ORNL-TM-1023.txt

 

OAK RIDGE NATIONAL LABORATORY
operated by

UNION CARBIDE CORPORATION
for the

U.S. ATOMIC ENERGY COMMISSION
ORNL- TM- 1023

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TUBE PLUGGING IN THE MOLTEN-SALT REACTOR EXPERIMENT

PRIMARY HEAT EXCHANGER

i 58 e A A A s, e, oy

R. G. Donnelly

NOTICE

This document contains information of a preliminary nature and was prepared
primarily for internal use at the Qak Ridge National Laboratory. It is subject
to revision or correction and therefore does not represent a final report. The
information is not to be abstracted, reprinted or otherwise given public dis-
semination without the approval of the ORNL patent branch, Legal and Infor-
mation Control Department.

 
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_LEGAL NOTICE

This report was prepared as an account of Gavarament sponsored work. Neither the United States, .

nor the Commission, nor any person acting on behalf of the Commission:

A. Makes any warranty or repfesénfufion, expressed or implied, with tespect to the accuracy,
completeness, or usefulness of the information centained in this report, or ihar,_r_he use of .

ony information, apparatus, methed,. or.process disclosed in this report mey not infringe
- privotely owned rights; or - S . o
B. Assumes any lisbilities with respset to the yse of, or for damages resulting from the wse of

any information, apparatus, method, or process disclesed In this report.

As used in the above, '‘person acting pb behalf of the Commission’ includes any employse or

contractor of the Commission, or employee of such contractor, to the extent that such employee
or contractor of the Commission, or smployee of such contractor prepares, disseminates, or

provides access to, any information pursuant to his smployment or contract with the Commission, - |-

or his employment with such cenfracter.

 

 

 

 

 

 

 

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ORNL-TM-1023

Contract No. W-7405-eng-26

METALS AND CERAMICS DIVISION

TUBE PLUGGING IN THE MOLTEN-SALT REACTOR EXPERIMENT
PRIMARY HEAT EXCHANGER

R. G. Donnelly

~ FEBRUARY 1965

OAK RIDGE NATIONAL LABORATORY
- Qak Ridge, Tennessee
~~ operated by
UNION CARBIDE CORPORATION
o for the
U. S. ATOMIC ENERGY COMMISSION

 
 

 

 

 

 

 

 

 

 

 

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TUBE PLUGGING IN THE MOLTEN-SALT REACTOR EXPERIMENT
- PRIMARY HEAT EXCHANGER

R. G. Donnelly

ABSTRACT

In order to reduce the pressure drop through the shell
side of the Molten-Salt Reactor Experiment primary heat
exchanger, it was decided to remove four of the outer U-tubes.
This required sealing the eight tube stubs produced.

A plug design and seal welding procedure were developed
to assure a high-integrity seal between the molten fuel salt
on the shell side and the coolant salt on the tube side of
the heat exchanger. The plugs were made to have a slight
interference fit (0.0000 to 0.0002 in.) with the tubes and
were machined for edge~welding. The plug material was
INOR-8 as was the entire heat exchanger.

The procedure for making the seals was to manually weld
the tube end to the plug with a gas tungsten-arc torch. The
welding conditions were adjusted to provide weld metal pene-
tration equivalent to at least the thickness of the tube wall.

Visual, dye-penetrant, and radiographic examinations of
the welds gave every indication that high-integrity welds
had been made that would successfully isolate the fuel salt
from the coolant salt during the planned operation of the
heat exchanger.

 

INTRODUCTION

During preinstallation flow testing of the primary heat exchanger
for the Molten-Salt Reactor Experiment (MSRE), the pressure drop through
the - shell side was found to be significantly greater than was desired

and required modlflcation - The heat exchanger is of the conventional

. U-tube design (See Fig. 1), with tubes being 0.50-in. OD X 0.042-in.

wall. A1l 326 tube ends are joined to a 17-in. diam X 1.50-in. thick
tube sheet by welding and back brazing.
The containment material is the commercially available alloy, INOR-8

 

IR. G. Don.nelly and'G."M."maughter, "Fabrication 6f'the Molten-Salt

“ReeetOr Experiment Heat Exchanger Core," Weldi J., 43(2), 118-124,

(1964) .

 
 

 

 

 

UNCLASSIFIED
ORNL-LR-OWG 32038R2

FUEL INLET

27

 
    
   
  
   
 
 
    

THERMAL-BARRIER PLATE CROSS BAFFLES

TUBE SHEET
COOLANT INLET

16.4-in. 0D 2 O.2-in. WALL x 8-t LONG

COOLANT-STREAM
SEPARATING BAFFLE

FUEL OUTLET

Fig. 1. Molten-Salt Reactor Experiment Primary Heat Exchanger.

(Ni—17 Mo—7 Cr—4 Fe, wt $).? This alloy is a nonage-hardenable high-

- strength material which possesses excellent corrosion resistance to the

molten salts and good oxidation resistance. It also exhibits good general
weldability.? However, it has shown & tendency toward microfissuring under
certain welding conditions.? Th\is , the welding procedure and joint design
were more critical than usual. |

 

?y. D. Manly, et al. pp. 164~79 in Progress in Nuclear Energy, Series
IV, Vol. 2 — Technology, Engineering and Safety, Pergamon Press, London,

3G. M. Slaughter, P. Patriarca, and R. E. Clausing, Welding J. 2_2(10),
39354008 (1959).

4MSRP Semiann. Progr. Rept. July 31, 1964, ORNL-3708, p. 363.

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The method approved for reducing the pressure drop was to remove the
four outer U-tubes which were situated directly at the mouth of ‘the shell-
side Inlet and outlet nozzles. After removal of the shell, the four tubes
were cut, leaving eight stubs to be plugged. This was accomplished by
pressing in INOR-8 plugs and seal welding.

The development of procedures, qualification of the welder, and the
actual tube plugging operation are described.

 DEVELOPMENTAIL WORK

Joint Design

Due to the low-pressure differential (50 psi) between the tube and
shell sides of the heat exchanger, the‘plug was not fequired to have
great strength; however, a lesk-tight joint was essential. Therefore,
with these facts in mingd and taking into acccuhfi the proximity of the
adjacent U-tubes, an edge~type weld preparation was made on the plugs
as shown in Fig. 2. '

~ UNCLASSIFIED
ORNL-DWG 64-11563

WELD

  
 

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TUBE WALL

 

Fig. 2. Schematic of Tube Plugging Design.

 
 

 

 

Each plug was positioned flush with the tube end. It was felt that a
qualified welder with a small gas tungsten-arc torch could more easily
make & high-~quality weld on this type of joint than on any other. Sincé
the edgeAfype weld design also readily allows the tube end to be Joined
to a member of similar thickness, adequate Joinfi penetration is more

easily assured.

Both the square-edge design and beveled-edge design were investigated..

The thickness of the lip on the plug was also varied; the amounts being
1, 1 1/4, and 1 1/2 times the tube-wall thickness. In addition, joints
were welded by single-pass fusion, single pass with filler, and by a |
combination of fusion and filler passes. In the end, no combination
proved better than the square-edge design with a lip thickness equal to
the tube-wall thickness welded in one fusion pass. However, a lip thick-
ness of 1 1/4T was ultimately chosen to assure a minimum of 1T weld
penetration in any section of the weld.

The plugs were tapered and machined for each individual tube to pro-
vide a slight interference fit (Fig. 3). An interference of 0.0005-in.
was found to cause difficulty in fitting in some cases, so a value of
ig:gggg-in. was chosen. With such a tight fit none of the 30 samples
examined metallographically exhibited any indication of root cracking.

Welding Procedures

With the joint and plug designs chosen, the welding parameters of
arc current, travel speed, and electrode orientation were varied in an
effort to determine the optimum combination. In addition, a copper chill
was fitted to the tubes at a distance of about 0.070 in. from the top of
the joint. It was felt that this would tend to even out the thermal
gradients between the tube and plug.

Welding current was varied from 30 to 45 amps, and time for one
revolution was varied from 40 to 77 sec. An overlap of 3/16 to 1/4-1in..
was made at full émperage, and a foot-operated current tapering device
was used at the beginning and end of each weld to eliminate weld craters.
The conditions giving the best penetration.and wéld configuration wére:

'.Electrode material: W + 2% thoria
Electrode diameter: 1/16-1in.

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UNCLASSIFIED
ORNL-DWG 64-11562

 

 

 

 

 

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NOTE f: MAXIMUM TUBE L.D. *8:0002
MATERIAL: INOR-8 ROD

Fig. 3. Tube Plug for MSRE Heat Exchanger.

 
 

 

 

 

 

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Inert gas: argon (99.995% purity)
Gas flow rate: 17 cfh
Welding current: (34 tl)amp
Welding speed: 40 to 55 sec/joint

The position of the electrode with respect to the Jjoint was found
to have an effect on the configuration and, therefore, the effective
penetration of the weld. The best results were obtalned by holding the
electrode tip on the ocutside edge of the tube. This required that the
welder touch the adjacént tubes with the electrically insulated parts of
the torch. A photomicrograph of a typical Joint welded under these con-
ditions is presented in Fig. 4.

e UNCLASSIFIED
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Fig. 4. Photomicrograph of a Qualification Test Sample. 40X,

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WELDER QUALIFICATION

The welder was qualifiedzon Joints in a setup which simulated the
configuration of the actual heat exchangér. The mockup is shown in
Fig. 5. The welder had been previously quaiified for gas tungstén-arc
welding of INOR-8 in accordance with ORNL Operator's Qualification Test
QTS-25.

. The qualification welds were required to have the following: (1) uni-
form weld contour with no visual evidence of cracking or porosity, (2) no
positive dye-penetrant indications, (3) weld penetration of at least one
times the tube-wall thickness as evidenced by metallographic examination,

‘and (4) no porosity of a size greater than 10% of the tube-wall thickness

when radiographed in accordance with paragraph UW-51, Section 8, ASME
Boller and Pressure Vessel Code. All requirements were met, and no

porosity of any size was revealed.

 

UNCLASSIFIED
PHOTO 65913

 

       

Flg. 5. Mockup of Heat Exchanger Used 1in Welder's'Qualificafiion Test.

 
 

 

Prior to msking the welds on the heat exchanger, the welder was also
required to make one or more sample welds for visual examination as re-

quired by the welding inspector.

THE TUBE PLUGGING OPERATION

The first item in the procedure was to remove the four outer-U-tubes.
Heavy metel bars were clamped to the four tubes to act as guides for the
pneumatically operated abrasive wheel that was used for the-cutting
operation. Metal plates were also used between the tubes being cut and
adjacent tubes to prevent any damage by the cutting wheel. The cuts
were made 1 1/8 in. from the barrier plate. This allowed sufficient room
for the chill block and also for repreparation of a tube end should this
be necessary .

In addition to the four U-tubes, four adjacent baffle support rods
were also removed to provide access for the welding torch. These rods
were not replaced.

The resultant eight tube stubs were squared off, deburred,'énd
cleaned of all oxides for at least 1 in. from their ends. The inside
diameter of each tube end was then measured, and the maximum reading for
each recorded. The results are listed in Table 1 along with the plug
diameters for their respective tubes. It will be noted that the plug
for tube "E" was 0.0002 in. greater than the ig:gggg-in. range desired.
Nevertheless, the plug was fitted without difficulty.

Immediately before fitting a plug, both the tube and the plug were
degreased with acetone. The plug was then set into the tube stub with a
cleaned punch. A shoulder on the punch stopped the plug flush with the
tube end. After fitting the chill block, the welding sequence discussed
previously was followed and the Jjoint visually inspected. The process
was repeated for each tube. |

Visual examination revealed two welds that had areas oflquestionable
penetration. After demonstrating repair operations on sample'wélds, the
welder went over the two areas at full amperage to assure sufficient

penetration.

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Table 1. Tube and Plug Measurements

 

 

Tube Designation. Tube Inside Diameter Plug Outside
; ' - Maximum - Diameter
(in.) (in.)
A 0.4151 0.4153
B 0.4157 0.4157
c 0.4176 0.4176
D 0.4133 0.4134
E 0.4142 0.4146>
F 0.,4162 0.4162
G 0.4156 1 0.4156
H 0.4134 0.4135

 

0. 0002 in, oversiae."

All welds werezthen'inspected.by dye-penetrant and3radiographic
methods. No,imperfectionsuyere'revealed. A photograph of four of the
sealed tubes 1is presented in Fig. 6.

~ CONCLUSIONS'

As a result of this work it is felt that the tube plugging pro-
cedures and Joint design developed are consistent with the operating
_requirements of the heat exchanger. ‘

The use of these procedures, coupled with a specially qualified

N welder, has resulted in. sound ‘welded joints which give every indication

of successfully sealing the molten fuel salt from the coolant salt during

" the planned qperation of the heat exchanger.-

di ACKNOWLEDGMENT | . ,
The author would like to thank the following people ‘who contributed

| to this work and whose personal interest in high-quality workmanship are

appreciated" R. G. Shooster of the Welding and Brazing Group, Metals and
Ceramics Division, T. R. Housley and J. W. Hunley of the Plant and

j“Equipment Division,rwelding;lnspection Group; and W. H, Brown, welder_with'

the'Plant and Eqpipment”DiviSion, Research Services Group. The metallo-
graphic assistance of E. R. Boyd of the Metals and Ceramics Division'
‘Metallography Group 1s also acknowledged
 

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Four of the Sealed Tubes Inverted from Welding Position

Fig. 6.
(see inset).

 
 

 

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19,
20,
21.
22.

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35.
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55~-56.
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11
ORNL-TM-1023
% INTERNAL DISTRIBUTION

Central Research Library 37. W. R. Gall
Reactor Division Library ‘ 38. R. G. Gilliland
ORNL — Y~12 Technical Library 39+41. M. R. Hill
‘Document Reference Section | 42, T. R. Housley
Laboratory Records Department 43. H. G. MacPherson
Laboratory Records, ORNL RC 44, = W. B, McDonald
ORNL Patent Office 45, W. R. Martin

G. M. Adamson ’ 46, R. W. McClung

E. S. Bettis : 47. P, Patriarca

G. E. Boyd | 48.  G. M. Slaughter
R. B. Briggs 49, A. Taboada

K. V. Cook - - 50, - J. R. Tallackson
J. E. Cunningham 51. W. C. Thurber
R. G. Donnelly 52. A. M. Weinberg

C. W. Fox S 53, J. H. Westsik
J. H Frye, dJr. :

EXTERNAL DISTRIBUTION

C. M. Adams, Jr., Massachusetts Institute of Technology
D. F. Cope, AEC, Osk Ridge Operations Office

J. L. Gregg, Bard Hall, Cornell University

J. Simmons, AEC, Washing;ton

E. E. Stansbury, University of Tennessee

Research and Development Division, AEC,ORO

Division of Technical Information Extension