OAK RIDGE NATIONAL LABORATORY operated by UNION CARBIDE CORPORATION for the U.S. ATOMIC ENERGY COMMISSION ORNL- TM- 1023 &l oo - Pl ( ;—-“1% s f-h“" [ e trapre 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. A e L g g i S P I i v ! _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. | et bt o bt bt bt e b 3 B vty {3 v 3 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 ot g 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. {,‘.‘. R N b = ——y eyl - 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 ? 7 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. o — P —] = P iy UNCLASSIFIED ORNL-DWG 64-11562 I | «———NOTE { ——= I 0.310 £ 0.004 o a0 o S { ¥ . T oW : 0. . =~ o : = NN et — — 2 * DIMENSIONS -ARE IN INCHES et ARIAs e v o +0.0002 NOTE f: MAXIMUM TUBE L.D. *8:0002 MATERIAL: INOR-8 ROD Fig. 3. Tube Plug for MSRE Heat Exchanger. # 6 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 T 088 Fig. 4. Photomicrograph of a Qualification Test Sample. 40X, () e b h sy c Qs o ..‘.—"\__.,,,__,‘f-‘. 7 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. G g \J € . ';‘, ot e Y L ‘fi'——h ‘ ) Wy -l .4 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 10 o« we — 3 v o v