerdit ® Californjh Radistion Laboratory, Livermore
240-257. Wright Air Development /ténter {WCOSI-3)
258-282. Technical Information gefivgcg Extension, Oak Ridge

283. Division of Research and_Dgwsprment AEC, ORO

  

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ABSTRACT

The selection of the proper structural material to be used in the
construction of an Aircraft Reactor is influenced by a number of important
requirements. The material must have an acceptable cross section, be
resistant to elevated-temperature corrosion by liquid metals, air, and
the molten salts, and be free of temperature-gradient mass transfer. It
must possess good creep properties 1n these environments and be free of
structural changes during service. Finally, it must be easy to form and
weld into relatively complicated shapes and be commercially available.
Inconel was selected as the structural material for the Aircraft Reactor
because it met these requirements.

The experimental work performed in the flelds of corrosion, creep,
and welding is described. Although a small amount of corrosion in the
salt and some mass transfer in both sodium and salt were observed, the
material performed quite satisfactorily during the lifetime of the reactor.

The creep properties of Inconel were adversely affected by the various
environments; however, by designing to limit the stress, successful
operation of the reactor was achieved.

Both the successful operation of the reactor and the relative ease
with which a multitude of welded joints in the plumbing circuit was
produced attested to the weldability of this alloy.

A major lesson learned in the construction of this reactor was the
importance of cooperation between designers, suppliers, construction and

procurement people, and materials people.
-1-

ATRCRAFT REACTOR EXPERIMENT - METALLURGICAL ASPECTS

 

The main role of the metallurgist in the design and construction of

* Aircraft Reactors is to assist in determining the proper structural

metal or alley for the containing vessel, in seeing that the chosen metal

or alloy is of suitable quality, and in establishing its limits of practicality.
Aircraft Reactors demend a great deal from the material of construction,

as can be seen from the following, somewhat formidable requirements:

(1) The alloy must have corrosion resistance to the fused salt fuel.

(2) It must have corrosion resistance to sodium and NaK.

(3) It must have resistance to temperature-gradient mass transfer in
both media.

(4) The material should have tolerable oxidation resistance, since it
will be operating at high temperatures in the ambient-air
conditions in the reactor pit.

(5) The material should have good high-temperature creep strength,

- which should be retained in various enviromments, such as the
fused salts and sodium.

. (6) The material should possess good high-temperature ductility to
withstand the high-thermal stresses and vibratory stresses that
will be imposed on it in addition to the static stresses.

(7) The material should be easy to weld so that the various complicated
components, such as heat exchangers, can easily be constructed.

(8) The material should be easy to form into the various intricate
shapes that are required in fabricating the pieces of plumbing
needed in the construction of the reactor.

(9) 1If at all possible, the alloy should be commercially available.
This would provide ample fabrication experience to produce various
sizes and shapes and a backlog of information on the stability
of the alloy, in addition to the information that would be
obtained from tests conducted by the Laboratory.

y (10) The material should be free of structural changes under the

operating conditions; otherwise the creep properties might be

adversely affected. This is quite important since the material

 
Wwill be stressed at a high temperature for long periods of time
under an intenge fielid of neutron radiatiocon.

(11) The material showuld not have a prohibitive cross section, so
that the critical mass for the reactor will not be unreasonable,
and, if at all possible, elements giving rise to strong capture
gamma. rays with long half lives should be avoided in order

that the reactor will be approachable after shutdown.

-6)

chosen as the structural material for the Aircraft Reactor Experiment.

The previous papers in this series(l have stated that Inconel was
Inconel is a nickel-base alloy with a nominal composition of 15% éhromium,
7% iron, and the balance nickel with residual quantities of titanium,

ailuminum, carbon, and silicen that come through from the melting practice.

A typical analysis (ir weight per cent) of Inconel is given below:

Cr 15

Fe 7

Mn 0.03

C 0.04-0.06
Al 0.15
Ti 0.25
Si 0.22
Ni Balance

This alloy, produced by the International Nickel Company at Huntington, West
‘irginia, has been commercially available for a long period of time. The
reasons for the choice of Inccnel as the construction material for the
Aircraft Reactor Experiment are given in the following sections of this report,
which explain the experimerntal work performmed in the fields of corrosion,

welding, and creep testing.

Corrosion of Inconel in Molten Salts
The most difficult requirement of the container meterial is good

cerrosion resistance to the molten fluorides used as fuel carriers. An

oxide coating or other type of protective film that is sometimes used for

 

 
protection against aqueous corrosion and in certain high-temperature
applications could not be used in the Aircraft Reactor, because the molten
fluorides are very good fluxing agents and are used every day in metallurgical
operations to clean up the surface of metals before various Jjoining and
brazing operations. Therefore, the choice of a container material must

depend upon finding an alloy that is, for the most part, thermodynamically
stable with the fluid at the operating. temperature range and conditions.

The corrosion testing program at the Oak Ridge National Laboratory to
determine the best structural material in contact with molten fluorides was
a cooperative effort of Chemistry, Experimental Engineering, and Metallurgy.
Many new corrosion testing techniques had to be developed during this study.
Techniques used included the static capsule test, the rocking or seesaw test,

(7)

Early in the molten salt corrosion program it was found, in static and seesaw

the thermal-convection-loop test, and the forced-circulation-loop test.

tests, that nickel-base alloys and the austenitic stainless steels exhibited
promising resistance to the corrosive action of the molten salts. Further
tests were then carried out on these alloys by means of low-velocity dynamic
tests, which were conducted in thermal convection loops. [A battery of such
loops is represented in Fig. 1 (21341).] By heating one leg of the loop
and cooling the other, a flow velocity of 2 to 6 fpm could be maintained
by the difference in density in the hot and cold leg. The usual operating
conditions were 1500°F on the hot leg, 1300 to 1350°F on the cold leg, and
an operating time of 500 hr. At the end of the operation the loop was cut
open for examination, and sections of piping were removed for metallographic
studies. The molten salt was examined for phase changes and metallic crystals,
and the amount of metallic constituents in solution was determined by chemical
analysis. In the first early screening tests the molten fluorides used were
mixtures of sodium, lithium, potassium, and uranium fluorides. The operating
characteristics of the various alloys with these mixtures were such that
nickel and nickel-base alloys would operate for the full 500 hr without
stoppage of flow, as shown in Fig. 2 (T-4865). The austenitic stainless
steels exhibited stoppage of flow as a result of the cold zone of the loop
being obstructed by a higfi—melting—point compound, identified as K2NaCrF6.
Other nickel-base-alloy loops, Hastelloy B and Hastelloy C, were also
tested but could not be properly evaluated because of difficulties with
welding. The A-nickel loop, even though it operated for 500 hr, showed a

 
 

 

Fig. 1 (Photo 21341) Battery of Thermal Convection Loops Used to Conduct Low Velocity Dynamic Tests.
 

 

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SECTION NO. 4 SECTION NO. 7

INCONEL THERMAL CONVECTION LOOP

S00 HRS. AT I500°F

T-3096 250X

SECTION NO. 8

rig. 2 (1-4865) HOT LEG OF STANDARD LOOP
metallic mass transfer deposit in the cold zone. Examination of sections
from the Inconel lcops showed rather deep corrosive attack in certain parts
of the loop, as can be seen in Fig. 2. While the attack on Inconel was
excessive under the tegt conditions employed, this alloy was preferred over
all other wetal-base ailoys tested because of its strength properties,
fabricability and availability. When it was found that this alloy had
better compatibility with salt)mixtures of sodium, zirconium, and uranium
fluorides (NaF-quFu_-»UFh, 50-46-4 mole %) than with other salt mixtures,
extensive studies with this mixture and Inconel were begun. Investigations
were made of the corrosion mechanisms and the various parameters affecting
corrosion, such as fluoride salt purity, time, surface area and volume
ratio, flow velocity, and temperature.

From the photomicrcgraphs in Fig. 2, i% can be observed that the attack
of the molten fluorides on Inconel resulted in subsurface voids. A careful
study was made to determine what caused this type of attack. Chemical
analyses of the moiten fluorides showed that the iron and nickel contents
had decreased and that the chromium content had increased during the 500-hr
operation. The greatest change found was the reduction in chromium content
of the Inconel sections taken from the hot-leg wall, and this was first
demonstrated by a magnetic metallographic examination technique. In Fig. 3
(Y-6549 and Y-5954) two photomicrographs are presented which were taken on
the same section of pipe remcved from the hot leg of a loop. One photo-
micrograph shows the general corrosion attack experienced with this
environment, and the other shows the same sample with a collecidal dispersion
of iron particles applied to the specimen. When the sample was made the
core of an electromagnet; the iron particles collected on the portion of the
metal exhibiting ferromagnetism. This indicates that there was a severe
leaching of chromium from the Inconel, since the Curie point was now below
room temperature, and this could only occur if the chromium content, of the
nominal 15% Inconel, is less than 8%. When this severe leaching was found,
sections of the pipe were removed from several thermal convection loops,
and turnings were machined from the inside diameter and analyzed for nickel,
iron, and chromium. The data from three such experiments are plotted in
Fig. 4 (T-3493), where it can be seen that the chromium content dropped

appreciably below the average level to a depth of 25 mils from the surface.

O

 
Loss

Chromium

ting

er Depict

Magnetic lay

 

 

 

I1lustration of the Magnetic Metallographic

(Y-5954 - 65L49)

Technique.

3

Fig.
 

CHROMIUM-CONCENTRATION

 

 

 

   

 

 
 

 

 

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0 10 20 30 40

Fig. 4 CHANGE
(T-3493)

DEPTH FROM SURFACE MILS

IN CHROMIUM CONCENTRATION WITH DEPTH HOT LEG OF
THERMAL CONVECTION LOOP
-0~

The depth of chromium removal could be correlated with a lower limit at
which the voids were visible by metallographic examination. Voids of this
same type have been developed in Inconel by high-temperature oxidation tests
and high-temperature vacuum tests in which the chromium is selectively
removedu(B) Holes similar to these have also been developed in copper-brass
diffusion couples and by the dezincification of brass, as shown in the

(9)

Careful metallographic examination indicated that the voids did not

experimental work of the Sylvania Electric Products Company.

inter-connect to the surface. Hot-helium leak tests on sections in which the
unattacked areas had been removed by machining indicated that no helium was
leaking through the attacked areas. ILiquid penetration tests indicated that
the penetrant had remained at the surface of the specimen instead of extending
into the holes or voids.

After the tests were completed and the data carefully examined, the
mechanism for the formation of subsurface voids in the solid solution Inconel
Jlattice was then explained by the following steps:

(1) The chromium is oxidized from the Inconel surface by reacting with
impurities or the constituents of the molten fluoride mixture in
contact with the Inconel wall. These reactions will be explained
in a separate part of this paper.

(2) When the chromium is removed from the surface, the chromium from
the bulk metallic section diffuses into the area of lower concentration,

‘ down the concentration gradient.

(3) Metallic diffusion is by a vacancy process, and when the diffusion
is monodirectional, 1t 1s possible to obtain an over-equilibrium
number of vacancies in the metal.

(L) The excess vacancies precipitate from the crystal lattice in areas
of disregistry in the metal to form voids or holes. The main areas
of disregistry are the grain boundaries and inclusions in the metal.

(5) The holes are empty and do not connect with the surface.

(6) The voids tend to agglomerate and grow in size with an increase in

time and/or temperature.

These observations showed that the corrosion process of the molten salt

on Inconel was mainly the selective removal of chromium from the solid-solution

lattice. An attempt was then made to determine the manner by which chromium

 

 

 
-10-

was oxidized from the alloy.

In examinations of various corrosion or chemical reactions that could
be responsible for the removal of chromium from.these alloys, it is well to
classify the reactions into three types, as explained in another paper of

(6)

this series.

Impurities in the Melt

 

An example of this type of reaction is

(1) FeF, + Cr ~ CrF, + Fe.

Other impurities that would also enter into chemical reaction of this type

are NiFe, CrF3, FeF3, HF, and UFS.

Oxide Films on Metal Surfaces

 

The fused fluorides would remove the oxide film from structural metals

by reactions of the type
(2) -2Fe203 + 3CrF) ~ 3Cr0, + MFeF3°
The ferric fluoride thus formed would dissolve in the melt and would then attack

the chromium of the Inconel by reaction (1).

Constituents in the Fuel

 

The chromium will also react with constituents of the fuel, and the most
important reaction of this type is

(3) Cr + 2UF) - 2UF3 + CrF,.

With the realization of the methods by which chromium was removed from
the Inconel, a study was then made on the effect of several variables on the
corrosion process. In the early days of the corrosion testing program, the
first, and the most important, variable studied was the effects of the
impurity level of the molten fluorides. This is seen in Fig. 5 (T-45S4L),
which indicates that variations in the impurity level of the molten salts with
NiFe, FeF2,
15 mils. Sample T-4L485 was taken from a loop filled with molten salt that had

and HF could cause an increase in corrosion level from 5-1/2 to

a nickel and iron concentration of 1850 ppm, which gave a maximum attack of
15 mils. Sample T-3322 is from a loop that had nickel and iron impurities of
710 ppm, which resulted in a maximum attack of 8 mils. The salt from which

sample T-4410 was taken had iron and nickel impurities of less than 120 ppm

 

 
 

 

Fig. 5

 

T-4485 250X T-3322

FLUORIDE BATCH HIGH IN FE & NI FLUORIDE BATCH MODERATE

T-4410 250X T-3920

FLUORIDE BATCH LOW IN FE & NI

INCONEL THERMAL CONVECTION LOOPS-500 HRS. AT I500°F
(e-1544) EFFEGT OF FLUORIDE BATCH PURITY

 

250X

IN FE & NI

 

250X

FLUORIDE BATCH HIGH IN HF

Ll
-12-

and HF impurities of less than 2 x lO"LL mole/liter in the exit purging gas,
which resulted in a maximum attack of 5~l/2 mils. Sample T-3920 was taken
from a loop operating a salt in which the iron and nickel impurities were

less than 600 ppm and the HF impurities were approximately 5 x lO_LL mole/liter
in the exit gas, resulting in an attack of 14 mils after only 250 hr of
operation.

This study dramatically showed that the wide variation in the depth of
corrosion was due to the impurities in the bath. Steps were then taken to
develop production procedures by which a controlled purity could be maintained
from batch to batch. This led to the development of the hydrofluorination
process,(6) which resulted in the corrosion level becoming more reproducible
from test to test.

Several thermal-—convection loops were operated at a 1500°F fluid
temperature for various times, and the data are presented in Fig. 6 (T~-384L4).
From this figure it can be seen that the corrosion was quite rapid at first,
leveled off after 250 hr, and then continued as a straight-line function of
attack versus time. The initial rapid attack is due to the reduction of
impurities in the molten fluoride salt, to the reduction of oxide film on the
metal surface, and to reactions with the constituents of the fuel. The second
phase of the corrosion process 1s caused by the mass transfer of chromium from
the hot portion of the loop to the cold portions. The relative depths of
attack, as evidenced in Fig. 6, can be changed by impurity level and temperature
level, but the over-all shape of the curve will remain approximately the same.
In this case, it is seen that the attack after 250 hr is 7 to 8 mils; if the
test continued to 3000 hr, the attack would increase to a depth of approximately
18 mils. At the 1000-hr level the depth of attack is in the neighborhood of
10 mils.

The phenomenon of mass transfer in heat transfer media may be described
as the transport of material from one portion of the loop to the other and
is usually revealed as removal of material from the hot leg and its deposition
in the cold leg. This was first observed with liquid metals when materials
went into solution in the hot leg and deposited on the loop walls or as
dendritic crystals in the cold zone of the heat transfer system. In this

case the driving force for the mass transfer process was the difference in
MAXIMUM DEPTH OF ATTACK
MILS

Fig. 6

 

 

l |

O 1000
TIME HOURS

   
  

® SAME FLUORIDE BATCH
A COMPARABLE BATCHES

| |

2000

(r-3844) EFFECT OF TIME ON CORROSION DEPTH FLUORIDE IN

INCONEL

£l

 

|

3000
-1h4-

solubility of the constituents of the solid metal in the liquid metal at the
temperature extremes of the heat transfer system.

With the fused fluorides the manner by which the material is transported
from the hot to the cold leg is somewhat different. The chromium of the
Inconel reacts with the UF&’ and the equilibrium constant for this reaction
varies as a function of the temperature. This results in a difference in the
concentration of chromium as CrF2 in the fused salts at the various temperatures
in the circuit. It is fortunate that the activity of chromium in Inconel
in contact with the fused salts at the higher temperature cannot support the
production of chromium at activity one at the lowest temperature in the
fused salt circuit. In the case of the Inconel—NaF-ZrFu-UFu system, the
mass transfer process then continues by the chromium coming out of solution
by diffusing into the Inconel wall at the lower temperature. Thus, the rate-
controlling step for the mass transfer process is diffusion-controlled in
the cold lega(6)

As the corrosion workers became more experienced and the purity of the
fused fluorides was made reproducible from batch to batch, the testing program
moved from thermal-convection loops into forced-circulation or pump loops,
which, from the standpoint of flow, more realistically reproduced the operating
conditions of the reactor.

The first parameter studied was time; a series of forced-cireculation loops
were operated for various times with a maximum fuel temperature of 1600°F
and a maximum wall temperature of 1700°F. The fuel temperature drop was 300°F
and the Reynolds number was 6000. The ratio of the hot-leg surface area to
the salt volume was 2.5 in.2/in.3. Data from these loops are presented as Fig. 7
(ORNL-~LR-DWG 17739). Examination of this plot indicates a rapid attack for
the first 15 hr and then a slower or relatively constant rate of attack thereafter.
The slope of the straight line that approximates the data of the plot indicates
that the rate of attack after the first 15 hr is approximately 1 mil in 280
hr. The corrosion after 1000 hr is approximately 7 mils.

The next variable studied was the effect of temperature on the corrosion
process. The importance of the Inconel-fuel interface temperature as the

controlling factor in fluoride corrosion became apparent during attempts to
MAXIMUM ATTACK (mils)

 

o

ORNL-LR-DWG 17739

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 7 Effect of Time on Corrosion Depth for Inconel Forced Circulation Loops

 

MAXIMUM TUBE WALL TEMPERATURE: 1700°F 9
MAXIMUM FUEL TEMPERATURE : 1600°F .
TEMPERATURE DROP: 300°F /
///
//
//
22 —
1/
25 |_—"]
e
23C /
a7
534
0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000
TIME (hr)

a1
-16-

develop a suitable pump loop design for corrosion experiments. Earlyw}oops
having hot-leg sections in the form of a coil and heated by direct re%astance .
were found to be attacked very unevenly around the cross section of tfi; wall,
as is seen in Fig. 8 (T-7246). Marked temperature differences were found -
to have occurred across the bend of the coil, which resulted in quite
different attacks on the inner radii and the. outer radii of the bends, although
the bulk fluoride temperature over any cross section was the same. Examination
of Fig. 8 will reveal that the outside of the bend is practically free of
attack but that the inside of the bend of the coil was attacked to a depth
of approximately 10 to 12 mils. When it was found that the hotter surface
would corrode at such a greater rate and act similar to a sacrificial anode
in the case of aqueous corrosion, additional tests, with straight heater sections,
were run to determine the true effect of temperature. In one series of tests
the maximum wall temperature and the fluid temperature drop were maintained
constant at 1T7O00°F and 200°F, respectively, while various fluoride temperatures
were achieved by changing the Reynolds number. The sample from a loop that
operated with a 1650°F fluid temperature with a Reynolds number of 7250 exhibited .
a maximum attack of 10 mils. The sample taken from a loop operated at a fluid
temperature of 1500°F and a Reynolds number of 6500 exhibited 9 mils of attack.
This information is presented in Fig. 9 (T-9509). From data like these, it
was found that the boundary-layer temperature was more significant than the
buik fluid temperature in the corrosion process.
Another series of experiments were conducted to determine the effect of
the wall temperature or the fused salts-Inconel interface temperature on the
corrosion depth, and these data are presented in Fig. 10 (T-9035). In this
case, varying the Reynolds number increased the wall temperature of various
loops from 1560°F to 1635°F, while the maximum fluid temperature was held
constant at 1500°F. The loop operated for 1000 hr with a témperature gradient
of 150°F. The 1500°F wall-temperature sample gave a corrosion depth of 3 to
5 mils, whereas, the sample from the 1635°F wall temperature exhibited an attack -
of 8 to 9 mils. These tests lend further support to the above conclusion
concerning the importance of the maximum Inconel-fused salt interface temperature

to the corrosion process. In general, attack in pump loop tests increased
  

 

T-T24€&
' - ¥ o "t L o u"cL
5 : . :'::' 2] = .
pon SR i e
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INSIDE OUTSIDE = i -

fe—— w1000 X09Z—l
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fe—— 4100 RODT—el

 

L/ o = I g -
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INSIDE OUTSIDE
l NOT TO SCALE

 

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DIRECTION OF ‘_*"_
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INSIDE OUTSIDE

rig. 8 (r-1206) EFFECT OF BEND IN ATV LOOP

 
 

1-8663

MAX.

3 e Fa
Fig. 9

 

 

250X
1650°F
Rg-2750

ATTACK- 10 MILS

EFFECT OF FLUID

epesf B R

 

 

 

 

EEEFERREL

T-9507 250X

1500°F
Re-6500

MAX. ATTACK- 9 MILS

TEMPERATURE

81
 

 

 

 

I i . gy >
....:lla ': " . 'u :-::-, .
: i ;: "";;'. - "-
& ol ] A
. s g e
_ vos) = .
’ | oosl g = ’ £ ’ &
T-90I19 250X T-8227 250X
WALL TEMP.- I550°F WALL TEMP - I1635°F

MAX. FLUID TEMP. — IS00°F

TIME — 1000 HRS.
AT — 200°F

= VARIATION IN ATTACK WITH WALL
TEMPERATURE

 

61
-20-

approximately 2 mils for a SO°F increase in wall temperature in the range
1550 to 1TO0°F.

In considering the over-all effect of temperature, two rather separate
and distinct observations must be discussed. The first concerns the entrance
of the salt in the as-received or unreacted form into the loop; and the
second concerns the effect of the "equilibrated salt," or the mass transfer
portion of the corrosion process. During the first or initial stage of
corrosion process, the salt will attack the entire loop. However, since the
diffusion of chromium from the loop wall into the salt occurs at a rate dependent
on the temperature, the attack proceeds more rapidly at the hottest portion
of the loop. During the second part of the corrosion process, or the mass
transfer step, the attack would theoretically occur over all the heated surfaces.
However, in actual practice, this 1s not observed since the salt in most
circuits does not reach equilibrium in the cold leg of the loop; consequently,
there would not be a driving fOrce For corrosion except in the hottest Inconel-
fused salt reglons. 1In all the experiments to date the mass transfer attack
has been restricted to a small section of the loop near the point of the
highest wall temperature. The amount of chromium taken into the salt also
seems to be a function of the maximum Inconel-fused salt interface temperature,
which is understandable since the diffusion rate of chromium in Inconel at
temperatures of 1500°F and greater is quite rapid compared with the rate at
which the chromium is being removed by diffusion into the colder portions of
the plumbing circuit.

The effect of the ratio of the structural-metal-surface area to the volume
of salt on the depth of the corrosion attack has been studied, and the results
are presented in Table I. These data were obtained by changing the pipe

Table I. Effect of the Surface-Area-To-Volume Ratio On Fluoride Corrosion In
Inconel Thermal Convection Loops

 

Surface-Area-To-~

 

Loop Volume Ratio

No. Pipe Size (in.2/in.>) Attack

359 1/2-in. tubing 10.5 Moderate to heavy to 4 mils .
352 1/2-in. pipe 6.5 - Moderate to 5-1/2 mils

349 1l-in. tubing 4.6 Iight to moderate to 9 mils

 

 
21-

size that was used to make thermal-convection loops. It is interesting to note
that the corrosion depth increases from 4 mils to 9 mils as the surface-area-
to-volume ratio decreases from 10.5 in.g/in.3 to 4.6 inog/in,3. This is to be
expected since there is less surface area to supply the chromium to saturate
the given volume of the fused fluoride. This relationship is not quite

linear, and there are probably inconsistencies in these results caused by the
variation in the flow velocity brought about by the changes in pipe sizes.

This same effect has been studied in forced-circulation loops, and it was
fcund that the ratio of the total hot-leg surface area to the total loop volume
was quite important in determining the depth of attack in the hot region. It
is essential to point out that during the mass transfer part of the corrosion
process the effect of the ratio of the hot-leg surface area to the total area
of the loop is extremely important since the rate-controlling step of mass
transfer is the diffusion of chromium into the cold zone. Therefore, if there
is a large sink to remove chromium from the saturated fused fluoride salt, there
will be a greater amount of corrosion in the hot zone to keep the salt saturated
with chromiuvm. If an isothermal system is being saturated, then the surface-area-
to-volume ratio would be the limiting factor on the depth of corrosion;
however, when a temperature gradient exists, the ratio of the hot-leg surface
area in relation to the rest of the loop surface area exposed to the fused
fluorides must be examined.

Another variable that was studied 1s the effect that the flow velocity has
on the over-all Inconel-fused fluoride corrosion. Comparison of the results
obtained from thermal-convection loops with the data obtained from forced-
circulation loops showed that the flow rate had a very minor effect on the
corrosion of Inconel by the NaF-ZrF) -UF), (50-46-4 mole %) molten salt fuel. 1In
order to further evaluate the effect of flow velocity, forcedecirculation loops
were operated under identical temperature conditions, but the flow velocity
was increased to give Reynolds numbers of 5,200, 9,620, and 14,250. These flow
rates produced depths of corrosion of 4, 3, and 6.5 mils, respectively. From
the small spread in the corrosion data and the lack of correlation between the
filcw rates and depths of attack and information from previous tests, it was

concluded that the flow velocity or Reynolds number has a very small effect

on corrosion.

 
- 22 .

Inconel-Sodium Corrosion

 

In addition to having corrosion resistance in the fused fluorides, the
structural material must also have corrosion resistance in sodium or NaK. In
early studies on the compatibility of various structural metals in high-
temperature sodium or NeK, it was found that the nickel-base alloys and
austenitic stainless steels could he considered as suitable containers for
sodium at temperatures up tc 1500°F. The main mode of attack of liquid
metals on solid metals is the even surface removal of the solid metal to
saturate the liquid metal.(lo) Therefore,in an isothermal system the metal
corrodes only to a depth necessary to satisfy the solubility requirements of
the 1liguid at the operating temperatures. In static corrosion experiments
with Inconel a 1/2- to 1 l/2—mil corrosion attack, primarily at the grain
boundaries, was experienced. This attack resulted from the selective removal
of elements such as silicon, titanium, aluminum, and carbon. It was also
found that the oxygen level in the sodium or NaK must be kept below
approximately 50 ppm to minimize the decarburization reaction of the sodium
oxide. This decarburization reaction would result in a decrease in the load-
carrying abilities of the alloy, as will be discussed in a later section of
this report.

One of the most difficult problems in the use of liquid metals at high
temperature is temperature-gradient mass transfer. It was found in corrosion
tests of Inconel-forced-circulation loops filled with sodium that the
oxygen level had to be kept as low as possible to minimize the amount of mass
transfer. The effect of temperature on the amount of material mass-
transferred to the cold leg of sodium-forced-circulation loops was studied,
and examination of Fig. 11 (T-10779) shows that with a minimum fluid
temperature of 1350°F, there is a very small amount, of deposit in the cold~-
leg section of & loop that operated for 1000 hr with a temperature drop of
300°F. When the maximum fluid temperature was increased to 1500°F, there
was a large cold-leg deposit approximately 10 to 12 mils in thickness.

The effect of the temperature gradient on the amount of mass transfer
experienced in Inconel sodium loops was also studied. Three forced-circulation
loops were operated, with oxide cold traps at 300°F and with hot-leg temperatures

of 1500°F for 1000 hr. The cold-leg temperature was varied to give temperature
T-10779
UNCLASSIFIED

sg F B

  

 

 

 

 

EEEEERBER ~oERE]

!
FEEFERRER ™

 

T-10425 250X T-10186 250X
MAX. FLUID TEMP-1350°F MAX. FLUID TEMP.-1500°F

EC

ECONOMIZER SECTIONS AFTER 1000 HRS. OPERATION

~uEFFEGT OF TEMPERATURE IN Na-
INCONEL ATV PUMP LOOPS

 
-2 .

gradients from 150 to 4O0°F. The data obtained from this series of experiments
are plotted in Fig. 12 (ORNL-IR-DWG 16735). Examination of this plot shows
that the 150°F temperature drop gave a 12-mil deposit that weighed 5.5 g.
As the temperature drop increased to 4OO°F, the mass transfer deposit grew to
20 mils in thickness and weighed 21.2 g. The weight of the material in the
deposit was obtained by scraping the loop walls, and the material was found
to be composed of approximately 90% Ni and 10% Cr, iron being virtually absent.
Since the maximum temperature of the sodium in the Aircraft Reactor
Experiment was 1350 to 1370°F and the coldest portion of the sodium circuit
was 1210 to 1250°F, it was felt there would be a minimum amount of mass transfer
in the sodium circuit, and that it would not interfere with the operation of
the reactor; experience showed this to be the case. However, in the reactors
of the future, in which the sodium metal interface temperature might be quite
high, use of nickel-base alloys may not be feasible because of excessive mass
transfer. Fortunately, the austenitic stainless steels could be substituted;
since they are much less prone to mass transfer, they can be operated at a
150°F higher temperature before they start showing the degree of mass transfer
experienced with nickel-base alloys.
It was found from these corrosion experiments that the choice of Inconel
was a sound one for this reactor system. The corrosion of Inconel by the
fused salt (NaFu-ZrFu-UFu) resulted in a depth of attack of approximately 7 to
10 mils for 1000-hr operation at 1500°F, and the only mass transfer that would
occur in the Inconel-fused fluoride system would be the plating of chromium
on the colder regions of the plumbing circuit, with the chromium then diffusing
into the bulk Inconel wall. This type of mass transfer would not have a
serious effect on the hydraulic conditions of the plumbing circuit, whereas,
a dendritic mass transfer deposit would soon cause stoppage of flow in high-
velocity plumbing circuits because of plugging. The Inconel-sodium corrosion
situation was also free of difficulties; however, at higher operating temperatures,

temperature-gradient mass transfer would be a problen.

MECHANICAL PROPERTIES

The engineering life of structural members and component parts of a

 

power-producing, high-temperature device is controlled mainly by the creep and

 
UNCLASSIFIED
ORNL~-LR—-DWG 16735
2> | | i |
MAXIMUM FLUID TEMPERATURE : 1500°F
MINIMUM FLUID TEMPERATURE: VARIABLE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

o ®
g /
20
'—
5 e
O
a. /
5 15 o
L /
O
% 10 //
L
= /
I 3
5 5 ///
= -
”
”
O < -
O 50 100 150 200 250 300 350 400

FLUID TEMPERATURE DROP (°F)

rig. 12 Effect of A7 on Mass Transfer in Sodium - Inconel Pump Loops.

 

G
- 26 -

fatigue strength of the material. The design and mode of operation of the
Aircraft Reactor Experiment tended to eliminate fatigue as a limiting factor;
therefore, the major emphasis was placed on the creep strength of the material.
Since the high-temperature, oxidation-resistant alloy Inconel was chosen,
data were already available on the creep characteristics of this material
throughout the temperature range of interest. However, these results had
been obtained primarily on wrought bar in an air environment. Since most of
the reactor would be constructed from plate, sheet, and tubing and be subjected
to the corrosive fused salt environment, it was imperative that many additional
parameters be explored before the reactor could be intelligently designed,
constructed, and operated.

A testing program was formulated to explore the effect of such parameters

as environment, geometry, multiaxial stresses, and thermal history.

Environment

In the preceding section it was shown that Inconel would be attacked by
the fused fluorides by the selective removal of chromium. In addition to the
subsurface voids formed, the second-phase carbides that increase the creep
strength of a metal would be removed as a result of the solubility of carbon in
the metal lattice being increased when the chromium content is decreased. The
over-all effect of environment on the rupture life of the material is dependent
to a considerable extent on such interrelated variables as time, temperature,
grain size, and depth of attack.

Figure 13 (ORNL-ILR-DWG 17911) illustrates some of these effects by comparing
the stress-rupture strength of Inconel in argon and the fused salts at three
temperatures and over a wide range of stresses. Argon was used as an environ-
ment for obtaining the base-line data because it is inert to metals and does not
introduce spurious effects, such as variable oxidation rates, which might tend
to make analysis of the results difficult. Having demonstrated the engineering
importance of obtaining the data in the service environment, a more complete
picture was obtained by determining the interdependence of stress, strain,
and time at the temperatures of interest. Data from this program are presented
in a separate paper of this series.(ll)

In addition to the fused salt enviromment the structural material was in

contact with sodium during the operation of the reactor. A similar testing
STRESS (psi)

UNCLASSIFIED
ORNL-LR-DWG 17944

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

20,000 N*--...________
~\
s Sl 30
S NN Qo
SN ap
y .\ GON
10,000 ’35-» \"‘n
Oop N
W
\ /0\b
5000 N
NN
}5\\\\
Oop /\
A
(/ \
\
2000
N
1000 | 500 {000 2000 5000 10,000
{ 2 5 10 20 50 100 200 ’
TIME (hr)

Fig. 13 Comparison of the Stress-Rupture Strength of Inconel in Argon and Fused Salts at 1300, 1500, 1650°F

 

L
 

- 28 -

procedure was carried out to demonstrate the compatibility of Inconel with this
environment. It was established early in the test program that sodium could
have a somewhat deleterious effect on the creep properties of Inconel. This
effect is depicted in Fig. 14 (ORNL-IR-DWG 20150}, which is a plot of data

from tests of Inconel sheet in argon (test No. 144) and sodium (test No. 117)

at 1500°F with a stress of 4000 psi. The accelerated creep and increased
elongation at rupture were caused by the decarburization of the material. This
effect is believed to be the result of oxide contamination of the sodium.
Likewise, it was demonstrated that Inconel could be readily carburized by
sodium if it was contaminated by carbonaceous material. Due to the difficulties
involved in analytically determining oxygen in alkali metals, the critical

oxide level at which the decarburizing reaction begins has not been determined.
However, in Fig. 15 (ORNL-ILR-DWG 20146) the results of numerous tests in a
sodium environment are superimposed on data obtained in argon, and the excellent
correlation of these data demonstrates that when commercially available high-
purity sodium is used and contamination in the system is avoided, sodium is

inert to Inconel.

Geometry

Geometry is an important parameter because of the desire to obtain the
most efficient heat transfer system possible without a serious sacrifice in the
strength characteristics of the structure. It was assumed that the thinner
the wall, the more pronounced would be the effect of any corrosive attack. This
was found to be the case from creep tests in fused fluorides in which various
section thicknesses were used. Examination of these data indicates that the
best compromise between strength and heat transfer efficiency would be a
0.060-in. wall thickness; therefore the fuel-bearing tubes were fabricated to
obtain a nominal wall thickness of 0.060 in.(e’ll)

Multiaxial Stresses

 

Creep tests provide information on the strength of metals at elevated
temperatures under uniaxial stress conditions. Unfortunately, it is impossible
to design complicated power-producing devices so that only uniaxial stress -
will be imposed. In order to provide some data which would tend to define
the magnitude of the effect of stress distribution, a test program was devised
to determine the stress-rupture strength of Inconel under biaxial stress

conditions. Closed-end tubes in which a reduced gage section was machined.
ELONGATION (%)

&

UNCLASSIFIED
ORNL-LR-DWG 20150

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

/
/
// /
/
/ /
/ /
/ e
/ 1~
/
/
/ _—
// P /
S
y

Fig. 14 Comparison of Inconel Sheet Tested in Sodium at 1500° F and 4000 psi.

 

6L
STRESS (psi)

UNCLASSIFIED
ORNL-LR-DWG 20t46

 

 

 

 

 

 

 

 

 

 

 

 

 

LTI I LT

LEGEND FOR SODIUM DATA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A RUPTURE
O 10 % ELONGATION
/* 107 ELONGATION 0 5% ELONGATION
Ao A 1% ELONGATION
S LSy N =fl<~4$‘74 ~_ ®  05% ELONGATION
s ~— — RUPTURE
# \‘--.."""- \ \\._k N N
~\~‘~. ™ V“\‘N ‘\\\\
5000 f R NG
TR
2 7o ELONGATION / o s X
| b4 Py
{ o ELONGATION / " fi o &
EREE , . * O
NG N
05 c7° ELONGATION _/ - \\ik\kfl‘
2000 SNOININNN
1000
4 2 5 10 20 50 100 200 500 4000 2000 5000
TIME (hr)

Fig. 15

Comparison of Stress-Rupture Properties of inconel Sheet Tested in Sodium and Argon at 1500° F.

10,000

ot
- 31 -

were internally pressurized. The resulting stress distribution provided a
biaxial stress condition, with the hoop stress being twice the axial stress.
The test apparatus was designed so that any desired environment could be used,
and results were obtained in argon, scdium, and fused salts. There is scme
decrease in final elongation at rupture under the multistress condition, but
in general it appears that stress distribution is not a serious problem under

conditions of operation of this reactor.

Thermal History

 

The thermal history of a metal can be an important factor in the high-
temperature propertles of the material either from the standpoint of grain size
or from recrystallization. Tests were run to determine the effect of grain
size, and it was found that at 1300°F a fine-grain size gave a better creep
resistance but that at a temperature of 1500°F or above the coarse-grain size
enhanced the creep resistance.

Complete elimination of cold-worked metal in structural parts is necessaxry
to prevent recrystallization from occurring during service. If recrystallization

(11)

The results of this creep testing program emphasize the importance of fully

does occur during the creep test, the creep rate will accelerate rapidly.

exploring all parameters affecting the behavior of a material at high temperatures.
The successful design and operation of all high-temperature reactors depend in
large part on the extent of our knowledge of the behavior of a material under

the various service conditions.

WELDING

It was recognized that the vessels, components, and piping of the Alrcraft
Reactor would be exposed to a corrosive environment at elevated temperature
ard to relatively severe and unpredictable stress conditions. The requirement
that the welded joints be flawless, in order to ensure leak tightness throughout
the life of the reactor, made the use of optimum welding procedures mandatory.
Inert arc welding had been found to be far superior to metallic arc welding for
fabrication of test assemblies and was adopted almost exclusively for fabri-

cation of the reactor. The critical nature of the work to be accomplished

 

 
 

- 32 -

made it necessary that the welders bve trained and qualified for the highest
quality of welding possible.

An inert-gas-shielded tungster-arc welding procedure was developed by
experiment which would ensure crack-free, porosity-free, full-penetration welds
in the reactor piping. Heretofcre, common practice for achievement of full-
penetration welds in piping was through the use of a backing ring, which in
most instances was impossible to remove. The crevices resulting from this joint
design constituted a built-in crack for possible propagation during thermal
stress at elevated temperatures. The procedure developed at the Osk Ridge
National Iaboratory incorporated an inert-gas backup, which eliminated the
need for a backing ring. Full peneiration was achieved by providing a root
opening, and an INCO No. 62 Inconel welding wire was used to deposit the root
pass. The tungsten arc was shielded with argon, while helium; because of its
lower cost, was used to purge the piping and to provide the shielding of the
underside of the root and subsequent passes during welding. A typical butt
weld in 3-in. dia., sched-40 pipe is shown, after welding, in Fig. 16 (Y-22363).

The welding procedure was prepared according to specification PS-l,(lE)

as was
a corresponding welder's qualification test, specification QTS—l,(l3)

A number of welding operators were trained and thoroughly tested. The
efficiency of the training was demonstrated by the relatively low number of
rejected welds despite the high visual, dye-penetrant, and radiographic
inspection standard used. Approximately 500 welds were used in the fuel
circuit of the reactor, with only 5% having to be reworked.

As a result of the success of the welding procedure in the reactor piping,

(1)

Joint designs by which full-penetration, inert-gas-backed, high-quality welds

a Jjoint design manual was prepared. The manual consists of sample weld

could be obtained in the fabrication of the various reactor components, including

valves and heat exchangers. The shortage of time and the lack of personnel

for training prevented the universal adoption of the ORNL welding procedure.

Several components which were fabricated by conventional inert arc wvelding

and metal arc welding procedures were found to be questionable and were virtually

rebuilt in the interest of maintaining the quality standards of the fuel piping.
A typical example was the rebuilding of the heat exchangers. Innumerable

flaws were detected, indicating inadequate workmanship. Typical was the
33

 

ONE INCH

Fig. 16 (Y-22363) Typical Full Penetration Weld in 3-in. Schedule 40
Pipe.

 
-~ 3 -

accidental arc strike shown in Fig. 17 (Y-12126). Metal arc welds, expressly
forbidden in the specifications, were used to attach lugs on the fluid piping
(see Fig. 18, Y-12127). These welds were found to be cracked, full of porosity,
and encrusted with a slag which at elevated temperatures would have rapidly
destroyed the piping. The heat exchangers were completely redesigned and

rebuilt at Oak Ridge National Iaboratory, and use of inert arc welding procedures
and a Joint design permitted full-peretration welds of the highest quality.

Several components were not subcontracted, partially because of the
difficulties that would have been encountered and because of the time element.
Typical of these was the fabrication of the impellers for the reactor pumps.
Castings were found to be quite imperfect, containing numerous sand inclusions,
cold shuts, and extensive porosity. The decision was made to manufacture
them by careful welding and machining with stress-relief anneals to ensure
dimensional control. A complete impeller is shown in Fig. 19 (Y-10794).

To assist i£ the alignment of the serpentine coils in the reactor core,
the weld Jjoint design initially specified did not incorpcrate the desirable
full-penetration welded joint. Since scheduling prevented changes, the most
skillful welding operator available was selected to perform the welding; the
operator was qualified on sample welds before he was assigned the fabrication
of the serpentine coil. The basic design used was the socket-type Jjoint
shown in microsection (Fig. 20, Y-8456) after welding. Numerous sample welds
were made, and it was found that with adequate gas shielding and with the
elimination of detrimental weld-metal dilution effects by use of INCO No. 62
filler metal, flawless welds could be obtained.

Because of the thickness of the Aircraft Reactor pressure shell, the
use of the inert-gas-shielded tungsten arc process was considered to be
impractical. It was recognized that the metal arc welding process, with the
use of INCO No. 132 coated electrodes, was best suited for the enormous
amount of welding required. It was agreed that inert arc welding would be
used for the root passes where the corrosive media would come into direct
contact with the weld and that metal arc welding would be used to complete

the welds. Porosity and slag inclusions were kept to a minimum by baking of

electrodes and meticulous interpass cleaning. In view of the thickness of

 
35

 

Fig. 17 (Y-12126) Crater Resulting from Accidental Arc Strike in

Heat Exchanger Tube.

 
36

 

| _k i

  

__... - = ..n.v..fn_.r. 3

% )

   
  

5

i ,___ ffir;fiéfé

 

Metal Arc Welds Used to Attach ILugs on the Fluid

(Y-12127)

Fig. 18

Piping.
37

 

Fig. 19 (Y-10794) Completed Impeller after Welding, Stress Relieving,

and Final Machining.

 
38

 

 

Fig. 20 (Y-8456) Socket Type Joint Selected for Serpentine Coils in

Reactor Core.
..39_

the shell, the inferiority of metal arc welding to inert arc welding was
thereby recognized and accepted.

The success of the careful welding and inspection procedures for the
construction of the Aircraft Reactor Experiment was proved by the absence
cf leaks or weld failures during the reactor's operation. The only weld
failure was experienced during the preliminary operation of the Aircraft
Reactor Experiment sodium circuit after the sodium had been in the system for
only 37 hr and the maximum temperature of the sodium had not exceeded 1150°F.

A section of austenitic stainless steel pipe leaked at a tube bend to
which a thermocouple. had been attached, shown in Fig. 21 (Y-13298). The inside
of the tube with the cracked weld is shown in Fig. 22 (Y-13310). The piping
in this system V@fied from 0.076 to 0.135 in., and consequently the thermocouple
wz2lding procedure and inert gas shielding required on the inside of the system
should have been modified for the various wall thicknesses. It had not been
considered necessary to exercise accurate and supervised control, primarily
because the system was not the fuel circuit and was not considered critical.

The cause of the excessive penetration and subseqguent cracking was attributed
to the welder's lack of knowledge of the variations in pipe wall thickness
in this section.

The experience gained here has influenced thinking in other reactor
construction at ORNL. It was recognized that attention to details was important,
that all welds must be of the highest quality possible, and that only the most
highly skilled and properly trained welders should be used. These requirements,
in conjunction with carefully prepared specifications and extensive use of the
most modern inspection methods, constitute the minimum necessity for ensuring

satisfactory construction of reactor plumbing circuits.

FABRICATION OF CONTROL ROD COMPONENTS

 

Safety Rod Components

 

The design selected for the safety rods and regulating rods for the
Aircraft Reactor ExXperiment called for the use of BMC as the absorber material.
Hcllow cylinders of pure th were desired for the safety rod, but purchase

ingquiries to industry indicated that the cost of such cylinders would be

 
40

 

Fig. 21 (Y-13298) Site of Improperly Attached Thermocouple and
Resulting Sodium Ieak.

 

 
4]

 

Fig., 22 (Y-13310) Underside of Improperly Attached Thermocouple Weld

Showing Crack.
- Lo _

prohibitive. Therefore a technique was developed for fabricating cylinders
of BMC by hot pressing with a suitable binder.

A series of compacts were prepared which had the equivalent of 20 vol %
of metallic binder; copper, nickel, and iron were used as the binder phase,
and the compacts were hot-pressed at various temperatures. The compositions
were prepared by ball-milling the mixtures of =325 mesh powders together for
8 hr, and the compacts were fabricated by hot pressing in induction-heated
graphite dies. Properties of typical compacts are presented in Table II. 1In

no case did the use of copper or nickel as the binder produce a compact with the

Table II. Properties of Hot-Pressed BAC Compacts With 20 vol % of Metallic Binders

 

 

Pressing
Binder Temperature Theoretical
Metal (°c) Density (%) Comments
Ni 1200 T2 Weak, scft
Cu 1080 76 Crumbly, loss of Cu
Fe 1050 69 Soft
1170 Th Sof't
1300 71 Soft
1525 78.5 Strong, hard structure
1730 69 Weak, excessive loss of Fe

 

desired properties. Nickel reacted with the BMC to form a liquid phase which
did not wet the BMC and did not promote densification. Compacts prepared with
copper had reasonably high density but were very weakly cemented together and
were too crumbly to be useful.

The series of compacts prepared with iron as the binder were also soft and
weak when pressed at moderate temperatures. It finally bécame necessary for
the pressing temperature to be increased sufficlently to completely convert the
iron to the high-melting phase FeB, which seemed to promote densification and
also to act as a cementing agent. Compacts with suitable properties were prepared
by hot pressing for 5 min at 1530°C at a pressure of 2500 psi. Only minor loss
of iron occurred during this cycle, and the small amount lost was probably
caused by eutectic melting. Typical weight loss ranged from 4 to 6%.

The hollow cylinders produced for the safety rod slugs are illustrated in

Fig. 23 (Y-7090), along with the stainless steel can components and a finished
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oLl -

slug. These cans were assembled by tack welding, followed by furnace brazing
with Nicrobraze. In order to prevent the slug from cracking during brazing,
because of differential expansion of the inner stainless steel tube, it
was necessary to leave a gap of about 0.010 in. on the inside diameter.
The diameter of the finished slugs was readily controlied, even when the
graphite dies were used as many as 12 times. The only finishing operation
required was that of grinding the end faces slightly to conform to the length
tolerances.

In fabricating the slugs, it was found that the core rod had to be
pushed out of the finished piece at the pressing temperature in order to prevent
the slugs from cracking during cooling because of shrinkage around the graphite
core rod. This was accomplished by using an undersized mandrel, which was

left in place during cooling.

Regulating Rod Components

 

The components for the regulating rod were desired to be in the form of
cylinders similar to the safety rod slugs, and to be composed of a dilute
suspension of th in an inert, refractory carrier. One set of 15 cylinders
was required with a B)C concentration of 0.022 to 0.024k g/cc, and a second
set with a B)C concentration of 0.005 g/cec.

One undersized slug composed of 10% th in refractory-grade A1203 was
fabricated to confirm the compatibility of the two materials. The required
number of slugs was then fabricated by a technique similar to that used in
producing the safety rod slugs, and the two rods were assembled and brazed as
illustrated in Fig. 24 (Y-7091). However, these regulating rods were not
used because of changes neéessary in the control rod thimbles to provide
increased reactivity; instead, a type 316 stainless steel regulating rod was

used.

Control Rod Thimbles

 

In order to reduce the structural poisons in the reactor, it was decided
to replace the three concentric Inconel sleeves around the four control rods
(one regulating and three safety) with a single stainless steel, Inconel-
clad sleeve in each hcle. Stainless steel was preferred because of its
lower cross section, approximately 3.3 barns compared with 4.6 barns for

Inconel. Unclad stainless steel could not be used because dissimilar-metal

 
§ ASSEMBLY FOR BORON CAR

OXIDE SLUGS

 

Fig. 24 (Y-TO9L)
- 46 -

transfer would occur at these temperatures; so 2.95-in.-0D duplex tubes with

a 0.082-in. wall of 0.072-in., type 316 stainless steel clad with 0.010-in.
Inconel were fabricated by the International Nickel Company at Huntington,
West Virginia. In operation the Inconel was in contact with the sodium

bath, and the stainless steel was in contact with the helium cooling system
for the control rods. The space between the outside diameter of this duplex
thimble and the BeO block was filled with BeO pellets. These changes resulted

in a 4b-to 5-1b decrease in the critical mass.

Fabrication of the Beryllium Oxide Blocks

The search for a material to be used as a moderator in the Aircraft
Reactor Experiment led to the choice of BeO, which has good corrosion resistance
to sodium, has good high-temperature stability, and is readily fabricated into
the desired shapes.

Early in the experimental work on determining the compatibility of BeO
to sodium, samples of BeO blocks from the Daniels power pile were used. These
blocks were made from multiply crystallized BeSOh and therefore was of extremely
high purity. The BeO content of the blocks was greater than 99.5% and they
had been hot-pressed to a density of 2.86 to 2.9% g/cc with no porous areas
detectable. In all testing of samples from this material the BeO was found
to have extremely good sodium corrosion resistance.

The BeO blocks for the Aircraft Reactor Experiment were fsbricated by
the Norton Company, Chippawa, Ontario, from BeO powder furnished them by
the Brush Beryllium Company. The blocks were hot-pressed to closely controlled
sizes and to a specified minimum density of 2.7 g/cc. Examination of sections
of the blocks after their arrival at the Oak Ridge National Laboratory, indicated
that they were quite porous and that they would absorb sodium. This was at
first thought to be due to cracks which had generated during the cutting of the
blocks; however, when several of the uncut blocks were soaked in fluorescent
dye and then examined under an ultraviolet light, a different physical structure
was found in the core of the block. The results of these experiments can be
seen in Fig. 25 (Y-79l3). The dark shaded areas outlined the porous core
which was penetrated by the fluorescent dye. Representative samples were cut
from two of these blocks for density and apparent porosity measurements. This

information is presented in Table III. In examining these data, it is evident

 
Fig. 25

47

 

(Y-7913)
by Fluores

Section of BeO Block Showing Porous Core Penetrated

cent Dye.
- 48 -

TABLE III. AVERAGE DENSITY AND APPARENT POROSITY MEASUREMENTS

 

 

1/2-in.-ID Block 1-1/8-in.-ID Block
Core ’
Average density, g/cc 2,34 2.61
Apparent porosity, % 20.6 12.1 |
Periphery
Average density, g/cc 2.82 2.80
Average porosity, % 0.0 2,0

 

that the core of the blocks is guite porous in comparison to the periphery.
This porosity defect is undcocubtedly due to pocr fabrication techniques, which
were brought about by the speed by which the blocks were procured and by the
effort to keep the cost of the blocks t0 a minimum. The BeQO powder for the
blocks was made by a single crystallization of the BeSO“ rather than by the
multiple crystallization necessary for higher purity. Consequently, the
quality of the blocks was quite objectionable from a chemistry standpoint and
the BeO content was only 97.6%, the balance being A1203, 510,, Na,0, and .
Fe203a The hot-pressing furnaces at the Norton Company were not large enough
to use the proper size of dies and core pins. In order to avoid the delay
that would have resulted from changing the furnaces, underdesigned dies and
pins were used. This necessitated use of a pressing pressure lower than that
desirable to obtain good hot-pressed density.

Samples from the blocks were examined for compatibility with sodium in
various static and dynamic corrosion tests. It was found that in the static
and low velocity dymamic tests the specimens showed a small weight loss;
however, when the specimens were tested at velocities of 400 fpm, they showed
a weight loss of approximately T0%. After all the tests of BeO and sodium
were carefully examined, it was concluded that the corrosion mechanism was
primarily the mechanical erosion of the BeO surface. In the Aircraft Reactor
Experiment, the sodium in contact with BeO was virtually stagnant, and the
outside surfaces of the block had a high-density skin. From these considerations
it was concluded that there would be no difficulty in the BeO-Na, moderator .
circuit. The examination cf the BeQO blccks and cf the sodium circuit after

the successful operation of the Aircraft Reactor has shown this conclusion

to be correct.

 
-~ 49 -

Conclusions

The choice of Inccnel as the commercially available structural material
for the Aircraft Reactor Experiment was substantiated by its fulfilling
the eleven reguirements for a structural material. From a corrosion standpoint
Iniconel exhibited a small amount of attack during the operating lifetime of the
Aircraft Reactor. The corrosion in the sodium circuit was of very minor
importance. Inconel did show mass transfer to small extent in both media.
Fortunately, in the fused fluoride circuit the mass transfer did not appear
as dendritic crystal growth but, rather, occurred by diffusion of the mass-
transferred material into the loop wall. At the temperatures reached in the
scdium circuit, mass transfer was not a problem. The oxidation resistance
of Tnconel is sufficiently good to present no problem under our experimental
ccnditions. The creep or stress-rupture properties of Inconel were adversely
affected by the various environmments when compared with the creep properties
of material tested in air; hovever, this was recognized early in the reactor
studies, and data were obtained in these environments for limiting, by
design, the stress values for the successful operation of the reactor. The
material had extremely good ductility at all temperatures. The lack of
difficulty with the multitude of welds in the circuit proved that use of a
proper weld joint design and welding procedures will enable leak-tight, high-
temperature plumbing circuits to be fabricated. Since Inconel had been an
article of commerce for years, considerable fabrication experience was available
to assist in obtaining the necessary items for the construction of the reactor.
Also, considerable operating experience was available to show that the alloy
would be stable under the stringent conditions of the reactor operation. The
nuclear properties of Inconel leave something to be desired since it does
have a cross section of 4.6 barns, and it has a residual cobalt content because
of the incomplete separation of nickel and cobalt during the refining processes
of the nickel.

One of the most important lessons learned in the construction of this
reactor was that cooperation must exist between the design engineer, the
construction and procurement people, the supplier, and the materials people.
Without this cooperation, the manufacturing processes would not have been
given the very careful control called for by the various welding procedures and

inspection procedures. Since the mechanical properties of the material were
- 50 -

affected by the environment, there had to be collaboration between the materials

people, the stress analysts, the design engineers, and the operating engineers

to assure that the reactor was designed to operate within the limits of safety.
For the construction of Aircraft Reactors of the future, there will

undoubtedly be a host of new metals and alloys and new fuel mixtures that will

evolve into a new genus of reactors, and the problems in welding, brazing,

fabrication, corrosion, and inspection will differ from those of the present.

It is hoped that the experience gained in the construction of the Aircraft

Reactor will be of benefit and that there will be a continued cooperative approach

to the design, construction, and operation of the reactor.

ADKNOWLEDGMENTS

 

The authors would like to acknowledge the very fine metallographic work
performed in the support of this research by the Metallographic Groups under
the direction of R. J. Gray and R. S. Crouse. The authors would also like
to thank the Reports Group of the Metallurgy Division and the Information and

Reports Division of the Iaboratory for their very fine cooperation.
- 51 -

REFERENCES

1. A. M. Weinberg, et al., "Molten Fluorides as Power Reactor Fuels,'" Nuclear
Sci. Eng. (in pressy

2. E. 5. Bettis,et al. "The Aircraft Reactor Experiment - Design and
Construction," Nuclear Sci. Eng. (in press).

 

5. W. K. Ergen, et al. "The Aircraft Reactor Experiment - Physics," Nuclear
Sci. Eng. (in ; press)

L. W. B. Cottrell, et al,, "The Aircraft Reactor Experiment - Operation,"
Nuclear Sci. Eng. (in press).

 

5. H. W. Savage, et al., Components of the Fused Salt and Sodium Circuits of
the Aircraft Reactor Experiment, ORNL-2348, SECRET (in press).

 

6. W. R. Grimes, et al., Aspects of Alrcraft Reactors, ORNL-2368, SECRET
(in press).

 

 

7. D. C. Vreeland, E. E. Hoffman, and W, D. Manly, "Corrosion Tests for Liquid
Metals, Fused Salts at High Temperatures,"” Nucleonics 11 (11), 36—39

(1953). —

8. A. de S. Brasunas, "Subsurface Porosity Developed in Sound Metals During
High-Temperature Corrosion,"” Metal Progr. 62 (6), 88 (1952).

9. R. W. Balluff and B. H. Alexander, Development of Porosity by Unequal
Diffusion in Substitutional Solutions, SEP-83 (Feb. 1952).

 

10. W. D. Manly, "Fundamentals of Liquid Metal Corrosion," Corrosion 12,
Le—52 (July 1956). —_— =

11. D. A. Douglas and J. R. Weir, The Effect of the Fused Salts on the Stress-
Rupture Properties of Inconel, CF-57-2-146, SECRET (Feb. 1957)-

 

12. P. Patriarca and T. R. Housley, Procedure Specification P. S. - 1 for D. C.
Inert Arc Welding of Inconel Pipe, Plate, and Fittings (ORNL unpublished
data).

 

 

13. P. Patriarca and T. R. Housley, Operator's Qualification Test Specification
QIS - 1 for Inert Arc Welding of Inconel Pipe, Plate, and Fittings (ORNL
unpublished data),

14. P. Patriarca and T. R. Housley, Joint Design for Inert Arc Welded Vessels
(ORNL unpublished data).